  We investigate the quantum phase transition in the dissipative anisotropic
quantum Rabi model in the framework of quantum dressed master equation. From
perspectives of both numerical and analytical analysis, we unravel the
implication of the giant photon-bunching feature on the first-order quantum
phase transition. The observed two-photon statistics can be well described
analytically within a few lowest eigenstates at the low temperature. Moreover,
such significant photon-bunching peak is generally exhibited at the deep-strong
qubit-photon coupling, which is however lacking in the dissipative isotropic
quantum Rabi model. Therefore, we suggest that the photon-bunching measurement
is helpful to characterize the first-order QPT of the qubit-photon hybrid
systems.
