Abstract
Ultrastrong coupling (USC) in the quantum Rabi model, characterized by the breakdown of the rotating-wave approximation (RWA), has emerged as a topic of considerable interest and study. This critical reevaluation of the validity of the RWA concludes that the accepted definition of USC in terms of a fixed ratio of coupling to field frequency is inadequate. Connecting an improved spectral validity criterion with the derivation of the semiclassical limit suggests that the dynamical validity of the quantum RWA should be linked to that of the corresponding semiclassical model. This, however, is not supported by numerical calculations of coherent-state dynamics, which unambiguously demonstrate that spectral validity does not imply dynamical validity and reveal surprisingly complicated dependence on coupling and field amplitude.
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