Exceptional Eigenvalues and Braak’s Conjecture in the Quantum Rabi Model

The spectrum of the quantum Rabi model can be separated as regular eigenvalues that need to be computed numerically and exceptional eigenvalues, that match the energies of a shifted quantum harmonic oscillator. Exceptional values can be separated further as Juddean, if they occur under certain algebraic conditions, and non-Juddean, if they obey more elusive transcendental conditions. In this paper (arXiv:2503.15572), we show that simple assumptions on these conditions imply and extend Braak’s conjecture on the distribution of the quantum Rabi spectrum.

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