Survival of the fittest in the coherent evolution of quantum ensembles

We report two novel effects in an ensemble of two-level systems driven by an external field. First, we observe a frequency rigidity where the dominant Rabi oscillation frequency does not change with detuning, in contrast to the well-known law according to which the Rabi frequency grows with increasing detuning of the driving field. Second, we observe a time-dependent frequency shift of the ensemble-averaged oscillation, as some frequency components decay faster than others. Hence, coherence emerges from long-lived oscillations in an inhomogeneous ensemble. Our general model shows that these effects are related to a surprising double peak structure in the frequency spectrum of the generalized Rabi frequency. 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, and set stringent limits for applications requiring accurate coherent quantum manipulation.