Coherent backscattering of light by saturated atoms

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The present project aims at the theoretical study of coherent backscattering (CBS) of intense laser light by cold atoms. It is important to understand how quantum interference, crucial for the emergence of this phenomenon, can be affected by fundamental quantum mechanical dephasing mechanisms - such as photon spin flips or inelastic scattering. This has important consequences for the transition from weak to strong, or Anderson, localization, of light within cold atomic gases, as well as for potential technological applications, such as random lasers. Previous studies explored the dependence of the underlying interference effect on the degeneracy of the atomic dipole transitions, on residual thermal motion of the atoms, or on inelastic scattering. Concerning the latter mechanism, many fundamental questions are still unresolved, such as how higher-order scattering contributions are affected by nonlinear saturation effects, or whether CBS leaves a detectable trace in the photocount statistics. Furthermore, the combined impact of inelastic scattering together with either one of the two other dephasing mechanisms (i.e., degeneracy and thermal motion) has so far not been considered theoretically, although there are already experimental results to be matched with. To address these questions, we shall develop a non-perturbative description of the atoms-laser interaction, combining numerical approaches based on master equations for a small number of atoms with new analytical tools for treating disordered samples of many atoms. Answering them will be an important step towards the ultimate goal of this project - a theory for multiple scattering of intense laser light in cold atomic gases.