Quantum criticality from spectral collapse in the two-photon Rabi model

• URL: http://arxiv.org/abs/2604.23164v1
• Date: Sat, 25 Apr 2026 06:31:02 GMT
• Title: Quantum criticality from spectral collapse in the two-photon Rabi model
• Abstract summary: Spectral collapse in the two-photon quantum Rabi model (tpQRM) has long been regarded as incompatible with quantum criticality.<n>We show that, in the anisotropic tpQRM, spectral collapse constitutes a genuine continuous quantum phase transition governed by a single soft mode.
• Score: 2.274915755738124
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Spectral collapse in the two-photon quantum Rabi model (tpQRM) has long been regarded as incompatible with quantum criticality due to the absence of a vanishing excitation gap. We show that, in the anisotropic tpQRM, spectral collapse constitutes a genuine continuous quantum phase transition governed by a single soft mode. The excitation gap within the same parity closes as $ε_{sp} \sim |g - g_c|^{zν}$ with $zν= 1/2$, placing the system in the same universality class as the standard QRM, while the gap between different parities reflects symmetry-induced level splitting rather than a critical excitation. This soft mode defines a unique energy scale that controls both equilibrium and nonequilibrium properties, including macroscopic observables, quantum Fisher information, and Kibble-Zurek dynamics. These results establish spectral collapse as an experimentally accessible realization of quantum criticality in a few-body system and demonstrate that universality is fully determined by the soft-mode structure rather than by microscopic details.

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