Spectral collapse—once thought impossible—now signals quantum criticality

Jiong Li, Jun-Ling Wang, Qing-Hu Chen, and Hai-Qing Lin of Zhejiang University report that spectral collapse, long considered incapable of establishing quantum criticality due to a persisting lowest excitation gap, can indeed signal a continuous quantum phase transition. Their work demonstrates this occurs at a specific qubit frequency within the anisotropic two-photon quantum Rabi model, governed by a single soft mode. The researchers found the same-parity gap, set by the vanishing effective oscillator frequency, closes with an exponent of zν = 1/2, placing the system in the same universality class as the standard quantum Rabi model and establishing spectral collapse as a potential path to experimentally accessible quantum criticality. Anisotropic interactions…