Spectrum of Two-Level Systems with Discrete Frequency Fluctuations

We study, theoretically and experimentally, an ensemble of two-level systems coupled to an environment which induces random jumps in their resonant frequency. We present a closed-form formula for the spectrum in terms of the resonant frequency distribution and the Poisson rate constant. For a normal distribution the spectrum deviates from a generalized Gumbel function, a well-known result for continuous stochastic Gaussian processes. We perform experiments with optically trapped cold 87Rb atoms and show that the predictions of our theory for a 3D harmonic trap match the measured spectra without fitting parameters.