The quantum Rabi model describes the fundamental mechanism of light-matter
interaction. It consists of a two-level atom or qubit coupled to a quantized
harmonic mode via a transversal interaction. In the weak coupling regime, it
reduces to the well-known Jaynes-Cummings model by applying a rotating wave
approximation (RWA). The RWA breaks down in the ultra-strong coupling (USC)
regime, where the effective coupling strength g is comparable to the energy
ω of the bosonic mode, and remarkable features in the system dynamics
are revealed. We demonstrate an analog quantum simulation of an effective
quantum Rabi model in the USC regime, achieving a relative coupling ratio of
g/ω∼0.6. The quantum hardware of the simulator is a superconducting
circuit embedded in a cQED setup. We observe fast and periodic quantum state
collapses and revivals of the initial qubit state, being the most distinct
signature of the synthesized model.Comment: 20 pages, 13 figure