  We report two novel effects in an inhomogeneous ensemble of two-level systems
driven by an external field. First, we observe a rigidity of the oscillation
frequency: the dominant Rabi oscillation frequency does not change with the
frequency of the driving field, in contrast to the well-known law of Rabi
frequency increase with growing detuning of the driving field. Second, we
observe a time-dependent frequency shift of the ensemble-averaged oscillation.
We show that these effects follow from the inhomogeneity of the two-level
splitting across the ensemble, allowing for a distribution of local
oscillations in which those with high frequencies interfere destructively and
decay faster than those with a low frequency, which are the only to survive in
the output signal. Hence, coherence emerges from long-lived oscillations in an
inhomogeneous ensemble. We analyze the Fourier spectrum of the time-dependent
oscillation signal and find a non-trivial spectral structure that is double
peaked for certain parameters. We show that the effects observed in alkali
vapor are universal and expected in any system with a moderate inhomogeneity
driven by an external field.
