Weak localization of light by cold atoms: The impact of quantum internal structure - art. no. 053804

Since the work of Anderson on localization, interference effects for the pr opagation of a wave in the presence of disorder have been extensively studi ed, as exemplified in coherent backscattering (CBS) of light. In the multip le scattering of light by a disordered sample of thermal atoms, interferenc e effects are usually washed out by the fast atomic motion. This is no long er true for cold atoms where CBS has recently been observed. However, the i nternal structure of the atoms strongly influences the interference propert ies. In this paper, we consider light scattering by an atomic dipole transi tion with arbitrary degeneracy and study its impact on coherent backscatter ing. We show that the interference contrast is strongly reduced. Assuming a uniform statistical distribution over internal degrees of freedom, we comp ute analytically the single- and double-scattering contributions to the int ensity in the weak-localization regime. The so-called ladder and crossed di agrams are generalized to the case of atoms and permit to calculate enhance ment factors and backscattering intensity profiles for polarized light and any closed atomic dipole transition.