Classical and quantum mechanical resonance fluorescence

We study resonance fluorescence from a two-level atom illuminated by coherent and incoherent light. Especially, we treat the case of an intense incoherent component which is broad band and chaotic in character.New insights into the phenomenon of resonance fluorescence are obtained by constructing certain analogies with the precession of a classical (Bloch) vector around a classical stochastic field. The analogies are based on a representation of the density operator of the two-level atoms as a diagonal mixture of directed angular momentum states.As long as the whole light field is an imposed one the weight function of the mixture mentioned above describes a random sequence of rotations of the Bloch vector and obeys a simple Fokker Planck equation. If, however, the incoherent component of the light field acts as a zero- or finite temperature heat bath, the equation of motion for the weight function is no longer a Fokker Planck equation. Nontheless, we find the exact solution and calculate the correlation functions relevant to a discussion of the spectrum and of antibunching effects.