Over the past decades many ground-breaking theoretical and experimental works in the field of quantum optics have led to a very precise understanding of light-matter interactions at the level of individual photons and atoms. These insights have not only revealed many intriguing quantum mechanical phenomena, they have also triggered the development of novel quantum communication and quantum computation technologies that might soon come into practical use. However, despite the immense theoretical and experimental progress in this field, there still exist puzzling open questions at the very fundamental level. This concerns in particular the regime of so-called ultrastrong light-matter interactions, where the coupling strength between a single photon and matter is so strong that it even exceeds the energy it takes to create the photon. In this regime most of our basic intuition about atom-photon interactions - even the notion of a ‘photon’ - breaks down.
The overall goal of this project is to develop a detailed theoretical basis for the physics of atom-photon interactions under such extreme coupling conditions. Although this regime is very hard to reach with real atoms and photons, it becomes accessible in artificial systems, where, for example, atoms are replaced by superconducting circuits and optical photons by quantized excitations of microwave resonators. In such artificial systems, which are currently studied in many laboratories around the world, the coupling between ‘atoms’ and ‘photons’ can be designed almost at will. From a theorist’s point of view, this flexibility allows one to analyze ultrastrong coupling phenomena in simpler as well as more complex configurations and thereby extract the most essential physical effects that can arise in this regime. These insights will not only lead to a deeper understanding of the nature of light-matter interactions on a fundamental level, but also provide the basis for first applications of such effect, for example, for new types of superconducting quantum information processing schemes.
