New ideas about energy levels in quantum dots

Researchers from Basel, Bochum and Copenhagen have gained new insights into the energy states of quantum dots. They are promising semiconductor nanostructures and building blocks for quantum communication. With their experiments, scientists confirmed certain energy transitions at quantum dots that had previously only been theoretically predicted: the so-called radiative Auger process. For their investigations, the researchers from Basel and Copenhagen used special samples that the team at the Ruhr-Universität Bochum Solid State Chair in Applied Physics had produced. The researchers report their results in the journal. Nanotechnology nature, published online June 15, 2020.

Block cargo carriers

To create a quantum dot, the Bochum researchers use self-organizing processes in crystal growth. In the process, they produce billions of nanometer crystals of, for example, indium arsenide. Charge carriers, such as a single electron, can trap them. This construction is interesting for quantum communication because the information can be encoded with the help of charge carrier turns. For this encoding, you need to be able to manipulate and read the turn from the outside. During reading, quantum information can be imprinted on the polarization of a photon, for example. This takes the information further 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 at the quantum dot when energy radiates from the outside into the artificial atom.

Special energy transitions demonstrated

Atoms consist of a positively charged nucleus that is surrounded by one or more negatively charged electrons. When an electron in the atom has high energy, it can reduce its energy by two well-known processes: in the first process, energy is released in the form of a single quantum of light (a photon), and the other electrons are not affected. . A second possibility is an Auger process, where the high-energy electron gives all of its energy to 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 theoretically described a third possibility: in the so-called radiative Auger process, the excited electron reduces its energy by transferring it to both a quantum of light and another electron in the atom. A semiconductor quantum dot resembles an atom in many ways. However, for quantum dots, the radiative Auger process had only been theoretically predicted until now. Now, the experimental observation has been carried out by researchers from Basel. Together with their colleagues from Bochum and Copenhagen, Basel-based researchers Dr. Matthias Löbl and Professor Richard Warburton have observed the radiative Auger process at the boundary of a single photon and an Auger electron. For the first time, the researchers demonstrated the connection between the radiative Auger process and quantum optics. They show that quantum optics measurements with Auger’s radiant emission can be used as a tool to investigate individual electron dynamics.

Quantum dot applications.

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

To find ideal quantum dots for different applications, questions such as the following must be answered: How long does an electron stay in the energetically excited state? What energy levels make up a quantum dot? And how can this be influenced by manufacturing processes?

Different quantum dots in stable environments.

The group observed the effect not only on quantum dots in indium arsenide semiconductors. The Bochum team of Dr. Julian Ritzmann, Dr. Arne Ludwig and Professor Andreas Wieck also managed to produce a quantum dot from the semiconductor gallium arsenide. In both material systems, the Bochum team has achieved a very stable quantum dot environment, which has been instrumental in the radiative auger process. For many years, the Ruhr-Universität Bochum group has been working on the optimal conditions for stable quantum dots.