2025/07/09 by Kang, Mingyu, Zhang, Yikang, Brown, Kenneth R. +1
#Atomic Physics (physics.atom-ph) #FOS: Physical sciences #Quantum Physics (quant-ph)
paper · doi:10.48550/arxiv.2507.07092
The open quantum Rabi model describes a two-level system coupled to a harmonic oscillator. A Gaussian phase transition for the nonequilibrium steady states has been predicted when the bosonic mode is soft and subject to damping. We show that oscillator dephasing is a relevant perturbation, which leads to a non-Gaussian phase transition and an intriguing cascade of instabilities for k-th order bosonic operators. For the soft-mode limit, the equations of motion form a closed hierarchy and spectral properties can be efficiently studied. To this purpose, we establish a fruitful connection to non-Hermitian Hamiltonians. The results for the phase diagram, stability boundaries, and relevant observables are based on mean-field analysis, exact diagonalization, perturbation theory, and Keldysh field theory.