Light transport in cold atoms: the fate of coherent backscattering in the weak localization regime

The recent observation of coherent backscattering (CBS) of light by atoms has emphasized the key role of the velocity spread and of the quantum internal structure of the atoms. Firstly, using highly resonant scatterers imposes very low temperatures of the disordered medium in order to keep the full contrast of the CBS interference. This criterion is usually achieved with standard laser cooling techniques. Secondly, a non trivial internal atomic structure leads to a dramatic decrease of the CBS contrast. Experiments with Rubidium atoms (with a non trivial internal structure) and with Strontium (with the simplest possible internal structure) show this behaviour and confirm theoretical calculations

Observation of coherent backscattering of light by cold atoms

Coherent backscattering (CBS) of light waves by a random medium is a signature of interference effects in multiple scattering. This effect has been studied in many systems ranging from white paint to biological tissues. Recently, we have observed CBS from a sample of laser-cooled atoms, a scattering medium with interesting new properties. In this paper we discuss various effects, which have to be taken into account for a quantitative study of coherent backscattering of light by cold atoms.Comment: 25 pages LaTex2e, 17 figures, submitted to J. Opt. B: Quant. Semicl. Op

Berry Phase of a Resonant State

We derive closed analytical expressions for the complex Berry phase of an open quantum system in a state which is a superposition of resonant states and evolves irreversibly due to the spontaneous decay of the metastable states. The codimension of an accidental degeneracy of resonances and the geometry of the energy hypersurfaces close to a crossing of resonances differ significantly from those of bound states. We discuss some of the consequences of these differences for the geometric phase factors, such as: Instead of a diabolical point singularity there is a continuous closed line of singularities formally equivalent to a continuous distribution of `magnetic' charge on a diabolical circle; different classes of topologically inequivalent non-trivial closed paths in parameter space, the topological invariant associated to the sum of the geometric phases, dilations of the wave function due to the imaginary part of the Berry phase and others.Comment: 28 pages Latex, three uuencoded postcript figure

A longitudinal Stern-Gerlach interferometer : the “beaded” atom

The principle on an atomic interferometer based on the longitudinal Stern-Gerlach effect is given. Possible realizations using beams of metastable rare gas atoms or metastable hydrogen atoms are described. Some examples of phase-objects are discussed and some possible applications are suggested. The atoms coming out of the interferometer exhibit uncommon properties, particulary a permanent multiple localisation regarding the external atomic variables (“beaded” atoms).On donne le principe d'un interféromètre atomique dans lequel est utilisé l'effet Stern-Gerlach longitudinal. Des réalisations possibles d'un tel interféromètre fonctionnant avec des atomes métastables de gaz rares ou d'hydrogène sont décrites. Quelques exemples d'objets de phase sont discutés, et quelques applications sont suggérées. A leur sortie de l'interféromètre, les atomes possèdent des propriétés inhabituelles, telles qu'une localisation multiple à l'égard des variables externes (atomes “ en chapelet ”)

Atomic quantum phase studies with a longitudinal Stern-Gerlach interferometer

A general description of atomic interferometers in terms of the scattering operator is given. The action of a longitudinal interferometer on an atom described by an incident polarized wavepacket is shown to be equivalent to a frame transformation, leading to a “beaded” atom. The special case of a pure longitudinal gradient of magnetic field and that of a spatially precessing field are examined. An experiment using metastable hydrogen atoms is presented and the results obtained in both situations mentioned above are discussed. Properties of beaded atoms produced by relatively strong fields ($\sim 10$ G) are investigated by means of the intensity diagram of their electrically induced radiative decay.On donne une description générale des interféromètres atomiques en termes d'opérateur de diffusion. On montre que l'action d'un interféromètre longitudinal sur un atome représenté par un paquet d'onde polarisé se ramène à une transformation de référentiel. On examine le cas particulier d'un champ magnétique à gradient longitudinal et celui d'un champ précessant spatialement. Une réalisation expérimentale utilisant des atomes métastables d'hydrogène est ensuite présentée et les résultats obtenus dans les deux situations précédentes sont discutés. On étudie enfin les propriétés des atomes “en chalelet” produits par des champs de quelques 10 G, à partir du rayonnement induit par un champ électrique

Angular correlation measurements in a thermal beam of H∗^* (2s) atoms using a Stern-Gerlach atomic axicon

The effect of transverse magnetic gradients in Stern-Gerlach atom interferometry is to make interfere plane waves the momenta of which differ in their directions. As a result the contrast of the interference pattern produced by the longitudinal gradient is attenuated by an angular auto-correlation function in the momentum space. This effect is studied experimentally on a thermal beam of metastable H$^*$ (2s) atoms, with a radial transverse gradient (atomic “axicon”).L'effet de gradients magnétiques transverses en interférométrie atomique de type Stern-Gerlach est de faire interférer des ondes planes ayant initialement des vecteurs d'onde différant par leurs directions. Il en résulte que, dans le signal d'interférences induit par le gradient longitudinal, le contraste est atténué par une fonction d'autocorrélation angulaire. Cet effet est étudié expérimentalement sur un jet thermique d'atomes métastables H$^*$ (2s), dans le cas d'un gradient transverse radial (“axicon” atomique)