Light mesoscopy in a disordered sample of cold atoms:
Coherent backscattering under a magnetic field

Olivier Sigwarth

O.Sigwarth, G.Labeyrie, T. Jonckheere, D.Delande, R.Kaiser and Ch. Miniatura
Universite Paris 6, Ecole Normale Superieure, Laboratoire Kastler-Brossel, case 74, 4, Place Jussieu, 75005 Paris, France

Coherent backscattering (CBS) is a paradigmatic example of interference effects surviving the configuration averaging in wave transport in disordered media. For light waves, it manifests itself as an enhanced diffuse reflection around exact backscattering due to the constructive interference between reversed light paths in the sample. By invoking the time-reversal symmetry for point-dipole scatterers, a perfect interference contrast is predicted and observed in the so-called helicity-preserving channel. However, this prediction is violated for atoms with a degenerate internal groundstate structure as evidenced by experiments with cold rubidium atoms (G. Labeyrie et al., PRL 83, 5266 (1999)). This is similar to the spin-flip scattering dephasing mechanism which alters interference effects in electron transport in solid-state samples. This contrast reduction can be associated with the existence of a coherence length in scattering. Adding a magnetic field however allows to split the atomic internal structure (Zeeman effect) and to design effective two-level atomic systems. This implies that full multiple interference contrast is restored or, equivalently, that the coherence length is increased in (seemingly) contradiction with the usual intuition (a magnetic field breaks the time-reversal invariance). The poster will present our most recent experimental and theoretical results in this field (O. Sigwarth et al., to appear in PRL (2004)). This work may offer an interesting route in the search of strong localization of light in cold atomic gases.