Dr. Arne Ludwig

Applied Solid State Physics
Ruhr-Universität Bochum

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  • A bright and fast source of coherent single photons
    Tomm, N. and Javadi, A. and Antoniadis, N.O. and Najer, D. and Löbl, M.C. and Korsch, A.R. and Schott, R. and Valentin, S.R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Nature Nanotechnology (2021)
    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.
    view abstract10.1038/s41565-020-00831-x
  • Bright Electrically Controllable Quantum-Dot-Molecule Devices Fabricated by In Situ Electron-Beam Lithography
    Schall, J. and Deconinck, M. and Bart, N. and Florian, M. and von Helversen, M. and Dangel, C. and Schmidt, R. and Bremer, L. and Bopp, F. and Hüllen, I. and Gies, C. and Reuter, D. and Wieck, A.D. and Rodt, S. and Finley, J.J. and Jahnke, F. and Ludwig, Ar. and Reitzenstein, S.
    Advanced Quantum Technologies 4 (2021)
    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
    view abstract10.1002/qute.202100002
  • Characterization of a surface plasmon antenna fabricated on a gate-defined lateral quantum dot
    Fukai, R. and Sakai, Y. and Nakagawa, T. and Fujita, T. and Kiyama, H. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A.
    Japanese Journal of Applied Physics 60 (2021)
    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
    view abstract10.35848/1347-4065/abd533
  • Charge tunable gaas quantum dots in a photonic n-i-p diode
    Babin, H.G. and Ritzmann, J. and Bart, N. and Schmidt, M. and Kruck, T. and Zhai, L. and Löbl, M.C. and Nguyen, G.N. and Spinnler, C. and Ranasinghe, L. and Warburton, R.J. and Heyn, C. and Wieck, A.D. and Ludwig, Ar .
    Nanomaterials 11 (2021)
    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.
    view abstract10.3390/nano11102703
  • Coherent Beam Splitting of Flying Electrons Driven by a Surface Acoustic Wave
    Ito, R. and Takada, S. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S. and Yamamoto, M.
    Physical Review Letters 126 (2021)
    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.
    view abstract10.1103/PhysRevLett.126.070501
  • Coherent Spin-Photon Interface with Waveguide Induced Cycling Transitions
    Appel, M.H. and Tiranov, A. and Javadi, A. and Löbl, M.C. and Wang, Y. and Scholz, S. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J. and Lodahl, P.
    Physical Review Letters 126 (2021)
    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.
    view abstract10.1103/PhysRevLett.126.013602
  • Distant spin entanglement via fast and coherent electron shuttling
    Jadot, B. and Mortemousque, P.-A. and Chanrion, E. and Thiney, V. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta, M. and Bäuerle, C. and Meunier, T.
    Nature Nanotechnology 16 (2021)
    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.
    view abstract10.1038/s41565-021-00846-y
  • Distinguishing persistent effects in an undoped GaAs/AlGaAs quantum well by top-gate-dependent illumination
    Fujita, T. and Hayashi, R. and Kohda, M. and Ritzmann, J. and Ludwig, Ar. and Nitta, J. and Wieck, A.D. and Oiwa, A.
    Journal of Applied Physics 129 (2021)
    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).
    view abstract10.1063/5.0047558
  • Electroabsorption in gated GaAs nanophotonic waveguides
    Wang, Y. and Uppu, R. and Zhou, X. and Papon, C. and Scholz, S. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    Applied Physics Letters 118 (2021)
    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).
    view abstract10.1063/5.0039373
  • Experimental Reconstruction of the Few-Photon Nonlinear Scattering Matrix from a Single Quantum Dot in a Nanophotonic Waveguide
    Le Jeannic, H. and Ramos, T. and Simonsen, S.F. and Pregnolato, T. and Liu, Z. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Rotenberg, N. and García-Ripoll, J.J. and Lodahl, P.
    Physical Review Letters 126 (2021)
    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.
    view abstract10.1103/PhysRevLett.126.023603
  • Experimental validation of formula for calculation thermal diffusivity in superlattices performed using a combination of two frequency-domain methods: Photothermal infrared radiometry and thermoreflectance
    Pawlak, M. and Kruck, T. and Spitzer, N. and Dziczek, D. and Ludwig, Ar. and Wieck, A.D.
    Applied Sciences (Switzerland) 11 (2021)
    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.
    view abstract10.3390/app11136125
  • Gate voltage dependence of noise distribution in radio-frequency reflectometry in gallium arsenide quantum dots
    Shinozaki, M. and Muto, Y. and Kitada, T. and Nakajima, T. and Delbecq, M.R. and Yoneda, J. and Takeda, K. and Noiri, A. and Ito, T. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S. and Otsuka, T.
    Applied Physics Express 14 (2021)
    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
    view abstract10.35848/1882-0786/abe41f
  • Homogeneous optical anisotropy in an ensemble of InGaAs quantum dots induced by strong enhancement of the heavy-hole band Landé parameter q
    Trifonov, A.V. and Akimov, I.A. and Golub, L.E. and Ivchenko, E.L. and Yugova, I.A. and Kosarev, A.N. and Scholz, S.E. and Sgroi, C. and Ludwig, Ar. and Wieck, A.D. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 104 (2021)
    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.
    view abstract10.1103/PhysRevB.104.L161405
  • Integrated Whispering-Gallery-Mode Resonator for Solid-State Coherent Quantum Photonics
    Brooks, A. and Chu, X.-L. and Liu, Z. and Schott, R. and Ludwig, Ar. and Wieck, A.D. and Midolo, L. and Lodahl, P. and Rotenberg, N.
    Nano Letters 21 (2021)
    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.
    view abstract10.1021/acs.nanolett.1c02818
  • Internal photoeffect from a single quantum emitter
    Lochner, P. and Kerski, J. and Kurzmann, A. and Wieck, A.D. and Ludwig, A. and Geller, M. and Lorke, A.
    Physical Review B 103 (2021)
    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.
    view abstract10.1103/PhysRevB.103.075426
  • New signatures of the spin gap in quantum point contacts
    Hudson, K.L. and Srinivasan, A. and Goulko, O. and Adam, J. and Wang, Q. and Yeoh, L.A. and Klochan, O. and Farrer, I. and Ritchie, D.A. and Ludwig, Ar. and Wieck, A.D. and von Delft, J. and Hamilton, A.R.
    Nature Communications 12 (2021)
    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).
    view abstract10.1038/s41467-020-19895-3
  • Optical spin control and coherence properties of acceptor bound holes in strained GaAs
    Linpeng, X. and Karin, T. and Durnev, M.V. and Glazov, M.M. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Fu, K.-M.C.
    Physical Review B 103 (2021)
    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.
    view abstract10.1103/PhysRevB.103.115412
  • Optically driving the radiative Auger transition
    Spinnler, C. and Zhai, L. and Nguyen, G.N. and Ritzmann, J. and Wieck, A.D. and Ludwig, Ar. and Javadi, A. and Reiter, D.E. and Machnikowski, P. and Warburton, R.J. and Löbl, M.C.
    Nature Communications 12 (2021)
    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).
    view abstract10.1038/s41467-021-26875-8
  • Probabilistic teleportation of a quantum dot spin qubit
    Kojima, Y. and Nakajima, T. and Noiri, A. and Yoneda, J. and Otsuka, T. and Takeda, K. and Li, S. and Bartlett, S.D. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    npj Quantum Information 7 (2021)
    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).
    view abstract10.1038/s41534-021-00403-4
  • Quantum Sensor for Nanoscale Defect Characterization
    Kerski, J. and Lochner, P. and Ludwig, Ar. and Wieck, A.D. and Kurzmann, A. and Lorke, A. and Geller, M.
    Physical Review Applied 15 (2021)
    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.
    view abstract10.1103/PhysRevApplied.15.024029
  • Suppression of Surface-Related Loss in a Gated Semiconductor Microcavity
    Najer, D. and Tomm, N. and Javadi, A. and Korsch, A.R. and Petrak, B. and Riedel, D. and Dolique, V. and Valentin, S.R. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Physical Review Applied 15 (2021)
    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.
    view abstract10.1103/PhysRevApplied.15.044004
  • Tuning the Mode Splitting of a Semiconductor Microcavity with Uniaxial Stress
    Tomm, N. and Korsch, A.R. and Javadi, A. and Najer, D. and Schott, R. and Valentin, S.R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Physical Review Applied 15 (2021)
    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.
    view abstract10.1103/PhysRevApplied.15.054061
  • Closed-loop control of a GaAs-based singlet-triplet spin qubit with 99.5% gate fidelity and low leakage
    Cerfontaine, P. and Botzem, T. and Ritzmann, J. and Humpohl, S.S. and Ludwig, Ar. and Schuh, D. and Bougeard, D. and Wieck, A.D. and Bluhm, H.
    Nature Communications 11 (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).
    view abstract10.1038/s41467-020-17865-3
  • Coherence of a Driven Electron Spin Qubit Actively Decoupled from Quasistatic Noise
    Nakajima, T. and Noiri, A. and Kawasaki, K. and Yoneda, J. and Stano, P. and Amaha, S. and Otsuka, T. and Takeda, K. and Delbecq, M.R. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Physical Review X 10 (2020)
    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.
    view abstract10.1103/PhysRevX.10.011060
  • Coherent control of individual electron spins in a two-dimensional quantum dot array
    Mortemousque, P.-A. and Chanrion, E. and Jadot, B. and Flentje, H. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta, M. and Bäuerle, C. and Meunier, T.
    Nature Nanotechnology (2020)
    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.
    view abstract10.1038/s41565-020-00816-w
  • Detection and amplification of spin noise using scattered laser light in a quantum-dot microcavity
    Kamenskii, A.N. and Petrov, M.Y. and Kozlov, G.G. and Zapasskii, V.S. and Scholz, S.E. and Sgroi, C. and Ludwig, Ar. and Wieck, A.D. and Bayer, M. and Greilich, A.
    Physical Review B 101 (2020)
    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.
    view abstract10.1103/PhysRevB.101.041401
  • Deterministic positioning of nanophotonic waveguides around single self-assembled quantum dots
    Pregnolato, T. and Chu, X.-L. and Schröder, T. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Rotenberg, N.
    APL Photonics 5 (2020)
    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).
    view abstract10.1063/1.5117888
  • Effect of electric current on the optical orientation of interface electrons in AlGaAs/GaAs heterostructures
    Ken, O.S. and Zhukov, E.A. and Akimov, I.A. and Korenev, V.L. and Kopteva, N.E. and Kalitukha, I.V. and Sapega, V.F. and Wieck, A.D. and Ludwig, Ar. and Schott, R. and Kusrayev, Y.G. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 102 (2020)
    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.
    view abstract10.1103/PhysRevB.102.045302
  • Electrical detection of excitonic states by time-resolved conductance measurements
    Ebler, C. and Labud, P.A. and Rai, A.K. and Reuter, D. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 101 (2020)
    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.
    view abstract10.1103/PhysRevB.101.125303
  • Electron tunneling dynamics between two-dimensional and zero-dimensional quantum systems: Contributions of momentum matching, higher subbands, and phonon-assisted processes
    Korsch, A.R. and Ebler, C. and Nguyen, G.N. and Scholz, S. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 102 (2020)
    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.
    view abstract10.1103/PhysRevB.102.035413
  • Fabrication and optical characterization of photonic crystal nanocavities with electrodes for gate-defined quantum dots
    Tajiri, T. and Sakai, Y. and Kuruma, K. and Ji, S.M. and Kiyama, H. and Oiwa, A. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D. and Ota, Y. and Arakawa, Y. and Iwamoto, S.
    Japanese Journal of Applied Physics 59 (2020)
    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.
    view abstract10.7567/1347-4065/ab5b62
  • Full counting statistics of spin-flip and spin-conserving charge transitions in Pauli-spin blockade
    Matsuo, S. and Kuroyama, K. and Yabunaka, S. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review Research 2 (2020)
    We investigate the full counting statistics (FCS) of spin-conserving and spin-flip charge transitions in Pauli-spin blockade regime of a GaAs double quantum dot. Experimentally, we executed real-time observation of charge transitions and constructed the FCS. A theoretical model is proposed to evaluate all spin-conserving and spin-flip tunnel rates. We enumerate advantages in FCS comparing to waiting time distribution for the evaluation with demonstration of the universal relation between FCS and waiting time distribution We report peculiar statistical features in the FCS, which appear in the system holding spin degeneracy and coexistence of slow and fast transitions. Our experimental results supported by the numerical calculation provide how the spin correlation plays on the full counting statistics. This study is potentially useful for elucidating the spin-related and other complex transition dynamics in classical and quantum systems. © 2020 authors.
    view abstract10.1103/PhysRevResearch.2.033120
  • Influence of molecular beam effusion cell quality on optical and electrical properties of quantum dots and quantum wells
    Nguyen, G.N. and Korsch, A.R. and Schmidt, M. and Ebler, C. and Labud, P.A. and Schott, R. and Lochner, P. and Brinks, F. and Wieck, A.D. and Ludwig, Ar.
    Journal of Crystal Growth 550 (2020)
    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.
    view abstract10.1016/j.jcrysgro.2020.125884
  • Low-noise GaAs quantum dots for quantum photonics
    Zhai, L. and Löbl, M.C. and Nguyen, G.N. and Ritzmann, J. and Javadi, A. and Spinnler, C. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Nature Communications 11 (2020)
    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).
    view abstract10.1038/s41467-020-18625-z
  • Measurement of Backaction from Electron Spins in a Gate-Defined GaAs Double Quantum dot Coupled to a Mesoscopic Nuclear Spin Bath
    Bethke, P. and McNeil, R.P.G. and Ritzmann, J. and Botzem, T. and Ludwig, Ar. and Wieck, A.D. and Bluhm, H.
    Physical Review Letters 125 (2020)
    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.
    view abstract10.1103/PhysRevLett.125.047701
  • Measurement of thermal transport properties of selected superlattice and thin films using frequency-domain photothermal infrared radiometry
    Pawlak, M. and Jukam, N. and Kruck, T. and Dziczek, D. and Ludwig, Ar. and Wieck, A.D.
    Measurement: Journal of the International Measurement Confederation 166 (2020)
    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
    view abstract10.1016/j.measurement.2020.108226
  • Microscopic model for the stacking-fault potential and the exciton wave function in GaAs
    Durnev, M.V. and Glazov, M.M. and Linpeng, X. and Viitaniemi, M.L.K. and Matthews, B. and Spurgeon, S.R. and Sushko, P.V. and Wieck, A.D. and Ludwig, Ar. and Fu, K.-M.C.
    Physical Review B 101 (2020)
    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.
    view abstract10.1103/PhysRevB.101.125420
  • Near Transform-Limited Quantum Dot Linewidths in a Broadband Photonic Crystal Waveguide
    Pedersen, F.T. and Wang, Y. and Olesen, C.T. and Scholz, S. and Wieck, A.D. and Ludwig, Ar. and Löbl, M.C. and Warburton, R.J. and Midolo, L. and Uppu, R. and Lodahl, P.
    ACS Photonics 7 (2020)
    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.
    view abstract10.1021/acsphotonics.0c00758
  • Observation of the Kondo screening cloud
    V. Borzenets, I. and Shim, J. and Chen, J.C.H. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S. and Sim, H.-S. and Yamamoto, M.
    Nature 579 (2020)
    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.
    view abstract10.1038/s41586-020-2058-6
  • On-chip deterministic operation of quantum dots in dual-mode waveguides for a plug-and-play single-photon source
    Uppu, R. and Eriksen, H.T. and Thyrrestrup, H. and Uğurlu, A.D. and Wang, Y. and Scholz, S. and Wieck, A.D. and Ludwig, Ar. and Löbl, M.C. and Warburton, R.J. and Lodahl, P. and Midolo, L.
    Nature Communications 11 (2020)
    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).
    view abstract10.1038/s41467-020-17603-9
  • On-Chip Nanomechanical Filtering of Quantum-Dot Single-Photon Sources
    Zhou, X. and Uppu, R. and Liu, Z. and Papon, C. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    Laser and Photonics Reviews 14 (2020)
    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
    view abstract10.1002/lpor.201900404
  • Quantitative STEM Imaging and Multislice Simulation of Stacking Fault Defects for Exciton Trapping in GaAs
    Spurgeon, S. and Matthews, B. and Sushko, P. and Linpeng, X. and Viitaniemi, M. and Durnev, M. and Glazov, M. and Wieck, A. and Ludwig, Ar. and Fu, K.-M.
    Microscopy and Microanalysis (2020)
    view abstract10.1017/S1431927620022904
  • Radiative Auger process in the single-photon limit
    Löbl, M.C. and Spinnler, C. and Javadi, A. and Zhai, L. and Nguyen, G.N. and Ritzmann, J. and Midolo, L. and Lodahl, P. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Nature Nanotechnology 15 (2020)
    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.
    view abstract10.1038/s41565-020-0697-2
  • Real-Time Detection of Single Auger Recombination Events in a Self-Assembled Quantum Dot
    Lochner, P. and Kurzmann, A. and Kerski, J. and Stegmann, P. and König, J. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Nano Letters 20 (2020)
    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.
    view abstract10.1021/acs.nanolett.9b04650
  • Scalable integrated single-photon source
    Uppu, R. and Pedersen, F.T. and Wang, Y. and Olesen, C.T. and Papon, C. and Zhou, X. and Midolo, L. and Scholz, S. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P.
    Science Advances 6 (2020)
    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.
    view abstract10.1126/sciadv.abc8268
  • Suspended Spot-Size Converters for Scalable Single-Photon Devices
    Uğurlu, A.D. and Thyrrestrup, H. and Uppu, R. and Ouellet-Plamondon, C. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    Advanced Quantum Technologies 3 (2020)
    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
    view abstract10.1002/qute.201900076
  • Ultra-bright source of indistinguishable single photons
    Javadi, A. and Tomm, N. and Antoniadis, N.O. and Najer, D. and Löbl, M.C. and Korsch, A. and Schott, R. and Valentin, S.R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Optics InfoBase Conference Papers (2020)
    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)
    view abstract10.1364/FIO.2020.JTu7B.1
  • A gated quantum dot strongly coupled to an optical microcavity
    Najer, D. and Söllner, I. and Sekatski, P. and Dolique, V. and Löbl, M.C. and Riedel, D. and Schott, R. and Starosielec, S. and Valentin, S.R. and Wieck, A.D. and Sangouard, N. and Ludwig, Ar. and Warburton, R.J.
    Nature 575 (2019)
    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.
    view abstract10.1038/s41586-019-1709-y
  • A machine learning approach for automated fine-tuning of semiconductor spin qubits
    Teske, J.D. and Humpohl, S.S. and Otten, R. and Bethke, P. and Cerfontaine, P. and Dedden, J. and Ludwig, Ar. and Wieck, A.D. and Bluhm, H.
    Applied Physics Letters 114 (2019)
    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).
    view abstract10.1063/1.5088412
  • Angular momentum transfer from photon polarization to an electron spin in a gate-defined quantum dot
    Fujita, T. and Morimoto, K. and Kiyama, H. and Allison, G. and Larsson, M. and Ludwig, Ar. and Valentin, S.R. and Wieck, A.D. and Oiwa, A. and Tarucha, S.
    Nature Communications 10 (2019)
    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).
    view abstract10.1038/s41467-019-10939-x
  • Author Correction: Contrast of 83% in reflection measurements on a single quantum dot (Scientific Reports, (2019), 9, 1, (8817), 10.1038/s41598-019-45259-z)
    Lochner, P. and Kurzmann, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Scientific Reports 9 (2019)
    In the original version of this Article, the authors Andreas D. Wieck and Arne Ludwig were incorrectly affiliated with ‘Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland’. The correct affiliation is listed below. Ruhr-Universität Bochum, Lehrstuhl für Angewandte Festkörperphysik, D-44780 Bochum, Germany This error has now been corrected in the PDF and HTML versions of the Article. © 2019, The Author(s).
    view abstract10.1038/s41598-019-54298-5
  • Ballistic rectification based on inhomogeneous magnetic stray fields
    Szelong, M. and Ludwig, Ar. and Wieck, A.D. and Kunze, U.
    Journal of Applied Physics 125 (2019)
    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).
    view abstract10.1063/1.5085714
  • Coherent Optical Control of a Quantum-Dot Spin-Qubit in a Waveguide-Based Spin-Photon Interface
    Ding, D. and Appel, M.H. and Javadi, A. and Zhou, X. and Löbl, M.C. and Söllner, I. and Schott, R. and Papon, C. and Pregnolato, T. and Midolo, L. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J. and Schröder, T. and Lodahl, P.
    Physical Review Applied 11 (2019)
    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.
    view abstract10.1103/PhysRevApplied.11.031002
  • Contrast of 83% in reflection measurements on a single quantum dot
    Lochner, P. and Kurzmann, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Scientific Reports 9 (2019)
    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).
    view abstract10.1038/s41598-019-45259-z
  • Correlations between optical properties and Voronoi-cell area of quantum dots
    Löbl, M.C. and Zhai, L. and Jahn, J.-P. and Ritzmann, J. and Huo, Y. and Wieck, A.D. and Schmidt, O.G. and Ludwig, Ar. and Rastelli, A. and Warburton, R.J.
    Physical Review B 100 (2019)
    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.
    view abstract10.1103/PhysRevB.100.155402
  • Difference in charge and spin dynamics in a quantum dot-lead coupled system
    Otsuka, T. and Nakajima, T. and Delbecq, M.R. and Stano, P. and Amaha, S. and Yoneda, J. and Takeda, K. and Allison, G. and Li, S. and Noiri, A. and Ito, T. and Loss, D. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review B 99 (2019)
    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.
    view abstract10.1103/PhysRevB.99.085402
  • Excitons in InGaAs quantum dots without electron wetting layer states
    Löbl, M.C. and Scholz, S. and Söllner, I. and Ritzmann, J. and Denneulin, T. and Kovács, A. and Kardynał, B.E. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Communications Physics 2 (2019)
    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).
    view abstract10.1038/s42005-019-0194-9
  • Irradiation Effects on Induced Electron Conductivity in an un-doped GaAs/AlGaAs Quantum Well Hall Bar
    Fujita, T. and Hayashi, R. and Kohda, M. and Ritzmann, J. and Ludwig, Ar. and Nitta, J. and Wieck, A.D. and Oiwa, A.
    2019 Compound Semiconductor Week, CSW 2019 - Proceedings (2019)
    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.
    view abstract10.1109/ICIPRM.2019.8819035
  • Nanomechanical single-photon routing
    Papon, C. and Zhou, X. and Thyrrestrup, H. and Liu, Z. and Stobbe, S. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    Optica 6 (2019)
    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.
    view abstract10.1364/OPTICA.6.000524
  • On-chip nano-electro-mechanical switching of deterministic single photons
    Zhou, X. and Papon, C. and Thyrrestrup, H. and Liu, Z. and Stobbe, S. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    2019 Conference on Lasers and Electro-Optics, CLEO 2019 - Proceedings (2019)
    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.
    view abstract10.23919/CLEO.2019.8749753
  • Optical Detection of Single-Electron Tunneling into a Semiconductor Quantum Dot
    Kurzmann, A. and Stegmann, P. and Kerski, J. and Schott, R. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A. and Geller, M.
    Physical Review Letters 122 (2019)
    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.
    view abstract10.1103/PhysRevLett.122.247403
  • Photogeneration of a single electron from a single Zeeman-resolved light-hole exciton with preserved angular momentum
    Kuroyama, K. and Larsson, M. and Chang, C.Y. and Muramoto, J. and Heya, K. and Fujita, T. and Allison, G. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Matsuo, S. and Oiwa, A. and Tarucha, S.
    Physical Review B 99 (2019)
    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.
    view abstract10.1103/PhysRevB.99.085203
  • Photon Noise Suppression by a Built-in Feedback Loop
    Al-Ashouri, A. and Kurzmann, A. and Merkel, B. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Nano Letters 19 (2019)
    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.
    view abstract10.1021/acs.nanolett.8b03486
  • Quantum non-demolition measurement of an electron spin qubit
    Nakajima, T. and Noiri, A. and Yoneda, J. and Delbecq, M.R. and Stano, P. and Otsuka, T. and Takeda, K. and Amaha, S. and Allison, G. and Kawasaki, K. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Nature Nanotechnology 14 (2019)
    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.
    view abstract10.1038/s41565-019-0426-x
  • Sound-driven single-electron transfer in a circuit of coupled quantum rails
    Takada, S. and Edlbauer, H. and Lepage, H.V. and Wang, J. and Mortemousque, P.-A. and Georgiou, G. and Barnes, C.H.W. and Ford, C.J.B. and Yuan, M. and Santos, P.V. and Waintal, X. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta, M. and Meunier, T. and Bäuerle, C.
    Nature Communications 10 (2019)
    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).
    view abstract10.1038/s41467-019-12514-w
  • Spin Detection in GaAs/AlGaAs Quantum Wells by Inverse Spin-Hall Effect
    Sakai, Y. and Chatani, T. and Nakagawa, T. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A.
    2019 Compound Semiconductor Week, CSW 2019 - Proceedings (2019)
    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.
    view abstract10.1109/ICIPRM.2019.8819356
  • Suspended epoxy polymer inverted tapers for scalable fibre-coupled single-photon devices
    Uǧurlu, A.D. and Thyrrestrup, H. and Uppu, R. and Ouellet-Plamondon, C. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)
    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.
    view abstract10.1109/CLEOE-EQEC.2019.8871552
  • Temperature and bias anomalies in the photoluminescence of InAs quantum dots coupled to a Fermi reservoir
    Korsch, A.R. and Nguyen, G.N. and Schmidt, M. and Ebler, C. and Valentin, S.R. and Lochner, P. and Rothfuchs, C. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 99 (2019)
    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.
    view abstract10.1103/PhysRevB.99.165303
  • Top-down fabrication and transformation properties of vanadium dioxide nanostructures
    Rastjoo, S. and Wang, X. and Ludwig, Al. and Kohl, M.
    Journal of Applied Physics 125 (2019)
    The fabrication of nanostructures of vanadium dioxide (VO 2)-based films with critical dimensions down to 100 nm and the characterization of their phase transformation properties are presented. Starting materials are VO 2 and V 0.99 Mo 0.01 O 2 films that are deposited by magnetron sputtering. For nanofabrication, two top-down processes are investigated, in which the substrate is nanomachined either before or after film deposition. Electrical resistance measurements on V 0.99 Mo 0.01 O 2 bridge nanostructures exhibit a semiconductor-metal transition similar to reference films. A detailed analysis of phase transition temperatures does not reveal any significant width-dependence as it may be expected when approaching the grain size of 100 nm. The absolute electrical resistance in the semiconducting state scales inversely proportional to the width reflecting homogeneous material characteristics. Yet, the resistance change at the semiconductor-metal transition tends to increase for decreasing width indicating reduced carrier scattering as the absolute number of grain boundaries decreases. © 2019 Author(s).
    view abstract10.1063/1.5085322
  • Wavelength locking of Er-doped random fiber laser
    Hu, B. and Zhang, W. and Ma, R. and Guo, J. and Ludwig, Ar. and Rao, Y.
    Laser Physics Letters 16 (2019)
    This paper demonstrates the wavelength locking of a coherent random lasing system, i.e. an Erbium-doped random fiber laser with a disordered array of fiber Bragg gratings. To lock lasing modes of the disordered system, an external seed light from a tunable laser was introduced into the cavity. It was found that different emission wavelengths/modes can be selected to emit separately through injection locking. The wavelength fluctuation of the output is less than 0.01%, and the power fluctuation is less than 4%. The proposed method is also applicable to general disordered systems, providing an efficient way to control or select light emission in these systems. © 2019 Astro Ltd.
    view abstract10.1088/1612-202X/aaff4d
  • A Charge-Tunable Quantum Dot Deep in the Strong Coupling Regime of Cavity QED
    Najer, D. and Sollner, I. and Loebl, M.C. and Riedel, D. and Petrak, B. and Starosielec, S. and Dolique, V. and Valentin, S.R. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    IEEE Photonics Society Summer Topicals Meeting Series, SUM 2018 (2018)
    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.
    view abstract10.1109/PHOSST.2018.8456757
  • A fast quantum interface between different spin qubit encodings
    Noiri, A. and Nakajima, T. and Yoneda, J. and Delbecq, M.R. and Stano, P. and Otsuka, T. and Takeda, K. and Amaha, S. and Allison, G. and Kawasaki, K. and Kojima, Y. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Nature Communications 9 (2018)
    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).
    view abstract10.1038/s41467-018-07522-1
  • Coherent transfer of electron spin correlations assisted by dephasing noise
    Nakajima, T. and Delbecq, M.R. and Otsuka, T. and Amaha, S. and Yoneda, J. and Noiri, A. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Hu, X. and Nori, F. and Tarucha, S.
    Nature Communications 9 (2018)
    Quantum coherence of superposed states, especially of entangled states, is indispensable for many quantum technologies. However, it is vulnerable to environmental noises, posing a fundamental challenge in solid-state systems including spin qubits. Here we show a scheme of entanglement engineering where pure dephasing assists the generation of quantum entanglement at distant sites in a chain of electron spins confined in semiconductor quantum dots. One party of an entangled spin pair, prepared at a single site, is transferred to the next site and then adiabatically swapped with a third spin using a transition across a multi-level avoided crossing. This process is accelerated by the noise-induced dephasing through a variant of the quantum Zeno effect, without sacrificing the coherence of the entangled state. Our finding brings insight into the spin dynamics in open quantum systems coupled to noisy environments, opening an avenue to quantum state manipulation utilizing decoherence effects. © 2018 The Author(s).
    view abstract10.1038/s41467-018-04544-7
  • Corrigendum to “Simultaneous measurement of thermal conductivity and diffusivity of an undoped Al0.33Ga0.67As thin film epitaxially grown on a heavily Zn doped GaAs using spectrally-resolved modulated photothermal infrared radiometry” [Thermochim. Acta 662 (2018), 69-74] (Thermochimica Acta (2018) 662 (69–74), (S0040603118300443) (10.1016/j.tca.2018.02.009))
    Pawlak, M. and Pal, S. and Scholz, S. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 669 (2018)
    The authors regret that in this article, equation 2 describing the model was presented not correctly in the text. In Eq. 2 instead of [Formula presented] should stand [Formula presented]. and the equation should read as follows: [Formula presented]where f, βopt,i, βIR,i, Li are the modulation frequency, optical absorption coefficient, infrared absorption coefficient, and thickness of the thin film (with index i = 1) and substrate (with i = 2). Moreover, a typing error occurred while transferring the equations for the temperature disibutions ΔT1(f,z,βopt,1) and ΔT2(f,z,βopt,2) from Ref. 1. Instead of the thermal diffusivity αi the complex thermal wave vector, defined as σi(f)=sqrt((i·2·π·f)/αi), should be given in equations 3 and 4. The temperature distributions are described by the following formulas: [Formula presented] The results and conclusions are not influenced by these typing errors, because all calculations were performed using the correct equations. The authors would like to apologize for any inconvenience caused. [1] M. Reichling and H. Groenbecka, Harmonic heat flow in isotropic layered systems and its use for thin film thermal conductivity measurements, J. Appl. Phys. 75 (4) 1914, (1994) © 2018 Elsevier B.V.
    view abstract10.1016/j.tca.2018.06.015
  • Dephasing of InAs quantum dot p -shell excitons studied using two-dimensional coherent spectroscopy
    Suzuki, T. and Singh, R. and Moody, G. and Aßmann, M. and Bayer, M. and Ludwig, Ar. and Wieck, A.D. and Cundiff, S.T.
    Physical Review B 98 (2018)
    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.
    view abstract10.1103/PhysRevB.98.195304
  • Far-field nanoscopy on a semiconductor quantum dot via a rapid-adiabatic-passage-based switch
    Kaldewey, T. and Kuhlmann, A.V. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Warburton, R.J.
    Nature Photonics 12 (2018)
    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).
    view abstract10.1038/s41566-017-0079-y
  • Four single-spin Rabi oscillations in a quadruple quantum dot
    Ito, T. and Otsuka, T. and Nakajima, T. and Delbecq, M.R. and Amaha, S. and Yoneda, J. and Takeda, K. and Noiri, A. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Applied Physics Letters 113 (2018)
    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).
    view abstract10.1063/1.5040280
  • Illumination-induced nonequilibrium charge states in self-assembled quantum dots
    Valentin, S.R. and Schwinger, J. and Eickelmann, P. and Labud, P.A. and Wieck, A.D. and Sothmann, B. and Ludwig, Ar.
    Physical Review B 97 (2018)
    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.
    view abstract10.1103/PhysRevB.97.045416
  • Laplace deep level transient spectroscopy on self-assembled quantum dots
    Schnorr, L. and Heinzel, T. and Scholz, S. and Ludwig, Ar. and Wieck, A.D.
    Journal of Applied Physics 124 (2018)
    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).
    view abstract10.1063/1.5028319
  • Near lifetime-limited emitter in a nanophotonic waveguide
    Thyrrestrup, H. and Kiršanskė, G. and Jeannic, H.L. and Pregnolato, T. and Zhai, L. and Midolo, L. and Rotenberg, N. and Javadi, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Löbl, M.C. and Söllner, I. and Warburton, R.J. and Lodahl, P.
    Optics InfoBase Conference Papers Part F93-CLEO_QELS 2018 (2018)
    Coherent light-matter interactions are key to a range of quantum optical technologies and experiments. We present measurements showing near-life-time limited transitions, for quantum dots embedded in nanoguides, demonstrating the robust suppression of environmental decoherence processes. © OSA 2018.
    view abstract10.1364/CLEO_QELS.2018.FM1H.7
  • Overcoming Ehrlich-Schwöbel barrier in (1 1 1)A GaAs molecular beam epitaxy
    Ritzmann, J. and Schott, R. and Gross, K. and Reuter, D. and Ludwig, Ar. and Wieck, A.D.
    Journal of Crystal Growth 481 (2018)
    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.
    view abstract10.1016/j.jcrysgro.2017.10.029
  • Quantum Optics with Near-Lifetime-Limited Quantum-Dot Transitions in a Nanophotonic Waveguide
    Thyrrestrup, H. and Kiršanske, G. and Le Jeannic, H. and Pregnolato, T. and Zhai, L. and Raahauge, L. and Midolo, L. and Rotenberg, N. and Javadi, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Löbl, M.C. and Söllner, I. and Warburton, R.J. and Lodahl, P.
    Nano Letters 18 (2018)
    Establishing a highly efficient photon-emitter interface where the intrinsic linewidth broadening is limited solely by spontaneous emission is a key step in quantum optics. It opens a pathway to coherent light-matter interaction for, e.g., the generation of highly indistinguishable photons, few-photon optical nonlinearities, and photon-emitter quantum gates. However, residual broadening mechanisms are ubiquitous and need to be combated. For solid-state emitters charge and nuclear spin noise are of importance, and the influence of photonic nanostructures on the broadening has not been clarified. We present near-lifetime-limited linewidths for quantum dots embedded in nanophotonic waveguides through a resonant transmission experiment. It is found that the scattering of single photons from the quantum dot can be obtained with an extinction of 66 ± 4%, which is limited by the coupling of the quantum dot to the nanostructure rather than the linewidth broadening. This is obtained by embedding the quantum dot in an electrically contacted nanophotonic membrane. A clear pathway to obtaining even larger single-photon extinction is laid out; i.e., the approach enables a fully deterministic and coherent photon-emitter interface in the solid state that is operated at optical frequencies. © 2018 American Chemical Society.
    view abstract10.1021/acs.nanolett.7b05016
  • Self-Organized Growth of Quantum Dots and Quantum Wires by Combination of Focused Ion Beams and Molecular Beam Epitaxy
    Scholz, S. and Schott, R. and Schmidt, M. and Mehta, M. and Ludwig, Ar. and Wieck, A.D.
    Physica Status Solidi (B) Basic Research (2018)
    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
    view abstract10.1002/pssb.201800375
  • Simultaneous measurement of infrared absorption coefficient of Carbon doped Al0.33Ga0.67As thin film and thermal boundary resistance between thin film and heavily Zn doped GaAs substrate using spectrally-resolved modulated photothermal infrared radiometry
    Pawlak, M. and Horny, N. and Scholz, S. and Ebler, C. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 667 (2018)
    In this paper, we investigated C doped Al0.33Ga0.67As thin film epitaxially grown on a Zn-doped GaAs substrate using spectrally resolved modulated photothermal infrared radiometry (SR-PTR). We assumed that thermal conductivity and diffusivity of the thin layer are known and estimate values of the infrared absorption coefficient of the thin layer and the thermal boundary resistance of the interface between the thin layer and the substrate. We found out that the thermal boundary resistance is two orders of magnitude greater than thermal boundary resistance of studied recently undoped AlGaAs/GaAs sample. We attribute this effect to formation of quasi 2-dimensional hole gas due to modulation doping. Finally, the infrared absorption coefficient of the thin layer decreases with increasing wavelength due to inter-valence band absorption. We found out that with increasing infrared absorption coefficient of the thin film, the sensitivity of the method for estimation of the infrared absorption coefficient increases, while the estimation error decreases. © 2018 Elsevier B.V.
    view abstract10.1016/j.tca.2018.06.016
  • Simultaneous measurement of thermal conductivity and diffusivity of an undoped Al0.33Ga0.67As thin film epitaxially grown on a heavily Zn doped GaAs using spectrally-resolved modulated photothermal infrared radiometry
    Pawlak, M. and Pal, S. and Scholz, S. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 662 (2018)
    In this paper, we propose a method for measuring thermal and infrared properties of infrared transparent and semi-transparent thin film. We have investigated an undoped Al0.33Ga0.67As thin film epitaxially grown on a heavily Zn doped GaAs substrate using spectrally-resolved modulated photothermal infrared radiometry (SR-PTR). We perform supplementary measurements in order to determine values of layer thickness and infrared absorption coefficient and estimate successively values of the thermal conductivity and diffusivity of the Al0.33Ga0.67As thin layer, using the SR-PTR method. The obtained values of the thermal conductivity and diffusivity of the Al0.33Ga0.67As thin layer demonstrate that PTR method can be used for the thermal characterization of infrared transparent layers deposited on a highly infrared absorbing substrate. Supplementary Fourier Transform Infrared (FTIR) Spectroscopy measurements yield information only about the thickness of the Al0.33Ga0.67As layer. The results demonstrate that the SR-PTR method is a very good method for characterizing the thermal, geometrical and infrared properties of infrared-transparent thin film samples. However, some of the layer properties should be known a priori. It is worth emphasizing that the spectrally resolved measurements increase the reliability in estimating parameters of the thin layer by introducing additional channels of information. Finally, we conclude that the SR-PTR method combines features of infrared spectroscopic and calorimetric methods. © 2018 Elsevier B.V.
    view abstract10.1016/j.tca.2018.02.009
  • Spin-photon interface and spin-controlled photon switching in a nanobeam waveguide
    Javadi, A. and Ding, D. and Appel, M.H. and Mahmoodian, S. and Löbl, M.C. and Söllner, I. and Schott, R. and Papon, C. and Pregnolato, T. and Stobbe, Sø. and Midolo, L. and Schröder, T. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J. and Lodahl, P.
    Nature Nanotechnology 13 (2018)
    The spin of an electron is a promising memory state and qubit. Connecting spin states that are spatially far apart will enable quantum nodes and quantum networks based on the electron spin. Towards this goal, an integrated spin-photon interface would be a major leap forward as it combines the memory capability of a single spin with the efficient transfer of information by photons. Here, we demonstrate such an efficient and optically programmable interface between the spin of an electron in a quantum dot and photons in a nanophotonic waveguide. The spin can be deterministically prepared in the ground state with a fidelity of up to 96%. Subsequently, the system is used to implement a single-spin photonic switch, in which the spin state of the electron directs the flow of photons through the waveguide. The spin-photon interface may enable on-chip photon-photon gates, single-photon transistors and the efficient generation of a photonic cluster state. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstract10.1038/s41565-018-0091-5
  • Spin-photon interface controlled optical switching in a nanobeam waveguide
    Schröder, T. and Javadi, A. and Ding, D. and Appel, M.H. and Mahmoodian, S. and Löbl, M.C. and Söllner, I. and Schott, R. and Papon, C. and Pregnolato, T. and Stobbe, S. and Midolo, L. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J. and Lodahl, P.
    Optics InfoBase Conference Papers Part F92-CLEO_AT 2018 (2018)
    We demonstrate a single InGaAs quantum dot electron spin controlled photon switch in a nanobeam waveguide with 4-fold switching ratio, a spin-state preparation fidelity of 96%, and a lifetime T1 approaching 5 μs. © OSA 2018.
    view abstract10.1364/CLEO_AT.2018.ATh3H.1
  • Tuning Methods for Semiconductor Spin Qubits
    Botzem, T. and Shulman, M.D. and Foletti, S. and Harvey, S.P. and Dial, O.E. and Bethke, P. and Cerfontaine, P. and McNeil, R.P.G. and Mahalu, D. and Umansky, V. and Ludwig, Ar. and Wieck, A. and Schuh, D. and Bougeard, D. and Yacoby, A. and Bluhm, H.
    Physical Review Applied 10 (2018)
    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.
    view abstract10.1103/PhysRevApplied.10.054026
  • Universal Ratio of Coulomb Interaction to Geometric Quantization in (In, Ga)As/GaAs Quantum Dots
    Bayer, M. and Ludwig, Ar. and Wieck, A.
    Physics of the Solid State 60 (2018)
    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.
    view abstract10.1134/S1063783418080024
  • Unveiling the bosonic nature of an ultrashort few-electron pulse
    Roussely, G. and Arrighi, E. and Georgiou, G. and Takada, S. and Schalk, M. and Urdampilleta, M. and Ludwig, Ar. and Wieck, A.D. and Armagnat, P. and Kloss, T. and Waintal, X. and Meunier, T. and Bäuerle, C.
    Nature Communications 9 (2018)
    Quantum dynamics is very sensitive to dimensionality. While two-dimensional electronic systems form Fermi liquids, one-dimensional systems - Tomonaga-Luttinger liquids - are described by purely bosonic excitations, even though they are initially made of fermions. With the advent of coherent single-electron sources, the quantum dynamics of such a liquid is now accessible at the single-electron level. Here, we report on time-of-flight measurements of ultrashort few-electron charge pulses injected into a quasi one-dimensional quantum conductor. By changing the confinement potential we can tune the system from the one-dimensional Tomonaga-Luttinger liquid limit to the multi-channel Fermi liquid and show that the plasmon velocity can be varied over almost an order of magnitude. These results are in quantitative agreement with a parameter-free theory and demonstrate a powerful probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors. © 2018 The Author(s).
    view abstract10.1038/s41467-018-05203-7
  • A linear triple quantum dot system in isolated configuration
    Flentje, H. and Bertrand, B. and Mortemousque, P.-A. and Thiney, V. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Applied Physics Letters 110 (2017)
    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).
    view abstract10.1063/1.4984745
  • A triangular triple quantum dot with tunable tunnel couplings
    Noiri, A. and Kawasaki, K. and Otsuka, T. and Nakajima, T. and Yoneda, J. and Amaha, S. and Delbecq, M.R. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Semiconductor Science and Technology 32 (2017)
    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.
    view abstract10.1088/1361-6641/aa7596
  • All-electrical measurement of the triplet-singlet spin relaxation time in self-assembled quantum dots
    Eltrudis, K. and Al-Ashouri, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
    Applied Physics Letters 111 (2017)
    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).
    view abstract10.1063/1.4985572
  • Classical information transfer between distant quantum dots using individual electrons in fast moving quantum dots
    Hermelin, S. and Bertrand, B. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Physica Status Solidi (B) Basic Research 254 (2017)
    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
    view abstract10.1002/pssb.201600673
  • Coherent and robust high-fidelity generation of a biexciton in a quantum dot by rapid adiabatic passage
    Kaldewey, T. and Lüker, S. and Kuhlmann, A.V. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Reiter, D.E. and Kuhn, T. and Warburton, R.J.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.161302
  • Coherent long-distance displacement of individual electron spins
    Flentje, H. and Mortemousque, P.-A. and Thalineau, R. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Nature Communications 8 (2017)
    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).
    view abstract10.1038/s41467-017-00534-3
  • Conversion from single photon to single electron spin using electrically controllable quantum dots
    Oiwa, A. and Fujita, T. and Kiyama, H. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Journal of the Physical Society of Japan 86 (2017)
    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.
    view abstract10.7566/JPSJ.86.011008
  • Demonstrating the decoupling regime of the electron-phonon interaction in a quantum dot using chirped optical excitation
    Kaldewey, T. and Lüker, S. and Kuhlmann, A.V. and Valentin, S.R. and Chauveau, J.-M. and Ludwig, Ar. and Wieck, A.D. and Reiter, D.E. and Kuhn, T. and Warburton, R.J.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.241306
  • Efficiency enhancement of the coherent electron spin-flip Raman scattering through thermal phonons in (In,Ga)As/GaAs quantum dots
    Debus, J. and Kudlacik, D. and Waldkirch, P. and Sapega, V.F. and Scholz, S. and Ludwig, Ar. and Wieck, A.D. and Bayer, M.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.201303
  • Electron dynamics in transport and optical measurements of self-assembled quantum dots
    Kurzmann, A. and Merkel, B. and Marquardt, B. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Physica Status Solidi (B) Basic Research 254 (2017)
    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.
    view abstract10.1002/pssb.201600625
  • Focused ion beam supported growth of monocrystalline wurtzite InAs nanowires grown by molecular beam epitaxy
    Scholz, S. and Schott, R. and Labud, P.A. and Somsen, C. and Reuter, D. and Ludwig, Ar. and Wieck, A.D.
    Journal of Crystal Growth 470 (2017)
    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
    view abstract10.1016/j.jcrysgro.2017.04.013
  • Higher-order spin and charge dynamics in a quantum dot-lead hybrid system
    Otsuka, T. and Nakajima, T. and Delbecq, M.R. and Amaha, S. and Yoneda, J. and Takeda, K. and Allison, G. and Stano, P. and Noiri, A. and Ito, T. and Loss, D. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Scientific Reports 7 (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).
    view abstract10.1038/s41598-017-12217-6
  • Ion-induced interdiffusion of surface GaN quantum dots
    Rothfuchs, C. and Semond, F. and Portail, M. and Tottereau, O. and Courville, A. and Wieck, A.D. and Ludwig, Ar.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 409 (2017)
    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.
    view abstract10.1016/j.nimb.2017.04.036
  • Mesoscopic phase behavior in a quantum dot around crossover between single-level and multilevel transport regimes
    Takada, S. and Yamamoto, M. and Bäuerle, C. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.241301
  • On measurement of the thermal diffusivity of moderate and heavily doped semiconductor samples using modulated photothermal infrared radiometry
    Pawlak, M. and Panas, A. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 650 (2017)
    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.
    view abstract10.1016/j.tca.2017.02.003
  • On the infrared absorption coefficient measurement of thick heavily Zn doped GaAs using spectrally resolved modulated photothermal infrared radiometry
    Pawlak, M. and Pal, S. and Ludwig, Ar. and Wieck, A.D.
    Journal of Applied Physics 122 (2017)
    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).
    view abstract10.1063/1.4989448
  • Positive centre voltage in T-branch junctions on n-type GaAs/AlGaAs based on hydrodynamics
    Szelong, M. and Ludwig, Ar. and Wieck, A. and Kunze, U.
    Semiconductor Science and Technology 32 (2017)
    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.
    view abstract10.1088/1361-6641/aa7d44
  • Robust Single-Shot Spin Measurement with 99.5% Fidelity in a Quantum Dot Array
    Nakajima, T. and Delbecq, M.R. and Otsuka, T. and Stano, P. and Amaha, S. and Yoneda, J. and Noiri, A. and Kawasaki, K. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Physical Review Letters 119 (2017)
    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.
    view abstract10.1103/PhysRevLett.119.017701
  • Synthesis and evaluation of new copper ketoiminate precursors for a facile and additive-free solution-based approach to nanoscale copper oxide thin films
    Karle, Sarah and Rogalla, Detlef and Ludwig, Arne and Becker, Hans-Werner and Wieck, Andreas Dirk and Grafen, Markus and Ostendorf, Andreas and Devi, Anjana
    Dalton Transactions 46 (2017)
    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.
    view abstract10.1039/c6dt04399b
  • Ultra-low charge and spin noise in self-assembled quantum dots
    Ludwig, Ar. and Prechtel, J.H. and Kuhlmann, A.V. and Houel, J. and Valentin, S.R. and Warburton, R.J. and Wieck, A.D.
    Journal of Crystal Growth 477 (2017)
    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.
    view abstract10.1016/j.jcrysgro.2017.05.008
  • Altering the luminescence properties of self-assembled quantum dots in GaAs by focused ion beam implantation
    Rothfuchs, C. and Kukharchyk, N. and Greff, M.K. and Wieck, A.D. and Ludwig, Ar.
    Applied Physics B: Lasers and Optics 122 (2016)
    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.
    view abstract10.1007/s00340-015-6305-8
  • Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition
    Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Nano Letters 16 (2016)
    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.
    view abstract10.1021/acs.nanolett.6b01082
  • Coherent Control of the Exciton-Biexciton System in an InAs Self-Assembled Quantum Dot Ensemble
    Suzuki, T. and Singh, R. and Bayer, M. and Ludwig, Ar. and Wieck, A.D. and Cundiff, S.T.
    Physical Review Letters 117 (2016)
    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.
    view abstract10.1103/PhysRevLett.117.157402
  • Coherent electron-spin-resonance manipulation of three individual spins in a triple quantum dot
    Noiri, A. and Yoneda, J. and Nakajima, T. and Otsuka, T. and Delbecq, M.R. and Takeda, K. and Amaha, S. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Applied Physics Letters 108 (2016)
    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.
    view abstract10.1063/1.4945592
  • Decoupling a hole spin qubit from the nuclear spins
    Prechtel, J.H. and Kuhlmann, A.V. and Houel, J. and Ludwig, Ar. and Valentin, S.R. and Wieck, A.D. and Warburton, R.J.
    Nature Materials 15 (2016)
    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.
    view abstract10.1038/nmat4704
  • Fast spin information transfer between distant quantum dots using individual electrons
    Bertrand, B. and Hermelin, S. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Nature Nanotechnology 11 (2016)
    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.
    view abstract10.1038/nnano.2016.82
  • Giant permanent dipole moment of two-dimensional excitons bound to a single stacking fault
    Karin, T. and Linpeng, X. and Glazov, M.M. and Durnev, M.V. and Ivchenko, E.L. and Harvey, S. and Rai, A.K. and Ludwig, Ar. and Wieck, A.D. and Fu, K.-M.C.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    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.
    view abstract10.1103/PhysRevB.94.041201
  • Injection of a single electron from static to moving quantum dots
    Bertrand, B. and Hermelin, S. and Mortemousque, P.-A. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Nanotechnology 27 (2016)
    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.
    view abstract10.1088/0957-4484/27/21/214001
  • Low-temperature behavior of transmission phase shift across a Kondo correlated quantum dot
    Takada, S. and Yamamoto, M. and Bäuerle, C. and Alex, A. and Von Delft, J. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    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.
    view abstract10.1103/PhysRevB.94.081303
  • Nanoscale nonlinear effects in Erbium-implanted Yttrium Orthosilicate
    Kukharchyk, N. and Shvarkov, S. and Probst, S. and Xia, K. and Becker, H.-W. and Pal, S. and Markmann, S. and Kolesov, R. and Siyushev, P. and Wrachtrup, J. and Ludwig, Ar. and Ustinov, A.V. and Wieck, A.D. and Bushev, P.
    Journal of Luminescence 177 (2016)
    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.
    view abstract10.1016/j.jlumin.2016.05.010
  • Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot
    Kurzmann, A. and Merkel, B. and Labud, P.A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
    Physical Review Letters 117 (2016)
    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.
    view abstract10.1103/PhysRevLett.117.017401
  • Optical visualization of radiative recombination at partial dislocations in GaAs
    Karin, T. and Linpeng, X. and Rai, A.K. and Ludwig, Ar. and Wieck, A.D. and Fu, K.-M.C.
    Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November (2016)
    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.
    view abstract10.1109/PVSC.2016.7749976
  • Photoelectron generation and capture in the resonance fluorescence of a quantum dot
    Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Applied Physics Letters 108 (2016)
    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).
    view abstract10.1063/1.4954944
  • Photoluminescence of gallium ion irradiated hexagonal and cubic GaN quantum dots
    Rothfuchs, C. and Kukharchyk, N. and Koppe, T. and Semond, F. and Blumenthal, S. and Becker, H.-W. and As, D.J. and Hofsäss, H.C. and Wieck, A.D. and Ludwig, Ar.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 383 (2016)
    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.
    view abstract10.1016/j.nimb.2016.06.004
  • Probing indirect exciton complexes in a quantum dot molecule via capacitance-voltage spectroscopy
    Pal, S. and Junggebauer, C. and Valentin, S.R. and Eickelmann, P. and Scholz, S. and Ludwig, Ar. and Wieck, A.D.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    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.
    view abstract10.1103/PhysRevB.94.245311
  • Quantum Dephasing in a Gated GaAs Triple Quantum Dot due to Nonergodic Noise
    Delbecq, M.R. and Nakajima, T. and Stano, P. and Otsuka, T. and Amaha, S. and Yoneda, J. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review Letters 116 (2016)
    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.
    view abstract10.1103/PhysRevLett.116.046802
  • Role of the electron spin in determining the coherence of the nuclear spins in a quantum dot
    Wüst, G. and Munsch, M. and Maier, F. and Kuhlmann, A.V. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Poggio, M. and Warburton, R.J.
    Nature Nanotechnology 11 (2016)
    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.
    view abstract10.1038/nnano.2016.114
  • Signatures of hyperfine, spin-orbit, and decoherence effects in a Pauli spin blockade
    Fujita, T. and Stano, P. and Allison, G. and Morimoto, K. and Sato, Y. and Larsson, M. and Park, J.-H. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A. and Tarucha, S.
    Physical Review Letters 117 (2016)
    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.
    view abstract10.1103/PhysRevLett.117.206802
  • Single-electron spin resonance in a quadruple quantum dot
    Otsuka, T. and Nakajima, T. and Delbecq, M.R. and Amaha, S. and Yoneda, J. and Takeda, K. and Allison, G. and Ito, T. and Sugawara, R. and Noiri, A. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Scientific Reports 6 (2016)
    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.
    view abstract10.1038/srep31820
  • Spatially indirect transitions in electric field tunable quantum dot diodes
    Rai, A.K. and Gordon, S. and Ludwig, Ar. and Wieck, A.D. and Zrenner, A. and Reuter, D.
    Physica Status Solidi (B) Basic Research 253 (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.
    view abstract10.1002/pssb.201552591
  • Thermal shift of the resonance between an electron gas and quantum dots: What is the origin?
    Brinks, F. and Wieck, A.D. and Ludwig, Ar.
    New Journal of Physics 18 (2016)
    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.
    view abstract10.1088/1367-2630/aa4f63
  • Density-chopped far-infrared transmission spectroscopy to probe subband-Landau splittings and tune intersubband transitions
    Pal, S. and Nong, H. and Valentin, S.R. and Kukharchyk, N. and Ludwig, Ar. and Jukam, N. and Wieck, A.D.
    Conference on Lasers and Electro-Optics Europe - Technical Digest 2015-August (2015)
    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.
    view abstract10.1364/CLEO_AT.2015.JW2A.43
  • Direct photonic coupling of a semiconductor quantum dot and a trapped ion
    Meyer, H.M. and Stockill, R. and Steiner, M. and Le Gall, C. and Matthiesen, C. and Clarke, E. and Ludwig, Ar. and Reichel, J. and Atatüre, M. and Köhl, M.
    Physical Review Letters 114 (2015)
    Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of a Yb+ ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σz projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network. © 2015 American Physical Society.
    view abstract10.1103/PhysRevLett.114.123001
  • Double-pulse injection seeding of a terahertz quantum cascade laser
    Markmann, S. and Nong, H. and Pal, S. and Hekmat, N. and Scholz, S. and Kukharchyk, N. and Ludwig, Ar. and Dhillon, S. and Tignon, J. and Marcadet, X. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves (2015)
    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.
    view abstract10.1109/IRMMW-THz.2015.7327478
  • Electrically tunable hole g factor of an optically active quantum dot for fast spin rotations
    Prechtel, J.H. and Maier, F. and Houel, J. and Kuhlmann, A.V. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Warburton, R.J.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    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.
    view abstract10.1103/PhysRevB.91.165304
  • Epitaxial lift-off for solid-state cavity quantum electrodynamics
    Greuter, L. and Najer, D. and Kuhlmann, A.V. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Starosielec, S. and Warburton, R.J.
    Journal of Applied Physics 118 (2015)
    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.
    view abstract10.1063/1.4928769
  • Extending the spectral range of CdSe/ZnSe quantum wells by strain engineering
    Finke, A. and Ruth, M. and Scholz, S. and Ludwig, Ar. and Wieck, A.D. and Reuter, D. and Pawlis, A.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    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.
    view abstract10.1103/PhysRevB.91.035409
  • Fast probe of local electronic states in nanostructures utilizing a single-lead quantum dot
    Otsuka, T. and Amaha, S. and Nakajima, T. and Delbecq, M.R. and Yoneda, J. and Takeda, K. and Sugawara, R. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Scientific Reports 5 (2015)
    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.
    view abstract10.1038/srep14616
  • Hybrid architecture for shallow accumulation mode AlGaAs/GaAs heterostructures with epitaxial gates
    Macleod, S.J. and See, A.M. and Hamilton, A.R. and Farrer, I. and Ritchie, D.A. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D.
    Applied Physics Letters 106 (2015)
    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.
    view abstract10.1063/1.4905210
  • Interplay of Electron and Nuclear Spin Noise in n -Type GaAs
    Berski, F. and Hübner, J. and Oestreich, M. and Ludwig, Ar. and Wieck, A.D. and Glazov, M.
    Physical Review Letters 115 (2015)
    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.
    view abstract10.1103/PhysRevLett.115.176601
  • Measurement of the transmission phase of an electron in a quantum two-path interferometer
    Takada, S. and Yamamoto, M. and Bäuerle, C. and Watanabe, K. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Applied Physics Letters 107 (2015)
    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.
    view abstract10.1063/1.4928035
  • Optical and microwave properties of focused ion beam implanted Erbium ions in Y2SiO5 crystals
    Kukharchyk, N. and Probst, S. and Pal, S. and Xia, K. and Kolesov, R. and Ludwig, Ar. and Ustinov, A.V. and Bushev, P. and Wieck, A.D.
    Conference on Lasers and Electro-Optics Europe - Technical Digest 2015-August (2015)
    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.
    view abstract10.1364/CLEO_AT.2015.JW2A.21
  • Optical properties of strain-compensated CdSe/ZnSe/(Zn,Mg)Se quantum well microdisks
    Ruth, M. and Finke, A. and Schmidt, G. and Reuter, D. and Scholz, S. and Ludwig, Ar. and Wieck, A.D. and Pawlis, A.
    Optics Express 23 (2015)
    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.
    view abstract10.1364/OE.23.029079
  • Quantum Manipulation of Two-Electron Spin States in Isolated Double Quantum Dots
    Bertrand, B. and Flentje, H. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Physical Review Letters 115 (2015)
    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.
    view abstract10.1103/PhysRevLett.115.096801
  • Spectral modification of the laser emission of a terahertz quantum cascade laser induced by broad-band double pulse injection seeding
    Markmann, S. and Nong, H. and Pal, S. and Hekmat, N. and Scholz, S. and Kukharchyk, N. and Ludwig, Ar. and Dhillon, S. and Tignon, J. and Marcadet, X. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    Applied Physics Letters 107 (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.
    view abstract10.1063/1.4930993
  • Strong coupling of intersubband resonance in a single triangular well to a THz metamaterial
    Pal, S. and Nongt, H. and Markmann, S. and Kukharchyk, N. and Valentin, S.R. and Scholz, S. and Ludwig, Ar. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves (2015)
    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.
    view abstract10.1109/IRMMW-THz.2015.7327816
  • The effect of charged quantum dots on the mobility of a two-dimensional electron gas: How important is the Coulomb scattering?
    Kurzmann, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Journal of Applied Physics 117 (2015)
    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.
    view abstract10.1063/1.4907217
  • Transform-limited single photons from a single quantum dot
    Kuhlmann, A.V. and Prechtel, J.H. and Houel, J. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Warburton, R.J.
    Nature Communications 6 (2015)
    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.
    view abstract10.1038/ncomms9204
  • Tuning the tunneling probability between low-dimensional electron systems by momentum matching
    Zhou, D. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
    Applied Physics Letters 106 (2015)
    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.
    view abstract10.1063/1.4922738
  • Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure
    Pal, S. and Nong, H. and Markmann, S. and Kukharchyk, N. and Valentin, S.R. and Scholz, S. and Ludwig, Ar. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    Scientific Reports 5 (2015)
    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.
    view abstract10.1038/srep16812
  • A linear relationship between the Hall carrier concentration and the effective absorption coefficient measured by means of photothermal radiometry in IR semi-transparent n-type CdMgSe mixed crystals
    Pawlak, M. and Maliński, M. and Firszt, F. and Pelzl, J. and Ludwig, Ar. and Marasek, A.
    Measurement Science and Technology 25 (2014)
    In this work we demonstrate the ability to measure the effective infrared absorption coefficient in semiconductors by a photothermal infrared radiometry (PTR) experiment, and its correlation with the Hall carrier concentration. The amplitude and phase of the PTR signal were measured for Cd1-xMg xSe mixed crystals, with the magnesium content varying from x = 0 to x = 0.15. The PTR experiments were performed at room temperature in thermal reflection and transmission configurations using a mercury cadmium telluride infrared detector. The PTR data were analyzed in the frame of the one-dimensional heat transport model for infrared semi-transparent crystals. Based on the variation of the normalized PTR phase and amplitude on the modulation frequency, the thermal diffusivity and the effective infrared absorption coefficient were obtained by fitting the theoretical expression to experimental data and compared with the Hall carrier concentration determined by supplementary Hall experiments. A linear relationship between the effective infrared absorption coefficient and the Hall carrier concentration was found which is explained in the frame of the Drude theory. The uncertainty of the measured slope was 6%. The value of the slope depends on (1) the sample IR absorption spectrum and (2) the spectral range of the infrared detector. It has to be pointed out that this method is suitable for use in an industrial environment for a fast and contactless carrier concentration measurement. This method can be used for the characterization of other semiconductors after a calibration procedure is carried out. In addition, the PTR technique yields information on the thermal properties in the same experiment. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-0233/25/3/035204
  • A small mode volume tunable microcavity: Development and characterization
    Greuter, L. and Starosielec, S. and Najer, D. and Ludwig, Ar. and Duempelmann, L. and Rohner, D. and Warburton, R.J.
    Applied Physics Letters 105 (2014)
    We report the realization of a spatially and spectrally tunable air-gap Fabry-Pérot type microcavity of high finesse and cubic-wavelength-scale mode volume. These properties are attractive in the fields of opto-mechanics, quantum sensing, and foremost cavity quantum electrodynamics. The major design feature is a miniaturized concave mirror with atomically smooth surface and radius of curvature as low as 10 μm produced by CO2laser ablation of fused silica. We demonstrate excellent mode-matching of a focussed laser beam to the microcavity mode and confirm from the frequencies of the resonator modes that the effective optical radius matches the physical radius. With these small radii, we demonstrate wavelength-size beam waists. We also show that the microcavity is sufficiently rigid for practical applications: in a cryostat at 4 K, the root-mean-square microcavity length fluctuations are below 5 pm. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4896415
  • Annulated and bridged tetrahydrofurans from alkenoxyl radical cyclization
    Schur, C. and Kelm, H. and Gottwald, T. and Ludwig, Ar. and Kneuer, R. and Hartung, J.
    Organic and Biomolecular Chemistry 12 (2014)
    4-Pentenoxyl radicals sharing two or more carbon atoms with a cycloalkane cyclize in a predictable manner stereoselectively and regioselectively to afford in solutions of bromotrichloromethane cycloalkyl-fused or -bridged 2-bromomethyltetrahydrofurans in up to 95% yield. Stereoselectivity in alkenoxyl radical ring closures arises from cumulative steric effects. The substituent positioned the closest to the alkene carbon, which is being attacked by the oxygen radical, exerts the strongest stereodirecting effect. This principal inductor guides 5-exo-cyclization 2,3-trans- or 2,4-cis-selectively. The substituent located further from the attacked π-bond is the secondary inductor. A secondary inductor in the relative trans-configuration enhances stereodifferentiation by the primary inductor; a cis-configured secondary inductor decreases this effect. A secondary inductor is not able to overrule the guiding effect of a similar sized primary inductor. Intramolecular 4-pentenoxyl radical additions to a cyclohexene-bound exo-methylene group or to endocyclic double bonds proceed cis-specifically, as exemplified by synthesis of a diastereomerically pure bromobicyclo[2.2.1]heptyl-annulated tetrahydrofuran from the verbenylethyloxyl radical. According to theory, the experimental 2,3-cis-specificity in alkoxyl radical cyclization to an endocyclic π-bond arises from strain associated with the 2,3-trans-ring closure. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4ob01266f
  • Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy
    Beckel, A. and Kurzmann, A. and Geller, M. P. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A.
    EPL 106 (2014)
    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.
    view abstract10.1209/0295-5075/106/47002
  • Direct quantitative electrical measurement of many-body interactions in exciton complexes in InAs quantum dots
    Labud, P.A. and Ludwig, Ar. and Wieck, A.D. and Bester, G. and Reuter, D.
    Physical Review Letters 112 (2014)
    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.
    view abstract10.1103/PhysRevLett.112.046803
  • Erratum: Manipulation of the nuclear spin ensemble in a quantum dot with chirped magnetic resonance pulses (Nature Nanotechnology (2014) 9 (671-675))
    Munsch, M. and Wüst, G. and Kuhlmann, A.V. and Xue, F. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Poggio, M. and Warburton, R.J.
    Nature Nanotechnology 9 (2014)
    view abstract10.1038/nnano.2014.212
  • Infrared transmission spectroscopy of charge carriers in self-assembled InAs quantum dots under surface electric fields
    Pal, S. and Valentin, S.R. and Kukharchyk, N. and Nong, H. and Parsa, A.B. and Eggeler, G. and Ludwig, Ar. and Jukam, N. and Wieck, A.D.
    Journal of Physics Condensed Matter 26 (2014)
    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.
    view abstract10.1088/0953-8984/26/50/505801
  • Investigation of carrier scattering mechanisms in n-Cd1- xMgxSe single crystals using Fourier Transform Infrared Spectroscopy
    Pawlak, M. and Maliński, M. and Firszt, F. and Łȩgowski, S. and Mȩczyńska, H. and Ollesch, J. and Ludwig, Ar. and Marasek, A. and Schulte-Braucks, C.
    Infrared Physics and Technology 64 (2014)
    This paper presents results of investigations of carrier scattering mechanisms in n-Cd1-xMgxSe mixed crystals with magnesium content varying from x = 0 to x = 0.33. Experimental results obtained by means of the Fourier Transform Infrared Spectroscopy (FT-IR) and Hall measurements are discussed in the frame of the Drude and the quantum theories. The character of the wavelength dependence of the optical absorption coefficient in investigated crystals was found to be of the type ∼λp, where 2 < p < 3.5. The p = 2 is expected from the Drude theory and the relaxation time approximation. The obtained experimental values of p parameter suggest that the optical phonon and impurity scattering mechanisms are dominating scattering mechanisms in these crystals. The calculated carrier concentration from optical absorption spectrum for a n-CdSe crystal is in a good agreement with this obtained from Hall measurement. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.infrared.2014.02.012
  • Magnetic properties of Gd-doped GaN
    Shvarkov, S. and Ludwig, As. and Wieck, A.D. and Cordier, Y. and Ney, A. and Hardtdegen, H. and Haab, A. and Trampert, A. and Ranchal, R. and Herfort, J. and Becker, H.-W. and Rogalla, D. and Reuter, D.
    Physica Status Solidi (B) Basic Research 251 (2014)
    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.
    view abstract10.1002/pssb.201350205
  • Manipulation of the nuclear spin ensemble in a quantum dot with chirped magnetic resonance pulses
    Munsch, M. and Wüst, G. and Kuhlmann, A.V. and Xue, F. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Poggio, M. and Warburton, R.J.
    Nature Nanotechnology 9 (2014)
    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.
    view abstract10.1038/nnano.2014.175
  • Monitoring of amorfization of the oxygen implanted layers in silicon wafers using photothermal radiometry and modulated free carrier absorption methods
    Maliński, M. and Pawlak, M. and Chrobak, Ł. and Pal, S. and Ludwig, Ar.
    Applied Physics A: Materials Science and Processing 118 (2014)
    This paper presents experimental results that characterize implanted layers in silicon being the result of a high energy implantation of O+6 ions. We propose a simple relation between attenuation of photothermal radiometry and/or modulated free carrier absorption amplitudes, the implanted layer thickness and its optical absorption coefficient. The thickness of the implanted layers was determined from capacitance–voltage characteristics and computations with the TRIM program. The obtained results allowed to estimate changes of the optical absorption coefficient of the oxygen implanted layers indicating the amorfization of the layers. © 2014, The Author(s).
    view abstract10.1007/s00339-014-8859-4
  • Photoluminescence of focused ion beam implanted Er3+: Y2SiO5 crystals
    Kukharchyk, N. and Pal, S. and Rödiger, J. and Ludwig, Ar. and Probst, S. and Ustinov, A.V. and Bushev, P. and Wieck, A.D.
    Physica Status Solidi - Rapid Research Letters 8 (2014)
    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.
    view abstract10.1002/pssr.201409304
  • Single photoelectron detection after selective excitation of electron heavy-hole and electron light-hole pairs in double quantum dots
    Morimoto, K. and Fujita, T. and Allison, G. and Teraoka, S. and Larsson, M. and Kiyama, H. and Haffouz, S. and Austing, D.G. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A. and Tarucha, S.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    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.
    view abstract10.1103/PhysRevB.90.085306
  • Time-resolved transconductance spectroscopy on self-assembled quantum dots: Spectral evolution from single- into many-particle states
    Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    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.
    view abstract10.1103/PhysRevB.89.155430
  • Transient and persistent current induced conductivity changes in GaAs/AlGaAs high-electron-mobility transistors
    Schulte-Braucks, C. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D.
    Applied Physics Letters 104 (2014)
    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.
    view abstract10.1063/1.4870422
  • Transmission phase in the Kondo regime revealed in a two-path interferometer
    Takada, S. and Bäuerle, C. and Yamamoto, M. and Watanabe, K. and Hermelin, S. and Meunier, T. and Alex, A. and Weichselbaum, A. and Von Delft, J. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review Letters 113 (2014)
    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.
    view abstract10.1103/PhysRevLett.113.126601
  • Tuning the electrically evaluated electron Landé g factor in GaAs quantum dots and quantum wells of different well widths
    Allison, G. and Fujita, T. and Morimoto, K. and Teraoka, S. and Larsson, M. and Kiyama, H. and Oiwa, A. and Haffouz, S. and Austing, D.G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    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.
    view abstract10.1103/PhysRevB.90.235310
  • A dark-field microscope for background-free detection of resonance fluorescence from single semiconductor quantum dots operating in a set-and-forget mode
    Kuhlmann, A.V. and Houel, J. and Brunner, D. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Warburton, R.J.
    Review of Scientific Instruments 84 (2013)
    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.
    view abstract10.1063/1.4813879
  • Charge noise and spin noise in a semiconductor quantum device
    Kuhlmann, A.V. and Houel, J. and Ludwig, Ar. and Greuter, L. and Reuter, D. and Wieck, A.D. and Poggio, M. and Warburton, R.J.
    Nature Physics 9 (2013)
    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.
    view abstract10.1038/nphys2688
  • Frequency-Stabilized Source of Single Photons from a Solid-State Qubit
    Prechtel, J. H. and Kuhlmann, A. V. and Houel, J. and Greuter, L. and Ludwig, Ar. and Reuter, D. and Wieck, A. D. and Warburton, R. J.
    Physical Review X 3 (2013)
    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.
    view abstract10.1103/PhysRevX.3.041006
  • High-resolution mass spectrometer for liquid metal ion sources
    Wortmann, M. and Ludwig, Ar. and Meijer, J. and Reuter, D. and Wieck, A.D.
    Review of Scientific Instruments 84 (2013)
    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.
    view abstract10.1063/1.4822275
  • Interaction effects and transport properties of Pt capped Co nanoparticles
    Ludwig, Ar. and Agudo, L. and Eggeler, G. and Ludwig, Al. and Wieck, A.D. and Petracic, O.
    Journal of Applied Physics 113 (2013)
    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.
    view abstract10.1063/1.4789422
  • Nondestructive real-time measurement of charge and spin dynamics of photoelectrons in a double quantum dot
    Fujita, T. and Kiyama, H. and Morimoto, K. and Teraoka, S. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A. and Tarucha, S.
    Physical Review Letters 110 (2013)
    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.
    view abstract10.1103/PhysRevLett.110.266803
  • Spin injection, transport, and relaxation in spin light-emitting diodes: Magnetic field effects
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8813 (2013)
    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.
    view abstract10.1117/12.2023324
  • Spin relaxation in spin light-emitting diodes: Effects of magnetic field and temperature
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8623 (2013)
    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.
    view abstract10.1117/12.2001511
  • Spin relaxation length in quantum dot spin LEDs
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Physica Status Solidi (C) Current Topics in Solid State Physics 10 (2013)
    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.
    view abstract10.1002/pssc.201200689
  • Electron-nuclei spin coupling in GaAs-Free versus localized electrons
    Huang, J. and Chen, Y.S. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Bacher, G.
    Applied Physics Letters 100 (2012)
    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.
    view abstract10.1063/1.3699261
  • Erratum: Probing single-charge fluctuations at a GaAs/AlAs interface using laser spectroscopy on a nearby InGaAs quantum dot (Physical Review Letters (2012) 108 (107401))
    Houel, J. and Kuhlmann, A.V. and Greuter, L. and Xue, F. and Poggio, M. and Gerardot, B.D. and Dalgarno, P.A. and Badolato, A. and Petroff, P.M. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Warburton, R.J.
    Physical Review Letters 108 (2012)
    view abstract10.1103/PhysRevLett.108.119902
  • Erratum: Room temperature spin relaxation length in spin light-emitting diodes (Applied Physics Letters (2011) 99 (051102))
    Soldat, H. and Li, M. and Gerhardt, N.C. and Hofmann, M.R. and Ludwig, Ar. and Ebbing, A. and Reuter, D. and Wieck, A.D. and Stromberg, F. and Keune, W. and Wende, H.
    Applied Physics Letters 100 (2012)
    view abstract10.1063/1.4728990
  • Magnetic field dependence of the spin relaxation length in spin light-emitting diodes
    Höpfner, H. and Fritsche, C. and Ludwig, As. and Ludwig, Ar. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Physics Letters 101 (2012)
    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.
    view abstract10.1063/1.4752162
  • Probing single-charge fluctuations at a GaAs/AlAs interface using laser spectroscopy on a nearby InGaAs quantum dot
    Houel, J. and Kuhlmann, A.V. and Greuter, L. and Xue, F. and Poggio, M. and Warburton, R.J. and Gerardot, B.D. and Dalgarno, P.A. and Badolato, A. and Petroff, P.M. and Ludwig, Ar. and Reuter, D. and Wieck, A.D.
    Physical Review Letters 108 (2012)
    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.
    view abstract10.1103/PhysRevLett.108.107401
  • Room temperature spin relaxation in quantum dot based spin-optoelectronic devices
    Höpfner, H. and Li, M. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8260 (2012)
    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).
    view abstract10.1117/12.907821
  • (1 0 0) GaAs/AlxGa1-xAs heterostructures for Zeeman spin splitting studies of hole quantum wires
    Trunov, K. and Reuter, D. and Ludwig, Ar. and Chen, J.C.H. and Klochan, O. and Micolich, A.P. and Hamilton, A.R. and Wieck, A.D.
    Journal of Crystal Growth 323 (2011)
    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.
    view abstract10.1016/j.jcrysgro.2010.11.060
  • Electrical spin injection in InAs quantum dots at room temperature and adjustment of the emission wavelength for spintronic applications
    Ludwig, Ar. and Roescu, R. and Rai, A.K. and Trunov, K. and Stromberg, F. and Li, M. and Soldat, H. and Ebbing, A. and Gerhardt, N.C. and Hofmann, M.R. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D.
    Journal of Crystal Growth 323 (2011)
    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.
    view abstract10.1016/j.jcrysgro.2010.09.087
  • Growth of GaN based structures on focused ion beam patterned templates
    Cordier, Y. and Tottereau, O. and Nguyen, L. and Ramdani, M. and Soltani, A. and Boucherit, M. and Troadec, D. and Lo, F.-Y. and Hu, Y.Y. and Ludwig, Ar. and Wieck, A.D.
    Physica Status Solidi (C) Current Topics in Solid State Physics 8 (2011)
    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.
    view abstract10.1002/pssc.201000882
  • Manipulation of nuclear spin dynamics in n-GaAs using an on-chip microcoil
    Chen, Y.S. and Huang, J. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Bacher, G.
    Journal of Applied Physics 109 (2011)
    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.
    view abstract10.1063/1.3530731
  • MOCVD of ZnO films from bis(ketoiminato)Zn(II) precursors: Structure, morphology and optical properties
    Bekermann, D. and Ludwig, Ar. and Toader, T. and MacCato, C. and Barreca, D. and Gasparotto, A. and Bock, C. and Wieck, A.D. and Kunze, U. and Tondello, E. and Fischer, R.A. and Devi, A.
    Chemical Vapor Deposition 17 (2011)
    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.
    view abstract10.1002/cvde.201006898
  • Optically detected nuclear magnetic resonance in n-GaAs using an on-chip microcoil
    Chen, Y.S. and Huang, J. and Reuter, D. and Ludwig, Ar. and Wieck, A.D. and Bacher, G.
    Applied Physics Letters 98 (2011)
    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.
    view abstract10.1063/1.3553503
  • Room temperature spin relaxation length in spin light-emitting diodes
    Soldat, H. and Li, M. and Gerhardt, N.C. and Hofmann, M.R. and Ludwig, Ar. and Ebbing, A. and Reuter, D. and Wieck, A.D. and Stromberg, F. and Keune, W. and Wende, H.
    Applied Physics Letters 99 (2011)
    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.
    view abstract10.1063/1.3622662
  • Epitaxial growth and interfacial magnetism of spin aligner for remanent spin injection: [Fe/Tb]n /Fe/MgO/GaAs -light emitting diode as a prototype system
    Schuster, E. and Brand, R.A. and Stromberg, F. and Lo, F.-Y. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Hövel, S. and Gerhardt, N.C. and Hofmann, M.R. and Wende, H. and Keune, W.
    Journal of Applied Physics 108 (2010)
    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.
    view abstract10.1063/1.3476265
  • magnetism

  • molecular beam epitaxy

  • nanostructures

  • quantum dots

  • semiconductors

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