Phase driven exact and unconventional superradiance phase transition in non-Hermitian cascaded Rabi cavities

This work reports the phase driven symmetry breaking and exact and unconventional superradiance phase transition in the non-Hermitian cascaded Rabi cavities. The non-Hermiticity is introduced in the coupling phase (denoted by φ) between the atom and the optical field. The exactness refers to the fact that the superradiance phase boundary is obtained analytically and verified by the observables. The unconventionality is reflected in that when |φ|=π/4 or |φ|=3π/4, the phase boundary is uniquely determined by J=1/2 (where J is the dimensionless cavity coupling strength) and is independent of the atom-optical field coupling strength g. For other φ, the phase boundary is determined by J and the dimensionless atom-optical field coupling strength g together. Besides, we find that there are phase driven first-order and second-order superradiance phase transitions, and the quantum criticality for the second-order superradiance phase transition is studied. In addition, the experimental feasibility is discussed. This work will stimulate the studies of non-Hermitian superradiance quantum phase transitions and their experimental realizations, as well as the underlying universality class of phase transitions.