Scientists at Ruhr-University Bochum, working with researchers based in Basel and Copenhagen, have demonstrated the connection between the radiative Auger process and quantum optics. The scientists experimentally confirmed the radiative Auger process in quantum dots, observing this process in the limit of a single photon and one Auger electron. They showed that quantum optics measurements with the radiative Auger emission can be used as a tool for investigating the dynamics of a single electron. Atoms consist of a positively charged core that is surrounded by one or more negatively charged electrons. When one electron in an atom has a high energy level, it can reduce its energy by two well-known processes: It can release energy in the form of a photon without affecting the other electrons, or through an Auger process, where the high-energy electron gives all its energy to other electrons in the atom. In a third possible process — the so-called radiative Auger process — the excited electron reduces its energy by transferring it to both a photon and another electron in the atom. A semiconductor quantum dot resembles an atom in many aspects. However, for quantum dots, energy transitions using the radiative Auger process until now have been only theoretically predicted. Schematic representation of a charged exciton, that is, an excited state consisting of two electrons and one hole within a quantum dot. Courtesy of RUB, Arne Ludwig. To create a quantum dot, the Bochum researchers used self-organizing processes in crystal growth. These processes led to the creation of billions of nm-size crystals made from indium arsenide and other materials. Charge carriers such as a single electron could be trapped in the nanocrystals. According to the Bochum team, this construct is interesting for quantum communication because information can be encoded with the help of charge carrier spins. For this coding, it would be necessary to manipulate and read the spin from the outside. During readout, quantum information could be imprinted into the polarization of a photon, which could carry the information at the speed of light and be used for quantum information transfer. The scientists demonstrated radiative Auger on trions in single quantum dots. For a trion, a photon was created on electron-hole recombination, leaving behind a single electron. The radiative Auger process promoted this additional electron to a higher shell of the quantum dot. The energy separations between the resonance fluorescence and the radiative Auger emission directly measured the single-particle splittings of the electronic states in the quantum dot with high precision. After the radiative Auger emission, the Auger carrier relaxed back to the lowest shell. An electron inside a quantum dot is raised by a photon (green waveform) to a higher energy level. The result is a so-called exciton, an excited state consisting of two electrons and one hole. By emitting a photon (green waveform), the system returns to the ground state (green path). In rare cases, a radiative Auger process takes place (red arrow): An electron stays in the excited state, while a photon of lower energy (red waveform) is emitted. Courtesy of RUB, Arne Ludwig. The radiative Auger effect was shown to be a powerful probe of the single electron. Using this effect, the scientists could precisely determine the structure of the quantum mechanical energy levels available to a single electron in a quantum dot. Until now, this has only been possible indirectly through calculations using a combination with optical methods. To find the most suitable quantum dots for different applications, several questions must be answered, the Bochum team believes. How much time does an electron remain in the energetically excited state? What energy levels form a quantum dot? How can the quantum dot’s energy levels be influenced by means of manufacturing processes? Julian Ritzmann made the samples for the measurements. Courtesy of RUB, Marquard. The Bochum team, comprising researchers Julian Ritzmann and Arne Ludwig and professor Andreas Wieck, produced quantum dots from two semiconductor materials: indium arsenide and gallium arsenide. In both systems, the team was able to achieve the ultrastable surroundings for the quantum dots that are necessary for the radiative Auger process. The research was published in Nature Nanotechnology (www.doi.org/10.1038/s41565-020-0697-2).