Non-Gaussian Phase Transition and Cascade of Instabilities in the Dissipative Quantum Rabi Model

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 lea...

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Bibliographic Details
Main Authors Kang, Mingyu, Zhang, Yikang, Brown, Kenneth R, Barthel, Thomas
Format Journal Article
LanguageEnglish
Published 09.07.2025
Subjects
Physics - Atomic Physics
Physics - Quantum Physics
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Summary: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.
DOI:10.48550/arxiv.2507.07092