Ultrastrong Light-Matter Coupling in Tailored Environments

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 theorists 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.

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. 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. The overall goal of this project was to investigate the physics of ultrastrongly coupled light-matter systems in order to develop a detailed theoretical understanding for the properties of these systems under such extreme coupling conditions. As one of the most important outcomes of this project, we have predicted for the first time the ground state phase diagram of an ultrastrongly coupled atom-photon system. This solved a long-standing open problem in this field, which has led to many controversial debates over the past 50 years. This analysis also showed how the ultrastrong coupling to photons can in principle even change phase transitions and other material properties. Apart from these and many other more conceptual findings, this project also addressed some of the practical implications of this new physics. For example, the superconducting circuits mentioned above, are currently seen as one of the most promising technologies for building a quantum computer. Our calculation show that by using ultrastrongly coupled superconducting quantum bits ("qubits") the speed of gate operations in future quantum computers can be increased by a factor 100-1000. Other practically relevant examples include ultrastrong-coupling modifications of chemical reactions, which have been observed recently in experiments, but which are not yet understood. Here, the models developed within this project can be used to develop further insights into these fascinating effects.