We study the diffusive propagation of multiply scattered light in an optically thick cloud of cold rubidium atoms illuminated by a quasiresonant laser beam. In the vicinity of a sharp atomic resonance, the energy transport velocity of the scattered light is almost 5 orders of magnitude smaller than the vacuum speed of light, reducing strongly the diffusion constant. We verify the theoretical prediction of a frequency-independent transport time around the resonance. We also observe the effect of the residual velocity of the atoms at long times.

We use coherent backscattering (CBS) of light by cold Strontium atoms to study the mutual coherence of light waves in the multiple scattering regime. As the probe light intensity is increased, the atomic optical transition starts to be saturated. Nonlinearities and inelastic scattering then occur. In our experiment, we observe a strongly reduced enhancement factor of the coherent backscattering cone when the intensity of the probe laser is increased, indicating a partial loss of coherence in multiple scattering.PACS numbers: 42.25.Fx, 32.80.Pj Wave coherence in the multiple scattering regime is a key ingredient to reveal the impact of interference on wave transport in strongly scattering media, with special interest in photonic crystals [1], random lasers [2], strong and weak localization [3,4]. It has been shown that wave coherence has some robust features which survive the spatial configuration average. A clean illustration is given by the coherent backscattering phenomenon (CBS) [5,6], a random two-wave zero path length interferometer. CBS manifests itself as an enhanced diffuse reflection peak in the backscattering direction. This signal is related to the Fourier transform of the configurationaveraged two-field correlation function (the mutual coherence) at two space-time points at the surface of the medium [7]. Hence the enhancement factor α (the peak to background ratio) provides a simple measure of spatial coherence properties of the disordered system after spatial averaging. For perfect contrast, α takes its maximal value of two, a symmetry property bearing on reciprocity [8]. In recent experiments, we have studied CBS in the elastic scattering regime with cold atomic vapours exposed to low intensity quasi-resonant monochromatic light. We have evidenced a loss of contrast due to the internal structure of atoms [9, 10, 11] and a full contrast restoration when non-degenerate atoms are used [12].Studying optical wave transport and localization effects with cold atoms offer several advantages. Indeed, they act as ideal point-like scatterers, where the scattering matrix can be fully described with ab initio calculations and no adjustable parameters. Moreover, the presence of sharp resonances results in large scattering cross-sections, easily tunable by few orders of magnitude, and large associated time scales, making resonant scattering systems different from non-resonant multiple scattering studied so far. If strong driving fields are used, atoms exhibit unusual scattering properties. First the atomic susceptibility shows up a dependence on the local field intensity. This non linearity alters both scattering (nonlinear reduction of the scattering cross-section) and propagation (generation of a nonlinear refractive index for the effective medium). Second, in addition to the usual elastic component, atoms radiate an inelastic spectrum component. For resonantly driven atoms with a non-degenerate groundstate, a characteristic frequency width of this spectrum is Γ (inverse of the excited-state lifetime) [13]. ...

We study light coherent transport in the weak localization regime using magneto-optically cooled strontium atoms. The coherent backscattering cone is measured in the four polarization channels using light resonant with a J(g) = 0-->J(e) = 1 transition of the strontium atom. We find an enhancement factor close to 2 in the helicity preserving channel, in agreement with theoretical predictions. This observation confirms the effect of internal structure as the key mechanism for the contrast reduction observed with a rubidium cold cloud [G. Labeyrie et al., Phys. Rev. Lett. 83, 5266 (1999)]. Experimental results are in good agreement with Monte Carlo simulations taking into account geometry effects.

The proposed mission "Space Atomic Gravity Explorer" (SAGE) has the scientific objective to investigate gravitational waves, dark matter, and other fundamental aspects of gravity as well as the connection between gravitational physics and quantum physics using new quantum sensors, namely, optical atomic clocks and atom interferometers based on ultracold strontium atoms.

The instabilities observed in the atomic cloud of a magneto-optical trap are experimentally studied through the dynamics of the center of mass location and the cloud population. Two dynamical components are identified: a slow, stochastic one affects both variables, and a fast, deterministic one affects only the center of mass location. A one-dimensional stochastic model taking into account the shadow effect is developed from these observations and reproduces the experimental behavior. It is shown that instabilities are driven by noise and present stochastic resonancelike characteristics.