Journée de Dynamique Non Linéaire (dans le cadre des Rencontres Nice-Marseille)

10h30   Robin Kaiser (Laboratoire Ondes et Désordre, Sophia-Antipolis)
Mesoscopic Quantum Physics: Coherent Backscattering of Light by Cold Atoms

11h30   Ricardo Lima (Centre de Physique Théorique)
Dynamique de réseaux génétiques

14h30   Débats - discussions

Résumé du séminaire de R. Kaiser

The propagation of a wave in a disordered medium can be seen as a random walk inside the medium, and a diffusion picture seems appropriate. Neglecting all interference phenomena, one predicts a total transmission of the medium inversely proportional to sample thickness (Ohm's law). However, interferences may have dramatic consequences, such as a vanishing diffusion constant: in this situation, the medium behaves like an insulator (strong or Anderson localization) and its total transmission decreases exponentially with the sample's thickness. This prediction has triggered a renewal of interest for the study of multiple scattering, leading to experiments on strong localization of microwaves and light. A more accessible experimental situation is the so-called weak localization regime, where interferences already hamper the diffusion process. Coherent backscattering (CBS) is a spectacular manifestation of interference effects in this multiple scattering regime, yielding an enhanced scattered intensity around the direction of backscattering. This phenomenon has been observed in a variety of systems ranging from white paint to biological tissues. Recently, we observed resonant coherent backscattering of light from a sample of laser-cooled atoms[1-5]. Indeed, multiple scattering of light is known to exist in such samples since it eventually limits the atomic density achievable in magneto-optical traps (MOTs). Direct manifestations of multiple scattering of light in cold atoms such as ''radiation trapping'' had already been observed, but our experiments now allow to probe the interference effects in this situation. In this respect, CBS is a powerful tool to study the properties of light scattered by cold atoms. Indeed, we observed some striking differences with what is reported in the literature for classical samples. Here we present recent results on CBS by our samples of laser cooled Rb and Sr atoms. We present various effects (polarization of the light, the internal structure of the atoms, the magnetic field), which illustrate the uniqueness of resonant coherent backscattering of light by cold atoms. Due to the specific features of our laser cooled samples (large optical thickness), we have also observed interesting features such as spontaneous formation of cloud instabilites. We will present experimental results on these experiments and some speculations on the mecanisms involved which are not yet understood.

[1] Coherent backscattering of light by cold atoms, G. Labeyrie, F. de Tomasi, J.-C. Bernard, C.A. Müller, Ch. Miniatura and R. Kaiser, Phys. Rev. Lett. 83, 5266 (1999).
[2] Multiple scattering of light by atoms in the weak localization regime, T. Jonckheere, C.A. Müller, R. Kaiser, Ch. Miniatura and D. Delande, Phys. Rev. Lett. 85, 4269 (2000).
[3] Coherent light transport in a cold Strontium cloud, Y. Bidel, B. Klappauf, J.C. Bernard, D. Delande, G. Labeyrie, C. Miniatura, D. Wilkowski, R. Kaiser, Phys. Rev. Lett. 88, 203902 (2002).
[4] Hanle effect in coherent backscattering, G. Labeyrie, C. Miniatura, C. A. Müller, O. Sigwarth, D. Delande, R. Kaiser, Phys. Rev. Lett. 89, 163901 (2002).
[5] http://www-lod.inln.cnrs.fr/

Résumé du séminaire de R. Lima

Apres une tres breve introduction, je decrirais un modele dynamique a temps discret que nous proposons pour l'etude des reseaux d'interaction genetique ainsi que les questions naturelles et les quelques reponses que l'on peut y apporter des maintenant. Le fait qu'il s'agisse d'un systeme d'agents sur un graphe (dont les fleches representent les interactions) peut rendre ce type de modele interessant dans d'autres contextes.