New Insights into the Energy Levels in Quantum Dots


Schematic representation of a charged exciton, i.e. an excited state consisting of two electrons and a hole in a quantum dot
© RUB, Arne Ludwig

Researchers have experimentally proven the theoretically predicted Auger effect in quantum dots. The findings help to understand the structures that could form the basis of quantum communication.
Researchers from Basel, Bochum and Copenhagen have gained new insights into the energy states of quantum dots. These semiconductor nanostructures are promising candidates for the basic information units for quantum communication. With their experiments, the scientists confirmed certain energy transitions in quantum dots that had previously only been predicted theoretically: the so-called radiative Auger process. For the investigations, the researchers in Basel and Copenhagen used special samples produced by the team from the Chair of Applied Solid State Physics at Ruhr Universität Bochum (RUB). They report on the results in the journal Nature Nanotechnology, published online on 15 June 2020.

Locking up charge carriers

In order to create a quantum dot, the Bochum researchers use self-organizing processes in crystal growth. In the process, they produce billions of nanometre-sized crystals from, for example, indium arsenide. In these they can trap charge carriers, such as a single electron. This construct is interesting for quantum communication because information can be encoded with the help of charge carrier spinning. For this coding it is necessary to be able to manipulate and read the spin from the outside. During readout, quantum information can be imprinted into the polarisation of a photon, for example. The photon then carries the information at the speed of light and can be used for quantum information transfer.

This is why scientists are interested, for example, in what exactly happens in the quantum dot when energy is irradiated from outside onto the artificial atom.

Special energy transitions demonstrated

Atoms consist of a positively charged nucleus surrounded by one or more negatively charged electrons. If an electron in an atom is excited, i.e. has an increased energy, it can reduce this energy in two ways: It can release the energy in the form of a single particle of light (photon) without affecting the other electrons. Or it comes to the Auger process, in which the high-energy electron transfers all its energy to the other electrons in the atom. This effect was discovered in 1922 by Lise Meitner and Pierre Victor Auger. About a decade later, physicist Felix Bloch described the so-called radiative Auger process. In this process the excited electron splits its energy between another electron in the atom and a photon.

A semiconductor quantum dot resembles an atom in many ways. However, the radiative Auger process had previously only been predicted theoretically for quantum dots. Experimental proof has now been provided by researchers from Basel. The physicists Dr. Matthias Löbl and Prof. Dr. Richard Warburton, together with colleagues from Bochum and Copenhagen, demonstrated the radiative Auger process in the smallest possible system of one electron and one photon. This was the first time they showed a connection between this process and quantum optics. They proved that quantum optical measurements with the radiating Auger process can be useful to study the dynamics of single electrons.

Applications for quantum dots

Using the radiant Auger effect, scientists can also precisely determine the structure of the quantum mechanical energy levels available to a single electron in the quantum dot. Until now, this has only been possible indirectly through calculations in combination with optical methods. Now a direct proof has been achieved. This helps to better understand the quantum mechanical system.

In order to find ideal quantum dots for different applications, questions such as How much time does an electron remain in the energetically excited state? What energy levels does a quantum dot form? And how can this be influenced by manufacturing processes?

Different quantum dots in stable environments

The group observed the effect not only in quantum dots in indium arsenide semiconductors. The Bochum team Dr. Julian Ritzmann, Dr. Arne Ludwig and Prof. Dr. Andreas Wieck also succeeded in producing a quantum dot from the semiconductor gallium arsenide. With both materials, the scientists were able to create quantum dots with very stable environments, which was crucial for demonstrating the radiative Auger process. For many years, the group at the Ruhr-Universität Bochum has been working on the optimal conditions for stable quantum dots.

The project was funded by the National Centre of Competence in Research "Quantum Science and Technology", the Swiss National Science Foundation (Grant No. 200020 156637), the European Union within the Horizon 2020 Programme (Grant Nos. 721394 and 840453), the German Research Foundation (Collaborative Research Centre TRR160, Project DFH/UFA CDFA05-06 and Project 383065199), the Federal Ministry of Education and Research (Project Q. Link.X/funding number 16KIS0867), the Danish Center of Excellence Hy-Q, (grant number DNRF139) and the European Research Council within the ERC Advanced Grant "Scale".

Original publication
Matthias C. Löbl, Clemens Spinnler, Alisa Javadi, Liang Zhai, Giang N. Nguyen, Julian Ritzmann, Leonardo Midolo, Peter Lodahl, Andreas D. Wieck, Arne Ludwig, Richard J. Warburton: Radiative Auger process in the single-photon limit, in: Nature Nanotechnology, 2020, DOI: 10.1038/s41565-020-0697-2

Press contact
Prof. Dr. Andreas Wieck
Chair of Applied Solid State Physics
Faculty of Physics and Astronomy
Ruhr University Bochum
Phone: +49 234 32 26726

Dr. Arne Ludwig
Chair of Solid State Physics
Faculty of Physics and Astronomy
Ruhr University Bochum
Phone: +49 234 32 25864