Light propagating in an optically thick sample experiences multiple scattering. It is now known that interferences alter this propagation, leading to an enhanced backscattering, a manifestation of weak localization of light in such diffuse samples. This phenomenon has been extensively studied with classical scatterers. In this letter we report the first experimental evidence for coherent backscattering of light in a laser-cooled gas of Rubidium atoms.Comment: 4 pages REVTEX, 1 page color image GIF, accepted for publication in Phys. Rev. Let

We present an investigation of the fast decompression of a three-dimensional (3D) Bose-Einstein condensate (BEC) at finite temperature using an engineered trajectory for the harmonic trapping potential. Taking advantage of the scaling invariance properties of the time-dependent Gross-Pitaevskii equation, we exhibit a solution yielding a final state identical to that obtained through a perfectly adiabatic transformation, in a much shorter time. Experimentally, we perform a large trap decompression and displacement within a time comparable to the final radial trapping period. By simultaneously monitoring the BEC and the non-condensed fraction, we demonstrate that our specific trap trajectory is valid both for a quantum interacting many-body system and a classical ensemble of non-interacting particles.

We have measured the attractive van der Waals force between a dielectric wall and an atom in its ground state. The method is a direct force measurement in which we use an evanescent wave atomic mirror to balance the van der Waals force and the inertia of the incident atom. [S0031-9007(96) PACS numbers: 42.50.Vk, For many years, the van der Waals interaction between a ground state atom and a wall-dielectric or conductor-has attracted a lot of theoretical attention. Even the simple Lennard-Jones model [1] based on the electrostatic interaction between the atomic dipole and its image involves the quantum fluctuations of the atomic dipole. It was recognized by Casimir and Polder [2] that when the atom-wall distance z is not small compared to the wavelengths of the dominant atomic transitions, the z 23 law associated with the instantaneous electrostatic interaction is no longer valid. The full quantum treatment of the van der Waals attraction, leading to the famous long distance z 24 law, is a fundamental QED problem [3] involving the quantized electromagnetic field and retardation effects. The van der Waals energy shift can be considered a modification of the Lamb shift resulting from the modification of the density of modes of the electromagnetic field due to the presence of the wall. In the case of a dielectric wall, this density must take into account not only modes associated with traveling waves incident on, and reflected from, the vacuum-dielectric interface, but also evanescent waves [4,5].In contrast to the theoretical work, few experimental results have been reported on the van der Waals interaction between an atom and a wall. The pioneering experiments of Ref.[6] studied the deflection of thermal atomic beam by a sharp metal or dielectric edge. The observed effect was extremely weak, because only a very small fraction of the atoms passed close enough to the interface to have an interaction energy comparable to their kinetic energy. Qualitative trends in agreement with the z 23 law were observed, but no precise quantitative comparison was possible. Recently, more precise data on the van der Waals interaction between an atom and a metal has been obtained by spectroscopic studies of Rydberg atoms in a micron-sized parallel-plate metallic cavity [7]. A study of the transmission of ground state atoms through a similar cavity also permitted the measurement of the Casimir Polder force on an atom in its ground state [8]. Another series of spectroscopic measurements on light reflected from the wall of a cell containing an atomic vapor has given information on the difference between the van der Waals shifts of various atomic levels, and interesting re-sults have been obtained on the role of the frequency dependence of the dielectric constant of the wall [9].In this paper, we report on new mechanical measurements for determining the van der Waals interaction between a ground state atom and a dielectric wall. The idea, first used in Ref.[10], is to release laser cooled atoms with a well-defined kinetic energy onto an ...

The rapidly developing field of optomechanics aims at the com- bined control of optical and mechanical modes1–3. In cold atoms, the spontaneous emergence of spatial structures due to opto- mechanical back-action has been observed in one dimension in optical cavities3–8 or highly anisotropic samples9. Extensions to higher dimensions that aim to exploit multimode configurations have been suggested theoretically10–16. Here, we describe a simple experiment with many spatial degrees of freedom, in which two continuous symmetries—rotation and translation in the plane orthogonal to a pump beam axis—are spontaneously broken. We observe the simultaneous long- range spatial structuring (with hexagonal symmetry) of the density of a cold atomic cloud and of the pump optical field, with adjustable length scale. Being based on coherent phenom- ena (diffraction and the dipole force), this scheme can poten- tially be extended to quantum degenerate gases

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 study, experimentally and theoretically, the controlled transfer of harmonically trapped ultracold gases between different quantum states. In particular we experimentally demonstrate a fast decompression and displacement of both a noninteracting gas and an interacting Bose-Einstein condensate which are initially at equilibrium. The decompression parameters are engineered such that the final state is identical to that obtained after a perfectly adiabatic transformation despite the fact that the fast decompression is performed in the strongly non-adiabatic regime. During the transfer the atomic sample goes through strongly out-of-equilibrium states while the external confinement is modified until the system reaches the desired stationary state. The scheme is theoretically based on the invariants of motion and scaling equations techniques and can be generalized to decompression trajectories including an arbitrary deformation of the trap. It is also directly applicable to arbitrary initial non-equilibrium states.

We demonstrate a technique based on invariants of motion for a time-dependent Hamiltonian, allowing a fast transition to a final state identical in theory to that obtained through a perfectly adiabatic transformation. This method is experimentally applied to the fast decompression of an ultracold cloud of Rubidium 87 atoms held in a harmonic magnetic trap, in the presence of gravity. We are able to decompress the trap by a factor of 15 within 35 ms with a strong suppression of the sloshing and breathing modes induced by the large vertical displacement and curvature reduction of the trap. When compared to a standard linear decompression, we achieve a gain of a factor of 37 on the transition time.Comment: 5 pages, 4 figures, an error in Eq. (2) has been correcte

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.