Spectral Determinant of the Two‐Photon Quantum Rabi Model

The various generalized spectral determinants ( G ‐functions) of the two‐photon quantum Rabi model are analyzed with emphasis on the qualitative aspects of the regular spectrum. Whereas all of them yield at least a subset of the exact regular eigenvalues, only the G ‐function proposed by Chen et al. in 2012 exhibits an explicitly known pole structure which dictates the approach to the collapse point. This function is derived rigorously employing the ‐symmetry of the model and shown that its zeros correspond to the complete regular spectrum.