Generalized coherent-squeezed-state expansion for the super-radiant phase transition
Xiang-You Chen, Yuyu Zhang, Libin Fu, Hang Zheng
Abstract
We develop a coherent-squeezed-state expansion for the ground state of the quantum phase transition in the quantum Rabi model. The wave function undergoes a transition from a localized state in the normal phase to a two-peaks delocalized state in the super-radiant phase. Interestingly, the two displaced wave packets are squeezed in the momentum space, and overlap strongly to form a correlated state. We develop a superposition of coherent-squeezed states to describe the phase transition correctly, which includes the higher-order excitations at the critical value. The corresponding ground-state energy and the average photon number are in agreement with numerical ones, exhibiting an improvement over previous coherent-state expansions. Moreover, our method successfully captures the reduced quantum fluctuations in momentum variable in the phase transition.