A quantum stochastic approach to the spectrum of a two-level atom
  By means of quantum stochastic calculus we construct a model for an atom with two degenerate levels and stimulated by a laser and we compute its fluorescence spectrum; let us stress that, once the model for the unitary atom-field dynamics has been given, then the spectrum is computed without further approximations. If only the absorption/emission term is included in the interaction, we reobtain the Mollow spectrum in the case of a monochromatic laser and the Kimble-Mandel spectrum in the case of a "phase diffusion model" for a non monochromatic laser. However, our model can describe also another type of light scattering, a "direct scattering" due to the response of the atom as a whole, which we expect to be small, but which interferes with the scattering due to the absorption/emission channel. When both the scattering channels are introduced we obtain a modification of the Mollow-Kimble-Mandel spectrum, which shares the main features with the usual case, but which presents some asymmetries even in the case of no detuning. 
Alberto Barchielli and Nicola Pero