The Vacuum in Ultrastrong Coupling Cavity Quantum Electrodynamics

This chapter presents a basic introduction to the ground‐state phases of ultrastrongly coupled light–matter systems, starting with a brief historical review of the Dicke model and the associated controversy about the superradiant phase transition. The main focus of this chapter is then placed on the derivation of effective cavity quantum electrodynamics (QED) Hamiltonians for systems of two‐level dipoles coupled to a single‐cavity mode, which can be used to explore the quantum many‐body states of light–matter systems at arbitrary interaction strength. This analysis clarifies some of the common misconceptions in this field, and also reveals intriguing new phases of light and matter that could become accessible in cavity QED systems where nonperturbative coupling conditions can be reached.