Strong field effects in the evolution of a two-level system

Evolution of a two-level system (TLS) in a strong resonant quantum field is considered beyond the rotating wave approximation (RWA). An analytical expression for the atomic population inversion that approximates numerical results with high accuracy is deduced. It allows us to describe new qualitative peculiarities of the TLS evolution in the strong field: i) suppression of the 'collapse-revival' effect, and ii) change of the spectrum and form of the population oscillations.