  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, and 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.
