Author(s): Jiong Li, Jun-Ling Wang, Qing-Hu Chen, and Hai-Qing Lin 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 c… [Phys. Rev. Lett. 137, 123602] Published Wed Sep 16, 2026