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arXiv · 2507.07092

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

Mingyu Kang·Yikang Zhang·Kenneth R. Brown·Thomas Barthel
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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 $k$-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.

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Mingyu Kang, Yikang Zhang, Kenneth R. Brown, Thomas Barthel. 2025-07-09. Non-Gaussian Phase Transition and Cascade of Instabilities in the Dissipative Quantum Rabi Model. https://doi.org/10.1103/q6xb-8t1j

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