  The Rabi model plays a fundamental role in understanding light-matter
interaction. It reduces to the Jaynes-Cummings model via the rotating-wave
approximation, which is applicable only to the cases of near resonance and weak
coupling. However, recent experimental breakthroughs in upgrading light-matter
coupling order require understanding the physics of the full quantum Rabi model
(QRM). Despite the fact that its integrability and energy spectra have been
exactly obtained, the challenge to formulate an exact wavefunction in a general
case still hinders physical exploration of the QRM. Here we unveil a
ground-state phase diagram of the QRM, consisting of a quadpolaron and a
bipolaron as well as their changeover in the weak-, strong- and
intermediate-coupling regimes, respectively. An unexpected overweighted
antipolaron is revealed in the quadpolaron state, and a hidden scaling behavior
relevant to symmetry breaking is found in the bipolaron state. An
experimentally accessible parameter is proposed to test these states, which
might provide novel insights into the nature of the light-matter interaction
for all regimes of the coupling strengths.
