Localisation faible de la lumière dans un gaz d´atomes froids

Recent experimental results show that the interference contrast
observed in coherent backscattering (CBS) of light by cold atoms is
drastically reduced with respect to classical disordered media. In
the present theoretical contribution, we study the impact of the
degeneracy of the atomic dipole transition on weak localisation of
light. The non-scalar components of the atomic scattering operator
are characteristic of the internal structure, and strongly modify
the interference properties of multiple light scattering. A
systematic analysis in terms of irreducible tensors permits to
calculate exact analytical expressions for the single and double
scattering contributions to the CBS signal for arbitrarily
degenerate atomic dipole transitions. Furthermore, we sum up the
series of ladder and crossed diagrams that describe the average
scattered light intensity and the weak localisation corrections,
respectively. We find that the degeneracy of the atomic transition
has negligeable impact on the average light amplitude, small impact
on the average intensity, but decisive impact on the interference
corrections. The internal degrees of freedom very effectively
reduce the interference leading to weak localisation and,
therefore, the CBS signal for any degenerate atomic dipole
transition.