Light-intensity susceptibility and "active" noise spectroscopy
  In this paper, we consider informative potentialities of the "active" optical noise spectroscopy, under which we understand, generally, spectroscopy of response of a multilevel quantum system to the resonant optical field with its intensity modulated by "white" noise. We show that calculations of such a response can be most conveniently performed, in the linear approximation, by introducing the notion of light-intensity susceptibility (LIS) whose spectrum is determined by Laplace transform of the response to a small step-wise change of the optical field intensity. The results of calculations for a simple four-level quantum system show that its LIS spectrum may provide information not only about the ground-state structure (like conventional Faraday-rotation-based spin noise spectroscopy), but also about properties of the optical transitions (including nutation frequencies in the applied optical field). From the experimental point of view, such a noise spectroscopy of the intensity-related susceptibility can be especially efficient in combination with the up-to-date spectrum analyzers providing extremely fast data processing. 
Gleb G. Kozlov and Valerii S. Zapasskii
physics.optics physics.atom-ph