FREQUENCY-MODULATED EXCITATION OF A 2-LEVEL ATOM

We present a detailed experimental study of the frequency-modulated ex citation of a two-level atom, using a microwave herd to drive transiti ons between two Rydberg Stark states of potassium. Zn the absence of a modulation the interaction is the standard model of the Rabi problem, producing sinusoidal oscillations of the population between the two s tates. In the presence of a frequency modulation of the interacting fi eld, however the time evolution of the system is significantly modifie d, producing square wave oscillations of the population, sinusoidal os cillations at a different frequency, or even sinusoidal oscillations b uilt up in a series of stair steps. The three responses described abov e are each found in a different regime for the frequency of the modula tion with respect to the unmodulated Rabi frequency: the low-, high-, and intermediate-frequency regimes, respectively.