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Disordered systems

A wave propagating in a thick disordered medium experiences a complicated process of multiple scattering and is rapidly turned into many partial waves which interfere, forming a speckle pattern (see the picture for a laser speckle). At first it was thought that the corresponding amplitudes had uncorrelated phases. As a consequence, even in the absence of any phase-breaking mechanism, disorder average was expected to wash out all interference effects, meaning that transport would ultimately be described by a diffusion process. As a matter of fact, this picture is now known to be incomplete, as certain interference processes survive such a disorder average, leading to deviations from the diffusive description of wave transport. Important examples include weak localization and universal conductance fluctuations in mesoscopic electronic systems, or coherent backscattering and intensity correlations in speckle patterns in the context of wave transport in disordered media. In strongly disordered systems, diffusive transport can even be completely suppressed and leave room to strong localization, a phenomenon discovered by Anderson in 1958 and since then under active theoretical and experimental investigation. The group is mainly interested in the behavior of cold atoms in the presence of optical random potentials, in particular in the presence of Anderson localization (see also our recent results on dynamical localization), as well as in the reverse problem of light scattering of photons by cold atoms. A recent direction of research also concerns the role of interactions and nonlinearities on quantum transport.

Principal investigators : Dominique Delande, Benoît Grémaud, Nicolas Cherroret

Ultracold atoms in random potentials

The study of ultracold atoms and in particular of coherent matter waves in the presence of random (optical) potentials has stirred considerable interest in the past few years. In these systems, stable disordered potentials can be conveniently tailored by laser light, interactions can be well controlled and dephasing processes like spontaneous emission can be easily handled. These are decisive advantages for studying coherent transport in disorder, especially with respect to solid-state (...)

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Light scattering by cold atoms

Light propagating in cold atomic media can undergo multiple scattering. The peculiarity of these systems is the strong "quantum" nature of the scatterers, which gives rise to original physical phenomena, like low-velocity diffusive transport, robust interference effects as coherent backscattering or fast coherent light emission. In certain conditions, the light excitation can also saturate atomic transitions, inducing a nonlinear dynamics that can dramatically affect the manifestation of (...)

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