Noise spectroscopy of randomly modulated two-level systems

Abstract

Summary form only given. The effect of environmental perturbations on a system of two-level atoms is to alter the coupling between the atoms and their surroundings. This manifests itself as a change in the relaxation rates of the atoms. One of the ways in which environmental fluctuations can affect an atom is to modulate its transition frequency in a stochastic manner. We study the response of such two-level atoms to irradiation by a monochromatic laser, and report the results of numerical experiments on various spectral properties, viz. the spectrum of scattered light, four-wave mixing, and fluctuations in fluorescence emission intensity. The spectrum of scattered light is a sensitive indicator of the parameters that describe the stochastic modulation. In addition, the spectra exhibit substantially different features depending on whether the stochastic process follows a diffusion or a jump-like mechanism. We also consider the modifications that are induced in four-wave mixing signals when the transition frequency of the atoms is stochastically modulated and describe how one can differentiate between Markov and non-Markov fluctuations in the transition frequency of the two-level-system through characteristics of the four-wave mixing signal. Lastly, we demonstrate that by using fluctuations in population inversion, i.e. the emission intensity, as the spectroscopic variable, one also can extract information on whether random modulation of the chromophores follows a diffusion process or a jump-like process. The population fluctuations discriminate between Markov and non-Markov fluctuations of the atomic frequency as well. In addition we show that the population inversion allows one to unambiguously determine, in certain regimes, whether the population fluctuations are due to atomic frequency noise or due to noise in the irradiating laser field. The work described here is based on a Monte-Carlo procedure, which can be applied to stochastic processes in a wide variety of biological, chemical, and physical processes.