Non-Gaussian Phase Transition and Cascade of Instabilities in the Dissipative Quantum Rabi Model
Abstract
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 -th order bosonic operators, as well as a jump in the steady-state qubit polarization. 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.
Keywords
Cite
@article{arxiv.2507.07092,
title = {Non-Gaussian Phase Transition and Cascade of Instabilities in the Dissipative Quantum Rabi Model},
author = {Mingyu Kang and Yikang Zhang and Kenneth R. Brown and Thomas Barthel},
journal= {arXiv preprint arXiv:2507.07092},
year = {2026}
}
Comments
5 pages main text, 13 pages appendix, 8 figures; additional appendices on interpretation in terms of Wigner function dynamics, Fock distributions, and convergence of data in the number of retained oscillator levels; minor improvements