Dynamical correlation functions and the quantum Rabi model
We study the quantum Rabi model within the framework of the analytical solution developed in Phys. Rev. Lett. 107 100401 (2011). In particular, through time-dependent correlation functions, we give a quantitative criterion for classifying two regions of the quantum Rabi model, involving the Jaynes-Cummings, the ultrastrong-coupling, and the deep strong-coupling regimes. In addition, we find a stationary qubit-field entangled basis that governs the whole dynamics as the coupling strength overcomes the mode frequency.