Adiabatic processes can keep the quantum system in its instantaneous eigenstate, which is robust to noises and dissipation. However, it is limited by sufficiently slow evolution. Here, we experimentally demonstrate the transitionless quantum driving (TLQD) of the shortcuts to adiabaticity in gate-defined semiconductor quantum dots (QDs) to greatly accelerate the conventional adiabatic passage for the first time. For a given efficiency of quantum state transfer, the acceleration can be more than twofold. The dynamic properties also prove that the TLQD can guarantee fast and high-fidelity quantum state transfer. In order to compensate for the diabatic errors caused by dephasing noises, the modified TLQD is proposed and demonstrated in experiment by enlarging the width of the counterdiabatic drivings. The benchmarking shows that the state transfer fidelity of 97.8% can be achieved. This work will greatly promote researches and applications about quantum simulations and adiabatic quantum computation based on the gate-defined QDs. © 2024 American Physical Society.

We report the nanofabrication and characterization of optical spot-size converter couplers based on curved GaAs cantilever waveguides. Using the stress mismatch between the GaAs substrate and deposited Cr-Ni-Au strips, single-mode waveguides can be bent out-of-plane in a controllable manner. A stable and vertical orientation of the out-coupler is achieved by locking the spot-size converter at a fixed 90° angle via short-range forces. The optical transmission is characterized as a function of temperature and polarization, resulting in a broad-band chip-to-fiber coupling extending over 150 nm wavelength bandwidth at cryogenic temperatures, with the lower bound for the coupling efficiency into the TE mode being 16± 2% in the interval 900-1050 nm. The methods reported here are fully compatible with quantum photonic integrated circuit technology with quantum dot emitters, and open opportunities to design novel photonic devices with enhanced functionality. © 2024 Optica Publishing Group.

A key challenge in quantum photonics today is the efficient and on-demand generation of high-quality single photons and entangled photon pairs. In this regard, one of the most promising types of emitters are semiconductor quantum dots, fluorescent nanostructures also described as artificial atoms. The main technological challenge in upscaling to an industrial level is the typically random spatial and spectral distribution in their growth. Furthermore, depending on the intended application, different requirements are imposed on a quantum dot, which are reflected in its spectral properties. Given that an in-depth suitability analysis is lengthy and costly, it is common practice to pre-select promising candidate quantum dots using their emission spectrum. Currently, this is done by hand. Therefore, to automate and expedite this process, in this paper, we propose a data-driven machine-learning-based method of evaluating the applicability of a semiconductor quantum dot as single photon source. For this, first, a minimally redundant, but maximally relevant feature representation for quantum dot emission spectra is derived by combining conventional spectral analysis with an autoencoding convolutional neural network. The obtained feature vector is subsequently used as input to a neural network regression model, which is specifically designed to not only return a rating score, gauging the technical suitability of a quantum dot, but also a measure of confidence for its evaluation. For training and testing, a large dataset of self-assembled InAs/GaAs semiconductor quantum dot emission spectra is used, partially labelled by a team of experts in the field. Overall, highly convincing results are achieved, as quantum dots are reliably evaluated correctly. Note, that the presented methodology can account for different spectral requirements and is applicable regardless of the underlying photonic structure, fabrication method and material composition. We therefore consider it the first step towards a fully integrated evaluation framework for quantum dots, proving the use of machine learning beneficial in the advancement of future quantum technologies. © The Author(s) 2024.

Surface acoustic waves (SAWs) are a reliable solution to the transport of single electrons with precision in piezoelectric semiconductor devices. Recently, highly efficient single-electron transport with a strongly compressed single-cycle acoustic pulse has been demonstrated. This approach, however, requires surface gates constituting the quantum dots, their wiring, and multiple gate movements to load and unload the electrons, which is very time-consuming. Here, on the contrary, we employ such a single-cycle acoustic pulse in a much simpler way - without any quantum dot at the entrance or exit of a transport channel - to perform single-electron transport between distant electron reservoirs. We observe the transport of a solitary electron in a single-cycle acoustic pulse via the appearance of the quantized acoustoelectric current. The simplicity of our approach allows for on-demand electron emission with arbitrary delays on a nanosecond time scale. We anticipate that enhanced synthesis of the SAWs will facilitate electron quantum optics experiments with multiple-electron flying qubits. © 2024 American Physical Society.

Quantum-dot-based single-photon sources are key assets for quantum information technology, supplying on-demand scalable quantum resources for computing and communication. However, long-lasting issues such as limited long-term stability and source brightness have traditionally impeded their adoption in real-world applications. Here, we realize a quantum key distribution field trial using true single photons across an 18-km-long dark fibre, located in the Copenhagen metropolitan area, using an optimized, state-of-the-art, quantum-dot single-photon source frequency-converted to the telecom wavelength. A secret key generation rate of > 2 kbits/s realized over a 9.6 dB channel loss is achieved with a polarization-encoded BB84 scheme, showing remarkable stability for more than 24 hours of continuous operation. Our results highlight the maturity of deterministic single-photon source technology while paving the way for advanced single-photon-based communication protocols, including fully device-independent quantum key distribution, towards the goal of a quantum internet. © 2024, The Author(s).

In quantum dots (QDs), the Auger recombination is a non-radiative scattering process in which the optical transition energy of a charged exciton (trion) is transferred to an additional electron leaving the dot. Electron tunneling from a reservoir is the competing process that replenishes the QD with an electron again. Here, we study the dependence of the tunneling and Auger recombination rate on the applied electric field using high-resolution time-resolved resonance fluorescence (RF) measurements. With the given p-i-n diode structure and a tunnel barrier between the electron reservoir and the QD of 45 nm, we measured a tunneling rate into the QD in the order of ms−1. This rate shows a strong decrease by almost an order of magnitude for decreasing electric field, while the Auger emission rate decreases by a factor of five in the same voltage range. Furthermore, we study in detail the influence of the Auger recombination and the tunneling rate from the charge reservoir into the QD on the intensity and linewidth of the trion transition. In addition to the well-known quenching of the trion transition, we observe in our time-resolved RF measurements a strong influence of the tunneling rate on the observed linewidth. The steady-state RF measurement yields a broadened trion transition of about 1.5 GHz for an Auger emission rate of the same order as the electron tunneling rate. In a non-equilibrium measurement, the Auger recombination can be suppressed, and a more than four times smaller linewidth of 340 MHz (1.4 μeV) is measured. © 2024 Author(s).

Triggered, indistinguishable single photons are crucial in various quantum photonic implementations. Here, we realize a novel n+-i-n++ diode structure embedding semiconductor quantum dots: the gated device enables spectral tuning of the transitions and deterministic control of the charged states. Blinking-free single-photon emission and high two-photon indistinguishability are observed. The line width’s temporal evolution is investigated across over 6 orders of magnitude time scales, combining photon-correlation Fourier spectroscopy, high-resolution photoluminescence spectroscopy, and two-photon interference (visibility of VTPI,2ns = (85.8 ± 2.2)% and VTPI,9ns = (78.3 ± 3.0)%). Most of the dots show no spectral broadening beyond ∼9 ns time scales, and the photons’ line width ((420 ± 30) MHz) deviates from the Fourier-transform limit by a factor of 1.68. The combined techniques verify that most dephasing mechanisms occur at time scales ≤2 ns, despite their modest impact. The presence of n-doping implies higher carrier mobility, enhancing the device’s appeal for high-speed tunable, high-performance quantum light sources. © 2023 The Authors. Published by American Chemical Society.

The GaAs/(Al,Ga)As heterostructure with short-period superlattice (SPSL) doping possesses ultrahigh mobility of its two-dimensional electron gas by placing donors within the remote GaAs layers. Here, we investigate its magnetotransport property under a heavily doped situation. After long enough illumination at cryogenic temperature, the change of the electron concentration inside the quantum well (QW) is only 5.9%. Meanwhile, the quantum lifetime τq,QW of the electron shows an anomalous behavior. It increases slightly and then exhibits an exponential decay until saturation. This is different from the monotonic increase of τq,QW under illumination for the conventional doping situation. The increase of τq,QW originates from the larger donor filling-fraction-enhanced screening effect. Meanwhile, the decrease of τq,QW may be caused by stronger scattering of ionized d+ states evolved from DX centers. The transfer of excess electrons between the AlAs layers can also cause the decrease of τq,QW. This work provides an insight into the mechanism of DX centers on the quantum transport properties of SPSL-doped heterostructures. © 2023 American Physical Society.

The Zeeman-split spin states of a single quantum dot can be used together with its optical trion transitions to form a spin-photon interface between a stationary (the spin) and a flying (the photon) quantum bit. In addition to long coherence times of the spin state itself, the limiting decoherence mechanisms of the trion states are of central importance. Here, we investigate in time-resolved resonance fluorescence the electron spin and trion dynamics in a single self-assembled quantum dot in an applied magnetic field of up to B = 10 T. The quantum dot is only weakly coupled to an electron reservoir with tunneling rates of about 1 ms − 1 . Using this sample structure, we can measure, in addition to the spin-flip rate of the electron and the spin-flip Raman rate of the trion transition, the Auger recombination process that scatters an Auger electron into the conduction band. The Auger effect destroys the radiative trion transition and leaves the quantum dot empty until an electron tunnels from the reservoir into the dot. The combination of an Auger recombination event with subsequent electron tunneling from the reservoir can flip the electron spin and thus constitutes another mechanism that limits the spin lifetime. © 2023 Author(s).

A major challenge in generating single photons with a single emitter is to excite the emitter while avoiding laser leakage into the collection path. Ideally, any scheme to suppress this leakage should not result in a loss in the efficiency of the single-photon source. Here, we investigate a scheme in which a single emitter, a semiconductor quantum dot, is embedded in a microcavity. The scheme exploits the splitting of the cavity mode into two orthogonally-polarised modes: one mode is used for excitation, the other for collection. By linking the experiment to theory, we show that the best population inversion is achieved with a laser pulse detuned from the quantum emitter. The Rabi oscillations exhibit an unusual dependence on pulse power. Our theory describes them quantitatively, enabling us to determine the absolute population inversion. By comparing the experimental results with our theoretical model, we determine a population inversion of 98 % − 5 % + 1 % for optimal laser detuning. The Rabi oscillations depend on the sign of the laser-pulse detuning, a phenomenon arising from the non-trivial effect of phonons on the exciton dynamics. The exciton-phonon interaction is included in the theory and gives excellent agreement with all the experimental results. © 2023 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

Rapid, high-fidelity single-shot readout of quantum states is a ubiquitous requirement in quantum information technologies. For emitters with a spin-preserving optical transition, spin readout can be achieved by driving the transition with a laser and detecting the emitted photons. The speed and fidelity of this approach is typically limited by low photon collection rates and measurement back-action. Here we use an open microcavity to enhance the optical readout signal from a semiconductor quantum dot spin state, largely overcoming these limitations. We achieve single-shot readout of an electron spin in only 3 nanoseconds with a fidelity of (95.2 ± 0.7)%, and observe quantum jumps using repeated single-shot measurements. Owing to the speed of our readout, errors resulting from measurement-induced back-action have minimal impact. Our work reduces the spin readout-time well below both the achievable spin relaxation and dephasing times in semiconductor quantum dots, opening up new possibilities for their use in quantum technologies. © 2023, The Author(s).

Coherently driven semiconductor quantum dots are one of the most promising platforms for non-classical light sources and quantum logic gates which form the foundation of photonic quantum technologies. However, to date, coherent manipulation of single charge carriers in quantum dots is limited mainly to their lowest orbital states. Ultrafast coherent control of high-orbital states is obstructed by the demand for tunable terahertz pulses. To break this constraint, we demonstrate an all-optical method to control high-orbital states of a hole via a stimulated Auger process. The coherent nature of the Auger process is proved by Rabi oscillation and Ramsey interference. Harnessing this coherence further enables the investigation of the single-hole relaxation mechanism. A hole relaxation time of 161 ps is observed and attributed to the phonon bottleneck effect. Our work opens new possibilities for understanding the fundamental properties of high-orbital states in quantum emitters and for developing new types of orbital-based quantum photonic devices. © 2023, The Author(s), under exclusive licence to Springer Nature Limited.

Quantum dot molecules (QDMs) are one of the few quantum light sources that promise deterministic generation of one- and two-dimensional photonic graph states. The proposed protocols rely on coherent excitation of the tunnel-coupled and spatially indirect exciton states. Here, we demonstrate power-dependent Rabi oscillations of direct excitons, spatially indirect excitons, and excitons with a hybridized electron wave function. An off-resonant detection technique based on phonon-mediated state transfer allows for spectrally filtered detection under resonant excitation. Applying a gate voltage to the QDM device enables a continuous transition between direct and indirect excitons and, thereby, control of the overlap of the electron and hole wave function. This does not only vary the Rabi frequency of the investigated transition by a factor of ≈3, but also allows to optimize graph state generation in terms of optical pulse power and reduction of radiative lifetimes. © 2023 American Physical Society.

Photon emission is the hallmark of light-matter interaction and the foundation of photonic quantum science, enabling advanced sources for quantum communication and computing. Although single-emitter radiation can be tailored by the photonic environment, the introduction of multiple emitters extends this picture. A fundamental challenge, however, is that the radiative dipole-dipole coupling rapidly decays with spatial separation, typically within a fraction of the optical wavelength. We realize distant dipole-dipole radiative coupling with pairs of solid-state optical quantum emitters embedded in a nanophotonic waveguide. We dynamically probe the collective response and identify both super- and subradiant emission as well as means to control the dynamics by proper excitation techniques. Our work constitutes a foundational step toward multiemitter applications for scalable quantum-information processing. © 2023 American Association for the Advancement of Science. All rights reserved.

We propose and demonstrate complete spin state readout of a two-electron system in a double quantum dot probed by an electrometer. The protocol is based on repetitive single-shot measurements using Pauli spin blockade and our ability to tune on fast timescales the detuning and the interdot tunnel coupling between the GHz and sub-Hz regime. A sequence of three distinct manipulations and measurements allows establishing if the spins are in S, T0, T+, or T- state. This work points at a procedure to reduce the overhead for spin readout, an important challenge for scaling up spin-qubit platforms. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Semiconductor quantum dot molecules are considered promising candidates for quantum technological applications due to their wide tunability of optical properties and coverage of different energy scales associated with charge and spin physics. While previous works have studied the tunnel-coupling of the different excitonic charge complexes shared by the two quantum dots by conventional optical spectroscopy, we here report on the first demonstration of a coherently controlled interdot tunnel-coupling focusing on the quantum coherence of the optically active trion transitions. We employ ultrafast four-wave mixing spectroscopy to resonantly generate a quantum coherence in one trion complex, transfer it to and probe it in another trion configuration. With the help of theoretical modeling on different levels of complexity, we give an instructive explanation of the underlying coupling mechanism and dynamical processes. © 2023 The Authors. Published by American Chemical Society.

Electron flying qubits are envisioned as potential information links within a quantum computer, but also promise—like photonic approaches—to serve as self-standing quantum processing units. In contrast to their photonic counterparts, electron-quantum-optics implementations are subject to Coulomb interactions, which provide a direct route to entangle the orbital or spin degree of freedom. However, controlled interaction of flying electrons at the single-particle level has not yet been established experimentally. Here we report antibunching of a pair of single electrons that is synchronously shuttled through a circuit of coupled quantum rails by means of a surface acoustic wave. The in-flight partitioning process exhibits a reciprocal gating effect which allows us to ascribe the observed repulsion predominantly to Coulomb interaction. Our single-shot experiment marks an important milestone on the route to realize a controlled-phase gate for in-flight quantum manipulations. © 2023, The Author(s), under exclusive licence to Springer Nature Limited.

We develop a quantum photonic platform that interconnects a high-quality quantum dot single-photon source and a low-loss photonic integrated circuit made in silicon nitride. The platform is characterized and programmed to demonstrate various multiphoton applications, including bosonic suppression laws and photonic entanglement generation. The results show a promising technological route forward to scale-up photonic quantum hardware. © 2023, Springer Nature Limited.

A spin-photon interface should operate with both coherent photons and a coherent spin to enable cluster-state generation and entanglement distribution. In high-quality devices, self-assembled GaAs quantum dots are near-perfect emitters of on-demand coherent photons. However, the spin rapidly decoheres via the magnetic noise arising from the host nuclei. Here, we address this drawback by implementing an all-optical nuclear-spin cooling scheme on a GaAs quantum dot. The electron-spin coherence time increases 156-fold from T2∗=3.9 ns to 0.608 μs. The cooling scheme depends on a non-collinear term in the hyperfine interaction. The results show that such a term is present even though the strain is low and no external stress is applied. Our work highlights the potential of optically active GaAs quantum dots as fast, highly coherent spin-photon interfaces. © 2023 American Physical Society.

Scalable photonic quantum computing architectures pose stringent requirements on photonic processing devices. The needs for low-loss high-speed reconfigurable circuits and near-deterministic resource state generators are some of the most challenging requirements. Here, we develop an integrated photonic platform based on thin-film lithium niobate and interface it with deterministic solid-state single-photon sources based on quantum dots in nanophotonic waveguides. The generated photons are processed with low-loss circuits programmable at speeds of several gigahertz.We realize a variety of key photonic quantum information processing functionalities with the high-speed circuits, including on-chip quantum interference, photon demultiplexing, and reprogrammability of a four-mode universal photonic circuit. These results show a promising path forward for scalable photonic quantum technologies by merging integrated photonics with solid-state deterministic photon sources in a heterogeneous approach to scaling up. ©2023 The Authors.

Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled to a photonic-crystal waveguide and individually controllable by applying a local electric Stark field. We present resonant transmission and fluorescence spectra in order to probe the coupling of the two emitters to the waveguide. We exploit the single-photon stream from one quantum dot to perform spectroscopy on the second quantum dot positioned 16 μm away in the waveguide. Furthermore, power-dependent resonant transmission measurements reveal signatures of coherent coupling between the emitters. Our work provides a scalable route to realizing multiemitter collective coupling, which has inherently been missing for solid-state deterministic photon emitters. © 2023 American Physical Society.

We demonstrate the resonant excitation of two quantum dots in a photonic integrated circuit for on-chip single-photon generation in multiple spatial modes. The two quantum dots are electrically tuned to the same emission wavelength using a pair of isolated p-i-n junctions and excited by a resonant pump laser via dual-mode waveguides. We demonstrate two-photon quantum interference visibility of (79±2)% under continuous-wave excitation of narrow-linewidth quantum dots. Our work solves an outstanding challenge in quantum photonics by realizing the key enabling functionality of how to scale up deterministic single-photon sources. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

The state of charge (SoC) dependent voltage and entropy of two lithium-ion battery cells with silicon–graphite blended anodes consisting of different material composition, were analysed and compared to a graphite only cell. Voltage and entropy showed a significant hysteresis regarding the charge/discharge history and both are increasing with the proportion of silicon. However, the share of silicon has a marginal impact on the general behaviour of the cell's entropy. This behaviour seems only to be influenced by the graphite. Furthermore, the entropy hysteresis of the anode vanishes, when plotting it against its half cell potential. An interpretation of this effect can be done, when regarding statistical thermodynamics. Thereby, a suggestion can be made that the measured entropy by [Formula presented] on a cell level is not equal to the effective entropy of the electrode, when the cell is operated. Through that effect, the amount of heat produced by the entropy is not in line with the formerly presumed measurements. © 2023 Elsevier Ltd

A single-photon source has been developed using a single quantum dot to mimic a two-level atom. Low noise is achieved by operation at low temperature, the use of very high quality material, and by embedding the quantum dots in a diode structure. A single quantum dot is tuned into resonance with an open microcavity, a highly miniaturised Fabry-Perot cavity. The Purcell factor is approximately ten resulting in a radiative lifetime of just 50 ps. An end-to-end efficiency of above 50% is achieved and the photons exhibit two-photon interference with a visibility of 98%. © 2023 SPIE.

Measurement of thermal properties of thin films is challenging. In particular, thermal characterization is very difficult in semi-transparent samples. Here, we use two photothermal methods to obtain information about the thermal diffusivity as well as thermal conductivity of azoheteroarene functionalized polymer thin layers. The photothermal beam deflection (PBD) method is employed to gather data directly on thermal conductivity and thermal diffusivity, while the thermal lens (TL) method is employed to measure the effective thermal diffusivity. Consequently, the thermal diffusivity of the layers is indirectly estimated from the effective thermal diffusivity using a well-established theoretical relationship. Despite the utilization of distinct methods, our study reveals a remarkable consistency in the highly accurate results obtained from both approaches. This remarkable agreement reaffirms the reliability and mutual compatibility of the employed methods, highlighting their shared ability to provide accurate and congruent outcomes. © 2023 by the authors.

Key elements in photonic quantum technologies and their implementations are triggered sources of single and indistinguishable photons. Among various candidates, semiconductor quantum dots are subject of intense research, and they represent a valid approach for the generation of bright quantum light. For semiconductor-based quantum light sources, the use of diode structures has been shown to be highly useful for improving the emitted photon coherence, stabilizing the source charge environment. Here we describe the implementation of a novel n-i-n diode structure embedding semiconductor quantum dots, combining molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE) growth (Fig. 1(a)). This new design allows for achieving spectral and charge state tuneability, as well as very low photon dephasing [1]. © 2023 IEEE.

The realization of on-chip quantum interfaces between flying photons and solid-state spins is a key building block for quantum-information processors, enabling, e.g., distributed quantum computing, where remote quantum registers are interconnected by flying photons. Self-assembled quantum dots integrated into nanostructures are one of the most promising systems for such an endeavor thanks to their near-unity photon-emitter coupling and fast spontaneous emission rate. Here we demonstrate high-fidelity on-chip entanglement between an incoming photon and a stationary quantum-dot hole spin qubit. The entanglement is induced by sequential scattering of the time-bin encoded photon interleaved with active spin control within a microsecond, two orders of magnitude faster than those achieved in other solid-state platforms. Conditioning on the detection of a reflected photon renders the entanglement fidelity immune to the spectral wandering of the emitter. These results represent a major step towards realizing a quantum node capable of interchanging information with flying photons and on-chip quantum logic, as required for quantum networks and quantum repeaters. © 2023, The Author(s).

The interaction between photons and a single two-level atom constitutes a fundamental paradigm in quantum physics. The nonlinearity provided by the atom leads to a strong dependence of the light–matter interface on the number of photons interacting with the two-level system within its emission lifetime. This nonlinearity unveils strongly correlated quasiparticles known as photon bound states, giving rise to key physical processes such as stimulated emission and soliton propagation. Although signatures consistent with the existence of photon bound states have been measured in strongly interacting Rydberg gases, their hallmark excitation-number-dependent dispersion and propagation velocity have not yet been observed. Here we report the direct observation of a photon-number-dependent time delay in the scattering off a single artificial atom—a semiconductor quantum dot coupled to an optical cavity. By scattering a weak coherent pulse off the cavity–quantum electrodynamics system and measuring the time-dependent output power and correlation functions, we show that single photons and two- and three-photon bound states incur different time delays, becoming shorter for higher photon numbers. This reduced time delay is a fingerprint of stimulated emission, where the arrival of two photons within the lifetime of an emitter causes one photon to stimulate the emission of another. © 2023, The Author(s).

Electron spins in gate-defined quantum dots (QDs) formed in semiconductor quantum wells (QWs) are promising stationary qubits for implementing large-scale quantum networks in a scalable manner. One key ingredient for such a network is an efficient photon-spin interface that converts any polarization state of a flying photonic qubit to the corresponding spins state of the electron in gate-defined QDs. A bull’s-eye cavity is an optical cavity structure that can enhance the photon absorption of an embedded gate-defined QD without polarization dependence. In this paper, we report the successful fabrication of air-bridge bull’s-eye cavities with metal electrodes and demonstrate the nearly polarization-independent optical absorption of a GaAs QW embedded in the cavities. This work marks an important step toward realizing an efficient photon-spin interface using gate-defined QDs. © 2023 The Japan Society of Applied Physics.

Efficient generation and detection of coherent single photons are key to advances in photonic quantum technologies such as quantum computation, quantum simulation, and quantum communication. Among many quantum emitters, semiconductor quantum dots are promising due to their deterministic and high-rate single-photon emission and the possibility of integration into nanostructures. However, poor quantum coherence between single photons created by independent emitters poses a major roadblock. Here, we present near-unity two-photon interference visibilities from two separate GaAs quantum dots [1], [2]. This high visibility (~93%) is achieved under rigorous conditions: there is no Purcell enhancement, no temporal post-selection, no narrow spectral filtering, nor frequency stabilization. One key component is the heterostructure, an n-i-p diode using material of excellent quality. The quantum dot charge is locked via Coulomb blockade; within a charging plateau, the exact emission frequency can be tuned via the bias applied to the gate; the charge noise is very low. A second key component is the quantum dot itself: the relatively large size confers multiple benefits. Our results suggest that GaAs quantum dots represent a versatile choice for generating identical photons from multiple emitters. © 2023 SPIE.

Combining the capabilities of gate-defined quantum transport devices in GaAs-based heterostructures and of optically addressed self-assembled quantum dots could open up broad perspectives in quantum technologies. For example, interfacing stationary solid-state qubits with photonic quantum states would open up a pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220-nm-thick membrane. We show that mobilities over 1×106cm2V-1s-1 can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single-photon sources, spin-photon interfaces and eventually quantum network nodes in GaAs quantum wells, realized entirely with a top-down fabrication process. © 2023 American Physical Society.

Gate layouts of spin-qubit devices are commonly adapted from previous successful devices. As qubit numbers and device complexity increase, modeling new device layouts and optimizing for yield and performance become necessary. The simulation tools used in the advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach to electrostatically modeling new spin-qubit-device layouts, considering gate voltages, heterostructures, doping, reservoirs, and an applied source-drain bias. We identify key challenges in spin-qubit-device design: validating the impact on quantum-dot parameters, considering cross-coupling among gates and reservoirs, and ensuring robustness of the design to fabrication limits. We select a demanding target potential to investigate and optimize examples of gate layouts under these challenges. We verify our model by fabricating two simulated designs and indirectly probing the potential landscape through transport measurements. © 2023 authors. Published by the American Physical Society.

We demonstrate an ultra-bright source of single photons rate at the GHz regime. By virtue of our record efficiency, a 20-photon quantum protocol will run seven orders of magnitude faster. Crucially, the coherence and the purity of our source match the state of the art. © 2023 IEEE.

Semiconductor self-assembled quantum dots (QDs) have garnered immense attention for their potential in various quantum technologies and photonics applications. Here, we explore a novel approach for fine-tuning the emission wavelength of QDs by building upon the indium flush growth method: Submonolayer variations in the capping thickness reveal a non-monotonic progression, where the emission energy can decrease even though the capping thickness decreases. indium flush, a well-known technique for inducing blue shifts in quantum dot emissions, involves the partial capping of QDs with GaAs followed by a temperature ramp-up. However, our findings reveal that the capping layer roughness, stemming from fractional monolayers during overgrowth, plays a pivotal role in modulating the emission energy of these QDs. We propose increased indium interdiffusion between the QDs and the surrounding GaAs capping layer for a rough surface surrounding the QD as the driving mechanism. This interdiffusion alters the indium content within the QDs, resulting in an additional emission energy shift, counterintuitive to the capping layer’s thickness increase. We utilize photoluminescence spectroscopy to generate wafer maps depicting the emission spectrum of the QDs. Using thickness gradients, we produce systematic variations in the capping layer thickness on 3″ wafers, resulting in modulations of the emission energy of up to 26 meV. © 2023 by the authors.

In a chiral one-dimensional atom, a photon propagating in one direction interacts with the atom; a photon propagating in the other direction does not. Chiral quantum optics has applications in creating nanoscopic single-photon routers, circulators, phase-shifters, and two-photon gates. Here, we implement chiral quantum optics using a low-noise quantum dot in an open microcavity. We demonstrate the non-reciprocal absorption of single photons, a single-photon diode. The non-reciprocity, the ratio of the transmission in the forward-direction to the transmission in the reverse direction, is as high as 10.7 dB. This is achieved by tuning the photon-emitter coupling in situ to the optimal operating condition (β = 0.5). Proof that the non-reciprocity arises from a single quantum emitter lies in the photon statistics—ultralow-power laser light propagating in the diode’s reverse direction results in a highly bunched output (g(2)(0) = 101), showing that the single-photon component is largely removed. © 2022, The Author(s).

On-demand single-photon sources emitting pure and indistinguishable photons at the telecommunication wavelength are critical assets toward the deployment of fiber-based quantum networks. Indeed, single photons may serve as flying qubits, allowing communication of quantum information over long distances. Self-assembled InAs quantum dots embedded in GaAs constitute an excellent nearly deterministic source of high-quality single photons, but the vast majority of sources operate in the 900–950 nm wavelength range, precluding their adoption in a quantum network. A quantum frequency conversion scheme is presented here for converting single photons from quantum dots to the telecommunication C band, around 1550 nm, achieving 40.8% end-to-end efficiency, while maintaining both high purity and a high degree of indistinguishability during conversion with measured values of (Formula presented.) and (Formula presented.), respectively. © 2022 The Authors. Advanced Quantum Technologies published by Wiley-VCH GmbH.

The full-cell entropy and its temperature dependency were measured for automotive lithium-ion cells with a graphite anode in dependence of the state of charge. Resulting entropy curves can be related to certain characteristic conditions of the graphite anode. Those characteristics are induced by a certain lithium-ion concentrations within the graphite. Comparing the entropy curves of fresh to aged cells shows a shift in the characteristics of these curves at a similar charge input. Those shifts were assigned to a change in the anodic net capacity, enabling an entropy based state of health estimation of the anode. The execution of the differential voltage analysis leads to similar results. © 2022 Elsevier Ltd

Recent demonstrations using electron spins stored in quantum dot array as qubits are promising for developing a scalable quantum computing platform. An ongoing effort is, therefore, aiming at the precise control of the quantum dot parameters in larger and larger arrays which represents a complex challenge. Partitioning of the system with the help of the inter-dot tunnel barriers can lead to a simplification for tuning and offers a protection against unwanted charge displacement. In a triple quantum dot system, we demonstrate a nanosecond control of the inter-dot tunnel rate permitting to reach the two extreme regimes, large GHz tunnel coupling, and sub-Hz isolation between adjacent dots. We use this development to isolate a subpart of the array in a metastable configuration while performing charge displacement and readout in the rest of the system. The degree of control over tunnel coupling achieved in a unit cell should motivate future protocol development for tuning, manipulation, and readout including this capability. © 2022 Author(s).

We present capacitance-voltage [C(V)] measurements of self-assembled wurtzite-GaN quantum dots (QDs). The QDs are embedded in a charge-tunable diode structure and were grown by molecular beam epitaxy in the Stranski-Krastanov growth method. The internal electric fields present in GaN and its alloys together with its wide bandgap make this material system an ideal candidate for high-temperature quantum applications. Charges and the internal electric fields influence the energy spacing in the QDs. We correlate photoluminescence measurements with C(V) measurements and show single-electron charging of the QDs and a Coulomb blockade energy of around 60 meV at room temperature. This finding demonstrates the possibility of quantum applications at room temperature. © 2022 Author(s).

The entropy coefficient is an important quantity to describe thermodynamic processes of battery cells and to model the temperature dependency of the open-circuit voltage. Determining the entropy via potentiometric measurements is often time-consuming. Therefore, several methods were developed to quickly estimate the entropy coefficient. This paper presents a new method, which is relatively simple in its execution and evaluation. The method is based on relating a dynamic temperature change to the cell's voltage change at an induced exponential temperature progression. Due to the dynamic of this process, the measurement time will be reduced compared to the potentiometric method by factor 13 for the executed experiments under its stated conditions. © 2022 Elsevier Ltd

Single photons role in the development of quantum science and technology. They can carry quantum information over extended distances to act as the backbone of a future quantum internet1 and can be manipulated in advanced photonic circuits, enabling scalable photonic quantum computing2,3. However, more sophisticated devices and protocols need access to multi-photon states with particular forms of entanglement. Efficient light–matter interfaces offer a route to reliably generating these entangled resource states4,5. Here we utilize the efficient and coherent coupling of a single quantum emitter to a nanophotonic waveguide to realize a quantum nonlinear interaction between single-photon wavepackets. We demonstrate the control of a photon using a second photon mediated by the quantum emitter. The dynamical response of the two-photon interaction is experimentally unravelled and reveals quantum correlations controlled by the pulse duration. Further development of this platform work, which constitutes a new research frontier in quantum optics6, will enable the tailoring of complex photonic quantum resource states. © 2022, The Author(s), under exclusive licence to Springer Nature Limited.

The choice of substrate orientation for semiconductor quantum dot circuits offers opportunities for tailoring spintronic properties such as g-factors for specific functionality. Here, we demonstrate the operation of a few-electron double quantum dot circuit fabricated from a (110)-oriented GaAs quantum well. We estimate the in-plane electron g-factor from the profile of the enhanced inter-dot tunneling (leakage) current near-zero magnetic field. Spin blockade due to Pauli exclusion can block inter-dot tunneling. However, this blockade becomes inactive due to hyperfine interaction mediated spin flip-flop processes between electron spin states and the nuclear spin of the host material. The g-factor of absolute value ∼0.1 found for a magnetic field parallel to the direction [1 ¯ 10] is approximately a factor of four lower than that for comparable circuits fabricated from a material grown on widely employed standard (001) GaAs substrates and is in line with reported values determined by purely optical means for quantum well structures grown on (110) GaAs substrates. © 2022 Author(s).

Deterministic sources of multiphoton entanglement are highly attractive for quantum information processing but are challenging to realize experimentally. In this Letter, we demonstrate a route toward a scaleable source of time-bin encoded Greenberger-Horne-Zeilinger and linear cluster states from a solid-state quantum dot embedded in a nanophotonic crystal waveguide. By utilizing a self-stabilizing double-pass interferometer, we measure a spin-photon Bell state with (67.8±0.4)% fidelity and devise steps for significant further improvements. By employing strict resonant excitation, we demonstrate a photon indistinguishability of (95.7±0.8)%, which is conducive to fusion of multiple cluster states for scaling up the technology and producing more general graph states. © 2022 American Physical Society.

A scale for magnetic field resilience of a superconductor is set by the paramagnetic limit. Comparing the condensation energy of the Bardeen-Cooper-Schrieffer (BCS) singlet ground state with the paramagnetically polarized state suggests that for an applied field μ 0 H > 1.8 T c (in SI), singlet pairing is not energetically favorable. Materials exceeding or approaching this limit are interesting from fundamental and technological perspectives. This may be a potential indicator of triplet superconductivity, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing, and other mechanisms involving topological aspects of surface states and may also allow Cooper pair injection at high magnetic fields. We have analyzed the microscopic composition of such a material arising from an unexpected source. A microjet of an organo-metallic gas, W [(CO) 6], can be decomposed by a gallium ion-beam, leaving behind a track of complex residue of gallium, tungsten, and carbon with remarkable superconducting properties, like an upper critical field, H c 2 > 10 T, above its paramagnetic limit. We carried out atomic probe tomography to establish the formation of nano-crystalline tungsten carbide (WC) in the tracks and the absence of free tungsten. Supporting calculations show that for Ga distributed on the surface of WC, its s,p-orbitals enhance the density of states near the Fermi energy. The observed variation of H c 2 (T) does not show features typical of enhancement of critical field due to granularity. Our observations may be significant in the context of some recent theoretical calculation of the band structure of WC and experimental observation of superconductivity in a WC-metal interface. © 2022 Author(s).

We present strategies for controlling growth parameters of local droplet etched GaAs quantum dots. We manage the local QD density and emission wavelength by gradient material deposition. We find regions with no light-emitting quantum dots sharply separated from regions with strong quantum dot luminescence. Close to this transition, we expect the lowest quantum dot densities possible under the used parameters. The maximum wavelength shift achieved due to a variation of GaAs hole filling level on a single 3-inch wafer ranges from 731 to 795 nm. By locally controlling the surface roughness, a profound additional influence on the emission wavelength and density is found. We show how to control this modulation on a millimeter-scale over the whole wafer. © 2022 Elsevier B.V.

The synthesis of single-cycle pulses of compressed light and microwave signals sparked novel areas of fundamental research. In the field of acoustics, however, such a generation has not been introduced yet. For numerous applications, the large spatial extent of surface acoustic waves (SAW) causes unwanted perturbations and limits the accuracy of physical manipulations. Particularly, this restriction applies to SAW-driven quantum experiments with single flying electrons, where extra modulation renders the exact position of the transported electron ambiguous and leads to undesired spin mixing. Here, we address this challenge by demonstrating single-shot chirp synthesis of a strongly compressed acoustic pulse. Employing this solitary SAW pulse to transport a single electron between distant quantum dots with an efficiency exceeding 99%, we show that chirp synthesis is competitive with regular transduction approaches. Performing a time-resolved investigation of the SAW-driven sending process, we outline the potential of the chirped SAW pulse to synchronize single-electron transport from many quantum-dot sources. By superimposing multiple pulses, we further point out the capability of chirp synthesis to generate arbitrary acoustic waveforms tailorable to a variety of (opto)nanomechanical applications. Our results shift the paradigm of compressed pulses to the field of acoustic phonons and pave the way for a SAW-driven platform of single-electron transport that is precise, synchronized, and scalable. © 2022 authors. Published by the American Physical Society.

We report a method for integrating GaAs waveguide circuits containing self-assembled quantum dots on a Si/SiO2 wafer, using die-to-wafer bonding. The large refractive-index contrast between GaAs and SiO2 enables fabricating single-mode waveguides without compromising the photon-emitter coupling. Anti-bunched emission from individual quantum dots is observed, along with a waveguide propagation loss <7 dB/mm, which is comparable with the performance of suspended GaAs circuits. These results enable the integration of quantum emitters with different material platforms, towards the realization of scalable quantum photonic integrated circuits. © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.

Entanglement is the fuel of advanced quantum technology, enabling, e.g., measurement-based quantum computing and loss-tolerant encoding of quantum information. In photonics, entanglement has traditionally been generated probabilistically, requiring massive multiplexing for scaling up to many photons. An alternative approach utilizing quantum emitters in nanophotonic devices can realize deterministic generation of entangled photons. However, such sources generate polarization entanglement that is incompatible with spatial dual-rail qubit encoding employed in scalable photonic quantum-computing platforms utilizing integrated circuits. Here we propose and experimentally realize an on-demand source of dual-rail photon pairs using a quantum dot in a planar nanophotonic waveguide. The source exploits the cascaded decay of a biexciton state and chiral light-matter coupling to achieve deterministic generation of spatial dual-rail Bell pairs with the amount of entanglement determined by the chirality. The operational principle can readily be extended to multiphoton entanglement generation required for efficient preparation of resource states for photonic quantum computing. © 2022 authors. Published by the American Physical Society.

In the version of this article initially published, there was a citation error in the sixth paragraph, now reading in part, “The calculation of the two-photon response was obtained following an approach as outlined in ref. 43,” where ref. 43 (Ramos, T. & García-Ripoll, J. J. Phys. Rev. Lett. 119, 153601 (2017)) replaces the originally cited ref. 44 (Houck, M. et al. Phys. Rev. Lett. 124, 16051 (2020)), which has been removed from the reference list. The change has been made to the HTML and PDF versions of the article © 2022 The Author(s), under exclusive licence to Springer Nature Limited.

Tunnel-coupled pairs of optically active quantum dots—quantum dot molecules (QDMs)—offer the possibility to combine excellent optical properties such as strong light-matter coupling with two-spin singlet–triplet ((Formula presented.)) qubits having extended coherence times. The (Formula presented.) basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter-dot orbital couplings, operation of the (Formula presented.) qubits under optimal conditions remains challenging. Here, an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand is presented. A four-phase optical and electric field control sequence facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter-dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. One- and two-hole charging efficiencies of (93.5 ± 0.8)% and (80.5 ± 1.3)% are achieved, respectively. Combining efficient charge state preparation and precise setting of inter-dot coupling allows for the control of few-spin qubits, as would be required for the on-demand generation of 2D photonic cluster states or quantum transduction between microwaves and photons. © 2022 The Authors. Advanced Quantum Technologies published by Wiley-VCH GmbH.

Photonic quantum technology provides a viable route to quantum communication1,2, quantum simulation3 and quantum information processing4. Recent progress has seen the realization of boson sampling using 20 single photons3 and quantum key distribution over hundreds of kilometres2. Scaling the complexity requires architectures containing multiple photon sources, photon counters and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single photons5–9. For applications, a roadblock is the poor quantum coherence on interfering single photons created by independent quantum dots10,11. Here we demonstrate two-photon interference with near-unity visibility (93.0 ± 0.8)% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero phonon line—only the weak phonon sideband is rejected; temporal post-selection is not employed. By exploiting quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity of (85.0 ± 1.0)% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual coherence of the photons stems from high-quality materials, diode structure and relatively large quantum dot size. Our results establish a platform—GaAs quantum dots—for creating coherent single photons in a scalable way. © 2022, The Author(s), under exclusive licence to Springer Nature Limited.

We report on experimental observations of charge-spin cooperative dynamics of two-electron states in a GaAs double quantum dot located in a nonequilibrium phonon environment. When the phonon energy exceeds the lowest excitation energy in the quantum dot, the spin-flip rate of a single electron strongly enhances. In addition, originated from the spatial gradient of phonon density between the dots, the parallel spin states become more probable than the antiparallel ones. These results indicate that spin is essential for further demonstrations of single-electron thermodynamic systems driven by phonons, which will greatly contribute to understanding of the fundamental physics of thermoelectric devices. © 2022 American Physical Society.

Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/µm2 and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer. © 2022, The Author(s).

A single-photon source is an enabling technology in device-independent quantum communication1, quantum simulation2,3, and linear optics-based4 and measurement-based quantum computing5. These applications employ many photons and place stringent requirements on the efficiency of single-photon creation. The scaling on efficiency is typically an exponential function of the number of photons. Schemes taking full advantage of quantum superpositions also depend sensitively on the coherence of the photons, that is, their indistinguishability6. Here, we report a single-photon source with a high end-to-end efficiency. We employ gated quantum dots in an open, tunable microcavity7. The gating provides control of the charge and electrical tuning of the emission frequency; the high-quality material ensures low noise; and the tunability of the microcavity compensates for the lack of control in quantum dot position and emission frequency. Transmission through the top mirror is the dominant escape route for photons from the microcavity, and this output is well matched to a single-mode fibre. With this design, we can create a single photon at the output of the final optical fibre on-demand with a probability of up to 57% and with an average two-photon interference visibility of 97.5%. Coherence persists in trains of thousands of photons with single-photon creation at a repetition rate of 1 GHz. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.

Self-organized semiconductor quantum dots represent almost ideal two-level systems, which have strong potential to applications in photonic quantum technologies. For instance, they can act as emitters in close-to-ideal quantum light sources. Coupled quantum dot systems with significantly increased functionality are potentially of even stronger interest since they can be used to host ultra-stable singlet-triplet spin qubits for efficient spin-photon interfaces and for deterministic photonic 2D cluster-state generation. An advanced quantum dot molecule (QDM) device is realized and excellent optical properties are demonstrated. The device includes electrically controllable QDMs based on stacked quantum dots in a pin-diode structure. The QDMs are deterministically integrated into a photonic structure with a circular Bragg grating using in situ electron beam lithography. A photon extraction efficiency of up to (24 ± 4)% is measured in good agreement with numerical simulations. The coupling character of the QDMs is clearly demonstrated by bias voltage dependent spectroscopy that also controls the orbital couplings of the QDMs and their charge state in quantitative agreement with theory. The QDM devices show excellent single-photon emission properties with a multi-photon suppression of (Formula presented.). These metrics make the developed QDM devices attractive building blocks for use in future photonic quantum networks using advanced nanophotonic hardware. © 2021 The Authors. Advanced Quantum Technologies published by Wiley-VCH GmbH

Quantum repeater composed of a quantum memory and an interface between photon qubits and memory qubits is indispensable for long-distance quantum communication. Gate-defined lateral quantum dots (QDs) can be a suitable platform for such quantum repeaters because of its aptitude for spin qubit and feasibility of quantum state transfer from photon polarization to electron spin. So far, the reported photoelectron excitation probabilities in such a QD are not high enough to implement practical repeater protocols. To improve the photoexcitation probability, we combine a surface plasmon antenna (SPA) with QDs. We fabricated a SPA designed to enhance the optical transmission to the QDs in a practical illumination setup in a refrigerator and characterized the fabricated antenna by measuring photocurrents at room temperature. © 2021 The Japan Society of Applied Physics

In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n-and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

We develop a coherent beam splitter for single electrons driven through two tunnel-coupled quantum wires by surface acoustic waves (SAWs). The output current through each wire oscillates with gate voltages to tune the tunnel coupling and potential difference between the wires. This oscillation is assigned to coherent electron tunneling motion that can be used to encode a flying qubit and is well reproduced by numerical calculations of time evolution of the SAW-driven single electrons. The oscillation visibility is currently limited to about 3%, but robust against decoherence, indicating that the SAW electron can serve as a novel platform for a solid-state flying qubit. © 2021 American Physical Society.

Solid-state quantum dots are promising candidates for efficient light-matter interfaces connecting internal spin degrees of freedom to the states of emitted photons. However, selection rules prevent the combination of efficient spin control and optical cyclicity in this platform. By utilizing a photonic crystal waveguide we here experimentally demonstrate optical cyclicity up to ≈15 through photonic state engineering while achieving high fidelity spin initialization and coherent optical spin control. These capabilities pave the way towards scalable multiphoton entanglement generation and on-chip spin-photon gates. © 2021 American Physical Society. All rights reserved.

Electron spins in gate-defined lateral quantum dots (QDs) are a suitable platform for qubits and have the capacity for coherent coupling to freespace photons. Superposition state transfer from single photon polarizations to single spins has been realized using a GaAs gate-defined QD. However, its efficiency is limited by the small size of the QDs. Here, we use a surface plasmon antenna on a GaAs gate-defined QD and demonstrate that the photoelectron detection probability is enhanced by 5.3-9.2 times. This result contributes to the development of photon-spin quantum interface using gate-defined lateral QDs. © 2021 The Japan Society of Applied Physics.

In the quest for large-scale quantum computing, networked quantum computers offer a natural path towards scalability. While recent experiments have demonstrated nearest neighbour entanglement for electron spin qubits in semiconductors, on-chip long-distance entanglement could bring more versatility to connect quantum core units. Here, we employ the moving trapping potential of a surface acoustic wave to realize the controlled and coherent transfer of a pair of entangled electron spins between two distant quantum dots. The subsequent electron displacement induces coherent spin rotations, which drives spin quantum interferences. We observe high-contrast interference as a signature of the preservation of the entanglement all along the displacement procedure, which includes a separation of the two spins by a distance of 6 μm. This work opens the route towards fast on-chip deterministic interconnection of remote quantum bits in semiconductor quantum circuits. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.

Persistent photoconductivity of GaAs/AlGaAs heterostructures has hampered the measurement of charge- and spin-related quantum effects in gate-defined quantum devices and integrated charge sensors due to Si-dopant-related deep donor levels (DX centers). In this study, this effect is overcome by using an undoped GaAs/AlGaAs heterostructure for photonic purposes. We also measure the electron transport before and after LED illumination at low temperatures. In addition to a regular rapid saturation showing the increased carrier density, a slow accumulation of illumination effects appeared when different top-gate voltages were applied during illumination, which indicated the redistribution of charge at the oxide-GaAs interface. This study provides interesting insights into the development of optically stable devices for efficient semiconductor quantum interfaces. © 2021 Author(s).

We report on the analysis of electroabsorption in thin GaAs/Al0.3Ga0.7As nanophotonic waveguides with an embedded p-i-n junction. By measuring the transmission through waveguides of different lengths, we derive the propagation loss as a function of electric field, wavelength, and temperature. The results are in good agreement with the Franz-Keldysh model of electroabsorption extending over 200 meV below the GaAs bandgap, i.e., in the wavelength range of 910-970 nm. We find a pronounced residual absorption in forward bias, which we attribute to Fermi-level pinning at the waveguide surface, producing over 20 dB/mm loss at room temperature. These results are essential for understanding the origin of loss in nanophotonic devices operating in the emission range of self-assembled InAs semiconductor quantum dots toward the realization of scalable quantum photonic integrated circuits. © 2021 Author(s).

The electron transfer dynamics between self-assembled quantum dots and their environment are measured under nonequilibrium conditions by time-dependent capacitance spectroscopy. The quantum dots are embedded in a wide spacer, which inhibits elastic tunneling to or from the reservoirs. At certain bias voltages, electron capture and emission are both significant. A rate equation model is used to determine the corresponding transfer rates and the average occupation numbers of the dots as a function of the bias voltage. ©2021 American Physical Society

The ability to shuttle coherently individual electron spins in arrays of quantum dots is a key procedure for the development of scalable quantum information platforms. It allows the use of sparsely populated electron spin arrays, envisioned to efficiently tackle the one- and two-qubit gate challenges. When the electrons are displaced in an array, they are exposed to site-dependent environment interactions such as hyperfine coupling with substrate nuclear spins. Here, we demonstrate that the electron multidirectional displacement in a 3×3 array of tunnel-coupled quantum dots enhances the spin-coherence time via the motional narrowing phenomenon. More specifically, up to ten charge configurations are explored by the electrons to study the impact of the displacement on spin dynamics. An increase of the coherence time by a factor up to 10 is observed in the case of fast and repetitive displacement. A simple model quantitatively captures the physical mechanism underlying this enhancement of the spin-coherence time induced by displacement. The implications on spin-coherence properties during the electron displacement are discussed in the context of large-scale quantum circuits. © 2021 authors.

Coherent photon-emitter interfaces offer a way to mediate efficient nonlinear photon-photon interactions, much needed for quantum information processing. Here we experimentally study the case of a two-level emitter, a quantum dot, coupled to a single optical mode in a nanophotonic waveguide.We carry out few-photon transport experiments and record the statistics of the light to reconstructthe scattering matrix elements of one- and two-photon components. This provides direct insight to the complex nonlinear photon interaction that contains rich many-body physics. © 2021 American Physical Society.

In this paper, we validate two theoretical formula used to characterize thermal transport of superlattices at different temperatures. These formulas are used to measure cross-plane thermal conductivity and thermal boundary resistance, when it is not possible to obtain heat capacity or thermal diffusivity and in-plane thermal conductivity. We find that the most common formula for calculating thermal diffusivity and heat capacity (and density) can be used in a temperature range of −50 °C to 50 °C. This confirms that the heat capacity in the very thin silicon membranes is the same as in bulk silicon, as was preliminary investigated using an elastic continuum model. Based on the obtained thermal parameters, we can fully characterize the sample using a new procedure for characterization of the in-plane and cross-plane thermal transport properties of thin-layer and superlattice semiconductor samples. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

We investigate gate voltage dependence of electrical readout noise in high-speed rf reflectometry using gallium arsenide quantum dots. The fast Fourier transform spectrum from the real time measurement reflects build-in device noise and circuit noise including the resonator and the amplifier. We separate their noise spectral components by model analysis. Detail of gate voltage dependence of the flicker noise is investigated and compared to the charge sensor sensitivity. We point out that the dominant component of the readout noise changes by the measurement integration time. © 2021 The Japan Society of Applied Physics

We reveal the existence of a large in-plane heavy-hole g factor in symmetric self-Assembled (001) (In,Ga)As/GaAs quantum dots due to the warping of valence-band states. This warping dominates over the well-established mechanism associated with a reduced symmetry of the quantum dots and the corresponding mixing of heavy-hole and light-hole states. The effect of band warping is manifested in a unique angular dependence of the trion photon echo signal on the direction of the external magnetic field with respect to the sample axes. It results in a uniform magnetic-field-induced optical anisotropy for the entire quantum dot ensemble which is a prerequisite for the realization of spin quantum memories and spin-photon entanglement in the ensemble. © 2021 American Physical Society.

Capacitance-voltage measurements on self-assembled quantum dot layers exposed to strong electric fields and with large distances to the reservoirs show a marked hysteretic behavior. It is shown that at low temperatures this hysteresis can be explained quantitatively in terms of state-dependent capture and emission rates that are obtained by a rate equation model, applied to the measured capacitance transients. The occupation dynamics and the steady-state configuration can be extracted from these data via a Markov chain model. © 2021 American Physical Society.

Surface acoustic waves (SAWs) have large potential to realize quantum-optics-like experiments with single flying electrons employing their spin or charge degree of freedom. For such quantum applications, highly efficient trapping of the electron in a specific moving quantum dot (QD) of a SAW train plays a key role. Probabilistic transport over multiple moving minima would cause uncertainty in synchronization that is detrimental for coherence of entangled flying electrons and in-flight quantum operations. It is thus of central importance to identify the device parameters enabling electron transport within a single SAW minimum. A detailed experimental investigation of this aspect is so far missing. Here, we fill this gap by demonstrating time-of-flight measurements for a single electron that is transported via a SAW train between distant stationary QDs. Our measurements reveal the in-flight distribution of the electron within the moving acousto-electric quantum dots of the SAW train. Increasing the acousto-electric amplitude, we observe the threshold necessary to confine the flying electron at a specific, deliberately chosen SAW minimum. Investigating the effect of a barrier along the transport channel, we also benchmark the robustness of SAW-driven electron transport against stationary potential variations. Our results pave the way for highly controlled transport of electron qubits in a SAW-driven platform for quantum experiments. © 2021 Author(s).

Tailored photonics cavities enhance light-matter interactions, ultimately enabling a fully coherent quantum interface. Here, we report an integrated microdisk cavity containing self-assembled quantum dots to coherently route photons between different access waveguides. We measure a Purcell factor of Fexp = 6.9 ± 0.9 for a cavity quality factor of about 10,000, allowing us to observe clear signatures of coherent scattering of photons by the quantum dots. We show how this integrated system can coherently reroute photons between the drop and bus ports and how this routing is controlled by detuning the quantum dot and resonator or through the strength of the excitation beam, where a critical photon number less than one photon per lifetime is required. We discuss the strengths and limitations of this approach, focusing on how the coherent scattering and single-photon nonlinearity can be used to increase the efficiency of quantum devices such as routers or Bell-state analyzers. © 2021 American Chemical Society.

We demonstrate by time-resolved resonance fluorescence measurements on a single self-assembled quantum dot an internal photoeffect that emits electrons from the dot by an intraband excitation. We find a linear dependence of the optically generated emission rate on the excitation intensity and use a rate equation model to deduce the involved rates. The emission rate is tunable over several orders of magnitude by adjusting the excitation intensity. Our findings show that a process that is well known in single atom spectroscopy (i.e., photoionization) can also be observed in the solid state. The results also quantify an important, but mostly neglected, mechanism that may fundamentally limit the coherence times in solid-state quantum optical devices. © 2021 American Physical Society.

One dimensional semiconductor systems with strong spin-orbit interaction are both of fundamental interest and have potential applications to topological quantum computing. Applying a magnetic field can open a spin gap, a pre-requisite for Majorana zero modes. The spin gap is predicted to manifest as a field dependent dip on the first 1D conductance plateau. However, disorder and interaction effects make identifying spin gap signatures challenging. Here we study experimentally and numerically the 1D channel in a series of low disorder p-type GaAs quantum point contacts, where spin-orbit and hole-hole interactions are strong. We demonstrate an alternative signature for probing spin gaps, which is insensitive to disorder, based on the linear and non-linear response to the orientation of the applied magnetic field, and extract a spin-orbit gap ΔE ≈ 500 μeV. This approach could enable one-dimensional hole systems to be developed as a scalable and reproducible platform for topological quantum applications. © 2021, The Author(s).

Single-shot readout of charge and spin states by charge sensors such as quantum point contacts and quantum dots are essential technologies for the operation of semiconductor spin qubits. The fidelity of the single-shot readout depends both on experimental conditions such as signal-to-noise ratio, system temperature, and numerical parameters such as threshold values. Accurate charge sensing schemes that are robust under noisy environments are indispensable for developing a scalable fault-tolerant quantum computation architecture. In this study, we present a novel single-shot readout classification method that is robust to noises using a deep neural network (DNN). Importantly, the DNN classifier is automatically configured for spin-up and spin-down traces in any noise environment by tuning the trainable parameters using the datasets of charge transition signals experimentally obtained at a charging line. Moreover, we verify that our DNN classification is robust under noisy environment in comparison to the two conventional classification methods used for charge and spin state measurements in various quantum dot experiments. © 2021, The Author(s).

The open circuit voltage of galvanic cells is temperature dependent and the effect responsible for this dependency is its entropy. While it is well known that the Nernst equation plays an important role in describing this temperature dependency of the open-circuit voltage, this paper displays another effect. Measurements of the entropy for lithium-ion batteries show a significant temperature dependency, which cannot be explained by the linear Nernst equation. But this temperature dependency can be described by the free electron potential adapting via Fermi–Dirac statistics. This approach results in a quadratic temperature dependence of the measured potentials, which in the here shown cases for commercial lithium ion cells, could explain the measured effect. © 2021 Elsevier B.V.

Hole spins in semiconductors are a potential qubit alternative to electron spins. In nuclear-spin-rich host crystals like GaAs, the hyperfine interaction of hole spins with nuclei is considerably weaker than that for electrons, leading to potentially longer dephasing times. Here we demonstrate optical pumping and coherent population trapping for acceptor-bound holes in a strained GaAs epitaxial layer. We find μs-scale longitudinal spin relaxation time T1 and an inhomogeneous dephasing time T2∗ of ∼7 ns. We attribute the spin relaxation mechanism to the combined effect of a hole-phonon interaction through the deformation potentials, and heavy-hole-light-hole mixing in an in-plane magnetic field. We attribute the short T2∗ to g-factor broadening due to strain inhomogeneity. T1 and T2∗ are calculated based on these mechanisms and compared with the experimental results. While the hyperfine-mediated decoherence is mitigated, our results highlight the important contribution of strain to relaxation and dephasing of acceptor-bound hole spins. © 2021 American Physical Society.

In a radiative Auger process, optical decay leaves other carriers in excited states, resulting in weak red-shifted satellite peaks in the emission spectrum. The appearance of radiative Auger in the emission directly leads to the question if the process can be inverted: simultaneous photon absorption and electronic demotion. However, excitation of the radiative Auger transition has not been shown, neither on atoms nor on solid-state quantum emitters. Here, we demonstrate the optical driving of the radiative Auger transition, linking few-body Coulomb interactions and quantum optics. We perform our experiments on a trion in a semiconductor quantum dot, where the radiative Auger and the fundamental transition form a Λ-system. On driving both transitions simultaneously, we observe a reduction of the fluorescence signal by up to 70%. Our results suggest the possibility of turning resonance fluorescence on and off using radiative Auger as well as THz spectroscopy with optics close to the visible regime. © 2021, The Author(s).

Electron spins in semiconductor quantum dots have been intensively studied for implementing quantum computation and high-fidelity single- and two-qubit operations have recently been achieved. Quantum teleportation is a three-qubit protocol exploiting quantum entanglement and it serves as an essential primitive for more sophisticated quantum algorithms. Here we demonstrate a scheme for quantum teleportation based on direct Bell measurement for a single-electron spin qubit in a triple quantum dot utilizing the Pauli exclusion principle to create and detect maximally entangled states. The single spin polarization is teleported from the input qubit to the output qubit. We find this fidelity is primarily limited by singlet–triplet mixing, which can be improved by optimizing the device parameters. Our results may be extended to quantum algorithms with a larger number of semiconductor spin qubits. © 2021, The Author(s).

Quantum polyspectra of up to fourth order are introduced for modeling and evaluating quantum transport measurements offering a powerful alternative to methods of the traditional full counting statistics. Experimental time traces of the occupation dynamics of a single quantum dot are evaluated via simultaneously fitting their second-, third-, and fourth-order spectra. The scheme recovers the same electron tunneling and spin relaxation rates as previously obtained from an analysis of the same data in terms of factorial cumulants of the full counting statistics and waiting-time distributions. Moreover, the evaluation of time traces via quantum polyspectra is demonstrated to be feasible also in the weak measurement regime even when quantum jumps can no longer be identified from time traces and methods related to the full counting statistics cease to be applicable. A numerical study of a double dot system shows strongly changing features in the quantum polyspectra for the transition from the weak measurement regime to the Zeno regime where coherent tunneling dynamics is suppressed. Quantum polyspectra thus constitute a general unifying approach to the strong and weak regime of quantum measurements with possible applications in diverse fields as nanoelectronics, circuit quantum electrodynamics, spin noise spectroscopy, or quantum optics. © 2021 authors. Published by the American Physical Society.

The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled (In,Ga)As quantum dot (QD) at low temperatures (4.2K). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot's vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale. © 2021 authors. Published by the American Physical Society.

The coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots in a transverse magnetic field is driven by periodic optical excitation at 1 GHz repetition frequency. Despite the strong inhomogeneity of the electron g factor, the spectral spread of optical transitions, and the broad distribution of nuclear spin fluctuations, we are able to push the whole ensemble of excited spins into a single Larmor precession mode that is commensurate with the laser repetition frequency. Furthermore, we demonstrate that an optical detuning of the pump pulses from the probed optical transitions induces a directed dynamic nuclear polarization and leads to a discretization of the total magnetic field acting on the electron ensemble. Finally, we show that the highly periodic optical excitation can be used as universal tool for strongly reducing the nuclear spin fluctuations and preparation of a robust nuclear environment for subsequent manipulation of the electron spins, also at varying operation frequencies. © 2021, The Author(s).

We present a surface-passivation method that reduces surface-related losses by almost 2 orders of magnitude in a highly miniaturized GaAs open microcavity. The microcavity consists of a curved dielectric distributed Bragg reflector with radius of approximately 10μm paired with a GaAs-based heterostructure. The heterostructure consists of a semiconductor distributed Bragg reflector followed by an n-i-p diode with a layer of quantum dots in the intrinsic region. Free-carrier absorption in the highly-n-doped and highly-p-doped layers is minimized by our positioning them close to a node of the vacuum electromagnetic field. The surface, however, resides at an antinode of the vacuum field and results in significant loss. These losses are much reduced by surface passivation. The strong dependence on wavelength implies that the main effect of the surface passivation is to eliminate the surface electric field, thereby quenching below-band-gap absorption via a Franz-Keldysh-like effect. An additional benefit is that the surface passivation reduces scattering at the GaAs surface. These results are important in other nanophotonic devices that rely on a GaAs-vacuum interface to confine the electromagnetic field. © 2021 authors.

A splitting of the fundamental optical modes in micro- and nanocavities comprising semiconductor heterostructures is commonly observed. Given that this splitting plays a role in light-matter interaction and hence quantum technology applications, a method for controlling the mode splitting is useful. In this work we use an open microcavity composed of a "bottom"semiconductor distributed Bragg reflector (DBR) incorporating a n-i-p heterostructure, paired with a "top"curved dielectric DBR. We measure the mode splitting as a function of wavelength across the stopband. We demonstrate a reversible in situ technique to tune the mode splitting by applying uniaxial stress to the semiconductor DBR. The method exploits the photoelastic effect of the semiconductor materials. We achieve a maximum tuning of approximately 11 GHz. The stress applied to the heterostructure is determined by observing the photoluminescence of quantum dots embedded in the sample, converting a spectral shift to a stress via deformation potentials. A thorough study of the mode splitting and its tuning across the stopband leads to a quantitative understanding of the mechanism behind the results. © 2021 authors. Published by the American Physical Society.

The electron mobility in shallow GaAs/AlxGa1−xAs heterostructures is strongly suppressed by charge wafer surface, which arises from native surface oxide layers formed when the wafer is removed from the crystal growth system. Here an in situ epitaxial aluminum gate, grown as part of the wafer, is used to eliminate surface charge scattering. Transmission electron microscope characterization shows that the in situ epitaxial aluminum is crystalline, and the wafer surface is free of native oxide. The influence of Al thickness and the use of different semiconductor wetting layers at the semiconductor-aluminum interface are examined and correlated with electron mobility. The electron mobility is found to strongly depend on aluminum thickness. For 8 nm thick aluminum, the electron mobility is also influenced by the wetting layer, with aluminum grown on GaAs producing higher mobility compared to AlAs or Al0.33Ga0.67As wetting layers. The suppression of surface charge scattering in these all-epitaxial devices allows for high mobilities across a wide density range despite the shallow conduction channel (35 nm below the gate). These measurements also provide a uniquely sensitive method of determining the electrical quality of the semiconductor–metal interface, relevant to the formation of hybrid semiconductor–superconductor devices. © 2021 Wiley-VCH GmbH

The study of electron transport in low-dimensional systems is of importance, not only from a fundamental point of view, but also for future electronic and spintronic devices. In this context heterostructures containing a two-dimensional electron gas (2DEG) are a key technology. In particular GaAs/AlGaAs heterostructures, with a 2DEG at typically 100 nm below the surface, are widely studied. In order to explore electron transport in such systems, low-resistance ohmic contacts are required that connect the 2DEG to macroscopic measurement leads at the surface. Here we report on designing and measuring a dedicated device for unraveling the various resistance contributions in such contacts, which include pristine 2DEG series resistance, the 2DEG resistance under a contact, the contact resistance itself, and the influence of pressing a bonding wire onto a contact. We also report here a recipe for contacts with very low resistance values that remain below 10 ω for annealing times between 20 and 350 s, hence providing the flexibility to use this method for materials with different 2DEG depths. The type of heating, temperature ramp rate and gas forming used for annealing is found to strongly influence the annealing process and hence the quality of the resulting contacts. © EDP Sciences, 2020.

Semiconductor spin qubits have recently seen major advances in coherence time and control fidelities, leading to a single-qubit performance that is on par with other leading qubit platforms. Most of this progress is based on microwave control of single spins in devices made of isotopically purified silicon. For controlling spins, the exchange interaction is an additional key ingredient which poses new challenges for high-fidelity control. Here, we demonstrate exchange-based single-qubit gates of two-electron spin qubits in GaAs double quantum dots. Using careful pulse optimization and closed-loop tuning, we achieve a randomized benchmarking fidelity of (99.50±0.04)% and a leakage rate of 0.13% out of the computational subspace. These results open new perspectives for microwave-free control of singlet-triplet qubits in GaAs and other materials. © 2020, The Author(s).

The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise. During the past decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit. However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood. We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by high-frequency charge noise rather than the quasistatic noise inherent to any semiconductor device. We employ a feedback-control technique to actively suppress the latter, demonstrating a π-flip gate fidelity as high as 99.04±0.23% in a gallium arsenide quantum dot. We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the 1/f noise observed in isotopically purified silicon qubits. © 2020 authors. Published by the American Physical Society.

The coherent manipulation of individual quantum objects organized in arrays is a prerequisite to any scalable quantum information platform. The cumulated efforts to control electron spins in quantum dot arrays have permitted the recent realization of quantum simulators and multielectron spin-coherent manipulations. Although a natural path to resolve complex quantum-matter problems and to process quantum information, two-dimensional (2D) scaling with a high connectivity of such implementations remains undemonstrated. Here we demonstrate the 2D coherent control of individual electron spins in a 3 × 3 array of tunnel-coupled quantum dots. We focus on several key quantum functionalities: charge-deterministic loading and displacement, local spin readout and local coherent exchange manipulation between two electron spins trapped in adjacent dots. This work lays some of the foundations to exploit a 2D array of electron spins for quantum simulation and information processing. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

Fundamental properties of the spin-noise signal formation in a quantum-dot microcavity are studied by measuring the angular characteristics of the scattered light intensity. A distributed Bragg reflector microcavity was used to enhance the light-matter interaction with an ensemble of n-doped (In,Ga)As/GaAs quantum dots, which allowed us to study subtle effects of coherent scattering at the quantum dot ensemble. Detecting the scattered light outside of the aperture of the transmitted light, we measured the basic electron spin properties, such as g factor and spin dephasing time. Further, we investigated the influence of the microcavity on the scattering distribution and possibilities of signal amplification by additional resonant excitation. © 2020 American Physical Society.

The capability to embed self-assembled quantum dots (QDs) at predefined positions in nanophotonic structures is key to the development of complex quantum-photonic architectures. Here, we demonstrate that QDs can be deterministically positioned in nanophotonic waveguides by pre-locating QDs relative to a global reference frame using micro-photoluminescence (μPL) spectroscopy. After nanofabrication, μPL images reveal misalignments between the central axis of the waveguide and the embedded QD of only (9 ± 46) nm and (1 ± 33) nm for QDs embedded in undoped and doped membranes, respectively. A priori knowledge of the QD positions allows us to study the spectral changes introduced by nanofabrication. We record average spectral shifts ranging from 0.1 nm to 1.1 nm, indicating that the fabrication-induced shifts can generally be compensated by electrical or thermal tuning of the QDs. Finally, we quantify the effects of the nanofabrication on the polarizability, the permanent dipole moment, and the emission frequency at vanishing electric field of different QD charge states, finding that these changes are constant down to QD-surface separations of only 70 nm. Consequently, our approach deterministically integrates QDs into nanophotonic waveguides whose light-fields contain nanoscale structure and whose group index varies at the nanometer level. © 2020 Author(s).

The effect of a lateral electric current on the photoluminescence H band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H band are studied by means of continuous-wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of a lateral electric field of ∼100-200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ∼100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (>40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons. © 2020 American Physical Society.

We present time-resolved conductance measurements and charge spectra for the conduction-band states of InAs quantum dots after creating metastable holes by illumination. We demonstrate an electrical way of measuring the conduction-band energy offset and inverse tunnel rates of electrons from a two-dimensional electron gas into neutral (X0), single positively (X1+), and double positively (X2+) charged exciton states. The experiment also gives information about the metastable hole storage time and discharge dynamics. © 2020 American Physical Society.

We investigate tunneling dynamics of electrons from an ensemble of self-assembled InAs quantum dots into the subbands of a two-dimensional electron gas (2DEG). LO-phonon-assisted tunneling processes and tunneling into higher subbands of the 2DEG electronic structure cause distinct resonances in the evolution of the tunneling rate as a function of the energy detuning between quantum dot and 2DEG ground state. By devising a semiquantitative model, we identify the momentum mismatch between the quantum dot and 2DEG wave function as the crucial quantity governing the evolution of the tunneling rate. In particular, we demonstrate that this mechanism along with the availability of tunneling into the second 2DEG subband allows for tuning of the tunneling rate by more than two orders of magnitude. © 2020 American Physical Society.

Quantum point contacts (QPCs) are fundamental building blocks of nanoelectronic circuits. For their emission dynamics as well as for interaction effects such as the 0.7 anomaly the details of the electrostatic potential are important, but the precise potential shapes are usually unknown. Here, we measure the one-dimensional subband spacings of various QPCs as a function of their conductance and compare our findings with models of lateral parabolic versus hard-wall confinement. We find that a gate-defined QPC near pinch-off is compatible with the parabolic saddle-point scenario. However, as the number of populated subbands is increased, Coulomb screening flattens the potential bottom and a description in terms of a finite hard-wall potential becomes more realistic. © 2020 American Physical Society.

Cross-correlated noise measurements are performed in etched Al x Ga 1 - xAs/GaAs based quantum rings in equilibrium at a bath temperature of T bath = 4.2 K. The measured white noise exceeds the thermal (Johnson-Nyquist) noise expected from the measured electron temperature T e and the electrical resistance R. This excess part of the white noise decreases as T bath increases and vanishes for T bath ≥ 12 K. Excess noise is neither observed if one arm of a quantum ring is depleted of electrons nor in one-dimensional-constrictions that have a length and width comparable to the quantum rings. A model is presented that suggests that the excess noise originates from the correlation of noise sources, mediated by phase-coherent propagation of electrons. © 2020 Author(s).

Among various solid-state systems, gate-defined quantum dots (QD) with high scalability and controllability for single electron spin qubits are promising candidates to realize quantum spin-photon interface. The efficiency of the spin-photon interface is expected to be significantly enhanced by optical coupling of gate-defined QDs with photonic crystal (PhC) nanocavities. As the first step towards this optical coupling, we designed and experimentally demonstrated a PhC nanocavity with electrodes. The electrodes, which can form a single QD, were introduced on the top surfaces of two-dimensional PhC nanocavities with a position accuracy of a few tens of nanometers. Despite the electrodes, a resonant mode was confirmed for the PhC nanocavities through micro-photoluminescence spectroscopy. This work marks a crucial step towards optical coupling between gate-defined QDs and PhC nanocavities. © 2020 The Japan Society of Applied Physics.

Quantum dot heterostructures with excellent low-noise properties became possible with high purity materials recently. We present a study on molecular beam epitaxy grown quantum wells and quantum dots with a contaminated aluminum evaporation cell, which introduced a high amount of impurities, perceivable in anomalies in optical and electrical measurements. We describe a way of addressing this problem and find that reconditioning the aluminum cell by overheating can lead to a full recovery of the anomalies in photoluminescence and capacitance–voltage measurements, leading to excellent low noise heterostructures. Furthermore, we propose a method to sense photo-induced trap charges using capacitance–voltage spectroscopy on self-assembled quantum dots. Excitation energy-dependent ionization of defect centers leads to shifts in capacitance–voltage spectra which can be used to determine the charge density of photo-induced trap charges via 1D band structure simulations. This method can be performed on frequently used quantum dot diode structures. © 2020 Elsevier B.V.

Quantum dots are both excellent single-photon sources and hosts for single spins. This combination enables the deterministic generation of Raman-photons—bandwidth-matched to an atomic quantum-memory—and the generation of photon cluster states, a resource in quantum communication and measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched in frequency to a rubidium-based photon memory, and have potentially improved electron spin coherence compared to the widely used InGaAs quantum dots. However, their charge stability and optical linewidths are typically much worse than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an n-i-p-diode specially designed for low-temperature operation. We demonstrate ultra-low noise behaviour: charge control via Coulomb blockade, close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity optical electron-spin initialisation and long electron-spin lifetimes for these quantum dots. Our work establishes a materials platform for low-noise quantum photonics close to the red part of the spectrum. © 2020, The Author(s).

Decoherence of a quantum system arising from its interaction with an environment is a key concept for understanding the transition between the quantum and classical world as well as performance limitations in quantum technology applications. The effects of large, weakly coupled environments are often described as a classical, fluctuating field whose dynamics is unaffected by the qubit, whereas a fully quantum description still implies some backaction from the qubit on the environment. Here we show direct experimental evidence for such a backaction for an electron-spin qubit in a GaAs quantum dot coupled to a mesoscopic environment of order 106 nuclear spins. By means of a correlation measurement technique, we detect the backaction of a single qubit-environment interaction whose duration is comparable to the qubit's coherence time, even in such a large system. We repeatedly let the qubit interact with the spin bath and measure its state. Between such cycles, the qubit is reinitialized to different states. The correlations of the measurement outcomes are strongly affected by the intermediate qubit state, which reveals the action of a single electron spin on the nuclear spins. © 2020 American Physical Society.

Thermal transport properties in multi-layered semiconductor samples are reported using modulated photothermal infrared radiometry (PTR). The cross-plane thermal conductivity and diffusivity of thin AlxGa(1-x)As layers and AlAs/GaAs superlattices were determined by fitting solutions of the heat equation for multi-layered systems to PTR data. The thermal conductivity of an AlxGa(1-x)As film with x = 0.5 was found to be lower than an AlxGa(1-x)As film with x = 0.33 which is expected as scattering from alloy disorder is maximized for x = 0.5. In addition, it was found that thermal conductivities of AlAs/GaAs superlattices decrease when the superlattice period is decreased for a constant total thickness which is expected since the number of AlAs/GaAs interfaces (which impede thermal transport) increases as the period decreases. The maximum PTR amplitude signal was found to occur when the diffusion length of the thermal wave is on other order of the thickness of the semiconductor layer. The accuracy and applicability of photothermal infrared radiometry to the study of semiconductor multilayer structures are further discussed in the paper. © 2020 Elsevier Ltd

Two-dimensional stacking fault defects embedded in a bulk crystal can provide a homogeneous trapping potential for carriers and excitons. Here we utilize state-of-The-Art structural imaging coupled with density-functional and effective-mass theory to build a microscopic model of the stacking-fault exciton. The diamagnetic shift and exciton dipole moment at different magnetic fields are calculated and compared with the experimental photoluminescence of excitons bound to a single stacking fault in GaAs. The model is used to further provide insight into the properties of excitons bound to the double-well potential formed by stacking fault pairs. This microscopic exciton model can be used as an input into models which include exciton-exciton interactions to determine the excitonic phases accessible in this system. © 2020 American Physical Society.

Planar nanophotonic structures enable broadband, near-unity coupling of emission from quantum dots embedded within, thereby realizing ideal single-photon sources. The efficiency and coherence of the single-photon source is limited by charge noise, which results in the broadening of the emission spectrum. We report suppression of the noise by fabricating photonic crystal waveguides in a gallium arsenide membrane containing quantum dots embedded in a p-i-n diode. Local electrical contacts in the vicinity of the waveguides minimize the leakage current and allow fast electrical control (≈4 MHz bandwidth) of the quantum dot resonances. Resonant linewidth measurements of 51 quantum dots coupled to the photonic crystal waveguides exhibit near transform-limited emission over a 6 nm wide range of emission wavelengths. Importantly, the local electrical contacts allow independent tuning of multiple quantum dots on the same chip, which together with the transform-limited emission are key components in realizing multiemitter-based quantum information processing. Copyright © 2020 American Chemical Society.

When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect1,2. Unlike electric-charge screening, the spin-screening cloud3–6 occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the Kondo cloud can theoretically spread out over several micrometres. The cloud has not so far been detected, and so its physical existence—a fundamental aspect of the Kondo effect—remains controversial7,8. Here we present experimental evidence of a Kondo cloud extending over a length of micrometres, comparable to the theoretical length ξK. In our device, a Kondo impurity is formed in a quantum dot2,9–11, coupling on one side to a quasi-one-dimensional channel12 that houses a Fabry–Pérot interferometer of various gate-defined lengths L exceeding one micrometre. When we sweep a voltage on the interferometer end gate—separated by L from the quantum dot—to induce Fabry–Pérot oscillations in conductance we observe oscillations in the measured Kondo temperature TK, which is a signature of the Kondo cloud at distance L. When L is less than ξK the TK oscillation amplitude becomes larger as L becomes smaller, obeying a scaling function of a single parameter L/ξK, whereas when L is greater than ξK the oscillation is much weaker. Our results reveal that ξK is the only length parameter associated with the Kondo effect, and that the cloud lies mostly within a length of ξK. Our experimental method offers a way of detecting the spatial distribution of exotic non-Fermi liquids formed by multiple magnetic impurities or multiple screening channels13–16 and of studying spin-correlated systems. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

A deterministic source of coherent single photons is an enabling device for quantum information processing. Quantum dots in nanophotonic structures have been employed as excellent sources of single photons with the promise of scaling up towards multiple photons and emitters. It remains a challenge to implement deterministic resonant optical excitation of the quantum dot required for generating coherent single photons, since residual light from the excitation laser should be suppressed without compromising source efficiency and scalability. Here, we present a planar nanophotonic circuit that enables deterministic pulsed resonant excitation of quantum dots using two orthogonal waveguide modes for separating the laser and the emitted photons. We report a coherent and stable single-photon source that simultaneously achieves high-purity (g(2)(0) = 0.020 ± 0.005), high-indistinguishability (V = 96 ± 2%), and >80% coupling efficiency into the waveguide. Such ‘plug-and-play’ single-photon source can be integrated with on-chip optical networks implementing photonic quantum processors. © 2020, The Author(s).

Semiconductor quantum dots in photonic integrated circuits enable scaling quantum-information processing to many single photons and quantum-optical gates. Obtaining high-purity and coherent single photons from quantum dots requires spectral filtering to select individual excitonic transitions. Here, an on-chip wavelength-tunable filter integrated with a single-photon source, which preserves the optical properties of the emitter, is demonstrated. Nanomechanical motion is used for tuning the resonant wavelength over 10 nm, enabling operation at cryogenic temperatures, and single-photon emission from a quantum dot under non-resonant excitation is demonstrated without resorting to free-space optical filters. These results are key for the development of fully integrated de-multiplexing, multi-path photon encoding schemes, and multi-emitter circuits. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover electrons are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state and a redshifted photon is created1–4. In a semiconductor quantum dot, radiative Auger is predicted for charged excitons5. Here we report the observation of radiative Auger on trions in single quantum dots. For a trion, a photon is created on electron–hole recombination, leaving behind a single electron. The radiative Auger process promotes this additional (Auger) electron to a higher shell of the quantum dot. We show that the radiative Auger effect is a powerful probe of this single electron: the energy separations between the resonance fluorescence and the radiative Auger emission directly measure the single-particle splittings of the electronic states in the quantum dot with high precision. In semiconductors, these single-particle splittings are otherwise hard to access by optical means as particles are excited typically in pairs, as excitons. After the radiative Auger emission, the Auger carrier relaxes back to the lowest shell. Going beyond the original theoretical proposals, we show how applying quantum optics techniques to the radiative Auger photons gives access to the single-electron dynamics, notably relaxation and tunnelling. This is also hard to access by optical means: even for quasi-resonant p-shell excitation, electron relaxation takes place in the presence of a hole, complicating the relaxation dynamics. The radiative Auger effect can be exploited in other semiconductor nanostructures and quantum emitters in the solid state to determine the energy levels and the dynamics of a single carrier. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

Auger recombination is a nonradiative process, where the recombination energy of an electron-hole pair is transferred to a third charge carrier. It is a common effect in colloidal quantum dots that quenches the radiative emission with an Auger recombination time below nanoseconds. In self-assembled QDs, the Auger recombination has been observed with a much longer recombination time on the order of microseconds. Here, we use two-color laser excitation on the exciton and trion transition in resonance fluorescence on a single self-assembled quantum dot to monitor in real-time single quantum events of the Auger process. Full counting statistics on the random telegraph signal give access to the cumulants and demonstrate the tunability of the Fano factor from a Poissonian to a sub-Poissonian distribution by Auger-mediated electron emission from the dot. Therefore, the Auger process can be used to tune optically the charge carrier occupation of the dot by the incident laser intensity, independently from the electron tunneling from the reservoir by the gate voltage. Our findings are not only highly relevant for the understanding of the Auger process but also demonstrate the perspective of the Auger effect for controlling precisely the charge state in a quantum system by optical means. © 2020 American Chemical Society.

Photonic qubits are key enablers for quantum information processing deployable across a distributed quantum network. An on-demand and truly scalable source of indistinguishable single photons is the essential component enabling high-fidelity photonic quantum operations. A main challenge is to overcome noise and decoherence processes to reach the steep benchmarks on generation efficiency and photon indistinguishability required for scaling up the source. We report on the realization of a deterministic single-photon source featuring near-unity indistinguishability using a quantum dot in an "on-chip"planar nanophotonic waveguide circuit. The device produces long strings of >100 single photons without any observable decrease in the mutual indistinguishability between photons. A total generation rate of 122 million photons per second is achieved, corresponding to an on-chip source efficiency of 84%. These specifications of the single-photon source are benchmarked for boson sampling and found to enable scaling into the regime of quantum advantage. © 2020 The Authors.

We have measured the resistivity of magnetically doped Ag:Mn mesoscopic wires as a function of temperature and magnetic field. The doping has been made using ion implantation, allowing a distribution of the dopants in the middle of the sample. Comparison with an undoped sample, used as a reference sample, shows that the resistivity of the doped sample exhibits nonmonotonic behavior as a function of both magnetic field and temperature, revealing the competition between the Kondo effect and the RKKY interactions between spins. This proves that transport measurements are still a reliable probe of the spin-glass transition in nanoscopic metallic wire doped using implantation. © 2020 American Physical Society.

The realization of a highly efficient optical spot-size converter for the end-face coupling of single photons from GaAs-based nanophotonic waveguides with embedded quantum dots is reported. The converter is realized using an inverted taper and an epoxy polymer overlay providing a 1.3 µm output mode field diameter. The collection of single photons from a quantum dot into a lensed fiber with a rate of 5.84 ± 0.01 MHz is demonstrated and a chip-to-fiber coupling efficiency of ≈48% is estimated. The stability and compatibility with cryogenic temperatures make the epoxy waveguides a promising material to realize efficient and scalable interconnects between heterogeneous quantum photonic integrated circuits. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fano resonances are observed in a composite metamaterial that consists of an electric split ring resonator eSRR and an I-shaped resonator ISR. By adjusting the length of the ISR the degree of asymmetry in the line shape of the composite metamaterial can be controlled and even made to be symmetric. In contrast to other methods to create Fano resonances, the individual modes of the eSRR and ISR have the same symmetry and are not evanescently coupled to each other. The transmission is simulated using the finite difference time domain method and a coupled oscillator model is used to obtain nominal values of the Fano asymmetry factor q. Composite metamaterials and individual eSRR and ISR metamaterials are fabricated, and their transmission is measured with terahertz time-domain spectroscopy. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

The photoluminescence of quasi two-dimensional (2D) electrons in modulation doped Al x Ga1 - x As-GaAs single heterostructure recombining with bound holes was studied at liquid helium temperatures in magnetic fields up to 9 T. Luminescence contributions from the ground and a weakly populated first excited 2D subband were observed as well as bulk-like contributions from the GaAs buffer. The 2D subband and the bulk signals are linked and oscillate with the magnetic field strength. It is not the filling factor that rules but the alignment of the two subband Landau-ladders paired with intersubband scattering that drives an oscillatory behavior of the subband populations. Asymmetric Landau-level transition line shapes indicate a perturbed 2D electron system. We performed a detailed analyses of Landau-level dependent line strengths, positions and half widths and compared the experiment with calculations of subband energies and transition matrix elements. The total electron density is not fixed in our experiment but decreases with magnetic field strength. This might indicate that the 2D system exchanges electrons with a reservoir and/or that the correlation between radiative and nonradiative recombination channels vary strongly with the magnetic field strength. © 2020 IOP Publishing Ltd.

We present a double-layer design for two-dimensional lateral surface superlattice systems in GaAs-AlGaAs heterostructures. Unlike previous studies, our device (1) uses an in situ gate, which allows a very short period superlattice in high mobility, shallow heterostructures and (2) enables independent control of the carrier density and the superlattice modulation potential amplitude over a wide range. We characterize this device design using low-temperature magneto-transport measurements and show that the fabrication process caused minimal damage to the system. We demonstrate the tuning of potential modulation from weak (much smaller than Fermi energy) to strong (larger than the Fermi energy) regimes. © 2020 Author(s).

We demonstrate an ultra-bright source of coherent single photons with an average end-to-end efficiency of 55%. By the virtue of our record efficiency, a 20-photon quantum protocol will run seven orders of magnitude faster than the state-of-the-art. © OSA 2020 © 2020 The Author(s)

The strong-coupling regime of cavity quantum electrodynamics (QED) represents the light–matter interaction at the fully quantum level. Adding a single photon shifts the resonance frequencies—a profound nonlinearity. Cavity QED is a test bed for quantum optics1–3 and the basis of photon–photon and atom–atom entangling gates4,5. At microwave frequencies, cavity QED has had a transformative effect6, enabling qubit readout and qubit couplings in superconducting circuits. At optical frequencies, the gates are potentially much faster; the photons can propagate over long distances and can be easily detected. Following pioneering work on single atoms1–3,7, solid-state implementations using semiconductor quantum dots are emerging8–15. However, miniaturizing semiconductor cavities without introducing charge noise and scattering losses remains a challenge. Here we present a gated, ultralow-loss, frequency-tunable microcavity device. The gates allow both the quantum dot charge and its resonance frequency to be controlled electrically. Furthermore, cavity feeding10,11,13–17, the observation of the bare-cavity mode even at the quantum dot–cavity resonance, is eliminated. Even inside the microcavity, the quantum dot has a linewidth close to the radiative limit. In addition to a very pronounced avoided crossing in the spectral domain, we observe a clear coherent exchange of a single energy quantum between the ‘atom’ (the quantum dot) and the cavity in the time domain (vacuum Rabi oscillations), whereas decoherence arises mainly via the atom and photon loss channels. This coherence is exploited to probe the transitions between the singly and doubly excited photon–atom system using photon-statistics spectroscopy18. The work establishes a route to the development of semiconductor-based quantum photonics, such as single-photon sources and photon–photon gates. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.

While spin qubits based on gate-defined quantum dots have demonstrated very favorable properties for quantum computing, one remaining hurdle is the need to tune each of them into a good operating regime by adjusting the voltages applied to electrostatic gates. The automation of these tuning procedures is a necessary requirement for the operation of a quantum processor based on gate-defined quantum dots, which is yet to be fully addressed. We present an algorithm for the automated fine-tuning of quantum dots and demonstrate its performance on a semiconductor singlet-triplet qubit in GaAs. The algorithm employs a Kalman filter based on Bayesian statistics to estimate the gradients of the target parameters as a function of gate voltages, thus learning the system response. The algorithm's design is focused on the reduction of the number of required measurements. We experimentally demonstrate the ability to change the operation regime of the qubit within 3-5 iterations, corresponding to 10-15 min of lab-time. © 2019 Author(s).

Gate-defined quantum dots (QDs) are such a highly-tunable quantum system in which single spins can be electrically coupled, manipulated, and measured. However, the spins in gate-defined QDs are lacking its interface to free-space photons. Here, we verify that a circularly-polarized single photon can excite a single electron spin via the transfer of angular momentum, measured using Pauli spin blockade (PSB) in a double QD. We monitor the inter-dot charge tunneling which only occur when the photo-electron spin in one QD is anti-parallel to the electron spin in the other. This allows us to detect single photo-electrons in the spin-up/down basis using PSB. The photon polarization dependence of the excited spin state was finally confirmed for the heavy-hole exciton excitation. The angular momentum transfer observed here is a fundamental step providing a route to instant injection of spins, distributing single spin information, and possibly towards extending quantum communication. © 2019, The Author(s).

We present a ballistic rectification effect in an orthogonal four-terminal cross junction where the symmetry is broken by local magnetic fields. The input current is injected between opposing branches and the current-free branches serve as potential probes. The local magnetic field is induced by two permalloy (Py) stripes with a magnetic single-domain structure, where one end of each stripe is positioned close to the junction center. The Py stripes are oriented such that an external in-plane magnetic field can magnetize them into two different main configurations having either equally or oppositely magnetized ends. Equal magnetic ends are expected to result in a Hall-effect device, while for opposite magnetic ends, the stray field should deflect the electrons into the same output lead for both current polarities, leading to a rectifying behavior. Here, we present the proof of concept for stray-field controlled transfer characteristics. First, we show by magnetic force microscopy that both configurations are stable and the Py stripes exhibit a remanent magnetic single-domain structure. Second, we demonstrate the influence of the remanent magnetization on the low-temperature dc characteristics which are superimposed by a parasitic background. Third, we present the extracted Hall and the rectified voltage which are, respectively, linearly and parabolically dependent on the input current up to ±55 μA. © 2019 Author(s).

Waveguide-based spin-photon interfaces on the GaAs platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here, we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals an inhomogeneous dephasing time of 2.2±0.1 ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms. © 2019 American Physical Society.

We report on a high optical contrast between the photon emission from a single self-assembled quantum dot (QD) and the back-scattered excitation laser light. In an optimized semiconductor heterostructure with an epitaxially grown gate, an optically-matched layer structure and a distributed Bragg reflector, a record value of 83% is obtained; with tilted laser excitation even 885%. This enables measurements on a single dot without lock-in technique or suppression of the laser background by cross-polarization. These findings open up the possibility to perform simultaneously time-resolved and polarization-dependent resonant optical spectroscopy on a single quantum dot. © 2019, The Author(s).

A semiconductor quantum dot (QD) can generate highly indistinguishable single photons at a high rate. For application in quantum communication and integration in hybrid systems, control of the QD optical properties is essential. Understanding the connection between the optical properties of a QD and the growth process is therefore important. Here, we show for GaAs QDs, grown by infilling droplet-etched nanoholes, that the emission wavelength, the neutral-to-charged exciton splitting, and the diamagnetic shift are strongly correlated with the capture-zone area, an important concept from nucleation theory. We show that the capture-zone model applies to the growth of this system even in the limit of a low QD density in which atoms diffuse over μm distances. The strong correlations between the various QD parameters facilitate preselection of QDs for applications with specific requirements on the QD properties; they also suggest that a spectrally narrowed QD distribution will result if QD growth on a regular lattice can be achieved. © 2019 American Physical Society.

We analyze time evolution of charge and spin states in a quantum dot coupled to an electric reservoir. Utilizing high-speed single-electron detection, we focus on dynamics induced by the first-order tunneling. We find that there is a difference between the spin and the charge relaxation: The former appears slower than the latter. The difference depends on the Fermi occupation factor and the spin relaxation becomes slower when the energy level of the quantum dot is lowered. We explain this behavior by a theory including the first-order tunneling processes and find a good agreement between the experiment and the theory. © 2019 American Physical Society.

The Stranski–Krastanov growth-mode facilitates the self-assembly of quantum dots (QDs) by using lattice-mismatched semiconductors, for instance, InAs and GaAs. These QDs are excellent photon emitters: the optical decay of QD-excitons creates high-quality single-photons, which can be used for quantum communication. One significant drawback of the Stranski–Krastanov mode is the wetting layer. It results in a continuum close in energy to the confined states of the QD. The wetting-layer-states lead to scattering and dephasing of QD-excitons. Here, we report a slight modification to the Stranski–Krastanov growth-protocol of InAs on GaAs, which results in a radical change of the QD-properties. We demonstrate that the new QDs have no wetting-layer-continuum for electrons. They can host highly charged excitons where up to six electrons occupy the same QD. In addition, single QDs grown with this protocol exhibit optical linewidths matching those of the very best QDs making them an attractive alternative to conventional InGaAs QDs. © 2019, The Author(s).

Fano resonances are produced by integrating an electric split ring resonator (eSRR) and an I shaped resonator (ISR) into a composite metasurface. By adjusting the length of the ISR the degree of asymmetry in the line shape can be controlled and even made to be symmetric. Metasurfaces of the composite structures are fabricated, and their transmission is measured with terahertz time-domain spectroscopy. The measurements show good agreement with finite difference time domain (FDTD) simulations. © 2019 IEEE.

We measure conductivity and light response of a top gated electric field-induced 2DEG carrier in a GaAs based quantum-well heterostructure to confirm their stability as a consequence of removing the Si doping layer. Etched sidewall contacting techniques and surface treatment studies allowed carrier induction in a quantum-dot-fabrication compatible device structure. We performed quantum Hall measurements to evaluate the conduction stability and observed temporal decays of the induced carriers and illumination at several conditions that could still be redeemed by choosing proper fabrication techniques and operation voltage conditions. Mesoscopic research on such un-doped devices have possibility of efficiently interfacing single photons and single electron spins whereas that of holes. © 2019 IEEE.

Neodymium sulphide (Nd 2 S 3 ) belongs to the exciting class of rare earth sulphides (RES) and is projected to have a serious potential in a wide spectrum of application either in pure form or as dopant. We demonstrate a facile and first growth of Nd 2 S 3 thin films via metal-organic chemical vapour deposition (MOCVD) at moderate process conditions using two new Nd precursors, namely tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)Nd(iii) and tris(N,N′-diisopropyl-acetamidinato)Nd(iii). The promising thermal properties and suitable reactivity of both Nd precursors towards elemental sulphur enabled the formation of high purity γ-Nd 2 S 3 . While the process temperature for film growth ranged from 400 °C to 600 °C, the films were crystalline above 500 °C. We also demonstrate that the as-deposited γ-Nd 2 S 3 are luminescent, with the optical bandgap ranging from 2.3 eV to 2.5 eV. The process circumvents post-deposition treatments such as sulfurisation to fabricate the desired Nd 2 S 3 , which paves the way for large scale synthesis and also opens up new avenues for exploring the potential of this class of materials with properties for functional applications. © 2019 The Royal Society of Chemistry.

The active routing of photons using rapid reconfigurable integrated circuits is a key functionality for quantum-in-formation processing. Typical waveguide-based optical switches rely on the modulation of the refractive index, producing a modest variation of the phase of the optical fields. Mechanical motion of nanophotonic structures, on the contrary, can be tailored to produce a much larger effect, without introducing loss or emitter decoherence and operating at a speed matching the quantum memory storage time of the on-chip quantum emitter. Here we demonstrate a compact and low-loss nano-opto-electromechanical single-photon router, based on two coupled waveguides whose distance is adjusted on demand by an external voltage. We show controllable two-port routing of single photons emitted from quantum dots embedded in the same chip. We report a maximum splitting ratio >23 dB, insertion loss of 0.67 dB, and sub-microsecond response time. The device is an essential building block for constructing advanced quantum photonic architectures on-chip, towards, e.g., coherent multi-photon sources, deterministic photon– photon quantum gates, quantum-repeater nodes, or scalable quantum networks. © 2019 Optical Society of America.

We demonstrate a nano-electro-mechanical single-photon router integrated with semiconductor quantum emitters, showing an extinction ratio of > 20 mathrm{dB} and operation speed of MHz with insertion loss of 0.67 dB and footprint < 30mu m-{2}. © 2019 The Author(s) 2019 OSA.

The maximum information of a dynamic quantum system is given by real-time detection of every quantum event, where the ultimate challenge is a stable, sensitive detector with high bandwidth. All physical information can then be drawn from a statistical analysis of the time traces. We demonstrate here an optical detection scheme based on the time-resolved resonance fluorescence on a single quantum dot. Single-electron resolution with high signal-to-noise ratio (4σ confidence) and high bandwidth of 10 kHz make it possible to record the individual quantum events of the transport dynamics. Full counting statistics with factorial cumulants gives access to the nonequilibrium dynamics of spin relaxation of a singly charged dot (γ↑↓=3 ms-1), even in an equilibrium transport measurement. © 2019 American Physical Society.

Quantum state transfer from a single photon to a single electron following selection rules can only occur for a spin-resolved light-hole excitation in GaAs quantum dots; however, these phenomena have yet to be experimentally realized. Here, we report on single-shot readout of a single electron spin via the Zeeman-resolved light-hole excitation using an optical spin blockade method in a GaAs quantum dot and a Pauli spin blockade method in a double GaAs quantum dot. The observed photoexcitation probability strongly depends on the photon polarization, an indication of angular momentum transfer from a single photon to an electron. Our demonstration will open a pathway to further investigation of fundamental quantum physics and applications of quantum networking technology. © 2019 American Physical Society.

Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization. © 2018 American Chemical Society.

Measurements of quantum systems inevitably involve disturbance in various forms. Within the limits imposed by quantum mechanics, there exists an ideal projective measurement that does not introduce a back action on the measured observable, known as a quantum non-demolition (QND) measurement1,2. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot. We entangle the single spin with a two-electron, singlet–triplet ancilla qubit via the exchange interaction3,4 and then read out the ancilla in a single shot. This procedure realizes a disturbance-free projective measurement of the single spin at a rate two orders of magnitude faster than its relaxation. The QND nature of the measurement protocol5,6 enables enhancement of the overall measurement fidelity by repeating the protocol. We demonstrate a monotonic increase of the fidelity over 100 repetitions against arbitrary input states. Our analysis based on statistical inference is tolerant to the presence of the relaxation and dephasing. We further exemplify the QND character of the measurement by observing spontaneous flips (quantum jumps)7 of a single electron spin. Combined with the high-fidelity control of spin qubits8–13, these results will allow for various measurement-based quantum state manipulations including quantum error correction protocols14. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.

Surface acoustic waves (SAWs) strongly modulate the shallow electric potential in piezoelectric materials. In semiconductor heterostructures such as GaAs/AlGaAs, SAWs can thus be employed to transfer individual electrons between distant quantum dots. This transfer mechanism makes SAW technologies a promising candidate to convey quantum information through a circuit of quantum logic gates. Here we present two essential building blocks of such a SAW-driven quantum circuit. First, we implement a directional coupler allowing to partition a flying electron arbitrarily into two paths of transportation. Second, we demonstrate a triggered single-electron source enabling synchronisation of the SAW-driven sending process. Exceeding a single-shot transfer efficiency of 99%, we show that a SAW-driven integrated circuit is feasible with single electrons on a large scale. Our results pave the way to perform quantum logic operations with flying electron qubits. © 2019, The Author(s).

In order to realize a novel detection method of a polarization of a photon, we propose the combination of spin Hall effect and quantum conversion from a polarization state of photon to a spin state of electron. Cross-shaped Hall bar of a quantum well was fabricated and was illuminated by circular polarized light. The optically generated electron spin polarization generates the inverse spin Hall voltage via spin-orbit interaction along the orthogonal direction to the applied voltage. We investigated the dependence of inverse spin Hall voltage on the excitation wavelength. The result show two peaks at energies that match to light and heavy hole excitation. © 2019 IEEE.

The combination of integrated nanophotonic circuits and single quantum emitters holds great promise for scalable quantum information processing (QIP) and for the realization of photonic quantum networks. Efficient and deterministic sources of highly-pure and indistinguishable single photons have been demonstrated using quantum dots (QDs) in GaAs nanostructures [1], enabling the integration with planar circuitry. However, the efficient out-coupling of light from the chip into optical fibres, required for communication with distant quantum nodes and photon storage, remains a challenging task. Here, we report a spot-size converter for the end-fire coupling between suspended GaAs waveguides with embedded QDs and lensed fibres. © 2019 IEEE.

We present anomalous behavior of temperature-dependent photoluminescence (PL) measurements on InAs quantum dot ensembles coupled to an electron reservoir in an n-i-p diode structure. When negative gate voltages are applied to the sample, an anomalous initial increase of the integrated PL signal with rising temperature is observed for the ground-state and first-excited-state emission peaks. In contrast, measurements at positive gate voltages show no such anomaly and are well described by the commonly used Arrhenius model. Unlike previous studies on uncoupled quantum dot ensembles, we show that in quantum dot diode structures the anomalous temperature dependence and its dependence on the applied bias voltage is dominated by electrons tunneling from the electron reservoir to the quantum dots. Tunneling electrons enhance the PL signal by recombining with holes stored in the quantum dots and the tunneling rate depends on temperature via the Fermi distribution in the electron reservoir. With the implementation of a rate-based tunnel coupling, we develop a modified Arrhenius model that takes the observed anomalies excellently into account. Gate voltage dependent PL measurements at 77 K are further compared to capacitance-voltage spectroscopy measurements on the same sample, supporting the proposed interpretation. The PL peak width shows a characteristic evolution as a function of temperature, which is discussed qualitatively in terms of our model. © 2019 American Physical Society.

We present high-cooperativity (C up to 140) strong coupling of a charge-Tunable InAs quantum dot embedded in a tunable Fabry-Pérot microcavity (Q=500,000). Via second-order correlation measurements we show high single-photon purity in the photon-blockade regime and pronounced vacuum Rabi oscillations in the photon-induced tunneling regime. © 2018 IEEE.

Single-spin qubits in semiconductor quantum dots hold promise for universal quantum computation with demonstrations of a high single-qubit gate fidelity above 99.9% and two-qubit gates in conjunction with a long coherence time. However, initialization and readout of a qubit is orders of magnitude slower than control, which is detrimental for implementing measurement-based protocols such as error-correcting codes. In contrast, a singlet-triplet qubit, encoded in a two-spin subspace, has the virtue of fast readout with high fidelity. Here, we present a hybrid system which benefits from the different advantages of these two distinct spin-qubit implementations. A quantum interface between the two codes is realized by electrically tunable inter-qubit exchange coupling. We demonstrate a controlled-phase gate that acts within 5.5 ns, much faster than the measured dephasing time of 211 ns. The presented hybrid architecture will be useful to settle remaining key problems with building scalable spin-based quantum computers. © 2018, The Author(s).

We describe the major requirements to experimentally perform and observe resonant spin-flip Raman scattering on excitonic resonances in low-dimensional semiconductors. We characterize in detail the properties of this resonant light scattering technique and evaluate the criteria, which must be fulfilled by the experimental setup and the semiconductor sample studied to be able to observe a spin-flip scattering process. We also demonstrate the influence of additional unpolarized laser illumination with energies, which exceed considerably the band gap energy of the semiconductor nanostructure under study, on the resonantly excited electron spin-flip scattering in InAs-based quantum dot ensembles as well as on the paramagnetic Mn-ion spin-flip in (Zn,Mn)Se/(Zn,Be)Se quantum wells. © 2018, Pleiades Publishing, Ltd.

Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor-normal metal-superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2 μm with a critical magnetic field B c ≈ 8 T at 1.6 K and T c ≈ 5 K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG (μ n &gt; 104 cm2 V-1 s-1 ). The current and voltage probes were superconducting tungsten-gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current (I c) large enough such that the Josephson coupling parameter . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a T c typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston-Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures. © 2018 IOP Publishing Ltd.

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the buildup of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes. © 2018 American Physical Society.

The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, whereas p-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both s-shell and p-shell exciton dephasing. We show that multiple p-shell states are also responsible for stronger phonon dephasing for these transitions. © 2018 American Physical Society.

We study the coherent evolution of an InAs self-assembled quantum dot (QD) ensemble in the ultrafast regime. The evolution of the entire frequency distribution is revealed by performing prepulse two-dimensional (2D) coherent spectroscopy. Charged and neutral QDs display distinct nonlinear responses arising from two-level trion and four-level exciton-biexciton systems, respectively, and each signal is clearly separated in 2D spectra. Whereas the signals for charged QDs are symmetric with respect to the detuning, those for neutral QDs are asymmetric due to the asymmetric four-level energy structure. Experimental results for charged and neutral QDs are well reproduced by solving the optical Bloch equations, including detuning and excitation-induced dephasing (EID) effects. The temperature dependence suggests that wetting-layer carriers play an important role in EID. © 2018 American Physical Society.

The diffraction limit prevents a conventional optical microscope from imaging at the nanoscale. However, nanoscale imaging of molecules is possible by exploiting an intensity-dependent molecular switch 1-3 . This switch is translated into a microscopy scheme, stimulated emission depletion microscopy 4-7 . Variants on this scheme exist 3,8-13, yet all exploit an incoherent response to the lasers. We present a scheme that relies on a coherent response to a laser. Quantum control of a two-level system proceeds via rapid adiabatic passage, an ideal molecular switch. We implement this scheme on an ensemble of quantum dots. Each quantum dot results in a bright spot in the image with extent down to 30 nm (λ/31). There is no significant loss of intensity with respect to confocal microscopy, resulting in a factor of 10 improvement in emitter position determination. The experiments establish rapid adiabatic passage as a versatile tool in the super-resolution toolbox. © 2018 The Author(s).

Scaling up qubits is a necessary step to realize useful systems of quantum computation. Here, we demonstrate coherent manipulations of four individual electron spins using a micro-magnet method in each dot of a quadruple quantum dot - the largest number of dots used for the single spin control in multiple quantum dots. We observe Rabi oscillations for each dot through electron spin resonance, evaluate the spin-electric coupling of the four dots, and finally discuss practical approaches to independently address single spins in multiple quantum dot systems containing even more quantum dots. © 2018 Author(s).

We report on capacitance-voltage spectroscopy of self-assembled InAs quantum dots under constant illumination. Besides the electronic and excitonic charging peaks in the spectrum reported earlier, we find additional resonances associated with nonequilibrium state tunneling unseen in C(V) measurements before. We derive a master-equation-based model to assign the corresponding quantum state tunneling to the observed peaks. C(V) spectroscopy in a magnetic field is used to verify the model-assigned nonequilibrium peaks. The model is able to quantitatively address various experimental findings in C(V) spectroscopy of quantum dots such as the frequency- and illumination-dependent peak height, a thermal shift of the tunneling resonances and the occurrence of the additional nonequilibrium peaks. © 2018 American Physical Society.

We studied the interlayer charge transfer in n-modulation doped Al1-xGaxAs-GaAs single heterostructures by photoluminescence and magnetotransport. Photoluminescence contributions from a high-mobility quasi-twodimensional electron system at the interface and GaAs bulk type excitons were analyzed, covering an excitation intensity range of nearly three orders of magnitude. The experiment was compared with selfconsistent band structure calculations that allowed to follow the charge redistribution in the sample. After sample cool down in the dark a parallel conducting quasi-twodimensional channel of low electron mobility appeared in the Al1-xGaxAs-layer, that gained in density up to a saturation value. There was only little interaction between the Al1-xGaxAs and GaAs sides in the persistent regime and a reduction of the depletion charge in the GaAs could largely account for the interface channel density enhancement. Contrary, the negative photoeffect present during continuous illumination strongly depends on the interaction between the interface and the parallel channel, which serves as a temporary sink for excess charges from the GaAs side. When the illumination is switched off the parallel channel acts as a source and the sample essentially returns to the prior illumination state of saturation. © 2018 IOP Publishing Ltd.

Self-assembled InAs quantum dots in a GaAs matrix are studied by Laplace deep level transient spectroscopy (LDLTS). This technique is demonstrated to be complementary to the well-established capacitance spectroscopy concepts and is particularly well suited for characterization of quantum dot layers with large separations from conductive layers. In comparison to conventional deep level transient spectroscopy, LDLTS can also be applied in the tunneling regime where the lifetimes of the confined states are independent of temperature, while in the thermal regime, LDLTS has a superior selectivity. The problems encountered hitherto with this technique are demonstrated to originate from the ill-posed character of the inverse Laplace transform and can be solved by a properly adapted choice of the regularization parameter. © 2018 Author(s).

In this work, we first study the effect of different growth parameters on the molecular beam epitaxy (MBE) growth of GaAs layers on (1 1 1)A oriented substrates. After that we present a method for the MBE growth of atomically smooth layers by sequences of growth and annealing phases. The samples exhibit low surface roughness and good electrical properties shown by atomic force microscopy (AFM), scanning electron microscopy (SEM) and van-der-Pauw Hall measurements. © 2017 Elsevier B.V.

The combination of focused ion beam (FIB) implantation and molecular beam epitaxy (MBE) as ultrahigh-vacuum (UHV) processes allows for nm-resolution fabrication both in lateral as well as in growth direction. The authors exploit self-organized growth of Stranski-Krastanov InxGa1-x. As quantum dots and III–V nanowire structures, both initiated by FIB-implantation of different ion species. Samples are transferred between the FIB and the MBE by UHV-tunnels or a separate UHV-suitcase which links instruments far away from each other. Since the whole process is within the UHV, no wet or dry chemistry deteriorates the solid-state interfaces which increases the purity and the reproducibility. Since the available FIB ion species are not only Gallium, but around 40 elements of the periodic table, this method is very versatile and covers even elements which are usually not introduced in a GaAs-MBE chamber due to purity reasons. Thus, beside site controlled growth any FIB doping, before, in between, due to UHV transfer and after the MBE-growth becomes possible. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The spin dynamics in a broad range of systems can be studied using circularly polarized optical excitation with alternating helicity. The dependence of spin polarization on the frequency of helicity alternation, known as the spin inertia effect, is used here to study the spin dynamics in singly charged (In,Ga)As/GaAs quantum dots (QDs), providing insight into spin generation and accumulation processes. We demonstrate that the dependence of spin polarization in n- and p-type QDs on the external magnetic field has a characteristic V- and M-like shape, respectively. This difference is related to different microscopic mechanisms of the resident carriers' spin orientation. It allows us to determine the parameters of the spin dynamics both for the ground and excited states of singly charged QDs. © 2018 American Physical Society.

We demonstrate superresolution imaging of single rare-earth emitting centers, namely, trivalent cerium, in yttrium aluminum garnet crystals by means of stimulated emission depletion (STED) microscopy. The achieved all-optical resolution is ≈50 nm. Similar results were obtained on H3 color centers in diamond. In both cases, STED resolution is improving slower than the conventional inverse square-root dependence on the depletion beam intensity. In the proposed model of this effect, the anomalous behavior is caused by excited state absorption and the interaction of the emitter with nonfluorescing crystal defects in its local surrounding. © 2018 American Physical Society.

The spin inertia measurement is a recently emerged tool to study slow spin dynamics, which is based on the excitation of the system by a train of circularly polarized pulses with alternating helicity. Motivated by the experimental results reported by Zhukov et al. [Phys. Rev. B 98, 121304(R) (2018)10.1103/PhysRevB.98.121304], we develop the general theory of spin inertia of localized charge carriers. We demonstrate that the spin inertia measurement in longitudinal magnetic field allows one to determine the parameters of the spin dynamics of resident charge carriers and of photoexcited trions, such as the spin relaxation times, longitudinal g factors, parameters of hyperfine interaction, and nuclear spin correlation times. © 2018 American Physical Society.

We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are developed for double quantum dots in GaAs heterostructures, but they can easily be adapted to other quantum-dot-based qubit systems. These tuning procedures include the characterization of the interdot tunnel coupling, the tunnel coupling to the surrounding leads, and the identification of various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot's parameters will only grow in the near future, once the multiqubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds and require almost no human interference. © 2018 American Physical Society.

We study the photoluminescence of self-assembled (In,Ga)As/GaAs quantum dot ensembles with varying confinement potential height. The low energy shift of the s-shell emission with increasing excitation power gives a measure of the Coulomb interaction in these structures as it results from carrier–carrier interactions between the optically injected exciton complexes. When dividing this shift by the dot level splitting, determined by the geometric confinement, we obtain a universal function of the number of involved excitons that is independent of the confinement potential height. This shows an identical scaling of Coulomb interaction and geometric quantization with varying confinement. © 2018, Pleiades Publishing, Ltd.

The scaling up of electron spin qubit based nanocircuits has remained challenging up till date and involves the development of efficient charge control strategies. Here, we report on the experimental realization of a linear triple quantum dot in a regime isolated from the reservoir. We show how this regime can be reached with a fixed number of electrons. Charge stability diagrams of the one, two, and three electron configurations where only electron exchange between the dots is allowed are observed. They are modeled with the established theory based on a capacitive model of the dot systems. The advantages of the isolated regime with respect to experimental realizations of quantum simulators and qubits are discussed. We envision that the results presented here will make more manipulation schemes for existing qubit implementations possible and will ultimately allow to increase the number of tunnel coupled quantum dots which can be simultaneously controlled. © 2017 Author(s).

A two-dimensional arrangement of quantum dots (QDs) with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple QD with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov-Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple QD from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages. © 2017 IOP Publishing Ltd.

We have measured the spin relaxation time of an excited two-electron spin-triplet state into its singlet ground state in self-assembled InAs/GaAs quantum dots. We use a time-resolved measurement scheme that combines transconductance spectroscopy with spin-to-charge conversion to address the |s↑,p↑〉 triplet state, where one electron is in the quantum dot s-shell and a second one in the p-shell. The evaluation of the state-selective tunneling times from the dots into a nearby two-dimensional electron gas allows us to determine the s- and p-shell occupation and extract the relaxation time from a rate equation model. A comparably long triplet-to-singlet spin relaxation time of 25 μs is found. © 2017 Author(s).

Luminescent erbium-based inorganic–organic hybrid materials play an important role in many frontier nano-sized applications, such as amplifiers, detectors and OLEDs. Here, we demonstrate the possibility to fabricate high-quality thin films comprising both erbium and an appropriate organic molecule as a luminescence sensitizer utilizing the combined atomic layer deposition and molecular layer deposition (ALD/MLD) technique. We employ tris(N,N′-diisopropyl-2-dimethylamido guanidinato)erbium(III) [Er(DPDMG)3] together with 3,5-pyridine dicarboxylic acid as precursors. With the appreciably high film deposition rate achieved (6.4 Å cycle−1), the guanidinate precursor indeed appears as an interesting new addition to the ALD/MLD precursor variety toward novel materials. Our erbium–organic thin films showed highly promising UV absorption properties and a photoluminescence at 1535 nm for a 325-nm excitation, relevant to possible future luminescence applications. © 2017 Springer Science+Business Media New York

Over the past two decades, lateral quantum dots have permitted a tremendous advancement in the manipulation of individual electrons. In order to have a complete toolbox for electronics at the single electron level, local manipulation in a quantum dot needs to be associated with the controlled transport of individual electrons. Here, we review results on the transfer of individual electrons and their spin degree of freedom between distant lateral quantum dots. The electron is transported in a surface acoustic wave-generated moving quantum dot, with an efficiency of 92%. Furthermore, we will review recent results showing that classical spin information/magnetization can be partially transferred using this method. The fidelity was proven to be limited by the current sample design and implementation, and no fundamental limitation was met. This transfer capability opens new avenues in spin-based quantum information processing and in the implementation of quantum optics experiments with flying electrons. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. Several approaches for nonresonant, resonant, and quasiresonant biexciton preparation exist, but all have their own disadvantages; for instance, low fidelity, timing jitter, incoherence, or sensitivity to experimental parameters. We demonstrate a coherent and robust technique to generate a biexciton in an InGaAs quantum dot with a fidelity close to 1. The main concept is the application of rapid adiabatic passage to the ground-state-exciton-biexciton system. We reinforce our experimental results with simulations which include a microscopic coupling to phonons. © 2017 American Physical Society.

Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite extensive experimental efforts to control the electron displacement over long distances, maintaining electron spin coherence after transfer remained elusive up to now. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 μm. This displacement is realized on a closed path made of three tunnel-coupled lateral quantum dots at a speed approaching 100 ms-1. We find that the spin coherence length is eight times longer than expected from the electron spin coherence without displacement, pointing at a process similar to motional narrowing observed in nuclear magnetic resonance experiments. The demonstrated coherent displacement will open the route towards long-range interaction between distant spin qubits. © 2017 The Author(s).

Nano device prototyping (NDP) is essential for realizing and assessing ideas as well as theories in the form of nano devices, before they can be made available in or as commercial products. In this review, application results patterned similarly to those in the semiconductor industry (for cell phone, computer processors, or memory) will be presented. For NDP, some requirements are different: thus, other technologies are employed. Currently, in NDP, for many applications direct write Gaussian vector scan electron beam lithography (EBL) is used to define the required features in organic resists on this scale. We will take a look at many application results carried out by EBL, self-organized 3D epitaxy, atomic probe microscopy (scanning tunneling microscope/atomic force microscope), and in more detail ion beam techniques. For ion beam techniques, there is a special focus on those based upon liquid metal (alloy) ion sources, as recent developments have significantly increased their applicability for NDP. © 2017 Author(s).

Polarization is a fundamental property of light and could provide various solutions to the development of secure optical communications with high capacity and high speed. In particular, the coherent quantum state conversion between single photons and single electron spins is a prerequisite for long-distance quantum communications and distributed quantum computation. Electrically defined quantum dots have already been proven to be suitable for scalable solid state qubits by demonstrations of single-spin coherent manipulations and two-qubit gate operations. Thus, their capacity for quantum information technologies would be considerably extended by the achievement of entanglement between an electron spin in the quantum dots and a photon. In this review paper, we show the basic technologies for trapping single electrons generated by single photons in quantum dots and for detecting their spins using the Pauli effect with sensitive charge sensors. © 2017 The Physical Society of Japan.

Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultrashort laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations. ©2017 American Physical Society.

We investigate the relationship between the Zeeman interaction and the inversion-asymmetry-induced spin-orbit interactions (Rashba and Dresselhaus SOIs) in GaAs hole quantum point contacts. The presence of a strong SOI results in the crossing and anticrossing of adjacent spin-split hole subbands in a magnetic field. We demonstrate theoretically and experimentally that the anticrossing energy gap depends on the interplay between the SOI terms and the highly anisotropic hole g tensor and that this interplay can be tuned by selecting the crystal axis along which the current and magnetic field are aligned. Our results constitute the independent detection and control of the Dresselhaus and Rashba SOIs in hole systems, which could be of importance for spintronics and quantum information applications. © 2017 American Physical Society.

The spin-flip Raman scattering efficiency of the resident electron is thermally enhanced in singly charged (In,Ga)As/GaAs quantum dots, for probing the s- or p-shell trions. The Raman shift, polarization characteristics, and spectral position of the resonant scattering profile are insensitive to the sample temperature up to 50 K. This indicates a thermally robust mechanism of the coherent electron spin-flip based on exchange interaction. The background of the scattering spectra, whose intensity increases also by about one order of magnitude with temperature, is associated with acoustic phonon scattering. We propose that acoustic phonons enhance the spin-flip probability of the resident electron with growing temperature. The coherent spin-flip Raman scattering is ultimately suppressed at temperatures, which are a few times lower than that required for thermal trion dissociation. © 2017 American Physical Society.

The tunneling dynamics between self-assembled quantum dots (QDs) and a charge reservoir can be measured in an all-electrical or optical detection scheme. In all-electrical transconductance spectroscopy, a two-dimensional electron gas is used to probe the evolution of the many-particle states inside an ensemble of QDs from non-equilibrium to equilibrium. The optical detection scheme measures the tunneling dynamic into a single self-assembled dot. The work done and results obtained using these different measurement techniques are reviewed and compared within this article. We will show that transconductance spectroscopy is sensitive to a time-dependent density of states and enables preparation of non-equilibrium charge and spin states for future applications in quantum information processing. The optical resonance fluorescence measurements on the electron dynamics demonstrates the influence of the exciton states on the charge-carrier dynamics and enables a systematic study of the Auger recombination in self-assembled dots. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate monocrystalline InAs nanowires (NWs) which are grown catalyst assisted by molecular beam epitaxy (MBE) and create the catalyst by focused ion beam (FIB) implanted Au spots. With this combination of methods an aspect ratio, i.e. the length to width ratio, of the grown NWs up to 300 was achieved. To control the morphology and crystalline structure of the NWs, the growth parameters like temperature, flux ratios and implantation fluence are varied and optimized. Furthermore, the influence of the used molecular arsenic species, in particular the As2 to As4 ratio, is investigated and adjusted. In addition to the high aspect ratio, this optimization results in the growth of monocrystalline InAs NWs with a negligible number of stacking faults. Single NWs were placed site-controlled by FIB implantation, which supplements the working field of area growth. © 2017

Understanding the dynamics of open quantum systems is important and challenging in basic physics and applications for quantum devices and quantum computing. Semiconductor quantum dots offer a good platform to explore the physics of open quantum systems because we can tune parameters including the coupling to the environment or leads. Here, we apply the fast single-shot measurement techniques from spin qubit experiments to explore the spin and charge dynamics due to tunnel coupling to a lead in a quantum dot-lead hybrid system. We experimentally observe both spin and charge time evolution via first-and second-order tunneling processes, and reveal the dynamics of the spin-flip through the intermediate state. These results enable and stimulate the exploration of spin dynamics in dot-lead hybrid systems, and may offer useful resources for spin manipulation and simulation of open quantum systems. © 2017 The Author(s).

Formation and dissolution of oxide on copper under transpassive conditions, i.e. during OER and transpassive dissolution, in alkaline electrolyte was investigated by a combination of electrochemical techniques and in situ and operando Raman and photoluminescence (PL) spectroscopy, as well as spectropscopic ellipsometry. Experiments were conducted under potentiodynamic and potentiostatic polarisation in 0.1M NaOH. In chronoamperometry experiments with steps between potentials, oxide thickness continued increasing beyond the onset of OER. The thickness dropped significantly from &gt;10 nm to &lt;5 nm ≈400 mV above the OER onset. The presence of CuO, Cu2O and Cu4O3 was observed by Raman spectroscopy after the onset of OER. Correlating with the thickness drop, strong PL was observed at 1.55 eV, indicating the formation of singly charged oxygen vacancies VO+, following the classical PL spectrum interpretation from the literature. PL observation speaks against vacancy pair coalescence as mechanism of dissolution. After electrochemical experiments, the films were n-type semiconductors, not p-type conductors as expected for copper oxides. Results indicate that transpassive dissolution may be triggered by the instability of the oxide with respect to defect formation. © 2017 Elsevier Ltd

We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasiresonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4% and a photon indistinguishability of up to 94±1% by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes, notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another means of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The paper opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip. © 2017 American Physical Society.

In the flourishing fields of quantum technology gallium nitride (GaN) quantum dots (QDs) have great appeal by providing high stability and room-temperature operation. Here, we report on the ion implantation of surface GaN QDs grown in the hexagonal crystal structure. An uncapped sample (S1) and two samples capped by 8 ML (S2) and 16 ML (S3) of AlN are subjected to a 100 keV gallium (S1, S2) and a 210 keV erbium (S3) ion beam. The fluence ranged from 5×1010 cm−2 to 1×1015 cm−2 (S1, S2) and from 5×1010 cm−2 to 5×1013 cm−2 (S3). QD characterization is performed by cathodoluminescence measurements at 77 K and atomic force microscopy and scanning electron microscopy. Strong interdiffusion processes upon ion impact at the interfaces are evidenced leading besides other effects to a quenching of the quantum confined Stark effect. Moreover, a model for the QD morphology based on a fluence-dependent diffusion coefficient is developed. © 2017 Elsevier B.V.

Herein we describe an efficient low temperature (60-160 °C) plasma enhanced atomic layer deposition (PEALD) process for gallium oxide (Ga2O3) thin films using hexakis(dimethylamido)digallium [Ga(NMe2)3]2 with oxygen (O2) plasma on Si(100). The use of O2 plasma was found to have a significant improvement on the growth rate and deposition temperature when compared to former Ga2O3 processes. The process yielded the second highest growth rates (1.5 Å per cycle) in terms of Ga2O3 ALD and the lowest temperature to date for the ALD growth of Ga2O3 and typical ALD characteristics were determined. From in situ quartz crystal microbalance (QCM) studies and ex situ ellipsometry measurements, it was deduced that the process is initially substrate-inhibited. Complementary analytical techniques were employed to investigate the crystallinity (grazing-incidence X-ray diffraction), composition (Rutherford backscattering analysis/nuclear reaction analysis/X-ray photoelectron spectroscopy), morphology (X-ray reflectivity/atomic force microscopy) which revealed the formation of amorphous, homogeneous and nearly stoichiometric Ga2O3 thin films of high purity (carbon and nitrogen &lt;2 at.%) under optimised process conditions. Tauc plots obtained via UV-Vis spectroscopy yielded a band gap of 4.9 eV and the transmittance values were more than 80%. Upon annealing at 1000 °C, the transformation to oxygen rich polycrystalline β-gallium oxide took place, which also resulted in the densification and roughening of the layer, accompanied by a slight reduction in the band gap. This work outlines a fast and efficient method for the low temperature ALD growth of Ga2O3 thin films and provides the means to deposit Ga2O3 upon thermally sensitive polymers like polyethylene terephthalate. © 2017 The Royal Society of Chemistry.

Nanoelectronic devices embedded in the two-dimensional electron system (2DES) of a GaAs/(Al,Ga)As heterostructure enable a large variety of applications ranging from fundamental research to high-speed transistors. Electrical circuits are thereby commonly defined by creating barriers for carriers by the selective depletion of a preexisting 2DES. We explore an alternative approach: we deplete the 2DES globally by applying a negative voltage to a global top gate and screen the electric field of the top gate only locally using nanoscale gates placed on the wafer surface between the plane of the 2DES and the top gate. Free carriers are located beneath the screen gates, and their properties can be controlled by means of geometry and applied voltages. This method promises considerable advantages for the definition of complex circuits by the electric-field effect, as it allows us to reduce the number of gates and simplify gate geometries. Examples are carrier systems with ring topology or large arrays of quantum dots. We present a first exploration of this method pursuing field effect, Hall effect, and Aharonov-Bohm measurements to study electrostatic, dynamic, and coherent properties. © 2017 American Physical Society.

The transmission phase across a quantum dot (QD) is expected to show mesoscopic behavior, where the appearance of a phase lapse between Coulomb peaks (CPs) as a function of the gate voltage depends on the orbital parity relation between the corresponding CPs. On the other hand, such mesoscopic behavior has been observed only in a limited QD configuration (a few-electron and single-level transport regime) and universal phase lapses by π between consecutive CPs have been reported for all the other configurations. Here, we report on the measurement of a transmission phase across a QD around the crossover between single-level and multilevel transport regimes employing an original two-path quantum interferometer. We find mesoscopic behavior for the studied QD. Our results show that the universal phase lapse, a longstanding puzzle of the phase shift, is absent for a standard QD, where several tens of successive well-separated CPs are observed. ©2017 American Physical Society.

We demonstrate full charge control, narrow optical linewidths, and optical spin pumping on single self-assembled InGaAs quantum dots embedded in a 162.5-nm-thin diode structure. The quantum dots are just 88nm from the top GaAs surface. We design and realize a p-i-n-i-n diode that allows single-electron charging of the quantum dots at close-to-zero applied bias. In operation, the current flow through the device is extremely small resulting in low noise. In resonance fluorescence, we measure optical linewidths below 2μeV, just a factor of 2 above the transform limit. Clear optical spin pumping is observed in a magnetic field of 0.5T in the Faraday geometry. We present this design as ideal for securing the advantages of self-assembled quantum dots - highly coherent single-photon generation, ultrafast optical spin manipulation - in the thin diodes required in quantum nanophotonics and nanophononics applications. © 2017 American Physical Society.

For the first time, synthesis of two new amidinate-ligand comprising heteroleptic indium complexes, namely [InCl(amd)2] (1) and [InMe(amd)2] (2), via salt-metathesis and their detailed characterization is reported. For comparison, the earlier reported homoleptic tris-amidinate [In(amd)3] (3) was also synthesized and analyzed in detail especially with respect to the thermal properties and molecular crystal structure analysis which are reported here for the first time. From nuclear magnetic resonance spectroscopy (NMR) and single-crystal X-ray diffraction (XRD), all three compounds were found to be monomeric with C2 (compound 1 and 2) and C3 symmetry (compound 3). Both halide-free compounds 2 and 3 were evaluated regarding their thermal properties using temperature-dependent 1H-NMR, thermogravimetric analysis (TGA) and iso-TGA, revealing suitable volatility and thermal stability for their application as potential precursors for chemical vapor phase thin film deposition methods. Indeed, metalorganic chemical vapor deposition (MOCVD) experiments over a broad temperature range (400 °C-700 °C) revealed the suitability of these two compounds to fabricate In2O3 thin films in the presence of oxygen on Si, thermally grown SiO2 and fused silica substrates. The as-deposited thin films were characterized in terms of their crystallinity via X-ray diffraction (XRD), morphology by scanning electron microscopy (SEM) and composition through complementary techniques such as Rutherford-backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). From UV/Vis spectroscopy, the deposited In2O3 thin films on fused silica substrates were found to be highly transparent (T &gt; 95% at 560 nm, compound 3). In addition, Hall measurements revealed high charge carrier densities of 1.8 × 1020 cm-3 (2) and 6.5 × 1019 cm-3 (3) with a Hall-mobility of 48 cm2 V-1 s-1 (2) and 74 cm2 V-1 s-1 (3) for the respective thin films, rendering the obtained thin films applicable as a transparent conducting oxide that could be suitable for optoelectronic applications. © 2017 The Royal Society of Chemistry.

The electron wave function experiences a phase modification at coherent transmission through a quantum dot. This transmission phase undergoes a characteristic shift of π when scanning through a Coulomb blockade resonance. Between successive resonances either a transmission phase lapse of π or a phase plateau is theoretically expected to occur depending on the parity of quantum dot states. Despite considerable experimental effort, this transmission phase behaviour has remained elusive for a large quantum dot. Here we report on transmission phase measurements across such a large quantum dot hosting hundreds of electrons. Scanning the transmission phase along 14 successive resonances with an original two-path interferometer, we observe both phase lapses and plateaus. We demonstrate that quantum dot deformation alters the sequence of phase lapses and plateaus via parity modifications of the involved quantum dot states. Our findings set a milestone towards an comprehensive understanding of the transmission phase of quantum dots. © 2017 The Author(s).

The spin noise in singly charged self-assembled quantum dots is studied theoretically and experimentally under the influence of a perturbation, provided by additional photoexcited charge carriers. The theoretical description takes into account generation and relaxation of charge carriers in the quantum dot system. The spin noise is measured under application of above barrier excitation for which the data are well reproduced by the developed model. Our analysis demonstrates a strong difference of the recharging dynamics for holes and electrons in quantum dots. © 2017 American Physical Society.

In this work, the accuracy of the thermal diffusivity estimation in moderately and heavily doped semiconductor samples using the modulated photothermal infrared radiometry is investigated. The studies were carried out on heavily doped Si and GaAs wafers, and on moderately doped Si and recently studied GaAs and CdSe samples. It is shown, that depending on the infrared properties of the semiconductor sample, the modulated photothermal infrared radiometry signal can yield information about thermal diffusivity, (effective) infrared absorption coefficient and electronic transport parameters (recombination lifetime, carrier diffusivity and surface recombination velocities). For the heavily doped samples, the modulated photothermal infrared radiometry signal consists only of the thermal response yielding information about the (effective) infrared absorption coefficient and thermal diffusivity. The relative expanded uncertainty with 0.95 level of confidence Ur of estimating the thermal diffusivity in this case is about Ur = 0.05. For moderately doped samples the modulated photothermal infrared radiometry signal consists of the thermal and of the photocarrier response. The relative expanded uncertainty with 0.95 level of confidence Ur of estimating the thermal diffusivity in this case varies between about Ur = 0.10 and about Ur = 0.30, depending on the existence of the maximum in the signal phase, but information about the electronic transport properties is derived. It is shown that not only infrared properties have the influence on the accuracy in estimating the thermal diffusivity of moderate doped semiconductor samples, but also the thermal, geometrical (thickness) and carrier recombination properties can play an important role. © 2017 Elsevier B.V.

In this paper, we report on measurements of the infrared absorption coefficient in the mid-infrared range of a heavily Zn-doped GaAs wafer using spectrally resolved modulated photothermal infrared radiometry (PTR). The method allows us to measure the infrared absorption coefficient of (i) much thicker samples as compared to the one used in Fourier Transform Infrared (FTIR) spectroscopy in transmission configuration and (ii) with non-mirror-like surfaces as would be required for measurements in the reflection configuration. From the best fits of the theoretical model to the PTR results, the values of the infrared absorption coefficient and thermal diffusivity of GaAs wafer are obtained. These values of infrared absorption coefficients are compared both with the literature values on very thin, similarly doped GaAs:Be sample and with infrared absorption coefficients calculated from FTIR specular reflectance measurements on the same sample. FTIR reflectance measurements demand additional assumptions for the evaluation of absorption coefficient and mirror-like surfaces. The results obtained from both experimental methods yield the same order of the infrared absorption coefficients. It is observed that the infrared absorption coefficient decreases with increasing wavelength because of inter-valence band transitions. However, only the infrared spectrum estimated using PTR exhibits free carrier absorption effect at a shorter wavelength as observed in previous works on very thin Be-doped GaAs samples. It is worth mentioning that the presented method is not limited to semiconductors, but can be used for other highly infrared absorbing samples. In addition, the spectrally resolved PTR measurements simultaneously provide the same values of thermal diffusivity of the GaAs wafer within estimation uncertainties thus demonstrating the reliability of the PTR method in the measurement of thermal diffusivity of such samples. © 2017 Author(s).

Nanoscale three-terminal T-branch junctions operated in pushpull fashion (VL=-VR = V0) commonly exhibit a nonzero voltage VC at the centre electrode. In principle, their sign corresponds to the conduction type of the semiconductor material. For example is VC &lt; 0 for n-type conduction, independent of the origin of the effect which could be ballistic or diffusive mode control, hot-electron thermopower or, in Y-shaped junctions, ballistic charging. We report on orthogonal four-terminal junctions on high-mobility n-type GaAs/AlGaAs, with currentcarrying branches of varying length at constant widths of 410 nm and 320 nm, respectively. When operated at low temperatures as three-terminal devices, we show that under sufficiently large gate voltage the result is VC &gt; 0. This is particularly pronounced at short branches where the mode effect is weak. Up to a current ILR ∼ 25 mA we observe VC ϵ ILR independent of the branch length, where VC ILR ∼ 70 W. Around V0 = 0 the centre voltage exhibits a parabolic behaviour, VC V0 , = k 2 where the curvature k is independent of electron density in the range n ≥ 4.4 × 1011 cm-2. As temperature rises k monotonically decreases, staying positive up to 77 K. The observation of the Gurzhi effect as a signature of the hydrodynamic transport regime suggests an explanation of the mechanism in terms of the electronic analogue of the Venturi effect. © 2017 IOP Publishing Ltd.

We demonstrate a new method for projective single-shot measurement of two electron spin states (singlet versus triplet) in an array of gate-defined lateral quantum dots in GaAs. The measurement has very high fidelity and is robust with respect to electric and magnetic fluctuations in the environment. It exploits a long-lived metastable charge state, which increases both the contrast and the duration of the charge signal distinguishing the two measurement outcomes. This method allows us to evaluate the charge measurement error and the spin-to-charge conversion error separately. We specify conditions under which this method can be used, and project its general applicability to scalable quantum dot arrays in GaAs or silicon. © 2017 American Physical Society.

Quantum entanglement between different forms of qubits is an indication of the universality of quantum mechanics. Entanglement transfer between light and matter, especially photon and spin, has long been studied as the central concept, but it remains technically challenging for single photons and spins. In this paper, we show paired generation of a single electron in a GaAs quantum dot and a single photon from a single polarization-entangled photon pair. We measure temporal coincidence between the single photo-electron detection and the single photon detection. Considering a single photon polarization is converted to an electron spin via an optical selection rule, the present result indicates the capability of photon to spin entanglement transfer. This may be useful to explore the physics of entanglement transfer and also for applications to quantum teleportation based quantum communication. © 2017 The Author(s).

Photoluminescence polarization is experimentally studied for samples with (In,Ga)As/GaAs self-assembled quantum dots in transverse magnetic field (Hanle effect) under slow modulation of the excitation light polarization from fractions of Hz to tens of kHz. The polarization reflects the evolution of strongly coupled electron-nuclear spin systems in the quantum dots. Strong modification of the Hanle curves under variation of the modulation period is attributed to the peculiarities of the spin dynamics of quadrupole nuclei, which states are split due to deformation of the crystal lattice in the quantum dots. Analysis of the Hanle curves is fulfilled in the framework of a phenomenological model considering a separate dynamics of a nuclear field BNd determined by polarization of the ±1/2 nuclear spin states and of a nuclear field BNq determined by polarization of the split-off states ±3/2, ±5/2, etc. It is found that the characteristic relaxation time for the nuclear field BNd is of order a fraction of a second, while the relaxation of the field BNq is faster by about two orders of magnitude. © 2017 American Physical Society.

Novel copper ketoiminate compounds were synthesized and for the first time applied for additive-free solution-based deposition of nanoscale copper oxide thin films. The two closely related compounds, namely the bis[4-(2-ethoxyethyl-imino)-3-pentanonato] copper, [Cu(EEKI)(2)], and bis[4-(3-methoxypropylimino)- 3-pentanonato] copper, [Cu(MPKI)(2)], were characterized by means of elemental and thermogravimetric analysis (TGA), as well as electron impact mass spectrometry (EI-MS). The advantages of these compounds are that they are liquid and possess excellent solubility in common organic solvents in addition to an optimum reactivity towards ambient moisture that enables a facile solution-based approach to nanoscale copper oxide thin films. Moreover, no additives or aging is needed to stabilize the solution processing of the copper oxide layers. [Cu(MPKI)(2)] was tested in detail for the deposition of copper oxide thin films by spin coating. Upon one-step annealing, high-quality, uniform, crystalline copper oxide thin films were deposited on Si, SiO2, as well as on quartz substrates. Structural, morphological and compositional characteristics of the copper oxide nanostructures were investigated in detail by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and a combined analysis using Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). It was possible to control the copper oxide phases (CuO and Cu2O) by systematic tuning of the post-deposition annealing conditions. The functional properties in terms of optical band gap were investigated using UV/Vis spectroscopy, while the transport properties, such as resistivity, mobility and carrier concentration were analyzed employing Hall measurements, which confirmed the p-type conductivity of the copper oxide layers.

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ωΤ = 0, ωt = ω0), (ωΤ = ω0, ωt = ω0), and (ωΤ = 2ω0, ωt = ω0) where ω0 denotes the lasing frequency. The peak at ωΤ = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery. © 2017 Optical Society of America.

Self-assembled InxGa1−xAs quantum dots (QDs) are promising hosts for spin qubits with excellent coupling to photons. Nuclear spin and charge fluctuations lead to dephasing and limit the applicability of QDs as qubits. We show that charge noise can be minimized by high quality MBE growth of well-designed heterostructures yielding natural optical linewidths down to 1.15 µeV. To minimize the nuclear spin noise, one direction would be to reduce the wave function overlap with the nuclei. We show that this is indeed the case for a single hole spin in a QD that we embedded in the intrinsic region of an n-i-p-diode. For random nuclei, the heavy-hole limit is achieved down to neV energies, equivalent to dephasing times of microseconds. © 2017 Elsevier B.V.

Spins in semiconductor quantum dots have been considered as prospective quantum bit excitations. Their coupling to the crystal environment manifests itself in a limitation of the spin coherence times to the microsecond range, both for electron and hole spins. This rather short-lived coherence compared to atomic states asks for manipulations on timescales as short as possible. Due to the huge dipole moment for transitions between the valence and conduction band, pulsed laser systems offer the possibility to perform manipulations within picoseconds or even faster. Here, we report on results that show the potential of optical spin manipulations with currently available pulsed laser systems. Using picosecond laser pulses, we demonstrate optically induced spin rotations of electron and hole spins. We further realize the optical decoupling of the hole spins from the nuclear surrounding at the nanosecond timescales and demonstrate an all-optical spin tomography for interacting electron spin sub-ensembles. © 2016, Springer-Verlag Berlin Heidelberg.

Using quantum dots (QDs) as single-photon sources draws the attention in many quantum communication technologies. One pathway towards manufacturing single-photon sources is focused ion beam (FIB) implantation in molecular beam epitaxy-grown QD samples to disable all QDs around an intentional one for single photoluminescence (PL) emission. In this paper, we investigate the lattice disorders in the vicinity of InAs/GaAs QDs introduced by FIB implantation of gallium and indium ions. For high fluences, we achieve total elimination of the QDs photoluminescence. The impact of the different ion species and fluences is studied by low-temperature PL measurements. Furthermore, we deduce a simple model based on the trap-assisted recombination for the description of the degradation of the PL emission. It allows the determination of the fluences at which the PL emission is suppressed. Moreover, we identify the implantation-induced non-radiative defects by temperature-dependent PL measurements. © 2016, Springer-Verlag Berlin Heidelberg.

In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots. © 2016 American Chemical Society.

Dispersion control is a key objective in the field of photonics and spectroscopy, since it enhances non-linear effects by both enabling phase matching and offering slow light generation. In addition, it is essential for frequency comb generation, which requires a phase-lock mechanism that is provided by broadband compensation of group velocity dispersion (GVD). At optical frequencies, there are several well-established concepts for dispersion control such as prism or grating pairs. However, terahertz dispersion control is still a challenge, thus hindering further progress in the field of terahertz science and technology. In this work, we present a hybrid waveguide with both broadband, tuneable positive and more than octave-spanning negative terahertz GVD on the order of 10-22 s2/m, which is suitable for either intra- or extra cavity operation. This new terahertz device will enable ultra-short pulse compression, allow soliton propagation, improve frequency comb operation and foster the development of novel non-linear applications. © 2016 Optical Society of America.

Coherent control of a strongly inhomogeneously broadened system, namely, InAs self-assembled quantum dots, is demonstrated. To circumvent the deleterious effects of the inhomogeneous broadening, which usually masks the results of coherent manipulation, we use prepulse two-dimensional coherent spectroscopy to provide a size-selective readout of the ground, exciton, and biexciton states. The dependence on the timing of the prepulse is due to the dynamics of the coherently generated populations. To further validate the results, we performed prepulse polarization dependent measurements and confirmed the behavior expected from selection rules. All measured spectra can be excellently reproduced by solving the optical Bloch equations for a 4-level system. © 2016 American Physical Society.

Quantum dot arrays provide a promising platform for quantum information processing. For universal quantum simulation and computation, one central issue is to demonstrate the exhaustive controllability of quantum states. Here, we report the addressable manipulation of three single electron spins in a triple quantum dot using a technique combining electron-spin-resonance and a micro-magnet. The micro-magnet makes the local Zeeman field difference between neighboring spins much larger than the nuclear field fluctuation, which ensures the addressable driving of electron-spin-resonance by shifting the resonance condition for each spin. We observe distinct coherent Rabi oscillations for three spins in a semiconductor triple quantum dot with up to 25 MHz spin rotation frequencies. This individual manipulation over three spins enables us to arbitrarily change the magnetic spin quantum number of the three spin system, and thus to operate a triple-dot device as a three-qubit system in combination with the existing technique of exchange operations among three spins. © 2016 AIP Publishing LLC.

A huge effort is underway to develop semiconductor nanostructures as low-noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath. A hole spin may circumvent the nuclear spin noise. In principle, the nuclear spins can be switched off for a pure heavy-hole spin. In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark-state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is < 1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to nanoelectronvolt energies; equivalently dephasing times up to a microsecond. The combination of large T2∗ and strong optical dipole makes the single hole spin in a GaAs-based device an attractive quantum platform. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

We present a new YAuSi Liquid Metal Alloy Ion Source (LMAIS) generating focused ion beams of yttrium ions, and its prospective applications for nanofabrication, sample preparation, lithographic and implantation processes. Working parameters of the AuSiY LMAIS are similar to other gold-silicon based LMAIS. We found anomalously high emission current of triple charged yttrium ions. Influence of yttrium implantation on optical qualities of the implanted ion-ensembles is shown in luminescence of co-implanted Erbium ions. © 2016, Springer-Verlag Berlin Heidelberg.

We studied the photoluminescence line shapes of free and bound excitons in a n-modulation doped Al1-xGaxAs-GaAs heterostructure with linearly increasing electric field in the p-doped buffer. At small laser excitation power the line shapes of the neutral donor bound and free excitons deviate strongly from a simple Lorentzian, whereas the neutral acceptor bound exciton is not obviously affected. Asymmetric lines of sawtooth-type form are observed for the donor bound and the free exciton. The line asymmetry could be traced back to the field dependent exciton binding energy and the field distribution in our heterostructure. A simple analytical model can account for the field dependent line shapes and a fit to the experimental lines gives a satisfactory agreement.

DC and AC electrical properties of hybrid films, consisting of carbon nanotubes and tungsten disulfide nanotubes (and fullerene like nanoparticles) were studied within the 2 - 300 K temperature range and over the 20 Hz - 1 MHz frequency range. The temperature dependences of the resistance R(T) exhibit behavior typical for the fluctuation-induced tunneling model in the intermediate temperature range. Analysis of the dependences of real and imaginary components of the impedance on the frequency (Z'(f) and Z ''(f)) demonstrates the rising role of the contact barriers between carbon nanotubes inside hybrid films, consisting of the carbon nanotubes and inorganic tungsten disulfide nanotubes as the temperature was decreased. The active component of the impedance was found to prevail in the AC electrical properties of the hybrid films, consisting of multi-wall carbon nanotubes and WS2 nanoparticles over the entire available temperature range.

Transporting ensembles of electrons over long distances without losing their spin polarization is an important benchmark for spintronic devices. It usually requires injecting and probing spin-polarized electrons in conduction channels using ferromagnetic contacts or optical excitation. In parallel with this development, important efforts have been dedicated to achieving control of nanocircuits at the single-electron level. The detection and coherent manipulation of the spin of a single electron trapped in a quantum dot are now well established. Combined with the recently demonstrated control of the displacement of individual electrons between two distant quantum dots, these achievements allow the possibility of realizing spintronic protocols at the single-electron level. Here, we demonstrate that spin information carried by one or two electrons can be transferred between two quantum dots separated by a distance of 4μm with a classical fidelity of 65%. We show that at present it is limited by spin flips occurring during the transfer procedure before and after electron displacement. Being able to encode and control information in the spin degree of freedom of a single electron while it is being transferred over distances of a few micrometres on nanosecond timescales will pave the way towards ' quantum spintronics' devices, which could be used to implement large-scale spin-based quantum information processing. © 2016 Macmillan Publishers Limited, All rights reserved.

We investigate the magneto-optical properties of excitons bound to single stacking faults in high-purity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy hole, and we confirm a vanishing in-plane hole g-factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultranarrow photoluminescence emission lines (with a full width at half-maximum 80μeV) and reveals a large magnetic nonreciprocity effect that originates from the magneto-Stark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment (e×10nm) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases. © 2016 American Physical Society.

Low-dimensional transport in semiconductor meso- and nanostructures is a topical field of fundamental research with potential applications in future quantum devices. However, thermal non-equilibrium may destroy phase-coherence and remains to be explored experimentally. Here, we present effects of thermal non-equilibrium in various implementations of low-dimensional (non-interacting) electron systems, fabricated by etching AlGaAs/GaAs heterostructures. These include narrow quasi-two-dimensional (2D) channels, quasi-one-dimensional (1D) waveguide networks, quantum rings (QRs), and single 1D constrictions, such as quantum point contacts (QPCs). Thermal non-equilibrium is realized by current heating. The charge carrier temperature is determined by noise thermometry. The electrical conductance and the voltage-noise are measured with respect to bath temperatures, heating currents, thermal gradients, and electric fields. We determine and discuss heat transport processes, electron-energy loss rates, and electron-phonon interaction, and our results are consistent with the Wiedemann-Franz relation. Additionally, we show how non-thermal current fluctuations can be used to identify electric conductance anomalies due to charge states. This article presents effects of thermal non-equilibrium in various implementations of low-dimensional electron systems, fabricated by etching AlGaAs/GaAs heterostructures. Measurements of the thermal noise allow one to access electron-energy relaxation mechanisms, to investigate nanostructures as thermal contacts and to identify the paths of heat flow in branched 1D waveguide networks. Additionally, non-thermal current fluctuations can be used to identify electric conductance anomalies due to charge states. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A hybrid mode section is integrated into the end of the metal-metal waveguide of a terahertz (THz) frequency quantum cascade laser (QCL) by removing sub-wavelength portions of the top metal layer. This allows a hybrid mode to penetrate into the air, which reduces the effective index of the mode and improves the out-coupling performance at the facet. The transmission of the hybrid section is further increased by ensuring its length fulfills the criterion for constructive interference. These simple modifications to a 2.5-THz metal-metal QCL waveguide result in a significant increase in the output emission power. In addition, simulations show that further improvements in out-coupling efficiency can be achieved for lower frequencies with effective refractive indices close to the geometric mean of the indices of the metal-metal waveguide and air. © 2016, Springer Science+Business Media New York.

We study the injection mechanism of a single electron from a static quantum dot into a moving quantum dot. The moving quantum dots are created with surface acoustic waves (SAWs) in a long depleted channel. We demonstrate that the injection process is characterized by an activation law with a threshold that depends on the SAW amplitude and on the dot-channel potential gradient. By sufficiently increasing the SAW modulation amplitude, we can reach a regime where the transfer has unity probability and is potentially adiabatic. This study points to the relevant regime to use moving dots in quantum information protocols. © 2016 IOP Publishing Ltd Printed in the UK.

We study the transmission phase shift across a Kondo correlated quantum dot in a GaAs heterostructure at temperatures below the Kondo temperature (T<TK), where the phase shift is expected to show a plateau at π/2 for an ideal Kondo singlet ground state. Our device is tuned such that the ratio Γ/U of level width Γ to charging energy U is quite large (0.5 rather than 1). This situation is commonly used in GaAs quantum dots to ensure Kondo temperatures large enough (≃100 mK here) to be experimentally accessible; however, it also implies that charge fluctuations are more pronounced than typically assumed in theoretical studies focusing on the regime Γ/U1 needed to ensure a well-defined local moment. Our measured phase evolves monotonically by π across the two Coulomb peaks, but without being locked at π/2 in the Kondo valley for TTK, due to a significant influence of large Γ/U. Only when Γ/U is reduced sufficiently does the phase start to be locked around π/2 and develops into a plateau at π/2. Our observations are consistent with numerical renormalization group calculations, and can be understood as a direct consequence of the Friedel sum rule that relates the transmission phase shift to the local occupancy of the dot, and thermal average of a transmission coefficient through a resonance level near the Fermi energy. © 2016 American Physical Society.

Doping of substrates at desired locations is a key technology for spin-based quantum memory devices. Focused ion beam implantation is well-suited for this task due to its high spacial resolution. In this work, we investigate ion-beam implanted Erbium ensembles in Yttrium Orthosilicate crystals by means of confocal photoluminescence spectroscopy. The sample temperature and the post-implantation annealing step strongly reverberate in the properties of the implanted ions. We find that hot implantation leads to a higher activation rate of the ions. At high enough fluences, the relation between the fluence and final concentration of ions becomes non-linear. Two models are developed explaining the observed behavior. © 2016 Elsevier B.V. All rights reserved.

Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir - in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton. © 2016 American Physical Society.

Individual dislocations in an ultra-pure GaAs epi-layer are investigated with spatially and spectrally resolved photoluminescence imaging at 5 K. We find that some dislocations act as strong non-radiative recombination centers, while others are efficient radiative recombination centers. We characterize luminescence bands in GaAs due to dislocations, stacking faults, and pairs of stacking faults. These results indicate that low-temperature, spatially-resolved photoluminescence imaging can be a powerful tool for identifying luminescence bands of extended defects. This mapping could then be used to identify extended defects in other GaAs samples solely based on low-temperature photoluminescence spectra. © 2016 IEEE.

We present phase sensitive cavity field measurements on photonic crystal microcavities. The experiments have been performed as autocorrelation measurements with ps double pulse laser excitation for resonant and detuned conditions. Measured E-field autocorrelation functions reveal a very strong detuning dependence of the phase shift between laser and cavity field and of the autocorrelation amplitude of the cavity field. The fully resolved phase information allows for a precise frequency discrimination and hence for a precise measurement of the detuning between laser and cavity. The behavior of the autocorrelation amplitude and phase and their detuning dependence can be fully described by an analytic model. Furthermore, coherent control of the cavity field is demonstrated by tailored laser excitation with phase and amplitude controlled pulses. The experimental proof and verification of the above described phenomena became possible by an electric detection scheme, which employs planar photonic crystal microcavity photo diodes with metallic Schottky contacts in the defect region of the resonator. The applied photo current detection was shown to work also efficiently at room temperature, which make electrically contacted microcavities attractive for real world applications. ©2016 Optical Society of America.

Time-resolved resonance fluorescence on a single self-assembled quantum dot (QD) is used to analyze the generation and capture of photoinduced free charge carriers. We directly observe the capture of electrons into the QD as an intensity reduction of the exciton transition. The exciton transition is quenched until the captured electron tunnels out of the dot again in the order of milliseconds. Our results demonstrate that even under resonant excitation, excited free electrons are generated and can negatively influence the optical properties of a QD. © 2016 Author(s).

We report on ion implantation into GaN QDs and investigate their radiation hardness. The experimental study is carried out by photoluminescence (PL) measurements on molecular beam epitaxy-grown GaN quantum dots after ion implantation. Both quantum dots grown in the hexagonal (H) and the cubic (C) crystal structure were subjected to gallium ions with an energy of 400 kV (H) and 75 kV (C) with fluences ranging from 5×1010 cm−2 to 1×1014 cm−2 (H) and to 1×1015 cm−2 (C), respectively. Low-temperature PL measurements reveal a PL quenching for which a quantitative model as a function of the ion fluence is developed. A high degradation resistance is concluded. A non-radiative trap with one main activation energy is found for all QD structures by temperature-dependent PL measurements. Further analysis of fluence-dependent PL energy shifts shows ion-induced intermixing and strain effects. Particular for the hexagonal quantum dots, a strong influence of the quantum confined Stark effect is present. © 2016 Elsevier B.V.

We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is tentatively attributed to polaron formation by carriers trapped in the quantum dots. © 2016 IOP Publishing Ltd.

Capacitance-voltage spectroscopy has proved to be a very powerful experimental technique towards the investigation of carrier-carrier interactions both qualitatively and quantitatively in complex coupled nanostructures. Here, we exploit this method to observe indirect exciton complexes in a quantum dot molecule and to quantify the electron-hole interactions between two dots in a quantum dot molecule, formed by vertical stacking of self-assembled quantum dot layers. While frequency-dependent measurements distinguish between the s- and p-charging behavior, under perpendicular magnetic fields, reordering of the quantized states charging sequence is observed along with the formation of a Landau fan in the wetting layer that is used to reconstruct the Fermi energy level. © 2016 American Physical Society.

Rare-earth (RE) ions doped into desired locations of optical crystals might enable a range of novel integrated photonic devices for quantum applications. With this aim, we have investigated the production yield of cerium and praseodymium by means of ion implantation. As a measure, the collected fluorescence intensity from both implanted samples and single centers was used. With a tailored annealing procedure for cerium, a yield up to 53% was estimated. Praseodymium yield amounts up to 91%. Such high implantation yield indicates a feasibility of creation of nanopatterned rare-earth doping and suggests strong potential of RE species for on-chip photonic devices. © 2016 AIP Publishing LLC.

We extract the phase coherence of a qubit defined by singlet and triplet electronic states in a gated GaAs triple quantum dot, measuring on time scales much shorter than the decorrelation time of the environmental noise. In this nonergodic regime, we observe that the coherence is boosted and several dephasing times emerge, depending on how the phase stability is extracted. We elucidate their mutual relations, and demonstrate that they reflect the noise short-time dynamics. © 2016 American Physical Society.

A microscopic study of the individual annealed (In,Ga)As/GaAs quantum dots is done by means of high-resolution transmission electron microscopy. The Cauchy-Green strain-tensor component distribution and the chemical composition of the (In,Ga)As alloy are extracted from the microscopy images. The image processing allows for the reconstruction of the strain-induced electric-field gradients at the individual atomic columns extracting thereby the magnitude and asymmetry parameter of the nuclear quadrupole interaction. Nuclear magnetic resonance absorption spectra are analyzed for parallel and transverse mutual orientations of the electric-field gradient and a static magnetic field. © 2016 American Physical Society.

A huge effort is underway to develop semiconductor nanostructures as low-noise qubits. A key source of dephasing for an electron spin qubit in GaAs and in naturally occurring Si is the nuclear spin bath. The electron spin is coupled to each nuclear spin by the hyperfine interaction. The same interaction also couples two remote nuclear spins via a common coupling to the delocalized electron. It has been suggested that this interaction limits both electron and nuclear spin coherence, but experimental proof is lacking. We show that the nuclear spin decoherence time decreases by two orders of magnitude on occupying an empty quantum dot with a single electron, recovering to its original value for two electrons. In the case of one electron, agreement with a model calculation verifies the hypothesis of an electron-mediated nuclear spin-nuclear spin coupling. The results establish a framework to understand the main features of this complex interaction in semiconductor nanostructures. © 2016 Macmillan Publishers Limited, part of Springer Nature.

We detect in real time interdot tunneling events in a weakly coupled two-electron double quantum dot in GaAs. At finite magnetic fields, we observe two characteristic tunneling times Td and Tb, belonging to, respectively, a direct and a blocked (spin-flip-assisted) tunneling. The latter corresponds to the lifting of a Pauli spin blockade, and the tunneling times ratio η=Tb/Td characterizes the blockade efficiency. We find pronounced changes in the behavior of η upon increasing the magnetic field, with η increasing, saturating, and increasing again. We explain this behavior as due to the crossover of the dominant blockade-lifting mechanism from the hyperfine to spin-orbit interactions and due to a change in the contribution of the charge decoherence. © 2016 American Physical Society.

Electron spins in semiconductor quantum dots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantum dot. A few-electron quadruple quantum dot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantum dots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantum dots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of each electron spin resonance is possible. © The Author(s) 2016.

We analyse an InAs/GaAs-based electric field tunable single quantum dot diode with a thin tunnelling barrier between a buried n+-back contact and a quantum dot layer. In voltage-dependent photoluminescence measurements, we observe rich signatures from spatially direct and indirect transitions from the wetting layer and from a single quantum dot. By analysing the Stark effect, we show that the indirect transitions result from a recombination between confined holes in the wetting or quantum dot layer with electrons from the edge of the Fermi sea in the back contact. Using a 17 nm tunnel barrier which provides comparably weak tunnel coupling allowed us to observe clear signatures of direct and corresponding indirect lines for a series of neutral and positively charged quantum dot states. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We experimentally demonstrate how coherent population trapping (CPT) for donor-bound electron spins in GaAs results in autonomous feedback that prepares stabilized states for the spin polarization of nuclei around the electrons. CPT was realized by excitation with two lasers to a bound-exciton state. Transmission studies of the spectral CPT feature on an ensemble of electrons directly reveal the statistical distribution of prepared nuclear-spin states. Tuning the laser driving from blue to red detuned drives a transition from one to two stable states. Our results have importance for ongoing research on schemes for dynamic nuclear-spin polarization, the central spin problem, and control of spin coherence. © 2016 American Physical Society.

We study in experiment and theory thermal energy and charge transfer close to the quantum limit in a ballistic nanodevice, consisting of multiply connected one-dimensional electron waveguides. The fabricated device is based on an AlGaAs/GaAs heterostructure and is covered by a global top-gate to steer the thermal energy and charge transfer in the presence of a temperature gradient, which is established by a heating current. The estimate of the heat transfer by means of thermal noise measurements shows the device acting as a switch for charge and thermal energy transfer. The wave-packet simulations are based on the multi-terminal Landauer-Büttiker approach and confirm the experimental finding of a mode-dependent redistribution of the thermal energy current, if a scatterer breaks the device symmetry. © 2016 Author(s).

The operation of quantum dots (QDs) at highest possible temperatures is desirable for many applications. Capacitance-voltage spectroscopy (C(V)-spectroscopy) measurements are an established instrument to analyse the electronic structure and energy levels of self-assembled QDs. We perform C(V) in the dark and C(V) under the influence of non-resonant illumination, probing exciton states up to X4+ on InAs QDs embedded in a GaAs matrix for temperatures ranging from 2.5 to 120 K. While a small shift in the charging spectra resonance is observed for the two spin degenerate electron s-state charging voltages with increasing temperature, a huge shift is visible for the electron-hole excitonic states resonance voltages. The s2-peak moves to slightly higher, the s1-peak to slightly lower charging voltages. In contrast, the excitonic states are surprisingly charged at much lower voltages upon increasing temperature. We derive a rate-model allowing to attribute and value different contributions to these shifts. Resonant tunnelling, state degeneracy and hole generation rate in combination with the Fermi distribution function turn out to be of great importance for the observed effects. The differences in the shifting behaviour is connected to different equilibria schemes for the peaks - s-peaks arise when tunnelling-in- and out-rates become equal, while excitonic peaks occur, when electron tunnelling-in- and hole-generation rates are balanced. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

All-optical addressing and coherent control of single solid-state based quantum bits is a key tool for fast and precise control of ground-state spin qubits. So far, all-optical addressing of qubits was demonstrated only in a very few systems, such as color centers and quantum dots. Here, we perform high-resolution spectroscopic of native and implanted single rare earth ions in solid, namely, a cerium ion in yttrium aluminum garnet (YAG) crystal. We find narrow and spectrally stable optical transitions between the spin sublevels of the ground and excited optical states. Utilizing these transitions we demonstrate the generation of a coherent dark state in electron spin sublevels of a single Ce3+ ion in YAG by coherent population trapping. © 2015 American Physical Society.

We study the half-field subband-Landau resonant splitting of a two-dimensional electron gas and demonstrate the possibility to tune the intersubband spacings via density-chopped far-infrared transmission spectroscopy in the absence of external magnetic field. © 2015 OSA.

Double-pulse injection seeding is used to modify the spectral emission of a terahertz quantum cascade laser (THz QCL). Two broad-band THz pulses delayed in time imprint a modulation on the single THz pulse spectrum. The resulting seed enables modification of the QCL emission spectrum, even though, the spectral bandwidth of each THz pulse is much broader than the QCL gain bandwidth. For a proper time delay between the pulses, the seeded THz QCL emission can even be switched from a multimode to a single mode regime. © 2015 IEEE.

We report a large g factor tunability of a single hole spin in an InGaAs quantum dot via an electric field. The magnetic field lies in the in-plane direction x, the direction required for a coherent hole spin. The electrical field lies along the growth direction z and is changed over a large range, 100 kV/cm. Both electron and hole g factors are determined by high resolution laser spectroscopy with resonance fluorescence detection. This, along with the low electrical-noise environment, gives very high quality experimental results. The hole g factor ghx depends linearly on the electric field Fz,dghx/dFz=(8.3±1.2)×10-4 cm/kV, whereas the electron g factor gex is independent of electric field dgex/dFz=(0.1±0.3)×10-4 cm/kV (results averaged over a number of quantum dots). The dependence of ghx on Fz is well reproduced by a 4×4 k·p model demonstrating that the electric field sensitivity arises from a combination of soft hole confining potential, an In concentration gradient, and a strong dependence of material parameters on In concentration. The electric field sensitivity of the hole spin can be exploited for electrically driven hole spin rotations via the g tensor modulation technique and based on these results, a hole spin coupling as large as ∼1 GHz can be envisaged. © 2015 American Physical Society.

We demonstrate an approach to incorporate self-assembled quantum dots into a Fabry-Pérot-like microcavity. Thereby, a 3λ/4 GaAs layer containing quantum dots is epitaxially removed and attached by van der Waals bonding to one of the microcavity mirrors. We reach a finesse as high as 4100 with this configuration limited by the reflectivity of the dielectric mirrors and not by scattering at the semiconductor-mirror interface, demonstrating that the epitaxial lift-off procedure is a promising procedure for cavity quantum electrodynamics in the solid state. As a first step in this direction, we demonstrate a clear cavity-quantum dot interaction in the weak coupling regime with a Purcell factor in the order of 3. Estimations of the coupling strength via the Purcell factor suggest that we are close to the strong coupling regime. © 2015 AIP Publishing LLC.

We demonstrate efficient room-temperature photoluminescence and spectral tuning of epitaxially grown ZnSe/CdSe quantum well structures almost over the whole visible spectrum (470-600 nm wavelength). The key element to achieve the observed high quantum efficiency and enormous tuning range was the implementation of a special strain engineering technique, which allows us to suppress substantial lattice relaxation of CdSe on ZnSe. Previous studies indicated that a CdSe coverage exceeding 3 ML on ZnSe results in the formation of extensive lattice defects and complete quenching of the photoluminescence at low and room temperature. In contrast, our approach of strain engineering enables the deposition of planar CdSe quantum wells with a thickness ranging from 1 to 6 ML with excellent optical properties. We attribute the observed experimental features to a controllable strain compensation effect that is present in an alternating system of tensile and compressively strained epitaxial layers and supported this model by calculations of the transition energies of the ZnSe/CdSe quantum wells. © 2015 American Physical Society.

We show that ballistic one-dimensional channels can be formed in an ambipolar device fabricated on a high mobility Al 0.34 Ga 0.66 As / GaAs heterostructure. Both electron and hole quantised conductances can be measured in the same one-dimensional channel. We have used this device to compare directly the subband spacings of the two charge carriers in the same confining potential and used this to compare the electron and hole effective masses. © 2015 AIP Publishing LLC.

Transport measurements are powerful tools to probe electronic properties of solid-state materials. To access properties of local electronic states in nanostructures, such as local density of states, electronic distribution and so on, micro-probes utilizing artificial nanostructures have been invented to perform measurements in addition to those with conventional macroscopic electronic reservoirs. Here we demonstrate a new kind of micro-probe: a fast single-lead quantum dot probe, which utilizes a quantum dot coupled only to the target structure through a tunneling barrier and fast charge readout by RF reflectometry. The probe can directly access the local electronic states with wide bandwidth. The probe can also access more electronic states, not just those around the Fermi level, and the operations are robust against bias voltages and temperatures.

Accumulation mode devices with epitaxially grown gates have excellent electrical stability due to the absence of dopant impurities and surface states. We overcome typical fabrication issues associated with epitaxially gated structures (e.g., gate leakage and high contact resistance) by using separate gates to control the electron densities in the Ohmic and Hall bar regions. This hybrid gate architecture opens up a way to make ultrastable nanoscale devices where the separation between the surface gates and the 2D electron gas is small. In this work, we demonstrate that the hybrid devices made from the same wafer have reproducible electrical characteristics, with identical mobility and density traces over a large range of 2D densities. In addition, thermal cycling does not influence the measured electrical characteristics. As a demonstration of concept, we have fabricated a hybrid single-electron transistor on a shallow (50 nm) AlGaAs/GaAs heterostructure that shows clear Coulomb blockade oscillations in the low temperature conductance. © 2015 AIP Publishing LLC.

Silicon diodes irradiated with helium ions with energies of 4.1, 6.8 and 8.9 MeV are studied. It is shown that the mechanism determining the behaviour of frequency dependence of complex electric module and correspondingly the behavior of impedance of diodes irradiated with helium ions in the frequency region 3200 kHz is a recharging of vacancy complexes localized in the space charge region.

We present the effect of post-implantation annealing conditions on the structural and optical quality of InAs quantum dots (QDs) grown by combination of focused ion beam (FIB) and molecular beam epitaxy (MBE) approach. A FIB of Ga+ ion was employed to pattern a homogeneously GaAs buffer layers and then, an in situ annealing step followed by InAs deposition was performed. Three different post-implantation annealing conditions were tested and under well-optimized conditions, a dislocation and defect-free InAs QDs growth on FIB patterned surface was successfully achieved. Furthermore, using photoluminescence (PL) study, we demonstrate that our best sample shows almost similar optical quality as MBE grown QDs on unimplanted GaAs surface. The patterning technique described here can presumably be applied to systems other than InAs/GaAs and highly interesting for site-controlled nucleation of QDs that finds its potential applications in nanooptoelectronic devices. © 2015 World Scientific Publishing Company.

We present spin-noise spectroscopy measurements on an ensemble of donor-bound electrons in ultrapure GaAs:Si covering temporal dynamics over 6 orders of magnitude from milliseconds to nanoseconds. The spin-noise spectra detected at the donor-bound exciton transition show the multifaceted dynamical regime of the ubiquitous mutual electron and nuclear spin interaction typical for III-V-based semiconductor systems. The experiment distinctly reveals the finite Overhauser shift of an electron spin precession at zero external magnetic field and a second contribution around zero frequency stemming from the electron spin components parallel to the nuclear spin fluctuations. Moreover, at very low frequencies, features related with time-dependent nuclear spin fluctuations are clearly resolved making it possible to study the intricate nuclear spin dynamics at zero and low magnetic fields. The findings are in agreement with the developed model of electron and nuclear spin noise. © 2015 American Physical Society.

The evolution of the gain of terahertz quantum cascade laser during injection seeding is probed as a function of time. Oscillations of the gain are commensurate with the variations of the field envelope. © OSA 2015.

Magnetotransport measurements of a ballistic quantum wire exposed to two magnetic barriers of opposite polarity in series are reported. We find two types of conductance resonances with quite different characteristic magnetic fields. Numerical simulations show that the first type, characterized by a larger fluctuation period of the magnetoconductance, originates from bound states localized at the magnetic barrier oriented in the same direction with the perpendicular component of the external magnetic field, and the second type of conductance fluctuation can be traced back to states that reside close to the second magnetic barrier with antiparallel alignment. The simulations furthermore show that the confinement mechanism for these states can be understood in terms of spatially varying diamagnetic shifts of the one-dimensional wire modes. © 2014 Elsevier Ltd

A quantum two-path interferometer allows for direct measurement of the transmission phase shift of an electron, providing useful information on coherent scattering problems. In mesoscopic systems, however, the two-path interference is easily smeared by contributions from other paths, and this makes it difficult to observe the true transmission phase shift. To eliminate this problem, multi-terminal Aharonov-Bohm (AB) interferometers have been used to derive the phase shift by assuming that the relative phase shift of the electrons between the two paths is simply obtained when a smooth shift of the AB oscillations is observed. Nevertheless, the phase shifts using such a criterion have sometimes been inconsistent with theory. On the other hand, we have used an AB ring contacted to tunnel-coupled wires and acquired the phase shift consistent with theory when the two output currents through the coupled wires oscillate with well-defined anti-phase. Here, we investigate thoroughly these two criteria used to ensure a reliable phase measurement, the anti-phase relation of the two output currents, and the smooth phase shift in the AB oscillation. We confirm that the well-defined anti-phase relation ensures a correct phase measurement with a quantum two-path interference. In contrast, we find that even in a situation where the anti-phase relation is less well-defined, the smooth phase shift in the AB oscillation can still occur but does not give the correct transmission phase due to contributions from multiple paths. This indicates that the phase relation of the two output currents in our interferometer gives a good criterion for the measurement of the true transmission phase, while the smooth phase shift in the AB oscillation itself does not. © 2015 AIP Publishing LLC.

A hybrid mode section is integrated into the end of the metal-metal (MM) waveguide of a terahertz (THz) frequency quantum cascade laser (QCL) by removing sub-wavelength portions of the top metal layer. This allows a hybrid mode to penetrate into the air, which reduces the effective index of the mode and improves the out-coupling performance at the facet. The transmission of the processed metal-metal hybrid section (MMHS) waveguide is further increased by ensuring its length fulfills the criterion for constructive interference. These simple modifications to a 2.5 THz MM QCL waveguide result in a significant increase in the output emission power. In addition, simulations show that further improvements in out-coupling efficiency can be achieved for lower frequencies with effective refractive indices close to the geometric mean of the indices of the MM waveguide and air. © 2015 IEEE.

Cross-correlated measurements of thermal noise are performed to determine the electron temperature in nanopatterned channels of a GaAs/AlGaAs heterostructure at 4.2K. Two-dimensional (2D) electron reservoirs are connected via an extended one-dimensional (1D) electron waveguide network. Hot electrons are produced using a current Ih in a source 2D reservoir, are transmitted through the ballistic 1D waveguide, and relax in a drain 2D reservoir. We find that the electron temperature increase, ΔTe, in the drain is proportional to the square of the heating current Ih, as expected from Joule's law. No temperature increase is observed in the drain when the 1D waveguide does not transmit electrons. Therefore, we conclude that electron-phonon interaction is negligible for heat transport between 2D reservoirs at temperatures below 4.2K. Furthermore, mode control of the 1D electron waveguide by application of a top-gate voltage reveals that ΔTe is not proportional to the number of populated subbands N, as previously observed in single 1D conductors. This can be explained with the splitting of the heat flow in the 1D waveguide network. © 2015 AIP Publishing LLC.

We present new photovoltage oscillation in a pure two dimensional electron gas (2DEG) and in the presence of circular or semicircular antidot lattices. Results were interpreted as EMPs-like photovoltage oscillations. We observed and explained the photovoltage oscillation amplitude enhancement in the presence of an antidot lattice with regard to the pure 2DEG. The microwave frequency excitation range is 139 - 350 GHz. The cyclotron and magnetoplasmon resonances take place in the magnetic field range 0.4 - 0.8 T. This original experimental condition allows edge magnetoplasmons EMPs interference like observation at low magnetic field, typically B < B-c where B-c is the magnetic field at which the cyclotron resonance takes place. The different oscillation periods observed and their microwave frequency dependence were discussed. For 139 and 158 GHz microwave excitation frequencies, a unique EMPs-like interference period was found in the presence of antidots whereas two periods were extracted for 295 or 350 GHz. An explanation of this effect is given taking account of strong electron interaction with antidot at low magnetic field. Indeed, electrons involved in EMPs like phenomenon interact strongly with antidots when electron cyclotron orbits are larger than or comparable to the antidot diameter. (C) 2015 Author(s).

We study the interplay between the dynamic nuclear spin polarization and resonant spin-flip Raman scattering of the resident electron in an ensemble of singly charged (In,Ga)As/GaAs quantum dots by using a two-color laser excitation scheme. The shift of the electron spin-flip Raman line gives a direct measure of the optically induced Overhauser shift, while the linewidth indicates nuclear spin fluctuations. The dynamic nuclear spin polarization leads only to a reduction in the electron spin splitting induced by wetting-layer excitation that is copolarized with the resonant quantum dot excitation. The respective mechanism of the two-color spin-flip Raman scattering is discussed together with the electron-nuclear hyperfine interaction and Pauli exclusion principle. The temporal evolution of the Overhauser shift further demonstrates a nuclear spin depolarization within several seconds depending strongly on the temperature. © 2015 American Physical Society.

The dynamic response of a terahertz quantum cascade laser is probed as a function of time. The gain of the THz QCL is saturated by injection seeding the laser with an initial THz seed pulse. The time-resolved gain of the injection seeded laser is then probed with a second THz pulse. © 2015 IEEE.

We present focused ion beam implantation as a prospective tool for realizing a patterned rare-earth spin-ensemble in a solid-state substrate. We demonstrate a successful implantation with 20% of luminescent ion activation for Erbium ions in Y2SiO5 crystals. © 2015 OSA.

Strain-compensated CdSe/ZnSe/(Zn,Mg)Se quantum well structures that were grown on (In,Ga)As allow for efficient roomtemperature photoluminescence and spectral tuning over the whole visible range. We fabricated microdisk cavities from these samples by making use of a challenging chemical structuring technique for selective and homogeneous removal of the (In,Ga)As sacrificial layer below the quantum structure. The observed whispering gallery modes in our microdisks are mainly visible up to photon energies of ∼ 2:3 eV due to strong selfabsorption. As extinction coefficients and effective refractive indices are dominated by the quantum well material CdSe, thick quantum wells (> 3 monolayer) are necessary to observe resonances in the corresponding quantum well emission. © 2015 Optical Society of America.

We report about the fabrication and analysis of high Q photonic crystal cavities with metallic Schottky-contacts. The structures are based on GaAs n-i membranes with an InGaAs quantum well in the i-region and nanostructured low ohmic metal top-gates. They are designed for photocurrent readout within the cavity and fast electric manipulations. The cavity structures are characterized by photoluminescence and photocurrent spectroscopy under resonant excitation. We find strong cavity resonances in the photocurrent spectra and surprisingly high Q-factors up to 6500. Temperature dependent photocurrent measurements in the region between 4.5 K and 310 K show an exponential enhancement of the photocurrent signal and an external quantum efficiency up to 0.26. © 2015 AIP Publishing LLC.

We studied experimentally the dynamics of the exchange interaction between two antiparallel electron spins in an isolated double quantum dot where coupling to the electron reservoirs can be ignored. We demonstrate that the level of control of such a double dot is higher than in conventional double dots. In particular, it allows us to couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo [Phys. Rev. A 57, 120 (1998)]. The present study demonstrates that isolated quantum dots are a possible route to increase the number of coherently coupled quantum dots. © 2015 American Physical Society. © 2015 American Physical Society.

We have observed Rabi oscillations in an InAs quantum dot ensemble by using optical pre-pulse 2D coherent spectroscopy. The polarization for 2D coherent spectroscopy is set to be cross-linear in order to obtain biexciton signal, which enables us to distinguish the signals from the ground and excited states. Furthermore, the spectral domain in 2D can reveal the coherent evolution in an inhomogeneously broadened ensemble. With increasing pre-pulse intensity, the signals attributed to the ground and excited states exhibit the sinusoidal decrease and increase, respectively. The observed excitation behavior is well reproduced by a damped oscillation model. From the fitting the pulse area achieved in this work is deduced to be 0.41p and the dipole moment is estimated as 29 Debye. © Springer International Publishing Switzerland 2015.

The coherent state preparation and control of single quantum systems is an important prerequisite for the implementation of functional quantum devices. Prominent examples for such systems are semiconductor quantum dots, which exhibit a fine structure split single exciton state and a V-type three level structure, given by a common ground state and two distinguishable and separately excitable transitions. In this work we introduce a novel concept for the preparation of a robust inversion by the sequential excitation in a V-type system via distinguishable paths.

A terahertz quantum cascade laser is injection seeded with narrow-band seed pulses generated from a periodically poled lithium niobate crystal. The spectral emission of the quantum cascade laser is controlled by the seed spectra. © OSA 2015.

We demonstrate by injection seeding that the spectral emission of a terahertz (THz) quantum cascade laser (QCL) can be modified with broad-band THz pulses whose bandwidths are greater than the QCL bandwidth. Two broad-band THz pulses delayed in time imprint a modulation on the single THz pulse spectrum. The resulting spectrum is used to injection seed the THz QCL. By varying the time delay between the THz pulses, the amplitude distribution of the QCL longitudinal modes is modified. By applying this approach, the QCL emission is reversibly switched from multi-mode to single mode emission. © 2015 AIP Publishing LLC.

We investigate the strong light-matter interactions of intersubband resonances (ISRs) in a triangular quantum well to a THz metamaterial. The large tuning possibility of ISRs with a high quality epitaxial gate enables the device to be electrically driven in-and-out of the coupling regime. © 2015 IEEE.

We have investigated the influence of a layer of charged self-assembled quantum dots (QDs) on the mobility of a nearby two-dimensional electron gas (2DEG). Time-resolved transconductance spectroscopy was used to separate the two contributions of the change in mobility, which are: (i) The electrons in the QDs act as Coulomb scatterers for the electrons in the 2DEG. (ii) The screening ability and, hence, the mobility of the 2DEG decreases when the charge carrier density is reduced by the charged QDs, i.e., the mobility itself depends on the charge carrier concentration. Surprisingly, we find a negligible influence of the Coulomb scattering on the mobility for a 2DEG, separated by a 30nm tunneling barrier to the layer of QDs. This means that the mobility change is completely caused by depletion, i.e., reduction of the charge carrier density in the 2DEG, which indirectly influences the mobility. © 2015 AIP Publishing LLC.

Developing a quantum photonics network requires a source of very-high-fidelity single photons. An outstanding challenge is to produce a transform-limited single-photon emitter to guarantee that single photons emitted far apart in the time domain are truly indistinguishable. This is particularly difficult in the solid-state as the complex environment is the source of noise over a wide bandwidth. A quantum dot is a robust, fast, bright and narrow-linewidth emitter of single photons; layer-by-layer growth and subsequent nano-fabrication allow the electronic and photonic states to be engineered. This represents a set of features not shared by any other emitter but transform-limited linewidths have been elusive. Here, we report transform-limited linewidths measured on second timescales, primarily on the neutral exciton but also on the charged exciton close to saturation. The key feature is control of the nuclear spins, which dominate the exciton dephasing via the Overhauser field. © 2015 Macmillan Publishers Limited. All rights reserved.

We demonstrate the possibility to tune the tunneling probability between an array of self- assembled quantum dots and a two-dimensional electron gas (2DEG) by changing the energy imbalance between the dot states and the 2DEG. Contrary to the expectation from Fowler-Nordheim tunneling, the tunneling rate decreases with increasing injection energy. This can be explained by an increasing momentum mismatch between the dot states and the Fermi-circle in the 2DEG. Our findings demonstrate momentum matching as a useful mechanism (in addition to energy conservation, density of states, and transmission probability) to electrically control the charge transfer between quantum dots and an electron reservoir. © 2015 AIP Publishing LLC.

The interaction between intersubband resonances (ISRs) and metamaterial microcavities constitutes a strongly coupled system where new resonances form that depend on the coupling strength. Here we present experimental evidence of strong coupling between the cavity resonance of a terahertz metamaterial and the ISR in a high electron mobility transistor (HEMT) structure. The device is electrically switched from an uncoupled to a strongly coupled regime by tuning the ISR with epitaxially grown transparent gate. The asymmetric potential in the HEMT structure enables ultrawide electrical tuning of ISR, which is an order of magnitude higher as compared to an equivalent square well. For a single heterojunction with a triangular confinement, we achieve an avoided splitting of 0.52THz, which is a significant fraction of the bare intersubband resonance at 2THz. © 2015, Nature Publishing Group. All rights reserved.

We demonstrate the potential of a periodic laser-pulse protocol for dynamic decoupling of hole spins in (In,Ga)As quantum dots from surrounding baths. When doubling the repetition rate of inversion laser pulses between two reference pulses for orienting the spins, we find that the spin coherence time is increased by a factor of 2. © 2014 American Physical Society.

We demonstrate the basic features of an all-optical spin tomography on picosecond time scale. The magnetization vector associated with a mode-locked electron spin ensemble in singly charged quantum dots is traced by ellipticity measurements using picosecond laser pulses. After optical orientation the spins precess about a perpendicular magnetic field. By comparing the dynamics of two interacting ensembles with the dynamics of a single ensemble we find buildup of a spin component along the magnetic field in the two-ensemble case. This component arises from a Heisenberg-like spin-spin interaction. © 2014 American Physical Society.

Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum-dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach. © CopyrightEPLA, 2014.

Rare-earth-doped crystals are excellent hardware for quantum storage of photons. Additional functionality of these materials is added by their waveguiding properties allowing for on-chip photonic networks. However, detection and coherent properties of rare-earth single-spin qubits have not been demonstrated so far. Here we present experimental results on high-fidelity optical initialization, effcient coherent manipulation and optical readout of a single-electron spin of Ce 3+ ion in a yttrium aluminium garnet crystal. Under dynamic decoupling, spin coherence lifetime reaches T 2 =2 ms and is almost limited by the measured spin-lattice relaxation time T 1 =4.5 ms. Strong hyperfine coupling to aluminium nuclear spins suggests that cerium electron spins can be exploited as an interface between photons and long-lived nuclear spin memory. Combined with high brightness of Ce 3+ emission and a possibility of creating photonic circuits out of the host material, this makes cerium spins an interesting option for integrated quantum photonics. © 2014 Macmillan Publishers Limited. All rights reserved.

We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK. © 2014 AIP Publishing LLC.

We fabricated an etched hole quantum dot in a Si-doped (311)A AlGaAs/GaAs heterostructure to study disorder effects via magnetoconductance fluctuations (MCF) at millikelvin temperatures. Recent experiments in electron quantum dots have shown that the MCF are sensitive to the disorder potential created by remote ionized impurities. We utilize this to study the temporal/thermal stability of Si acceptors in p-type AlGaAs/GaAs heterostructures. In particular, we use a surface gate to cause charge migration between Si acceptor sites at T=40 mK, and detect the ensuing changes in the disorder potential using the MCF. We show that Si acceptors are metastable at T=40 mK and that raising the device to a temperature T=4.2 K and returning to T=40 mK is sufficient to produce complete decorrelation of the MCF. The same decorrelation occurs at T∼165 K for electron quantum dots; by comparing with the known trap energy for Si DX centers, we estimate that the shallow acceptor traps in our heterostructures have an activation energy EA∼3 meV. Our method can be used to study charge noise and dopant stability towards optimization of semiconductor materials and devices. © 2014 American Physical Society.

We present capacitance-voltage spectra for the conduction band states of InAs quantum dots obtained under continuous illumination. The illumination leads to the appearance of additional charging peaks that we attribute to the charging of electrons into quantum dots containing a variable number of illumination-induced holes. By this we demonstrate an electrical measurement of excitonic states in quantum dots. Magnetocapacitance-voltage spectroscopy reveals that the electron always tunnels into the lowest electronic state. This allows us to directly extract, from the highly correlated many-body states, the correlation energy. The results are compared quantitatively to state of the art atomistic configuration interaction calculations, showing very good agreement for a lower level of excitations and also limitations of the approach for an increasing number of particles. Our experiments offer a rare benchmark to many-body theoretical calculations. © 2014 American Physical Society.

We report on the electrical and optical characterisation of the high-permittivity (high-κ) TiO2 thin films grown by plasma enhanced atomic layer deposition on Si (100) and glass substrates, respectively. TiO2 films were incorporated in metal-oxide semiconductor (MOS) capacitor structures with an Al metal gate electrode. The as-deposited films were amorphous; however upon annealing in the temperature range 500-900°C, crystalline TiO2 in the anatase phase was formed. This was further confirmed by performing Raman measurements where the characteristic features corresponding to the anatase phase were observed. Transmittance and absorption spectra of the as-deposited and annealed films were performed by UV-Vis measurements showing more than 70% of transmittance. The formation of stoichiometric TiO2 was revealed by X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS) analysis performed on annealed samples (500-900°C). The dielectric constants were calculated from capacitance-voltage (C-V) curves of the MOS structure on the as-deposited film and annealed films revealing a significant improvement of the dielectric constants from 10 to 75 at AC frequencies of 100 kHz for the 700°C annealed TiO2 thin films. The increase in the dielectric constant for annealed films could be attributed to the transformation of film structure from amorphous to polycrystalline (anatase). However, the transformation of amorphous to crystalline phase, leads to an increase in the leakage current which was also found best fitted with Schottky emission mechanism at moderated electric fields. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Semiconductor nanostructures are of interest in the field of spintronics, because they allow us to employ special features of low-dimensional transport for spin polarization, manipulation, and detection. Proposals for spintronic devices are based on coherent transport and interference effects in extended electron waveguide (EWG) structures. We present the state-of-the-art of low-dimensional EWGs and Aharonov-Bohm (AB) interferometers, fabricated in GaAs/AlGaAs heterostructures. Low-temperature measurements in etched quantum point contacts (QPCs) show large subband spacing and, in the presence of a source-drain DC bias, the differential conductance is resonantly enhanced due to a many-body modification of the barrier potential because of spin fluctuations. Multiterminal asymmetric quantum wire rings (QRs) have been fabricated and AB conductance oscillations are used to detect electrostatically induced continuous phase shifts and π-phase jumps, due to resonances and reflections at cross-junctions. The effects of measurement configuration and non-equilibrium in QR structures are also studied. By embedding a QPC into the leads of a QR, coherent mode-filtered injection of electrons into a few-mode AB interferometer is demonstrated. Single-and few-mode spin transport in EWG interferometers is discussed and the opportunities offered by embedding quantum dots for studying interaction and spin effects are presented. The results encourage the investigation of spin-dependent quantum transport in extended EWG interferometers. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Samarium (Sm) ions of 200 keV in energy were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a focused-ion-beam implanter at room temperature. The implantation doses range between 1014 and 1016 cm-2. X-ray diffraction revealed Sm incorporation into GaN matrix without secondary phase. Raman-scattering spectroscopy identified impurity-independent defect-related oscillation modes. Slight decrease in band gap and significant reduction in transmittance were observed by optical transmission spectroscopy. Photoluminescence spectra showed emission peaks related to background p-type impurity. Ferromagnetic hysteresis loops were recorded from GaN implanted with highest Sm dose, and magnetic ordering was observed from Sm-implanted GaN with dose of and above 1015 cm-2. The long-range magnetic ordering can be attributed to interaction of Sm ions through the implantation-induced Ga vacancy. © 2013 Elsevier B.V. All rights reserved.

We exploit the flexibility offered by an (In,Ga)As/GaAs quantum dot spin ensemble to demonstrate that complex dynamic evolutions can be excited in the ensemble magnetization and accessed by tailored pulsed laser protocols. The modes for spin precession about a magnetic field are adapted to the periodic excitation protocol such that at specific times the magnetization can effectively be decomposed in two, three, or four equal components with angles of π,2π/3, or π/2 between them. Optical orientation of these components by an additional laser pulse leads to the generation of higher harmonics in the spin precession, as evidenced by time-resolved ellipticity measurements. © 2014 American Physical Society.

We report on hybrid circuit quantum electrodynamics experiments with focused ion beam implanted Er3+ ions in Y2SiO5 coupled to an array of superconducting lumped element microwave resonators. The Y2SiO5 crystal is divided into several areas with distinct erbium doping concentrations, each coupled to a separate resonator. The coupling strength is varied from 5 MHz to 18.7 MHz, while the linewidth ranges between 50 MHz and 130 MHz. We confirm the paramagnetic properties of the implanted spin ensemble by evaluating the temperature dependence of the coupling. The efficiency of the implantation process is analyzed and the results are compared to a bulk doped Er:Y2SiO5 sample. We demonstrate the integration of these engineered erbium spin ensembles with superconducting circuits. © 2014 AIP Publishing LLC.

We present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots. We use Fourier transform infrared transmission spectroscopy via a density chopping scheme for direct experimental observation of the intersublevel spacings of electrons without any external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as a back contact. Spacings of the hole sublevels are indirectly calculated from the photoluminescence spectrum by using a simple model given by Warburton et al [1]. Additionally, we observe that the intersubband resonances of the 2DEG are enhanced due to the quantum dot layer on top of the device. © 2014 IOP Publishing Ltd.

We studied experimentally the coherent exchange interaction between two antiparallel electron spins in a double quantum dot with an increasing magnetic field gradient between the two dots. We observed an increase of the oscillation frequency and a reduction of the oscillation contrast. Both observations are consistent with a simple model of two spins interacting via exchange interaction. The demonstrated control of the two-electron spin states is interesting to produce gates for individual spin qubits. © 2014 American Physical Society.

Metal-organic (MO)CVD of ZrO2 thin films is performed using the precursor [Zr(NMe2)2(guan)2] (guan=η2-(iPrN)2CNMe2) as the Zr source, together with oxygen. Film deposition is carried out on both Si(100) and glass substrates at various deposition temperatures. The resulting films are characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) for investigating the crystallinity and morphology, respectively. Optical properties are measured by ellipsometry and UV-vis on Si substrates and glass substrates, respectively, showing a high average refractive index of 2.14 and transmittance of more than 80% in visible light for the film deposited at 500°C. The potential of ZrO2 thin films as gate dielectrics is verified by carrying out capacitance-voltage (C-V) and current-voltage (I-V) measurements. Dielectric constants are estimated from the accumulation capacitance, and found to be in the range 12 - 19 at an AC frequency of 1MHz, and a leakage current of the order of 10-6 A cm-2 at the applied field of 1 to 2 MV cm-1 is measured for the films deposited at temperatures from 500 to 700°C. The low leakage current and high dielectric constant implies the good quality of the film, relevant for high-k applications. The hardness of the film ranges from 4.2 to 6.3GPa for the 400nm thick film, as determined by nano-indentation measurements. The optimum dielectric and hardness is found for the film deposited at 600°C, while the highest refractive index is found to be 2.14 for the film deposited at 500°C, due to higher density of the layers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this paper, we discuss the magnetic properties of Gd-doped GaN. This diluted magnetic semiconductor shows hysteretic magnetization behavior at room temperature, which is attributed to ferromagnetism with a Curie temperature well above 300K. However, the experimental results regarding the magnetic properties are not completely consistent and the microscopic origin for the reported magnetic properties is still unclear. We discuss the role of the growth method of the GaN comparing molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) including GaN doped during the growth process with Gd as well as Gd-implanted material. It seems that in general it is easier to obtain hysteretic magnetization behavior for MBE-grown material probably due to the higher oxygen and lower hydrogen content. An exception is Gd-implanted GaN MBE-grown on Si(111) where we observe no ferromagnetism. We will present experiments where by oxygen implantation and annealing the impurity concentration was manipulated. The role of native defects is addressed and new experiments where additional defects have been introduced by nitrogen implantation in MBE-grown GaN:Gd are discussed. We present our results on anomalous Hall effect observed in a Gd-implanted GaN/AlGaN heterostructure. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The nuclear spins in nanostructured semiconductors play a central role in quantum applications. The nuclear spins represent a useful resource for generating local magnetic fields but nuclear spin noise represents a major source of dephasing for spin qubits. Controlling the nuclear spins enhances the resource while suppressing the noise. NMR techniques are challenging: the group III and V isotopes have large spins with widely different gyromagnetic ratios; in strained material there are large atom-dependent quadrupole shifts; and nanoscale NMR is hard to detect. We report NMR on 100,000 nuclear spins of a quantum dot using chirped radiofrequency pulses. Following polarization, we demonstrate a reversal of the nuclear spin. We can flip the nuclear spin back and forth a hundred times. We demonstrate that chirped NMR is a powerful way of determining the chemical composition, the initial nuclear spin temperatures and quadrupole frequency distributions for all the main isotopes. The key observation is a plateau in the NMR signal as a function of sweep rate: we achieve inversion at the first quantum transition for all isotopes simultaneously. These experiments represent a generic technique for manipulating nanoscale inhomogeneous nuclear spin ensembles and open the way to probe the coherence of such mesoscopic systems. © 2014 Macmillan Publishers Limited. All rights reserved.

A new heteroleptic titanium precursor with a mixed oxygen/nitrogen coordination sphere [Ti(dmap)2(NMe2)2] (Hdmap=1-dimethylamino-2-propanol) is synthesized by a simple elimination reaction on tetrakis-dimethylaminotitanium(IV) [Ti(NMe2)4]. The compound shows encouraging results in terms of chemical and thermal stability compared to the parent alkyl amide [Ti(NMe2)4], and is therefore more suitable for MOCVD applications. TiO2 thin films are grown on Si(100) and ITO-coated borosilicate glass substrates via MOCVD in the temperature range 500-800°C. The deposition temperature has a significant effect on the phase and microstructure of the TiO2 films obtained, which influences the functional properties. The optical bandgaps of the films are in the range 2.92-3.36eV. The best photocurrent response (1.5mAcm-2 under AM 1.5G conditions) in aqueous electrolytes is observed for films grown at 700°C having improved crystallinity and porous columnar structure. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Narrowband THz pulses generated from a periodically poled lithium niobate crystal are used to injection seed a terahertz quantum cascade laser. The phase locked spectral emission from the quantum cascade laser is significantly influenced by the spectrum of the seed pulse. © 2014 IEEE.

A periodically poled lithium niobate (PPLN) crystal with multiple poling periods is used to generate tunable narrow-bandwidth THz pulses for injection seeding a quantum cascade laser (QCL). We demonstrate that longitudinal modes of the quantum cascade laser close to the gain maximum can be selected or suppressed according to the seed spectrum. The QCL emission spectra obtained by electro-optic sampling from the quantum cascade laser, in the most favorable case, shows high selectivity and amplification of the longitudinal modes that overlap the frequency of the narrow-band seed. Proper selection of the narrow-band THz seed from the PPLN crystal discretely tunes the longitudinal mode emission of the quantum cascade laser. Moreover, the THz wave build-up within the laser cavity is studied as a function of the round-trip time. When the seed frequency is outside the maximum of the gain spectrum the laser emission shifts to the preferential longitudinal mode. © 2014 AIP Publishing LLC.

The photoluminescence polarizations of (In,Ga)As/GaAs quantum dots annealed at different temperatures are studied as a function of external magnetic field (Hanle curves). In these dependencies, remarkable resonant features appear due to all-optical nuclear magnetic resonances (NMR) for optical excitation with modulated circular polarization. Application of an additional radio-frequency field synchronously with the polarization modulation strongly modifies the NMR features. The resonances can be related to transitions between different nuclear spin states split by the strain-induced gradient of the crystal field and by the externally applied magnetic field. A theoretical model is developed to simulate quadrupole and Zeeman splittings of the nuclear spins in a strained quantum dot. Comparison with the experiment allows us to uniquely identify the observed resonances. The large broadening of the NMR resonances is attributed to variations of the quadrupole splitting within the quantum dot volume, which is well described by the model. © 2014 American Physical Society.

Resonant transmission through electronic quantum states that exist at the zero points of a magnetic field gradient inside a ballistic quantum wire is reported. Since the semiclassical motion along such a line of zero magnetic field takes place in the form of unidirectional snake trajectories, these states have no classical equivalence. The existence of such quantum states has been predicted more than a decade ago by theoretical considerations. We further show how their properties depend on the amplitude of the magnetic field profile as well as on the Fermi energy. © 2014 American Physical Society.

Erbium-doped low symmetry Y<inf>2</inf>SiO<inf>5</inf> crystals attract a lot of attention in perspective of quantum information applications. However, only doping of the samples during growth is available up to now, which yields a quite homogeneous doping density. In the present work, we deposit Er3+-ions by the focused ion beam technique at yttrium sites with several fluences in one sample. With a photoluminescence study of these locally doped Er3+:Y<inf>2</inf>SiO<inf>5</inf> crystals, we are able to evaluate the efficiency of the implantation process and develop it for the highest efficiency possible. We observe the dependence of ion activation after the post-implantation annealing on the fluence value. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Many photocatalyst systems catalyze chemical reactions under ultraviolet (UV) illumination, because of its high photon energies. Activating inexpensive, widely available materials as photocatalyst using the intense visible part of the solar spectrum is more challenging. Here, nanorod arrays of the wide-band-gap semiconductor zinc oxide have been shown to act as photocatalysts for the aerobic photo-oxidation of organic dye Methyl Orange under illumination with red light, which is normally accessible only to narrow-band semiconductors. The homogeneous, 800-1000-nm-thick ZnO nanorod arrays show substantial light absorption (absorbances >1) throughout the visible spectral range. This absorption is caused by defect levels inside the band gap. Multiple scattering processes by the rods make the nanorods appear black. The dominantly crystalline ZnO nanorod structures grow in the (0001) direction, i.e., with the c-axis perpendicular to the surface of polycrystalline zinc. The room-temperature preparation route relies on controlled cathodic delamination of a weakly bound polymer coating from metallic zinc, an industrially produced and cheaply available substrate. Cathodic delamination is a sequential synthesis process, because it involves the propagation of a delamination front over the base material. Consequently, arbitrarily large sample surfaces can be nanostructured using this approach. © 2014 American Chemical Society.

We demonstrate the real-time detection of single photogenerated electrons in lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells with either a thin (20 nm) or a thick (100 nm) AlGaAs barrier layer. The observed photon energy and power dependencies of the photoelectron detection efficiency both indicate that the trapped photoelectrons are predominantly generated in the buffer layer followed by tunneling into one of the two dots for the thin barrier sample, whereas they are directly generated in the well in the thick barrier sample. Single photoelectron detection after selective excitation of the heavy- and light-hole state in the dot is well resolved in the latter case. This ensures the applicability of our quantum well-based quantum dot systems for the coherent projection from single photon polarization to single electron spin states. © 2014 American Physical Society.

Highly efficient spin-flip Raman scattering of the resident electron spin is found in singly charged (In,Ga)As/GaAs quantum dots. The applied magnetic field induces a symmetry reduction for the negatively charged exciton, which serves as intermediate scattering state, thus making the spin-flip Raman scattering of the resident electron allowed. Electron-electron exchange interaction mediates the electron spin-flip. Above a threshold magnetic field that depends on the dot size and experiment geometry, the efficiency of the scattering cross section is spectrally shifted with increasing field. This shift, which follows the electron cyclotron energy, is assigned to a hybridization of s-shell singlet and p-shell triplet states of the negatively charged exciton. © 2014 American Physical Society.

We report on the quantized conductance through side- and top-gated InAs quantum point contacts and discuss its dependence on the temperature and on a magnetic field applied perpendicular to the sample plane. Even in the absence of a magnetic field we observe besides the integer steps in units of 2e 2/h spin-resolved steps in units of e2/h up to the highest occupied mode. A conductance anomaly at 0.7 × 2e2/h is found as well. © 2014 IOP Publishing Ltd.

Quantum memories are integral parts of both quantum computers and quantum communication networks. Naturally, such a memory is embedded into a hybrid quantum architecture, which has to meet the requirements of fast gates, long coherence times, and long distance communication. Erbium-doped crystals are well suited as a microwave quantum memory for superconducting circuits with additional access to the optical telecom C band around 1.55 μm. Here, we report on circuit QED experiments with an Er3+:YAlO3 crystal and demonstrate strong coupling to a superconducting lumped element resonator. The low magnetic anisotropy of the host crystal allows for attaining the strong coupling regime at relatively low magnetic fields, which are compatible with superconducting circuits. In addition, Ce3+ impurities were detected in the crystal, which showed strong coupling as well. © 2014 American Physical Society.

Samarium ions of 200 keV in energy were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a focused-ion-beam implanter at room temperature. The implantation doses range from 1 × 1014 to 1 × 1016cm-2. Structural properties studied by x-ray diffraction and Raman-scattering spectroscopy revealed Sm incorporation into GaN matrix without secondary phase. The optical measurements showed that the band gap and optical constants changed very slightly by the implantation. Photoluminescence measurements showed emission spectra similar to p-type GaN for all samples. Magnetic investigations with a superconducting quantum interference device identified magnetic ordering for Sm dose of and above 1 × 1015cm-2 before thermal annealing, while ferromagnetism was only observed after thermal annealing from the sample with highest Sm dose. The long-range magnetic ordering can be attributed to interaction of Sm ions through the implantation-induced Ga vacancy. © 2014 AIP Publishing LLC.

We review on time and spatially resolved two-color pump-probe magneto-optical Kerr effect (MOKE) microscopy studies of electron spins in bulk n-GaAs and GaAs (110) quantum wells (QWs) at low lattice temperatures. The influence of photocarrier heating by above-bandgap optical spin excitation on the spatially resolved magneto-optical spin detection is considered in detail. We demonstrate that a continuous-wave (cw) measurement of the local Kerr rotation at a fixed arbitrary probe wavelength in general does not correctly reveal the local spin polarization when hot electrons are present. For bulk GaAs we determine the true lateral electron spin polarization profile from cw measurements of the spatial dependence of the full excitonic Kerr rotation spectrum. For the (110) QWs, we directly obtain the electron spin diffusion coefficient from picosecond real space imaging of the time evolution of an optically excited electron spin packet, which we observe with a spectrally broad probe pulse. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Using transconductance spectroscopy we study the tunneling dynamics of electrons from a two-dimensional electron gas (2DEG) into excited and ground states of a layer of self-assembled InAs quantum dots (QDs). From an initially selected nonequilibrium condition, we observe the charging dynamics of the QD states and their spectral evolution for one- and two-electron configurations. Furthermore, we measure the electron emission from the QD states into the 2DEG for the corresponding evolution of the QD-hydrogen and QD-helium spectra. The comparison with theoretically predicted energies, as well as the evaluation of the dynamics in charging and emission, allows us to separate and identify ground and excited electron configurations in the spectral evolution and discuss in detail the observed maxima in the different spectra. © 2014 American Physical Society.

We report the observation of a current induced change of the low temperature conductivity of two-dimensional electron gases in GaAs/AlGaAs-high-electron-mobility transistors. By applying voltage pulses on the ohmic contacts of a Hall bar-mesa-structure, both sheet-carrier-density n2D and electron mobility μ are decreased. At temperatures below 50K, a persistent change combined with a partial transient recovery of n 2D has been observed. The transient behaviour and the lateral spreading of the effect are studied. Moreover, a temperature dependent investigation has been done in order to get insight into the addressed defect energy levels. A model based on the phenomenology of the effect is proposed. The observed effect is not a permanent degradation as the original carrier concentration can be restored by warming up the sample to room temperature and recooling it. © 2014 AIP Publishing LLC.

We report on the direct observation of the transmission phase shift through a Kondo correlated quantum dot by employing a new type of two-path interferometer. We observed a clear π/2-phase shift, which persists up to the Kondo temperature TK. Above this temperature, the phase shifts by more than π/2 at each Coulomb peak, approaching the behavior observed for the standard Coulomb blockade regime. These observations are in remarkable agreement with two-level numerical renormalization group calculations. The unique combination of experimental and theoretical results presented here fully elucidates the phase evolution in the Kondo regime. © 2014 American Physical Society.

We evaluate the Landé g factor of electrons in quantum dots (QDs) fabricated from GaAs quantum well (QW) structures of different well width. We first determine the Landé electron g factor of the QWs through resistive detection of electron spin resonance and compare it to the enhanced electron g factor determined from analysis of the magnetotransport. Next, we form laterally defined quantum dots using these quantum wells and extract the electron g factor from analysis of the cotunneling and Kondo effect within the quantum dots. We conclude that the Landé electron g factor of the quantum dot is primarily governed by the electron g factor of the quantum well suggesting that well width is an ideal design parameter for g-factor engineering QDs. © 2014 American Physical Society.

Spin noise spectroscopy is an optical technique for probing electron and hole spin dynamics that is based on detecting their intrinsic fluctuations while in thermal equilibrium. Here we show that fluctuation correlations can be further exploited in multi-probe noise studies to reveal information that in general cannot be accessed by conventional linear optical spectroscopy, such as the underlying homogeneous linewidths of individual constituents within inhomogeneously broadened systems. This is demonstrated in singly charged (In,Ga)As quantum-dot ensembles using two weak probe lasers: When the lasers have similar wavelengths, they probe the same quantum dots in the ensemble and show correlated spin fluctuations. In contrast, mutually detuned probe lasers measure different subsets of quantum dots, giving uncorrelated fluctuations. The noise correlation versus laser detuning directly reveals the quantum dot homogeneous linewidth even in the presence of a strong inhomogeneous broadening. Such noise-based correlation techniques are not limited to semiconductor spin systems, but are applicable to any system with measurable intrinsic fluctuations. © 2014 Macmillan Publishers Limited.

Optically active quantum dots, for instance self-assembled InGaAs quantum dots, are potentially excellent single photon sources. The fidelity of the single photons is much improved using resonant rather than non-resonant excitation. With resonant excitation, the challenge is to distinguish between resonance fluorescence and scattered laser light. We have met this challenge by creating a polarization-based dark-field microscope to measure the resonance fluorescence from a single quantum dot at low temperature. We achieve a suppression of the scattered laser exceeding a factor of 107 and background-free detection of resonance fluorescence. The same optical setup operates over the entire quantum dot emission range (920-980 nm) and also in high magnetic fields. The major development is the outstanding long-term stability: once the dark-field point has been established, the microscope operates for days without alignment. The mechanical and optical designs of the microscope are presented, as well as exemplary resonance fluorescence spectroscopy results on individual quantum dots to underline the microscope's excellent performance. © 2013 AIP Publishing LLC.

We present low-temperature transport experiments on Aharonov-Bohm (AB) rings fabricated from two-dimensional hole gases in p-type GaAs/AlGaAs heterostructures. Highly visible h/e (up to 15%) and h/2e oscillations, present for different gate voltages, prove the high quality of the fabricated devices. Like in previous work, a clear beating pattern of the h/e and h/2e oscillations is present in the magnetoresistance, producing split peaks in the Fourier spectrum. The magnetoresistance evolution is presented and discussed as a function of temperature and gate voltage. It is found that sample specific properties have a pronounced influence on the observed behaviour. For example, the interference of different transverse modes or the interplay between h/e oscillations and conductance fluctuations can produce the features mentioned above. In previous work they have occasionally been interpreted as signatures of spin-orbit interaction (SOI)-induced effects. In the light of these results, the unambiguous identification of SOI-induced phase effects in AB rings remains still an open and challenging experimental task. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Low-temperature electrical conductance spectroscopy measurements of quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are used to study the Zeeman splitting of 1D subbands for both in-plane and out-of-plane magnetic field orientations. The resulting in-plane g-factors agree qualitatively with those of previous experiments on quantum wires while the quantitative differences can be understood in terms of the enhanced quasi-1D confinement anisotropy. The influence of confinement potential on the anisotropy is discussed and an estimate for the out-of-plane g-factor is obtained which, in contrast to previous experiments, is close to the theoretical prediction. © Copyright EPLA, 2013.

The effects of confinement on biexciton renormalization in self-assembled InAs and interfacial GaAs quantum dot (QD) ensembles are studied using two-dimensional Fourier-transform spectroscopy. We find that in thermally annealed InAs QDs, changes in the biexciton transition energy are strongly correlated with those of the exciton and that the biexciton binding energy is similar for all QDs in the ensemble. These results are in contrast to those from GaAs QDs formed from interfacial fluctuations of a narrow quantum well (QW). In both the GaAs QW and QDs, correlation is reduced and the biexciton binding exhibits a strong dependence on localization. Comparison with simulations reveals how confinement and Coulomb interactions modify biexciton renormalization. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report collection of precise photoluminescence maps (emission intensity versus energy and magnetic field) of superior-quality two-dimensional hole gases. The maps reveal field evolution of both direct and cyclotron-assisted recombination lines attributed to various excitonic complexes, either moving nearly freely in the plane or bound to the acceptors placed inside or outside the quantum well. Under two-beam illumination (with photon energies below and above the band-gap in the barrier) we were able to control hole concentration (in the same well), and in particular to decrease it beyond the point of p- to n-type conversion. Our results demonstrate contrast between charge conversion of free and acceptor-bound positive trions resulting from the breaking of charge reflection symmetry by a fixed impurity. © Published under licence by IOP Publishing Ltd.

Improving the quantum coherence of solid-state systems that mimic two-level atoms, for instance spin qubits or single-photon emitters using semiconductor quantum dots, involves dealing with the noise inherent to the device. Charge noise results in a fluctuating electric field, spin noise in a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and decoherence of optical and spin states. We investigate noise in an ultrapure semiconductor device using a minimally invasive, ultrasensitive local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise through a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived from 0.1 Hz to 100 kHz. The charge noise dominates at low frequencies, spin noise at high frequencies. The noise falls rapidly with increasing frequency, allowing us to demonstrate transform-limited quantum-dot optical linewidths by operating the device above 50 kHz. © 2013 Macmillan Publishers Limited.

We report on polarization-resolved photoluminescence studies of diverse excitonic complexes formed in GaAs quantum wells with a high-mobility two-dimensional hole gas in magnetic fields up to 23 T. Using two-beam illumination we decrease the hole concentration beyond the point of conversion from p- to n-type structures. We have demonstrated charge conversion between positive and negative exciton complexes (both free and bound to acceptors in the well). The switch between the electron and hole gases allowed us to distinguish between the emission lines from positive trions moving almost freely in the quantum well and bound to ionized acceptors in the barrier, which indicate their coexistence in the same well. © 2013 AIP Publishing LLC.

Experimental results for the carrier capture and relaxation dynamics in self-organized semiconductor quantum dots are analyzed using a microscopic theory. Time-resolved differential transmission changes of the quantum-dot transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by quantum-dot polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of configuration-picture and single-particle-picture descriptions, both with nonperturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence. © 2013 American Physical Society.

Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization. © 2013 American Physical Society.

Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1μeV/meV. Dephasing rates are compared to population decay rates to reveal that pure dephasing causes the exciton optical coherences to decay faster than the radiative limit at low temperature, independent of excitation density. Fluctuations of the bright state transition energies are nearly perfectly correlated, protecting the non-radiative coherence from interband dephasing mechanisms. © 2013 Elsevier Ltd. All rights reserved.

Nanoparticles (NPs) of cobalt are synthesized in shallow layers of polyimide using 40 keV implantation of Co+ ions with a few different fluences at various ion current densities. Nucleation of individual NPs at low fluencies and their percolation at high fluencies are crucial processes governing the electrical and magnetic properties of the metal/polymer nanocomposites that can be controlled by the implantation regimes. In particular, one can tune the magnetoresistance between negative and positive through appropriate choice of ion fluence and current density. The found non-monotonous dependence of the magnetoresistance on the applied magnetic field allows suggestion of spin-dependent domain wall scattering affecting the electron transport. The samples implanted with low fluencies demonstrate superparamagnetic behavior down to very low blocking temperatures. For high fluence (1.25 × 1017 cm-2) the transition to ferromagnetic ordering is observed that is related to the increased magnetic interaction of NPs. Nanoparticles of cobalt are synthesized in shallow layers of polyimide using low-energy implantation of cobalt ions. Nucleation of individual particles and their percolation are crucial processes governing the electrical and magnetic properties of the metal/polymer nanocomposites. By tuning the implantation regimes magnetoresistance and transitions between the superparamagnetic and ferromagnenic behavior can be controlled. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Low-temperature transport experiments on a p-type GaAs quantum dot capacitively coupled to a quantum point contact are presented. The time-averaged as well as time-resolved detection of charging events of the dot are demonstrated and they are used to extract the tunneling rates into and out of the quantum dot. The extracted rates exhibit a super-linear enhancement with the bias applied across the dot, which is interpreted in terms of a dense spectrum of excited states contributing to the transport, characteristic for heavy hole systems. The full counting statistics of charge transfer events and the effect of back action is studied. The normal cumulants as well as the recently proposed factorial cumulants are calculated and discussed in view of their importance for interacting systems. © 2013 American Physical Society.

Diodes manufactured on the wafers of single-crystalline silicon uniformly doped with phosphorus are studied. The wafer resistivity was 90 cm. Xenon ions were implanted into the diodes from the side of the p+-region (implantation energy 170 MeV, fluence ψ from 5×107 to 109 cm-2). It is shown that the formation of a continuous irradiation damaged layer with the thickness of the order of magnitude of the average projective range creates prerequisites for the negative differential resistance in the current-voltage characteristics of the irradiated diodes.

The degree of circular polarization of photoluminescence of (In,Ga)As quantum dots as a function of magnetic field applied perpendicular to the optical axis (Hanle effect) is experimentally studied. The measurements have been performed at various regimes of the optical excitation modulation. The analysis of experimental data has been performed in the framework of a vector model of regular nuclear spin polarization and its fluctuations. The analysis allowed us to evaluate the magnitude of nuclear polarization and its dynamics at the experimental conditions used. © 2013 AIP Publishing LLC.

We report on the measurement of phase coherence length in a high-mobility two-dimensional electron gas patterned in two different geometries, a wire and a ring. The phase coherence length is extracted both from the weak localization correction in long wires and from the amplitude of the Aharonov-Bohm oscillations in a single ring, in a low-temperature regime when decoherence is dominated by electronic interactions. We show that these two measurements lead to different phase coherence lengths, namely LΦwire α T -1/3 and LΦring α T-1 /2. This difference reflects the fact that the electrons winding around the ring necessarily explore the whole sample (ergodic trajectories), while in a long wire the electrons lose their phase coherence before reaching the edges of the sample (diffusive regime). © 2013 American Physical Society.

Lateral quantum dots are a promising system for quantum information processing devices. The required basic manipulations of a single electron spin have indeed been demonstrated. However, a stringent requirement is the ability to transfer quantum information from place to place within one sample. In this work, we explore and demonstrate the possibility to transfer a single electron between two distant quantum dots in a fast and reliable manner. © 2013 American Institute of Physics.

Exciton, trion, and biexciton dephasing rates are measured for an ensemble of InAs quantum dots using two-dimensional Fourier-transform spectroscopy. The two-dimensional spectra reveal that the dephasing rate of each excitonic state is similar for all dots in the ensemble and the rates are independent of excitation density. An additional spectral feature (too weak to be observed in the time-integrated four-wave mixing signal) appears at high excitation density and is attributed to the χ(5) biexcitonic nonlinear response. ©2013 American Physical Society.

In this work formation of the very first layers of carbon nanofilms on single crystal quartz substrates is studied. Films where grown by molecular beam growth, and have been characterized by Raman spectroscopy and atomic force microscopy. Formation of a non-conductive carbon layer of low crystallinity on the initial stage of the growth process is reported. Ab-initio calculations with an atom-by-atom approach have been performed to explain the experimental data. © 2013 Elsevier B.V.

Single quantum dots are solid-state emitters that mimic two-level atoms but with a highly enhanced spontaneous emission rate. A single quantum dot is the basis for a potentially excellent single-photon source. One outstanding problem is that there is considerable noise in the emission frequency, making it very difficult to couple the quantum dot to another quantum system. We solve this problem here with a dynamic feedback technique that locks the quantum-dot emission frequency to a reference. The incoherent scattering (resonance fluorescence) represents the single-photon output, whereas the coherent scattering (Rayleigh scattering) is used for the feedback control. The fluctuations in emission frequency are reduced to 20 MHz, just approximately 5% of the quantum-dot optical linewidth, even over several hours. By eliminating the 1/f-like noise, the relative fluctuations in quantum-dot noise power are reduced to approximately 10(-5) at low frequency. Under these conditions, the antibunching dip in the resonance fluorescence is described extremely well by the two-level atom result. The technique represents a way of removing charge noise from a quantum device.

Grazing-incidence X-ray diffraction measurements on single GaAs nanowires (NWs) grown on a (111)-oriented GaAs substrate by molecular beam epitaxy are reported. The positions of the NWs are intentionally determined by a direct implantation of Au with focused ion beams. This controlled arrangement in combination with a nanofocused X-ray beam allows the in-plane lattice parameter of single NWs to be probed, which is not possible for randomly grown NWs. Reciprocal space maps were collected at different heights along the NW to investigate the crystal structure. Simultaneously, substrate areas with different distances from the Au-implantation spots below the NWs were probed. Around the NWs, the data revealed a 0.4% decrease in the lattice spacing in the substrate compared with the expected unstrained value. This suggests the presence of a compressed region due to Au implantation. © 2013 International Union of Crystallography Printed in Singapore - all rights reserved.

Chemical vapor deposition of graphene-like (GL) films (nano-crystalline graphite films) from methane on single crystal quartz substrates is reported. The GL films have been characterized by means of Raman spectroscopy, atomic force microscopy and electrical measurements. The thinnest GL films obtained so far have a thickness of 1.5 nm, a relatively rough surface structure and an electrical conductivity in the range of 20 kΩ. The high chemical sensitivity of GL films has been demonstrated by measuring the change of their conductance during exposure to a NO2-containing atmosphere. The optimal electrical conductance, high chemical sensitivity as well as the simple growth method make the GL films promising for practical applications as a chemically sensitive material. © 2013 Elsevier B.V. All rights reserved.

The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As/GaAs quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified when changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model considering cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electron-nuclear spin system of the studied quantum dots. © 2013 American Physical Society.

Recently, a mass spectrometer for liquid metal ion sources (LMIS) has been built and set into operation. This device uses an E × B-filter as mass dispersive element and provides sufficient resolution to analyse the emission of clusters from LMIS to much higher mass ranges (&gt;2000 amu) than commercially available mass filters for focused ion beam systems. It has also been shown that for small masses the composition of clusters from different isotopes can be resolved. Furthermore, a rather high fluence of monodisperse clusters in the range of 106-107 clusters/s can be achieved with this setup. This makes it a promising tool for the preparation of mass selected clusters. In this contribution, theoretical considerations as well as technical details and the results of first measurements are presented. © 2013 AIP Publishing LLC.

We report on spatially resolved two-color pump-probe Hanle-magneto-optical Kerr effect studies of low-temperature electron spin diffusion in bulk n-type GaAs (nD=1.4×1016cm-3). The influence of the lattice temperature on electron spin diffusion is investigated and it is shown that simple drift-diffusion models do not describe the spin diffusion for above-band-gap optical spin injection correctly. Variation of lattice temperature and excitation energy allow us to trace this discrepancy to the influence of the pump-induced local overheating of the electron system with respect to the lattice which persists over length scales comparable to the spin diffusion length. Consideration of this hot carrier effect is crucial for a reliable extraction of spin propagation parameters from optical experiments. © 2013 American Physical Society.

We report on spatially resolved pump-probe MOKE spectroscopy measurements of electron spins in bulk n-type GaAs. For low lattice temperatures these measurements are significantly compromised by local changes in the excitonic magneto-optical response which are not related to the electron spin polarization. These local changes in the excitonic Kerr spectrum are due to carrier heating caused by above-band-gap optical excitation. When hot electrons are present, a measurement of the local Kerr rotation at a fixed arbitrary probe wavelength does not necessarily correctly reveal the local electron spin polarization. By analyzing the spatial dependence of the full excitonic Kerr rotation spectrum we determine the local excitonic spin splitting energy from which we obtain the true lateral electron spin polarization profile. © 2013 American Physical Society.

The emission energy dependence of the biexciton binding energy is investigated in three semiconductor quantum dot (QD) systems that exhibit different quantum well → QD confinement. Using two-dimensional Fourier-transform spectroscopy, we demonstrate that in strongly confining InAs QDs, the binding energy is independent of exciton emission energy and fluctuations in the ground state → exciton transition energy are strongly correlated with those of the exciton → biexciton. In contrast, the biexciton binding energy increases with emission energy in weakly confining interfacial GaAs QDs, and the level of correlation of exciton-biexciton broadening is reduced. A comparison with simulations reveals the significance of the strength and nature of confinement on Coulomb interactions responsible for biexciton renormalization. ©2013 American Physical Society.

The photoluminescence of an electron inversion layer with a monolayer of carbon acceptors in GaAs was investigated at liquid helium temperatures. In the limit of high laser illumination the luminescence line approaches a hat shape, the expected form if the recombination center is a single isolated acceptor. At medium illumination the line takes on a trapezoidal form, and in the small illumination limit a triangular shape emerges. The line shape variations could be traced back to the interaction of the carbon acceptors which is ruled by the illumination strength. Acceptor-acceptor interactions spread the distribution of transition energies and initiate a transfer of oscillator strength. The triangular line shape at low illumination is a clear signature of an impurity band formed in the carbon monolayer. © 2013 IOP Publishing Ltd.

We present a comparison between experimental and theoretical values of transport τt and quantum τq scattering lifetimes in high-quality undoped Al0.34Ga0.66As/GaAs heterostructures. We obtain excellent agreement between the experimental and modeled scattering times using three scattering processes: background impurity, interface roughness, and remote ionized impurity scattering from surface states. We show that despite the high mobility (μpeak=5. 6×106cm2V-1s-1), the quantum lifetime τq is significantly reduced by small-angle scattering from remote surface charge. We further show that in shallow devices scattering from surface charges will be a limiting factor for both transport and quantum lifetimes. © 2013 American Physical Society.

We studied the magnetic and transport properties of Co nanoparticles (NPs) being capped with varying amounts of Pt. Beside field and temperature dependent magnetization measurements, we performed δΜ measurements to study the magnetic interactions between the Co NPs. We observe a transition from demagnetizing towards magnetizing interactions between the particles for an increasing amount of Pt capping. Resistivity measurements show a crossover from giant magnetoresistance towards anisotropic magnetoresistance. © 2013 American Institute of Physics.

The impact of Ga and Au ion implantation on the electron spin dynamics in bulk wurtzite GaN is studied by time-resolved Kerr-rotation spectroscopy. The spin relaxation time increases strongly by up to a factor of 20 for increasing implantation doses. This drastic increase is caused by a transition from delocalized to localized electrons. We find a characteristic change in the magnetic field dependence of spin relaxation that can be used as a sensitive probe for the degree of localization. © 2013 AIP Publishing LLC.

Gd-implanted wurtzite GaN as a candidate for a ferromagnetic dilute magnetic semiconductor is investigated by time-resolved magneto-optical spectroscopy. We observe a strong increase of the electron spin lifetimes for increasing Gd doses, while the electron spin Larmor precession frequency is independent of the Gd concentration. These findings are well explained by carrier localization at defects and a negligible interaction with Gd ions. The data show that Gd-implanted GaN cannot be used for an electron spin aligner. © 2013 AIP Publishing LLC.

We study the photoluminescence (PL) of a two-dimensional liquid of oriented dipolar excitons in InxGa1-xAs coupled double quantum wells confined to a microtrap. Generating excitons outside the trap and transferring them at lattice temperatures down to T=240 mK into the trap we create cold quasiequilibrium bosonic ensembles of some 1000 excitons with thermal de Broglie wavelengths exceeding the excitonic separation. With decreasing temperature and increasing density n5×10101cm2 we find an increasingly asymmetric PL line shape with a sharpening blue edge and a broad red tail which we interpret to reflect correlated behavior mediated by dipolar interactions. From the PL intensity I(E) below the PL maximum at E0 we extract at T< 5 K a distinct power law I(E)∼(E 0-E) -|α| with -|α|≈-0.8 in the range E0-E of 1.5-4 meV, comparable to the dipolar interaction energy. © 2013 American Physical Society.

We demonstrate one and two photoelectron trapping and the subsequent dynamics associated with interdot transfer in double quantum dots over a time scale much shorter than the typical spin lifetime. Identification of photoelectron trapping is achieved via resonant interdot tunneling of the photoelectrons in the excited states. The interdot transfer enables detection of single photoelectrons in a nondestructive manner. When two photoelectrons are trapped at almost the same time we observed that the interdot resonant tunneling is strongly affected by the Coulomb interaction between the electrons. Finally the influence of the two-electron singlet-triplet state hybridization has been detected using the interdot tunneling of a photoelectron. © 2013 American Physical Society.

We report the observation of the Kondo effect in a spin-3/2 hole quantum dot formed near pinch-off in a GaAs quantum wire. We clearly observe two distinctive hallmarks of quantum dot Kondo physics. First, the zero-bias peak in the differential conductance splits an in-plane magnetic field and the splitting is independent of gate voltage. Second, the splitting rate is twice as large as that for the lowest one-dimensional subband. We show that the Zeeman splitting of the zero-bias peak is highly anisotropic and attribute this to the strong spin-orbit interaction for holes in GaAs. © 2013 AIP Publishing LLC.

A quantum point contact (QPC) is a basic nanometre-scale electronic device: a short and narrow transport channel between two electron reservoirs. In clean channels, electron transport is ballistic and the conductance is then quantized as a function of channel width with plateaux at integer multiples of 2e 2/h (where e is the electron charge and h is Planck's constant). This can be understood in a picture where the electron states are propagating waves, without the need to account for electron-electron interactions. Quantized conductance could thus be the signature of ultimate control over nanoscale electron transport. However, even studies with the cleanest QPCs generically show significant anomalies in the quantized conductance traces, and there is consensus that these result from electron many-body effects. Despite extensive experimental and theoretical studies, understanding these anomalies is an open problem. Here we report that the many-body effects have their origin in one or more spontaneously localized states that emerge from Friedel oscillations in the electron charge density within the QPC channel. These localized states will have electron spins associated with them, and the Kondo effect-related to electron transport through such localized electron spins-contributes to the formation of the many-body state. We present evidence for such localization, with Kondo effects of odd or even character, directly reflecting the parity of the number of localized states; the evidence is obtained from experiments with length-tunable QPCs that show a periodic modulation of the many-body properties with Kondo signatures that alternate between odd and even Kondo effects. Our results are of importance for assessing the role of QPCs in more complex hybrid devices and for proposals for spintronic and quantum information applications. In addition, our results show that tunable QPCs offer a versatile platform for investigating many-body effects in nanoscale systems, with the ability to probe such physics at the level of a single site. © 2013 Macmillan Publishers Limited. All rights reserved.

Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/Al xGa1 -xAs heterostructures are often realized by annealing of AuGe/Ni/Au that is deposited on its surface. We studied how the quality of this type of ohmic contact depends on the annealing time and temperature, and how optimal parameters depend on the depth of the 2DEG below the surface. Combined with transmission electron microscopy and energy-dispersive x-ray spectrometry studies of the annealed contacts, our results allow for identifying the annealing mechanism. We use this for proposing a model that can predict the optimal annealing time when our commonly applied recipe is used for a certain heterostructure at a certain temperature. © 2013 IOP Publishing Ltd.

Spontaneous fluctuations of the magnetization of a spin system in thermodynamic equilibrium (spin noise) manifest themselves as noise in the Faraday rotation of probe light. We show that the correlation properties of this noise over the optical spectrum can provide clear information about the composition of the spin system that is largely inaccessible for conventional linear optics. Such optical spectroscopy of spin noise, e.g., allows us to clearly distinguish between optical transitions associated with different spin subsystems, to resolve optical transitions that are unresolvable in the usual optical spectra, to unambiguously distinguish between homogeneously and inhomogeneously broadened optical bands, and to evaluate the degree of inhomogeneous broadening. These new possibilities are illustrated by theoretical calculations and by experiments on paramagnets with different degrees of inhomogeneous broadening of optical transitions [atomic vapors of K41 and singly charged (In,Ga)As quantum dots]. © 2013 American Physical Society.

Low-temperature transport measurements on a p-GaAs quantum point contact are presented which reveal the presence of a conductance anomaly that is markedly different from the conventional "0.7 anomaly." A lateral shift by asymmetric gating of the conducting channel is utilized to identify and separate different conductance anomalies of local and generic origins experimentally. While the more generic 0.7 anomaly is not directly affected by changing the gate configuration, a model is proposed which attributes the additional conductance features to a gate-dependent coupling of the propagating states to localized states emerging due to a nearby potential imperfection. Finite bias conductivity measurements reveal the interplay between the two anomalies consistently with a two-impurity Kondo model. © 2013 American Physical Society.

The photoluminescence of inversion electrons with carbon acceptors in a modulation-doped AlxGa1-xAs-GaAs single heterostructure was studied for different temperatures and excitation intensities. Despite the essentially homogeneous distribution of residual acceptors in the GaAs buffer, well-resolved luminescence profiles are observed for the ground and first excited two-dimensional electron subbands. This is due to the fact that the inversion layer interface potential restricts the overlap of the electron and acceptor wave functions to limited spatial regions close to the Al xGa1-xAs-GaAs interface, where the transition energies are weakly dispersive. A simple model will be presented for the calculation of the luminescence line shape that allows a quantitative analysis of the experiment. © 2013 American Physical Society.

We use time-resolved two-color Kerr microscopy to study low-temperature electron spin diffusion in bulk GaAs with high spatial and temporal resolution. This method enables direct real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet, which is strongly reduced with increasing time. Comparison with continuous-wave Kerr spectroscopy demonstrates that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient. © 2013 American Physical Society.

We explore the photoluminescence of spatially indirect, dipolar Mahan excitons in a gated double quantum well diode containing a mesoscopic electrostatic trap for neutral dipolar excitons at low temperatures down to 250 mK and in quantizing magnetic fields. Mahan excitons in the surrounding of the trap, consisting of individual holes interacting with a degenerate two-dimensional electron system confined in one of the quantum wells, exhibit strong quantum Hall signatures at integer filling factors and related anomalies around filling factor ν=23,35, and 12, reflecting the formation of composite fermions. Interactions across the trap perimeter are found to influence the energy of the confined neutral dipolar excitons by the presence of the quantum Hall effects in the two-dimensional electron system surrounding the trap. © 2013 American Physical Society.

We have measured the zero-bias peak in differential conductance in a hole quantum dot. We have scaled the experimental data with applied bias and compared to real-time renormalization group calculations of the differential conductance as a function of source-drain bias in the limit of zero temperature and at finite temperatures. The experimental data show deviations from the T=0 calculations at low bias, but are in very good agreement with the finite-T calculations. The Kondo temperature TK extracted from the data using T=0 calculations, and from the peak width at 23 maximum, is significantly higher than that obtained from finite-T calculations. © 2013 American Physical Society.

Efficient electrical spin injection into semiconductor based devices at room temperature is one of the most important requirements for the development of applicable spintronic devices in the near future and is thus an important and very active research field. Here we report experimental results for the electrical spin injection in spin light-emitting diodes (spin-LEDs) without external magnetic fields at room temperature. Our devices consist of a Fe/Tb multilayer spin injector with remanent out-of-plane magnetization, an MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. Using a series of samples with different injection path lengths allows us to experimentally determine the spin relaxation during vertical transport from the spin injector to the active region at room temperature. In combination with our concept for remanent spin injection, we are additionally able to investigate the influence of an external magnetic field on the spin relaxation process during transport. While the spin relaxation length at room temperature without external magnetic field is determined to be 27 nm, this value almost doubles if an external magnetic field of 2 Tesla is applied in Faraday geometry. This demonstrates that the results for spin injection and spin relaxation obtained with or without magnetic field can hardly be compared. The efficiency of spin-induced effects is overestimated as long as magnetic fields are involved. Since strong magnetic fields are not acceptable in application settings, this may lead to wrong conclusions and potentially impairs proper device development. © 2013 Copyright SPIE.

We report experimental results on the electron spin relaxation length during vertical transport in spin lightemitting diodes (LEDs). Our devices are GaAs based LEDs with InAs quantum dots in the active region, an MgO tunnel barrier and an Fe/Tb multilayer spin injector with perpendicular magnetic anisotropy, i.e. remanent out-of-plane magnetization, enabling efficient electrical spin injection in magnetic remanence. Additionally, our devices can be operated at room temperature. A series of samples with different injection path lengths allows us to experimentally determine the spin relaxation length in our devices. In combination with operation in magnetic remanence, we are able to determine the spin relaxation length without the influence of external magnetic fields and at room temperature and find it to be 27 nm. Applying an additional external magnetic field, we find that at a field strength of 2 T, this relaxation length almost doubles, which is in good agreement with spin relaxation times in GaAs. Temperature control of our samples allows us to measure the temperature dependence of the spin relaxation length. At 200 K, the spin relaxation length doubles to 50 nm and reaches 80 nm at 30 K, in good agreement with theoretic calculations. Our results show that polarization values obtained with spin-LEDs inside strong magnetic fields and at low temperatures are not comparable to those in remanence and at room temperature. However, the transfer of efficient spintronic devices to such applicationenabling settings is absolutely necessary and will be a major challenge considering the enormous differences in spin relaxation. © 2013 SPIE.

We analyse the spin relaxation length during vertical electron transport in spin light-emitting diode devices at room temperature. We obtain a spin relaxation length of 27 nm in remanence. When a magnetic field is applied, spin relaxation is significantly reduced during transport to the active region of the device. This results in a nearly doubled spin relaxation length at 2T magnetic field strength. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on developing split-gate quantum point contacts (QPCs) that have a tunable length for the transport channel. The QPCs were realized in a GaAs/AlGaAs heterostructure with a two-dimensional electron gas (2DEG) below its surface. The conventional design uses 2 gate fingers on the wafer surface which deplete the 2DEG underneath when a negative gate voltage is applied, and this allows for tuning the width of the QPC channel. Our design has 6 gate fingers and this provides additional control over the form of the electrostatic potential that defines the channel. Our study is based on electrostatic simulations and experiments and the results show that we developed QPCs where the effective channel length can be tuned from about 200 nm to 600 nm. Length-tunable QPCs are important for studies of electron many-body effects because these phenomena show a nanoscale dependence on the dimensions of the QPC channel. © 2013 American Institute of Physics.

A strong enhancement of Eu3+ luminescence in europium-implanted GaN samples is obtained by codoping with silicon (Si) and magnesium (Mg), simultaneously. The Eu3+ intensity in the 5D0 to 7F2 transition region is found to be 30 times higher compared to europium-implanted undoped GaN. The major contribution to this overall enhancement is due a weak peak present only in europium-implanted Mg-doped GaN at 2.0031 eV (618.9 nm) which is strongly enhanced by codoping both Mg and Si. The excitation process of europium ions is proposed to take place through a donor-acceptor pair related energy transfer mechanism. © 2013 American Institute of Physics.

The temperature dependence of the coherence time of hole spins confined in self-assembled (In,Ga)As/GaAs quantum dots is studied by spin-mode-locking and spin-echo techniques. Coherence times limited to about a microsecond are measured for temperatures below 8 K. For higher temperatures, a fast drop occurs down to a few nanoseconds over a 10-K range. The hole-nuclear hyperfine interaction appears too weak to account for these limitations. We suggest that spin-orbit-related interactions are the decisive sources for hole spin decoherence. © 2013 American Physical Society.

We have studied the efficacy of (NH4)2Sx surface passivation on the (311)A GaAs surface. We report XPS studies of simultaneously-grown (311)A and (100) heterostructures showing that the (NH 4)2Sx solution removes surface oxide and sulfidizes both surfaces. Passivation is often characterized using photoluminescence measurements; we show that while (NH4)2Sx treatment gives a 40-60 × increase in photoluminescence intensity for the (100) surface, an increase of only 2-3 × is obtained for the (311)A surface. A corresponding lack of reproducible improvement in the gate hysteresis of (311)A heterostructure transistor devices made with the passivation treatment performed immediately prior to gate deposition is also found. We discuss possible reasons why sulfur passivation is ineffective for (311)A GaAs, and propose alternative strategies for passivation of this surface. © 2013 IOP Publishing Ltd.

The out-of-plane g-factor ga⊥* for quasi two-dimensional (2D) holes in a (100) GaAs heterostructure is studied using a variable width quantum wire. A direct measurement of the Zeeman splitting is performed in a magnetic field applied perpendicular to the 2D plane. We measure an out-of-plane g-factor up to ga⊥* = 5, which is larger than previous optical studies of ga⊥* and is approaching the long predicted but never experimentally verified out-of-plane g-factor of 7.2 for heavy holes. © 2012 American Chemical Society.

We report the realization of a quadruple quantum dot device in a square-like configuration where a single electron can be transferred on a closed path free of other electrons. By studying the stability diagrams of this system, we demonstrate that we are able to reach the few-electron regime and to control the electronic population of each quantum dot with gate voltages. This allows us to control the transfer of a single electron on a closed path inside the quadruple dot system. This work opens the route towards electron spin manipulation using spin-orbit interaction by moving an electron on complex paths free of electrons. © 2012 American Institute of Physics.

Photoactive materials are highly promising candidates for novel applications as they enable all-optical control of photonic devices. Photochromic molecules exhibit a reversible change of their dielectric function upon irradiation with light of proper wavelength. The trans- and cis-isomers of azobenzene exhibit different absorption properties due to the effect of the configuration on the polarizability of the molecule. Here, we introduce a novel molecular/semiconductor hybrid device which is fully tunable by all-optical means via the integration of a semiconductor microdisk into a photo-adressable polyelectrolyte material. We demonstrate that such polyelectrolyte superlattices can be used to tune semiconductor photonic resonators with high precision and without any significant degeneration of device performance. Moreover, we demonstrate an all-optically tunable laser based on this hybrid concept. © 2012 Optical Society of America.

Avoided crossing of the emission lines of a nearly free positive trion and a cyclotron replica of an exciton bound to an interface acceptor has been observed in the magnetophotoluminescence spectra of p-doped GaAs quantum wells. Identification of the localized state depended on the precise mapping of the anticrossing pattern. The underlying coupling is caused by an exciton transfer combined with a resonant cyclotron excitation of an additional hole. The emission spectrum of the resulting magnetically tunable coherent state probes weak localization in the quantum well. © 2012 American Physical Society.

Solid-state approaches to quantum information technology are attractive because they are scalable. The coherent transport of quantum information over large distances is a requirement for any practical quantum computer and has been demonstrated by coupling super-conducting qubits to photons. Single electrons have also been transferred between distant quantum dots in times shorter than their spin coherence time. However, until now, there have been no demonstrations of scalable 'flying qubit' architectures - systems in which it is possible to perform quantum operations on qubits while they are being coherently transferred - in solid-state systems. These architectures allow for control over qubit separation and for non-local entanglement, which makes them more amenable to integration and scaling than static qubit approaches. Here, we report the transport and manipulation of qubits over distances of 6 μm within 40 ps, in an Aharonov - Bohm ring connected to two-channel wires that have a tunable tunnel coupling between channels. The flying qubit state is defined by the presence of a travelling electron in either channel of the wire, and can be controlled without a magnetic field. Our device has shorter quantum gates (< μm), longer coherence lengths (∼86 μm at 70 mK) and higher operating frequencies (∼100 GHz) than other solid-state implementations of flying qubits. © 2012 Macmillan Publishers Limited. All rights reserved.

We report on an experimental evidence of a significantly different dynamic nuclear polarization (DNP) for localized and itinerant electrons in n-GaAs. Optically injected spin-polarized electrons are used to generate dynamic nuclear polarization via electron-nucleus hyperfine interaction. Using time resolved Kerr rotation measurements for probing the transient Overhauser field, the DNP time constants for itinerant and localized electrons are extracted to be 10 min and less than 1 min, respectively. This is attributed to a rapid DNP occurring in the vicinity of the donors followed by a delayed nuclear spin polarization in between the donor sites. © 2012 American Institute of Physics.

We have fabricated AlGaAs/GaAs heterostructure devices in which the conduction channel can be populated with either electrons or holes simply by changing the polarity of a gate bias. The heterostructures are entirely undoped, and carriers are, instead, induced electrostatically. We use these devices to perform a direct comparison of the scattering mechanisms of two-dimensional electrons (μ peak 4 10 6 cm 2/Vs) and holes (μ peak 0.8 10 6 cm 2/Vs) in the same conduction channel with nominally identical disorder potentials. We find significant discrepancies between electron and hole scattering, with the hole mobility being considerably lower than expected from simple theory. © 2012 American Institute of Physics.

We present the growth optimization and the doping by the metal organic chemical vapor deposition of lattice-matched Al 0.82In 0.18N bottom optical confinement layers for edge emitting laser diodes. Due to the increasing size and density of V-shaped defects in Al 1-xIn xN with increasing thickness, we have designed an Al 1-xIn xN/GaN multilayer structure by optimizing the growth and thickness of the GaN interlayer. The Al 1-xIn xN and GaN interlayers in the multilayer structure were both doped using the same SiH 4 flow, while the Si levels in both layers were found to be significantly different by SIMS. The optimized 8×(Al 0.82In 0.18N/GaN=54/6 nm) multilayer structures grown on free-standing GaN substrates were characterized by high resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy, along with the in-situ measurements of stress evolution during growth. Finally, lasing was obtained from the UV (394 nm) to blue (436 nm) wavelengths, in electrically injected, edge-emitting, cleaved-facet laser diodes with 480 nm thick Si-doped Al 1-xIn xN/GaN multilayers as bottom waveguide claddings. © 2011 Elsevier B.V. All rights reserved.

Coherent collective phenomena of hole spins confined in an ensemble of (In,Ga)As quantum dots are studied by monitoring their Larmor precession about a magnetic field. Variation of the field strength drives the hole spins from a regime of resonant spin amplification, in which their precession frequency is a unique multiple of the laser repetition rate, to spin mode locking, in which several precession modes are commensurable with the laser repetition rate. In this regime the spin coherence time of individual holes is determined to be 0.7 μs. In contrast, electron spins in the quantum dots are always trapped in the mode-locking regime due to the strong hyperfine interaction with nuclear spins. © 2012 American Physical Society.

The population dynamics of dark and bright excitons in (In,Ga)As/GaAs quantum dots is studied by two-color pump-probe spectroscopy in an external magnetic field. With the field applied in Faraday geometry and at T<20 K, the dark excitons decay on a ten nanoseconds time scale unless the magnetic field induces a resonance with a bright exciton state. At these crossings their effective lifetime is drastically shortened due to spin flips of either electron or hole by which the dark excitons are converted into bright ones. Due to the quasielastic character we attribute the origin of these flips to the hyperfine interaction with the lattice nuclei. We compare the exciton spin relaxation times in the two resonances and find that the spin flip involving an electron is approximately 25 times faster than the one of the hole. A temperature increase leads to a considerable, nonmonotonic decrease of the dark exciton lifetime. Here phonon-mediated spin flips due to the spin-orbit interaction gradually become more important. © 2012 American Physical Society.

Impedance measurements of single-wall carbon nanotube (SWCNT) fibers were carried out in the frequency range of 2010 6 Hz at temperatures 4.2 < T < 300 K and in the range of applied bias voltage 05 V. It was found that in the low frequency range (f 1-20 kHz) at low temperatures and at bias voltage U &gt; 2 V sign of the imaginary part of impedance was changed from negative to positive, indicating the existence of the crossover from capacitive reactance to inductive one. This crossover was induced by the decrease of height of the energy barriers between nanotubes at the increase of bias voltage. As a result decrease of the impedance of the fibers is accompanied by the rising of the role of kinetic inductance of separate nanotubes. © 2012 Copyright Taylor and Francis Group, LLC.

The problem of how single central spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying time scales and dynamical responses (exponential, power-law, Gaussian, etc.). Here we detect the small random fluctuations of central spins in thermal equilibrium [holes in singly charged (In,Ga)As quantum dots] to reveal the time scales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at a zero magnetic field that increase to ∼5μs as dominant hole-nuclear relaxation channels are suppressed with small (100G) applied fields. Concomitantly, the noise line shape evolves from Lorentzian to power law, indicating a crossover from exponential to slow [∼1/log(t)] dynamics. © 2012 American Physical Society.

We investigate the spin relaxation length during vertical electron transport in spin-light emitting diode devices as a function of magnetic field strength at room temperature. In most publications on spin relaxation in optoelectronic devices, strong magnetic fields are used to achieve perpendicular-to-plane magnetization of the spin injection contacts. We show experimentally that high magnetic field strengths significantly reduce spin relaxation during transport to the active region of the device. We obtain a spin relaxation length of 27(3) nm in magnetic remanence and at room temperature, which nearly doubles at 2 T magnetic field strength. © 2012 American Institute of Physics.

We investigate the tunneling rates from a 2-dimensional electron gas (2DEG) into the ground state of self-assembled InGaAs quantum dots. These rates are strongly affected by a magnetic field perpendicular to the tunneling direction. Surprisingly, we find an increase in the rates for fields up to 4 T before they decrease again. This can be explained by a mismatch between the characteristic momentum of the quantum dot ground state and the Fermi momentum k F of the 2DEG. Calculations of the tunneling probability can account for the experimental data and allow us to determine the dot geometry as well as k F. © 2012 American Institute of Physics.

Thermal voltage noise measurements are performed in order to determine the electron temperature in nanopatterned channels of a GaAs/AlGaAs heterostructure at bath temperatures of 4.2 and 1.4 K. Two narrow two-dimensional (2D) heating channels are connected by a quasi-1D quantum interferometer. Under dc current heating of the electrons in one heating channel, we perform cross-correlated noise measurements locally in the directly heated channel and nonlocally in the other channel, which is indirectly heated by hot electron diffusion across the quasi-1D connection. The temperature dependence of the electron energy-loss rate is reduced compared to wider 2D systems. Under nonlocal current heating, which establishes a thermal gradient across the quantum interferometer, we show the decoherence in this structure by Aharonov-Bohm measurements. © 2012 American Physical Society.

Two-color pump probe experiments reveal the possibility to use non-resonant pulsed laser excitation to create mode-locking of the ground state electron spin precessions in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The mode-locking shows a resonance for excitation into the first excited shell while for excitation into higher shells or barriers it disappears; however, spin coherence can still be induced. We conclude that the optically excited carriers relax spin-conserved from the p-shell into their ground states on a picosecond time scale, much shorter than the spin revolution period about the magnetic field. © 2012 American Institute of Physics.

Gate instability/hysteresis in modulation-doped p-type AlGaAs/GaAs heterostructures impedes the development of nanoscale hole devices, which are of interest for topics from quantum computing to novel spin physics. We present an extended study conducted using custom-grown, matched modulation-doped n-type and p-type heterostructures, with and without insulated gates, aimed at understanding the origin of the hysteresis. We show the hysteresis is not due to the inherent "leakiness" of gates on p-type heterostructures, as commonly believed. Instead, hysteresis arises from a combination of GaAs surface-state trapping and charge migration in the doping layer. Our results provide insights into the physics of Si acceptors in AlGaAs/GaAs heterostructures, including widely debated acceptor complexes such as Si-X. We propose methods for mitigating the gate hysteresis, including poisoning the modulation-doping layer with deep-trapping centers (e.g., by codoping with transition metal species) and replacing the Schottky gates with degenerately doped semiconductor gates to screen the conducting channel from GaAs surface states. © 2012 American Physical Society.

Photoluminescence lineshape properties of quasi-two-dimensional electron systems in setback δ-doped GaAs heterostructures are studied at liquid helium temperature. Contributions from the ground and the first excited two-dimensional subband are clearly observed. A simple fit to the lineshape including broadening demonstrates that there is an exponential low-energy tail associated with the ground subband. No such tail is observed for the first excited subband. The fit precisely reveals the subband bottom energies, the Fermi energy, the electron temperature and the recombination intensities. A self-consistent calculation of subband properties including the potential contribution of the setback δ-doping reproduces well the subband properties and the recombination intensities. © 2012 IOP Publishing Ltd.

We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5nm resolution. The results identify and quantify the main source of charge noise in the commonly used optical field-effect devices. © 2012 American Physical Society.

Spin-optoelectronic devices have become a field of intensive research in the past few years. Here we present electrical spin injection into spin light-emitting diodes both at room temperature and in magnetic remanence. Our devices consist of a Fe/Tb multilayer spin injection structure with remanent out-of-plane magnetization, a MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. The ground state emission and first excited state emission both show circularly polarized emission in remanence, i.e. without external magnetic fields which is due to spin injection from our ferromagnetic contact. Using a series of samples with varying transport path lengths between the spin injector and the active region, we investigate the spin relaxation length during vertical carrier transport through our devices. Due to our spin injector with remanent out-of-plane magnetization this spin relaxation can be investigated without the need for external magnetic fields which would possibly influence the spin relaxation process. The decrease in circular polarization with increasing injection path length is found to be exponential, indicating drift-based transport which is in accordance with theoretic calculations. From the exponential decay the spin relaxation length of 26 nm as well as a lower bound for the spin injection efficiency of 25% are calculated. Additionally, influences of magnetic field, temperature and current density in the devices on the spin relaxation process are discussed. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

We report on recent progress in the acousto-electrical control of self-assembled quantum dot and quantum post using radio frequency surface acoustic waves (SAWs). We show that the occupancy state of these optically active nanostructures can be controlled via the SAW-induced dissociation of photogenerated excitons and the resulting sequential bipolar carrier injection which strongly favors the formation of neutral excitons for quantum posts in contrast to conventional quantum dots. We demonstrate high fidelity preparation of the neutral biexciton which makes this approach suitable for deterministic entangled photon pair generation. The SAW driven acoustic charge conveyance is found to be highly efficient within the wide quantum well surrounding the quantum posts. Finally we present the direct observation of acoustically triggered carrier injection into remotely positioned, individual quantum posts which is required for a low-jitter SAW-triggered single photon source. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gate instability and hysteresis in Si-doped p-type AlGaAs/GaAs heterostructures impedes the development of nanoscale hole devices, which are of interest for topics from quantum computing to novel spin physics. We report an extended study conducted using matched n-type and p-type heterostructures, with and without insulated gates, aimed at understanding the origin of the hysteresis. We show the hysteresis is not due to the inherent 'leakiness' of gates on p-type heterostructures, as commonly believed. Instead, hysteresis arises from a combination of GaAs surface-state trapping and charge migration in the doping layer. © 2012 IEEE.

We report low-temperature transport measurements through a double-quantum-dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot tunneling coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak-coupling regime can be understood by considering a single level on each dot, in the intermediate- and strong-coupling regimes, the multilevel nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong- and weak-coupling regimes, the double-quantum-dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate-coupling regime does the device present a single dotlike molecular behavior as the molecular wave functions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed. © 2012 American Physical Society.

We demonstrate tunable transverse rectification in a density-modulated two-dimensional electron gas (2DEG). The density modulation is induced by two surface gates, running in parallel along a narrow stripe of 2DEG. A transverse voltage in the direction of the density modulation is observed, i.e., perpendicular to the applied source-drain voltage. The polarity of the transverse voltage is independent of the polarity of the source-drain voltage, demonstrating rectification in the device. We find that the transverse voltage U y depends quadratically on the applied source-drain voltage and nonmonotonically on the density modulation. The experimental results are discussed in the framework of a diffusion thermopower model. © 2012 American Physical Society.

We have developed undoped (1 0 0) GaAs/Al0.34Ga0.66As heterostructures, in which a 2D hole system is introduced by a heavily carbon doped field effect gate. We compare transport and mobility data from these (1 0 0) undoped devices with conventional Si modulation doped p-type devices grown on (3 1 1) substrates. Finally we present conductance quantization data for hole quantum wires made of these (1 0 0) heterostructures. © 2010 Elsevier B.V. All rights reserved.

We demonstrate an all-electrical prepartion and detection of many-particle spin states in self-assembled QDs. The excited states of the first three 'QD elements' are measured, i. e. the degenerate s-, p- and d-shells of QD-Hydrogen (one electron), the many-particle triplet and singlet states of QD-Helium (two electrons) and the many-particle states of QD-Lithium (three electrons). Comparison with exact diagonalization calculations fully accounting for the effects of Coulomb interaction enables us to reveal unambiguously the different charge and spin configurations. © 2011 American Institute of Physics.

Ultrathin anodic alumina with a film thickness of 11nm was implanted by Au atoms with low energy of 2, 5 or 10keV. Stopping range simulations yielded three essentially different geometries ranging from surface near implantation over well penetrated oxide to near oxide metal interface implantation, covering the entire range of possible implantation modifications. This work aims at demonstrating how to perform band gap engineering in alumina not only on an energetic level but also targeting a certain geometrical position of the doping atoms by means of the implantation parameters. Beside the intended implantation the oxide destruction in the implantation path and its possible repair was of interest. The repassivation behaviour was considerably different showing a significant redox contribution of the gold nanoclusters on top of the simple oxide repassivation. Near surface implanted Au remained electrochemically active for low repassivation potentials. Higher repassivation potentials always buried the implanted Au atoms under anodic alumina. The repassivation charge determined allowed determining the volume destructed by the implantation. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electron spin lifetimes are investigated in bulk wurtzite n-GaN up to very high doping densities by time-resolved Kerr-rotation spectroscopy. The doping densities range from 5×1015 to 1.5×1019 cm-3, corresponding to Fermi temperatures as high as 1200 K. The spin lifetime shows a maximum at the onset of degeneracy. The additional determination of momentum scattering times allows for a direct comparison with an analytical expression of the spin relaxation tensor in wurtzite semiconductors for a degenerate electron gas following Dyakonov-Perel theory. We find good agreement with the experiment up to the highest densities without any fitting parameter. The only known theoretical value αe= 9.0 meV of the k-linear contribution to spin-orbit coupling in the conduction band is shown to be valid up to the highest doping densities. © 2011 American Physical Society.

The dynamics of optically detected nuclear magnetic resonance is studied in n-GaAs via time-resolved Kerr rotation using an on-chip microcoil for rf field generation. Both optically allowed and optically forbidden NMR are observed with a dynamics controlled by the interplay between dynamic nuclear polarization via hyperfine interaction with optically generated spin-polarized electrons and nuclear spin depolarization due to magnetic resonance absorption. Comparing the characteristic nuclear spin relaxation rate obtained in experiment with master equation simulations, the underlying nuclear spin depolarization mechanism for each resonance is extracted. © 2011 American Physical Society.

The spectrally resolved terahertz photoconductivity between two separately contacted edge channels of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a not-simply-connected sample geometry which is topologically equivalent to a Corbino disk. Due to the high sensitivity of our sample structure, a weak resonance situated on the high-energy side of the well known cyclotron resonance is revealed. The magnetic field as well as the carrier density dependence of this weak resonance, in comparison with different models, suggests that the additional resonance is an edge-induced magnetoplasmon. © 2011 American Physical Society.

We have observed room temperature (RT) electrical spin injection in an InAs quantum dot (QD) light emitting diode (LED) grown on a p-type GaAs substrate from a ferromagnetic Fe/Tb electrode with strong out-of-plane anisotropy in remanence, i.e. without applied magnetic field. The QDs in the LED emit at 1275 nm (ground state luminescence), which is beyond the range for highly sensitive detectors, and therefore not optimum for various applications, e.g. quantum information studies. We will present two different ways to blue-shift the emission wavelength and discuss the advantages and drawbacks of the experiments. © 2010 Elsevier B.V. All rights reserved.

Using a combined all-ultra-high-vacuum process employing lateral patterning with focused ion beams and molecular beam epitaxy, site-selective growth of single (In,Ga)As quantum dots is achieved. We have embedded such a layer of intentionally positioned quantum dots in the intrinsic region of a p-i-n junction so that the quantum dots can be driven electrically. In this contribution, we will present our results on the morphological properties of the ion-beam modified surface on which the quantum dot nucleation occurs together with a characterization of the electrical and optoelectronic properties. We will demonstrate that a single, individual quantum dot can directly be electrically addressed. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report the observation of electromagnetically induced transparency (EIT) with an ensemble of donor-bound electrons in low-doped n-GaAs. We used pure GaAs layers with Si doping at very low concentration in a strong magnetic field. EIT was implemented with the two optical transitions that exist for the three-level system that is formed by the two electron spin states and a donor-bound trion state. Our results show that EIT with n-GaAs can serve as a platform for studies of nonlocal quantum entanglement with spins in semiconductors, as well as for controlling and probing dynamical nuclear polarization with coherent electron spins. © 2011 American Institute of Physics.

Electrons in a metal are indistinguishable particles that interact strongly with other electrons and their environment. Isolating and detecting a single flying electron after propagation, in a similar manner to quantum optics experiments with single photons, is therefore a challenging task. So far only a few experiments have been performed in a high-mobility two-dimensional electron gas in which the electron propagates almost ballistically. In these previous works, flying electrons were detected by means of the current generated by an ensemble of electrons, and electron correlations were encrypted in the current noise. Here we demonstrate the experimental realization of high-efficiency single-electron source and detector for a single electron propagating isolated from the other electrons through a one-dimensional channel. The moving potential is excited by a surface acoustic wave, which carries the single electron along the one-dimensional channel at a speed of 3 μm ns -1. When this quantum channel is placed between two quantum dots several micrometres apart, a single electron can be transported from one quantum dot to the other with quantum efficiencies of emission and detection of 96% and 92%, respectively. Furthermore, the transfer of the electron can be triggered on a timescale shorter than the coherence time T 2 * of GaAs spin qubits. Our work opens new avenues with which to study the teleportation of a single electron spin and the distant interaction between spatially separated qubits in a condensed-matter system. ©2011 Macmillan Publishers Limited. All rights reserved.

We report on photoluminescence experiments on spatially indirect excitons in an InGaAs coupled double quantum well device in which semitransparent gates are employed to tune the in-plane potential landscape. We introduce a trapping configuration in which exciton generation is spatially separated from the excitonic trapping potential. Suitably biased gates control the flow of indirect dipolar excitons from the generation area to the electrostatically defined trap. Thus the trap is filled only with indirect excitons precooled to the lattice temperature. Using a confocal microscope at liquid helium temperatures we map the in-plane distribution of excitons at various gate voltages and illumination conditions. Our small and strongly confining traps with precooled excitons demonstrate interesting many-body effects which can be interpreted in terms of the electrostatic screening, the Coulomb binding, and excitonic flows. Gate voltage dependencies of PL energy in our samples are not monotonic and can be explained by considering the nonlinear exciton flows between the elements of our structure. At strong illumination hysteretic switching of the trapped exciton population reflects a nonlinear character of the self-consistent trapping potential. An unusual nonlinear increase of the emission of the trap is likely coming from the many-body interactions in a dense exciton gas in the presence of a disorder potential at high light intensity. The designs of electrostatic traps proposed and realized here allow for stronger confinements and lower temperatures and will be used to search for coherent phenomena in dense exciton gases. © 2011 American Physical Society.

Using optical pulses of variable duration up to 80 ps, we report on spin coherence initialization and its subsequent detection in n-type singly charged quantum dots, subject to a transverse magnetic field, by pump-probe techniques. We demonstrate experimentally and theoretically that the spin coherence generation and readout efficiencies are determined by the ratio of laser pulse duration to spin precession period: An increasing magnetic field suppresses the spin coherence signals for a fixed duration of pump and/or probe pulses, and this suppression occurs for smaller fields, the longer is the pulse duration. The reason for suppression is the varying spin orientation due to precession during pulse action. © 2011 American Physical Society.

Focused ion beam technique is a powerful tool for defining patterns within a semiconductor film. In this paper, we show that it is possible to realize patterns such as disks and columns within thick GaN templates and that it is compatible with the regrowth of GaN based heterostructures. We study the effect of the pattern size and shape on the regrowth by molecular beam epitaxy. We show that the growth using ammonia as the nitrogen source with flux at temperature optimized for 2-dimensional growth leads to the apparition of well defined growth planes. We show that the development of these planes is dependent with the initial pattern size and shape. These results confirm the difficulty for realizing micro or nano-columns with axial heterostructures. At the opposite, these growth conditions seem favourable for core-shell heterostructures column with well defined m-plane and eventually a-plane lateral facets. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present an approach for electrically manipulating nuclear spins in n-GaAs using an on-chip microcoil. Optically injected spin-polarized electrons are used to generate a dynamic nuclear polarization via electron-nucleus hyperfine interaction with a characteristic time constant of ∼10 min. The saturated Overhauser field amplitude is on the order of several 10 mT and proportional to the spin polarization degree of the injected electrons. Applying an rf field resonant for the 75As nuclei, complete depolarization of 75As nuclear spins is observed. © 2011 American Institute of Physics.

We measure a strong enhancement of the nonlinear differential conductance (g=dI/dV), the amplitude of which exceeds the universal quantum conductance (2e2/h), under finite bias voltage in quantum point contacts (QPCs). By developing a spin-based model in the low-electron-density limit, we demonstrate that this resonance is an intrinsic nonequilibrium phenomenon that arises from many-body induced modifications to the QPC potential. A comparison with the linear conductance (G=I/V) shows that this phenomenon is driven by many-body dynamics within a single one-dimensional sub-band. © 2011 American Physical Society.

Two closely related bis(ketoiminato) zinc precursors, which are air stable and possess favorable properties for metal-organic (MO)CVD, are successfully employed for the growth of ZnO films on silicon and borosilicate glass substrates at temperatures between 400 and 700 °C. The as-deposited films are investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), nuclear reaction analysis (NRA), as well as by UV-vis absorption spectroscopy and photoluminescence (PL) measurements. The structure, morphology, and composition of the as-grown films show a strong dependence on the substrate temperature. The formation of pure and (001)-oriented wurtzite-type stoichiometric ZnO is observed. PL measurements are performed both at room temperature and 77 K, revealing a defect-free emission of ZnO films. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Injection of mode-filtered electrons into a phase-sensitive four-terminal waveguide Aharonov-Bohm (AB) ring is studied. An individually tuneable quantum point contact (QPC) in a waveguide lead of the GaAs/AlGaAs-ring allows to selectively couple to one-dimensional modes in the ring. Thus, we demonstrate single-mode transport in a multimode waveguide structure. Coherent mode-filtering by the lowest QPC subband is verified by nonlocal bend resistance and phase-sensitive AB interference measurements. © 2011 American Institute of Physics.

Using vibrational imaging techniques including Fourier-transform infrared (FTIR) synchrotron microscopy, Raman microscopy, and scattering scanning near-field infrared microcscopy (s-SNIM), we mapped a sample of phosphor and copper ions implanted in a high-purity silicon wafer. While Raman microscopy monitors the structural disorder within the implantation fields, the aforementionedinfrared techniques provide a detailed picture of the distribution of the free carriers. On a large scale (tens of micrometers), we visualized the channeling effects of phosphor dopants in silicon using FTIR microscopy. In comparison, using s-SNIM we were able to image, on a nanometer scale, local variations of the dielectric properties of the silicon substrate due to the activation of copper dopants. © 2011 American Institute of Physics.

We report the observation of Kondo physics in a spin-32 hole quantum dot. The dot is formed close to pinch-off in a hole quantum wire defined in an undoped AlGaAs/GaAs heterostructure. We clearly observe two distinctive hallmarks of quantum dot Kondo physics. First, the Zeeman spin splitting of the zero-bias peak in the differential conductance is independent of the gate voltage. Second, this splitting is twice as large as the splitting for the lowest one-dimensional subband. We show that the Zeeman splitting of the zero-bias peak is highly anisotropic and attribute this to the strong spin-orbit interaction for holes in GaAs. © 2011 American Physical Society.

Coherent interactions between spins in quantum dots are a key requirement for quantum gates. We have performed pump-probe experiments in which pulsed lasers emitting at different photon energies manipulate two distinct subsets of electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin dynamics are monitored through their precession about an external magnetic field. These measurements demonstrate spin precession phase shifts and modulations of the magnitude of one subset of oriented spins after optical orientation of the second subset. The observations are consistent with results from a model using a Heisenberg-like interaction with μeV strength. © 2011 American Physical Society.

Optically detected nuclear magnetic resonance (NMR) with micrometer resolution is demonstrated in n-GaAs using an on-chip microcoil. To trace the Overhauser field, the electron Larmor frequency is monitored via time-resolved magneto-optical Kerr rotation. Sweeping the frequency of the rf magnetic field induced by an on-chip microscale current loop, nuclear spin depolarization is achieved for each isotope species. The experimental data indicate an impact of a local quadrupole field, most likely caused by ionized donors, on the amplitude and linewidth of the NMR spectrum. By applying rf pulse sequences, the Rabi oscillation of 75As nuclear spins is obtained with an effective dephasing time of ∼200 μs. © 2011 American Institute of Physics.

In this work, we study the temperature-dependent spin relaxation of exciton-bound carriers in (In,Ga)As/GaAs quantum dots. The exciton population mapped by a time-resolved differential transmission signal reveals a decay on two different time scales, reflecting the fractions of optically active and inactive excitons. The underlying exciton states are split from each other by the exchange interaction. The phonon-assisted spin-orbit interaction induces spin flips of an exciton-bound electron or hole which convert the exciton populations into each other. The temperature-dependent relaxation rate follows a thermal phonon distribution. Deviations indicate that two-phonon processes involving higher orbitals may contribute significantly to the total relaxation process. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film. © 2011 American Institute of Physics.

In a comment by Gehlhoff et al. on our work on "Paramagnetic and ferromagnetic resonance studies on dilute magnetic semiconductors based on GaN" [Phys. Status Solidi A 205, 1872 (2008)] the authors claim a contradictory explanation of angular dependent magnetic resonance measurements. Here we discuss their - solely based on computer simulations - claims. We show that an interpretation limited on magnetic impurities can hardly be used to explain the complex magnetic behavior. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present resistively detected NMR measurements in induced and modulation-doped electron quantum point contacts, as well as induced hole quantum point contacts. While the magnitude of the resistance change and associated NMR peaks in n-type devices is in line with other recent measurements using this technique, the effect in p-type devices is too small to measure. This suggests that the hyperfine coupling between holes and nuclei in this type of device is much smaller than the electron hyperfine coupling, which could have implications in quantum information processing. © 2011 American Chemical Society.

We report on the observation of resonant optical pumping of nuclear spin polarization in an ensemble of singly charged (In,Ga)As/GaAs quantum dots subject to a transverse magnetic field. Electron spin orientation by circularly polarized light with the polarization modulated at the nuclear spin transition frequency is found to create a significant nuclear spin polarization, precessing about the magnetic field. Synchronous rf field application along the optical excitation axis considerably enhances the effect. Nuclear spin resonances for all isotopes in the quantum dots are found in that way. In particular, transitions between states split off from the |±1/2 doublets by the nuclear quadrupole interaction are identified. © 2011 American Physical Society.

We investigate the spin relaxation length in GaAs spin light-emitting diode devices under drift transport at room temperature. The spin-polarised electrons are injected through a MgO tunnel barrier from a Fe/Tb multilayer in magnetic remanence. The decrease in circular polarization with increasing injection path length is investigated and found to be exponential, supporting drift-based transport. The spin relaxation length in our samples is 26 nm, and a lower bound for the spin injection efficiency at the spin injector/GaAs interface is estimated to be 25±2%. © 2011 American Institute of Physics.

We have investigated polarisation resolved magneto-photoluminescence from 22 nm wide asymmetric GaAs quantum well. In contrast to previous studies we observed abrupt decrease of photoluminescence intensity of the neutral exciton line with increasing temperatures whereas photoluminescence intensity of the positively charged exciton remains unchanged. The effect is a clear evidence of different mechanisms of neutral and charged exciton recombination. We have also detected a coupling of two different radiative states: an acceptor-bound and an essentially free trions.

Ho ions were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a 100kV focused-ion-beam implanter at room temperature (RT). The implantation doses of Ho ions ranges from 1014 to 10 16 cm-2. Without thermal annealing, the structural, optical, and magnetic properties of the Ho-implanted thin films were investigated. Structural properties studied by x-ray diffraction revealed Ho incorporation into GaN matrix without secondary phase. The overall photoluminescence of any implanted sample is weaker than that of the non-implanted one. The spectra show neutral-donor-bound exciton emission and defect-related blue luminescence. Blocked superparamagnetic behavior was identified from Ho-implanted samples at temperatures below RT by measurements with a superconducting quantum interference device. The highest ordering temperature is 100 K. © Published under licence by IOP Publishing Ltd.

Using time-resolved transport spectroscopy, we investigate the influence of charge-tunable InAs quantum dots (QDs) on the conductance of a nearby two-dimensional electron gas (2DEG). Loading successively electrons into the self-assembled QDs decreases the carrier concentration and mobility in the 2DEG. We are able to quantify how these transport properties change for each additional charge in the s- or p-shell. It is found that mobility and carrier concentration contribute equally to the overall change in conductance. © 2011 American Institute of Physics.

We report the experimental observation of magnetically bound electronic states in a quantum wire with a magnetic barrier, as a function of a superimposed homogeneous perpendicular magnetic field which displaces the magnetic barrier along the magnetic field axis. © 2011 American Institute of Physics.

Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.

We demonstrate the possibility to influence the shape of the wave functions in semiconductor quantum dots by the application of an external magnetic field Bz. The states of the so-called p shell, which show distinct orientations along the crystal axes for Bz=0, can be modified to become more and more circularly symmetric with an increasing field. Their changing probability density can be monitored using magnetotunneling wave function mapping. Calculations of the magnetotunneling signals are in good agreement with the experimental data and explain the different tunneling maps of the p+ and p- states as a consequence of the different sign of their respective phase factors. © 2010 The American Physical Society.

The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using time-resolved conductance measurements of a nearby two-dimensional electron gas (2DEG). The coupling strength (tunneling time) between the QDs and the 2DEG is adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance on the 2DEG, even for very weak coupling, where standard C-V spectroscopy is unsuitable to investigate the electronic structure of these QDs. © Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 2010.

A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients. © 2010 The Author(s).

The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using a time-resolved conductance measurement of a nearby two-dimensional electron gas (2DEG). The investigated heterostructures consist of a layer of QDs with different coupling strengths to a 2DEG, adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG, even for very weak coupling between the QD layer and the 2D system, where standard capacitance (C)voltage (V) spectroscopy is unsuitable to investigate the electronic structure of these QDs. © 2009 Elsevier B.V. All rights reserved.

We present a voltage-tunable in-plane diode in a nanoscale, two-dimensional electron system, whose functionality is mainly determined by the sample geometry. The diode consists of a narrow semiconductor channel, confined by etched insulating trenches. An applied voltage along the channel modulates the effective width of the conducting channel, depending on the sign of the applied voltage. This behavior results in a diode-like IV -characteristic. The tunability of the device is achieved by two in-plane side gates, which are able to widely tune the I (V) -characteristic of this rectifier. In the normally-off regime, this tunable in-plane diode works as a half-wave rectifier with sharply defined turn-on voltage. The value of the turn-on voltage depends on the side-gate voltage. © 2009 Elsevier B.V. All rights reserved.

We investigate coherent electron transport in a four-terminal asymmetric waveguide quantum ring with the focus on a controllable electrostatic Aharonov-Bohm (AB) oscillation phase shift. In the AlGaAs/GaAs device, electron wave interference has been detected in all possible four-terminal measurement configuration at a temperature as high as T = 1.5 K. We present a series of AB measurements in small magnetic fields for successive global gate voltages, which strongly suggest an electrostatic AB phase shift. Such a phase shift can be explained by the asymmetry of the ring and indicates that the design and positioning of voltage and current probes to the ring have a significant influence. © 2009 Elsevier B.V. All rights reserved.

We have integrated individual (In,Ga)As quantum dots (QDs) using site-controlled molecular beam epitaxial growth into the intrinsic region of a p-i-n junction diode. This is achieved using an in situ combination of focused ion beam prepatterning, annealing, and overgrowth, resulting in arrays of individually electrically addressable (In,Ga)As QDs with full control on the lateral position. Using microelectroluminescence spectroscopy we demonstrate that these QDs have the same optical quality as optically pumped Stranski-Krastanov QDs with random nucleation located in proximity to a doped interface. The results suggest that this technique is scalable and highly interesting for different applications in quantum devices. © 2010 American Institute of Physics.

We have fabricated and studied ballistic one-dimensional hole quantum wires using a high mobility undoped (1 0 0)-oriented AlGaAs/GaAs heterostructure. The devices exhibit clear quantized conductance plateaus with highly stable gate characteristics. These devices provide a platform for studying one-dimensional phenomena in hole systems with minimal crystallographic effects. © 2009 Elsevier B.V. All rights reserved.

We have developed a strategy for the easy fabrication of quantum devices on AlGaAs/GaAs heterostructures featuring gates insulated from the heterostructure surface by a thin oxide layer deposited by atomic layer deposition. We will present the results of comparative studies of devices made with and without oxide-insulated gates to establish whether this leads to a significant enhancement in device stability and reduced gate hysteresis and noise. © 2010 IEEE.

We have performed capacitance-voltage (C-V) spectroscopy on the valence band states of InAs quantum dots (QDs) in magnetic fields tilted with respect to the sample plane. We observe an unexpected behaviour for the height of the capacitance signal as function of the in-plane magnetic field, when varying the perpendicular field component, for the first two charging peaks corresponding both to an s-like ground state. With increasing perpendicular field component, the signal height for the first peak decreases strongly and for the second peak the maximum signal height is observed at finite in-plane field values. These observations are not in agreement with the usual assumption that the height-vs.-in-plane-field traces reflect the in-plane k-space probability distribution of the charged QD level because an s-like state should not vary its general shape with perpendicular field. The qualitatively different behaviour of the two peaks is attributed tentatively to correlation effects when the second electron is charged. © 2010 IOP Publishing Ltd.

A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf-21 at.% Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal through an intermediate amorphous region. The local oxidation of Hf-Ta parent metal alloys at different compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric constants were mapped for the entire composition range and XPS depth profiles allowed investigation of the oxide compositions. © 2010 Elsevier Ltd. All rights reserved.

Four different liquid metal ion sources (LMIS), working with pure Bi as well as with Bi containing alloys (Au13Bi87, Ga 38Bi62, Ga35Bi60Li5) were investigated with respect to the emission behavior as a function of current and temperature, the mass spectra and the energy distribution of the individual ion species. Additionally, for the pure Bi-LMIS the sputtering rates for Bi ions and clusters on Si, SiO2 and Ge substrates were compared with that of Ga projectile ions using a mass separating focused ion beam system. © Springer-Verlag 2010.

Phase sensitivity and thermal dephasing in coherent electron transport in quasi-one-dimensional (1D) waveguide rings of an asymmetric four-terminal geometry are studied by magnetotransport measurements. We demonstrate the electrostatic control of the phase in Aharonov-Bohm resistance oscillations and investigate the impact of the measurement circuitry on decoherence. Phase rigidity is broken due to the ring geometry: orthogonal waveguide cross junctions and 1D leads minimize reflections and resonances between leads allowing for a continuous electron transmission phase shift. The measurement circuitry influences dephasing: thermal averaging dominates in the nonlocal measurement configuration while additional influence of potential fluctuations becomes relevant in the local configuration. © 2010 The American Physical Society.

The confined electronic states of mesoscopic structures in a magnetic field are arranged in Landau levels consisting of spatially discrete eigenstates. These Landau orbits are the quantum mechanical analogue of classical cyclotron orbits. Here we present magnetoconductance oscillations in semiconductor rings, which visualize the spatial discreteness of the Landau orbits in high magnetic fields (typically B>2 T). We will show that these oscillations are caused by the flux-quantized, discrete electronic size of the ring leading to a corresponding modulation of its two-point conductance. The oscillation period is given by the number of flux quanta penetrating the conducting area of the structure. These high-field oscillations are distinctively different from the well-known Aharonov-Bohm effect, where, most generally, the penetration of individual flux quanta h/e through a nanostructure causes periodic crossings of field-dependent energy levels, which give rise to magneto-quantum oscillations in its conductance. © 2010 Macmillan Publishers Limited. All rights reserved.

We have studied the Faraday rotation and ellipticity signals in ensembles of singly charged (In,Ga)As/GaAs quantum dots by pump-probe spectroscopy. For degenerate pump and probe we observe that the Faraday rotation signal amplitude first grows with increasing the time separation between pump and probe before a decay is observed for large temporal separations. The temporal behavior of the ellipticity signal, on the other hand, is regular: its amplitude decays with the separation. By contrast, for detuned pump and probe the Faraday rotation and ellipticty signals both exhibit similar and conventional behavior. The experimental results are well described in the frame of a recently developed microscopic theory. The comparison between calculations and experimental data allows us to provide insight into the spectral dependence of the electron spin precession frequencies and extract the electron g factor dependence on energy. © 2010 The American Physical Society.

Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission. © 2010 IOP Publishing Ltd.

We present measurements of electromagnetically induced transparency with an ensemble of donor-bound electrons in low-doped n-GaAs. We used optical transitions from the Zeeman-split electron-spin states to a bound trion state in samples with optical densities of 0.3 and 1.0. The electron-spin dephasing time T2-2ns was limited by hyperfine coupling to fluctuating nuclear spins. We also observe signatures of dynamical nuclear polarization but find these effects to be much weaker than in experiments that use electron-spin resonance and related experiments with quantum dots. © 2010 The American Physical Society.

Self-assembled InGaAs quantum rings, embedded in a GaAs matrix, were investigated using magneto-capacitance-voltage spectroscopy. The magnetic-field dispersion of the charging energies exhibits characteristic features for both the first and second electron, which can be attributed to a ground state transition from l=0 into l=-1, and a ground state transition from l=-1 into l=-2, respectively. Furthermore, using a combination of capacitance-voltage spectroscopy and one-dimensional numerical simulations, the conduction band structure of these InGaAs quantum rings was determined. © 2010 American Institute of Physics.

Individual self-assembled quantum dots and quantum posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For quantum posts, switching occurs continuously from predominantely charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent of whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is nonmonotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which quantum dots nucleate. This limitation can be overcome for quantum posts where acoustically induced charge transport is highly efficient in a wide lateral matrix-quantum well. © 2010 American Chemical Society.

We have successfully grown and characterized [Fe/Tb]10 /Fe (001) / 57Fe (001) /MgO (001) multilayer contacts on a GaAs-based light emitting diode. Using 57Fe conversion-electron Mössbauer spectroscopy at room temperature (RT) and at 4.2 K, we provide atomistic proof of large perpendicular Fe spin components in zero external field at and below RT at the 57Fe (001) /MgO (001) interface. Further, indirect evidence of large interfacial Fe atomic moments is provided. Our contacts serve as a prototype spin aligner for remanent electrical spin injection at RT. © 2010 American Institute of Physics.

Silicon diodes with a p+-n junction made in a 48-μμ m-thick phosphorus-doped silicon epilayer (resistivity ρρ = 30 ΩΩ cm) grown on antimony-doped Si(111) wafers (ρρ = 0.01 ΩΩ cm) are studied. The diodes are irradiated by high-energy (3. 5 MeV) electrons with fluences from 5 ×× 1015 to 2 ×× 1016 cm-2. It is shown that the conventional equivalent circuit of the diode that consists of a parallel RC network and a series-connected resistor inadequately describes the dependence of the dielectric loss tanδδ on variable current frequency f in the range 1 ×× 102-3 ×× 107 Hz. Another equivalent circuit is suggested that includes not only the capacitance and resistance of the n-base (the latter increases because radiation-induced defects are compensated for by shallow donors) but also the f dependence of the capacitance of the space-charge region, which is due to retarded charge exchange between deep-level radiation-induced defects. ©© 2010 Pleiades Publishing, Ltd.

Quantum point contacts (QPCs) implemented in p-type GaAs/AlGaAs heterostructures are investigated by low-temperature electrical conductance spectroscopy. Besides onedimensional conductance quantization, a pronounced extra plateau is found at about 0.7(2e2/h) which possesses the characteristic properties of the so-called "0.7 anomaly" known from experiments with n-type samples. The evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and is consistent with the explanation of the 0.7 anomaly based on self-consistent charge localization. These observations are robust when lateral electrical fields are applied which shift the relative position of the electron wave function in the QPC, testifying to the intrinsic nature of the underlying physics. Copyright © EPLA, 2010.

In this study a metal-insulator semiconductor field effect transistor (MISFET) was fabricated on an undoped (100)-oriented AlGaAs/GaAs heterostructure. This device has the ability to switch the charge carriers in the conduction channel between electrons and holes by switching the sign of the applied top gate voltage. The device was characterised at 0.3K and electron/hole transport measurements were conducted. In the future these devices could potentially be used as lateral p-i-n junctions for fabrication of low dimensional electroluminescent devices. © 2010 IEEE.

The hot-electron thermopower of a quantum point contact (QPC) is exploited for full-wave rectification at low temperatures. In a nanoscale AlGaAs/GaAs cross junction with orthogonal current and voltage leads the QPC is embedded into one voltage lead. The transfer resistance RT, given by the output voltage divided by input current, exhibits a distinct maximum at finite current and at gate voltages close to the QPC pinch-off voltage. Values in excess of RT =7k and output voltages up to 60% of the input voltages indicate an efficient ballistic rectification process. © 2010 American Institute of Physics.

Characteristics of Si p+n diodes with non-uniformly distributed compensating defects, which were introduced by implantation with Xe23+ ions, have been studied. The layer with the maximum concentration of the compensating defects was located in the vicinity of the metallurgical p-n junction. It is found that the presence of the defect layer results in non-monotonic dependences of the imaginary part of impedance (-Z″) and differential conductance (G = -dI/dU) of the implanted diodes on reverse bias voltage U. An equivalent circuit of the irradiated diode is proposed, which allows us to approximate the measured frequency dependences of capacitance and conductance of the irradiated diodes and to determine values of diode barrier capacitance Cpn at different reverse bias voltages. © 2010 Elsevier Ltd. All rights reserved.

In-plane gate (IPG) transistors with various channel geometries have been fabricated by focus ion beam implantation technique in positive pattern definition mode on GaAs/Al xGa 1-xAs heterostructures. Both n- and p-type channel transistors were obtained for either n- or p-doped heterostructures, by implantation of the channel or the gate regions with the complementary dopant type to compensate the initial doping. The current-voltage characteristics showed that the channel can be fully controlled by a gate bias, i.e., it can be completely depleted or enhanced, but two gates are needed to control the channel. Different channel geometries were studied, the most efficient being the "Z-shaped" one. The n-type channel IPG transistors present source-drain currents of few hundreds of microamperes, which is two to three orders of magnitudes higher than that for the p-type channel ones. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Silicon diodes with a p+-n junction irradiated with 3. 5-MeV electrons (the fluence ranged from 1015 to 4 × 1016 cm-2) have been studied. It is established that the dependence of the tangent of the angle of electrical losses tanδ on the frequency f of alternating current in the range f = 102-106 Hz is a nonmonotonic function with two extrema: a minimum and a maximum. Transformation of the dependences tanδ(f) as the electron fluence and annealing temperature are increased is caused by a variation in the resistance of n-Si (the base region of the diodes) as a result of accumulation (as the fluence is increased) or disappearance and reconfiguration (in the course of annealing) of radiation defects. The role of time lag of the defect recharging in the formation of tanδ(f) is insignificant. © 2010 Pleiades Publishing, Ltd.

An intentional positioning of optically active quantum dots using site-selective growth by a combination of molecular beam epitaxy (MBE) and focused ion beam (FIB) implantation in an all-ultra-high-vacuum (UHV) setup has been successfully demonstrated. A square array of periodic holes on GaAs substrate was fabricated with FIB of 30 keV Ga ions followed by an in situ annealing step. Subsequently, the patterned holes were overgrown with an optimized amount of InAs in order to achieve site-selective growth of the QDs on the patterned holes. Under well-optimized conditions, a selectivity of single quantum dot growth in the patterned holes of 52% was achieved. Thereafter, carrier injection and subsequent radiative recombination from the positioned InAs/GaAs self-assembled QDs was investigated by embedding the QDs in the intrinsic part of a GaAs-based pin junction device. Electroluminescence spectra taken at 77 K show interband transitions up to the fifth excited state from the QDs. © 2009 Elsevier B.V. All rights reserved.

The artificial creation of shallow nitrogen-vacancy (NV) centres in diamond with 25 nm lateral resolution is performed by collimated implantation of low-energy nitrogen ions. The electron spin associated to this defect is the most promising qubit for a scalable quantum computer working at room temperature. Individual optical addressing of two single centres separated by only 16 nm is demonstrated with stimulated emission depletion (STED) microscopy.

A nanofabrication technique is presented which enables the fabrication of highly tunable devices on p-type, C-doped GaAs/AlGaAs heterostructures containing shallow two-dimensional hole systems. The high tunability of these structures is provided by the complementary electrostatic effects of intrinsic in-plane gates and evaporated metallic top-gates. Quantum point contacts fabricated with this technique were tested by electrical conductance spectroscopy. © 2010 American Institute of Physics.

We examine the resonance fluorescence from localized excitons in GaAs/AlGaAs quantum wells with increasing excitation power and for different temperatures. Extending our experimental setup by a microscope objective with high numerical aperture in the cryostat, the detection of the emission from localized excitons is possible. We find a nonlinear behaviour of the emitted intensity with increasing laser power, which is explained as a transition of the emission from excitonic to electron-hole plasma states due to many-body effects between excited carriers. This is supported by our theoretical description based on the semiconductor Bloch equations. © 2010 IOP Publishing Ltd.

In this paper, We study the Zeeman spin-splitting in hole quantum wires oriented along the [011] and [011̄] crystallographic axes of a high mobility undoped (100)-oriented AlGaAs/GaAs heterostructure. Our data show that the spin-splitting can be switched 'on' (finite g*) or 'off' (zero g*) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the g-factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the g-factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a k -dependent Zeeman splitting that arises from the spin-| nature of holes. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

The electron spin dynamics in a GaAs/AlGaAs heterojunction system containing a high-mobility two-dimensional electron gas (2DEG) have been studied in this paper by using pump-probe time-resolved Kerr rotation experiments. Owing to the complex layer structure of this material, the transient Kerr response contains information about electron spins in the 2DEG, an epilayer and the substrate. We analyzed the physics that underlies this Kerr response, and established the conditions under which it is possible to unravel the signatures of the various photo-induced spin populations. This was used to explore how the electron spin dynamics of the various populations depend on the temperature, magnetic field and pump-photon density. The results show that the D'Yakonov-Perel' mechanism for spin dephasing (by spin-orbit fields) plays a prominent role in both the 2DEG and bulk populations over a wide range of temperatures and magnetic fields. Our results are of importance for future studies on the 2DEG in this type of heterojunction system, which offers interesting possibilities for spin manipulation and control of spin relaxation via tunable spin-orbit effects. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Nuclear-magnetic resonances were detected optically in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The resonances were found in the magnetic field dependence of photoluminescence polarization (Hanle curve) when applying a radio frequency (RF) field. The frequency dependences of the resonances allow us to ascribe them to transitions between Zeeman states of the G 71 a and A 75 s nuclei perturbed by quadrupole interaction. Applying an RF-field sweep over a wide frequency range results in a notable narrowing of the Hanle curve, which can be explained by suppression of dynamic nuclear polarization that otherwise is stabilized by the quadrupole splitting. © 2010 The American Physical Society.

Quantum coherent properties of electrons can be studied in Aharonov-Bohm (AB) interferometers. We investigate both experimentally and theoretically the transmission phase evolution in a four-terminal quasi-one-dimensional AlGaAs/GaAs-based waveguide AB ring. As main control parameter besides the magnetic field, we tune the Fermi wave number along the pathways using a top-gate. Our experimental results and theoretical calculations demonstrate the strong influence of the measurement configuration upon the AB-resistance- oscillation phase in a four-terminal device. While the nonlocal setup displays continuous phase shifts of the AB oscillations, the phase remains rigid in the local voltage-probe setup. Abrupt phase jumps are found in all measurement configurations. We analyze the phase shifts as functions of the magnetic field and the Fermi energy and provide a detailed theoretical model of the device. Scattering and reflections in the arms of the ring are the source of abrupt phase jumps by π. © 2010 The American Physical Society.

Ballistic quantum wires are exposed to longitudinal profiles of perpendicular magnetic fields composed of a spike and a homogeneous part. An asymmetric magnetoconductance peak as a function of the homogeneous magnetic field is found, comprising quantized conductance steps in the interval where the homogeneous magnetic field and the magnetic barrier have identical polarities, and a characteristic shoulder with several resonances in the interval of opposite polarities. The observations are interpreted in terms of inhomogeneous diamagnetic shifts of the quantum wire modes leading to magnetic confinement. © 2010 The American Physical Society.

We study the disorder dependence of the phase coherence time of quasi-one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semiballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi-liquid theory, without any sign of low-temperature saturation. Surprisingly, in the semiballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime. © 2010 The American Physical Society.

GaAs-based semiconductor microdisks with high quality whispering gallery modes (Q>4000) have been fabricated. A layer of self-organized InAs quantum dots (QDs) served as a light source to feed the optical modes at room temperature. In order to achieve frequency tuning of the optical modes, the microdisk devices have been immersed in 4-cyano-4′-pentylbiphenyl (5CB), a liquid crystal (LC) with a nematic phase below the clearing temperature of TC≈34°C. We have studied the device performance in the temperature range of T=2050°C, in order to investigate the influence of the nematicisotropic phase transition on the optical modes. Moreover, we have applied an AC electric field to the device, which leads in the nematic phase to a reorientation of the anisotropic dielectric tensor of the liquid crystal. This electrical anisotropy can be used to achieve electrical tunability of the optical modes. Using the finite-difference time domain (FDTD) technique with an anisotropic material model, we are able to describe the influence of the liquid crystal qualitatively. © 2009 Elsevier B.V. All rights reserved.

The carrier spin dynamics in a n -doped (In,Ga)As/GaAs quantum well has been studied by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 milliKelvin. These techniques give data with very different spectral dependencies, from which nonetheless consistent information on the spin dynamics can be obtained, in agreement with theoretical predictions. The mechanisms of long-lived spin coherence generation are discussed for the cases of trion and exciton resonant excitation. We demonstrate that carrier localization leads to a saturation of spin relaxation times at 45 ns for electrons below 4.5 K and at 2 ns for holes below 2.3 K. The underlying spin relaxation mechanisms are discussed. © 2010 The American Physical Society.

We measure the frequency spectra of random spin fluctuations, or "spin noise," in ensembles of (In,Ga)As/GaAs quantum dots (QDs) at low temperatures. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array, to measure and average noise spectra from 0-1 GHz continuously in real time with subnanoradian/Hz sensitivity. Both electron and hole spin fluctuations generate distinct noise peaks, whose shift and broadening with magnetic field directly reveal their g factors and dephasing rates within the ensemble. A large, energy-dependent anisotropy of the in-plane hole g factor is clearly exposed, reflecting systematic variations in the average QD confinement potential. © 2010 The American Physical Society.

We report a study of suppressed spin dephasing for quasi-one-dimensional electron ensembles in wires etched into a GaAs/AlGaAs heterojunction system. Time-resolved Kerr-rotation measurements show a suppression that is most pronounced for wires along the [110] crystal direction. This is the fingerprint of a suppression that is enhanced due to a strong anisotropy in spin-orbit fields that can occur when the Rashba and Dresselhaus contributions are engineered to cancel each other. A surprising observation is that this mechanism for suppressing spin dephasing is not only effective for electrons in the heterojunction quantum well but also for electrons in a deeper bulk layer. © 2010 The American Physical Society.

The detailed studies of two-dimensional hole gas in an asymmetric 22 nm wide GaAs/Ga1-xAlxAs quantum well in polarization-resolved photoluminescence in high magnetic fields (up to B 20 T) and at low temperatures (down to T 50 mK) are reported. Additionally to the previously detected in symmetric quantum wells dominant emission channels of various free and acceptor-bound trions, the high-energy hole cyclotron replicas of the bound states are now also observed, corresponding to a the exciton-cyclotron resonance. The identification of transitions in the reach spectra was performed by the analysis of optical selection rules and comparison of the experimental spectra with numerical calculations of the real structure. © 2010 IOP Publishing Ltd.

The circular polarization degree of (In,Ga)As/GaAs quantum dot photoluminescence is experimentally studied as function of magnetic field applied perpendicular to the optical excitation direction (Voigt geometry). The measurements are performed using various modulation timing protocols for the excitation polarization and intensity. The experimental data obtained allow us to estimate the characteristic times of polarization and relaxation processes for the nuclear spin system. © 2010 IOP Publishing Ltd.

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator. (C) 2010 Optical Society of America