Transformation of a Rabi oscillation spectrum for a two-level system in a strong resonant field

We have found an analytical expression describing the evolution of a two-level system in a strong resonant quantum field beyond the rotating wave approximation. The solution obtained has allowed us for the first time to identify the qualitative features of the behavior of the system in this case: suppression of the "collapse–revival" effect, and a change in the spectrum and shape of Rabi oscillations for an inverted population. The results are of interest for applied spectroscopy, quantum optics, and nanomechanics.