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At sufficiently high intensity, an atom responds nonlinearly to incoming radiation. On the one hand, the atomic transition gets "saturated", i.e. the population of the excited state approaches its limit value ½. At the same time, saturation also implies inelastic scattering, with energy being redistributed among subsequently scattered photons. At lowest order, these nonlinear effects, which may significantly affect coherence between multiply scattered light waves, originate from two-photon scattering.
After a brief reminder of some known results for single atoms, I will discuss scattering of two photons by two distant atoms in free space - the simplest system exhibiting coherent backscattering. The coherence between the doubly scattered light emitted by both atoms is shown to be reduced by inelastic scattering processes. In analogy with the double slit experiment, the coherence loss can be explained by a which-path argument. In the final part of the talk, I will address the case of a finite, disordered atomic sample, and will outline future perspectives of this work. |