  The ground state of quantum Rabi model (QRM) exhibits rich nonclassical
states including squeezed state, cat state, and entangled state in different
parameter regimes. In this paper, we firstly use the polaron picture to figure
out the process of superradiant phase transition (SPT) in view of the
nonclassical states. According to the polaron wave functions, the squeezed
state extends to whole parameter regimes, and a small but non-zero weighted
antipolaron is necessary to form novel semi-cat states. Moreover, the behavior
of superradiance in the QRM can be viewed as the same displacement of the cat
states from zero to a finite value, while the ground state becomes entangled
state resulting from the emergence of spin-up state. On the other hand, the
nonclassical states can be intuitively characterized by the Wigner functions
with analytical expressions in the polaron picture, and the Wigner negativity
is also evaluated to measure the nonclassicality. Based on the squeezing and
superradiance, a classification of the coupling strength for the nonclassical
states containing in the ground state is presented, and the process of the SPT
is also revealed by the photon number distribution in Fock space, which is
important to understand the fundamental quantum physics in the QRM. Our work
provides a guidance for preparing the nonclassical states in experiments based
on the QRM.
