Ultrastrong coupling regimes of light-matter interaction

In the ultrastrong coupling regime, light and matter can no longer be distinguished: their coupling strengths are as large as their own energy scales. Even for simple two-level systems, this represents a new regime to explore within the quantum Rabi model. There has been considerable theoretical and experimental progress on the physics of these systems in recent years, as presented in this review. Key experimental results are discussed in a set of current platforms arising in quantum information research, including superconducting and semiconducting devices, and other hybrid quantum systems.