Ultra-Strong Coupling in Cavity Quantum Electrodynamics: From Single to Multiple Emitters

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In recent years a new regime of cavity quantum electrodynamics, where the coupling strength between a quantum emitter and a cavity photon becomes comparable to the photon energy or the emitter transition energy, has been reached in experiments. In this regime of ultra-strong coupling novel, to date unexplored effects of light-matter interaction are expected. In this project we will first investigate how information about the intra-cavity physics can be experimentally accessed in that regime. For this experimentally very crucial aspect we will develop approaches to a full tomography of the intra-cavity states. To explore the rich physics of the ultra-strong coupling regime we will then investigate suitable setups in circuit quantum electrodynamics as one of the most advanced technological platforms for our interests. In particular we will explore large resonators or open transmission lines that couple ultra-strongly to a single or to multiple qubits. For a single qubit we will investigate the scattering of light pulses at the qubit and its capability to act as a quantum simulator for spin-boson physics. Qubits that couple ultra-strongly to an open transmission line could form photon number and frequency selective switchable mirrors and we will thus explore these applications in detail. For multiple qubits we expect the system to give rise to a generalized ultra-strong coupling, multimode Dicke model that shows novel physics. We will throughly investigate its properties and dynamics, including possible quantum phase transitions.