Quantum Criticality from Spectral Collapse in the Two-Photon Rabi Model

Abstract:Spectral collapse in the two-photon quantum Rabi model (tpQRM) has long been regarded as insufficient to establish quantum criticality because, under generic conditions, the lowest excitation gap remains finite. We show that, at a special qubit frequency, spectral collapse in the anisotropic tpQRM constitutes a continuous quantum phase transition governed by a single soft mode. The same-parity gap, set by the vanishing effective oscillator frequency, closes with exponent $z\nu = 1/2$, whereas the lower different-parity gap arises from symmetry-induced splitting. Parity symmetry excludes the different-parity excitation from the quantum Fisher information response and restricts Kibble-Zurek dynamics to excitations within the ground-state parity sector. Together, these results identify the same-parity excitation as the soft mode and its gap as the characteristic energy scale. The corresponding critical exponents, $\nu=1/4$ and $z=2$, place the anisotropic tpQRM in the same universality class as the standard QRM. Our results establish spectral collapse as a potential route to experimentally accessible quantum criticality in a few-body system and show that universality is determined by the soft-mode structure rather than by the closing of the lowest excitation gap.

arXiv:2604.23164 [quant-ph]

(or arXiv:2604.23164v2 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2604.23164 Focus to learn more arXiv-issued DOI via DataCite

Phys. Rev. Lett. 137, 123602 (2026)

https://doi.org/10.1103/ctp9-k77q Focus to learn more DOI(s) linking to related resources

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