20 research outputs found
Creation and counting of defects in a temperature quenched Bose-Einstein Condensate
We study the spontaneous formation of defects in the order parameter of a
trapped ultracold bosonic gas while crossing the critical temperature for
Bose-Einstein Condensation (BEC) at different rates. The system has the shape
of an elongated ellipsoid, whose transverse width can be varied to explore
dimensionality effects. For slow enough temperature quenches we find a
power-law scaling of the average defect number with the quench rate, as
predicted by the Kibble-Zurek mechanism. A breakdown of such a scaling is found
for fast quenches, leading to a saturation of the average defect number. We
suggest an explanation for this saturation in terms of the mutual interactions
among defects.Comment: 9 pages, 10 figure
Emergence of coherence in a uniform quasi-two-dimensional Bose gas
Phase transitions are ubiquitous in our three-dimensional world. By contrast
most conventional transitions do not occur in infinite uniform two-dimensional
systems because of the increased role of thermal fluctuations. Here we explore
the dimensional crossover of Bose-Einstein condensation (BEC) for a weakly
interacting atomic gas confined in a novel quasi-two-dimensional geometry, with
a flat in-plane trap bottom. We detect the onset of an extended phase
coherence, using velocity distribution measurements and matter-wave
interferometry. We relate this coherence to the transverse condensation
phenomenon, in which a significant fraction of atoms accumulate in the ground
state of the motion perpendicular to the atom plane. We also investigate the
dynamical aspects of the transition through the detection of topological
defects that are nucleated in a quench cooling of the gas, and we compare our
results to the predictions of the Kibble-Zurek theory for the conventional BEC
second-order phase transition.Comment: main text = 24 pages, 6 figures + supplementary material = 10 pages,
5 figure
Compensation of Beer-Lambert attenuation using non-diffracting Bessel beams
We report on a versatile method to compensate the linear attenuation in a
medium, independently of its microscopic origin. The method exploits
diffraction-limited Bessel beams and tailored on-axis intensity profiles which
are generated using a phase-only spatial light modulator. This technique for
compensating one of the most fundamental limiting processes in linear optics is
shown to be efficient for a wide range of experimental conditions (modifying
the refractive index and the attenuation coefficient). Finally, we explain how
this method can be advantageously exploited in applications ranging from
bio-imaging light sheet microscopy to quantum memories for future quantum
communication networks
Transmission of near-resonant light through a dense slab of cold atoms
The optical properties of randomly positioned, resonant scatterers is a
fundamentally difficult problem to address across a wide range of densities and
geometries. We investigate it experimentally using a dense cloud of rubidium
atoms probed with near-resonant light. The atoms are confined in a slab
geometry with a sub-wavelength thickness. We probe the optical response of the
cloud as its density and hence the strength of the light-induced dipole-dipole
interactions are increased. We also describe a theoretical study based on a
coupled dipole simulation which is further complemented by a perturbative
approach. This model reproduces qualitatively the experimental observation of a
saturation of the optical depth, a broadening of the transition and a blue
shift of the resonance
Controlled Dicke Subradiance from a Large Cloud of Two-Level Systems
Dicke superradiance has been observed in many systems and is based on
constructive interferences between many scattered waves. The counterpart of
this enhanced dynamics, subradiance, is a destructive interference effect
leading to the partial trapping of light in the system. In contrast to the
robust superradiance, subradiant states are fragile and spurious decoherence
phenomena hitherto obstructed the observation of such metastable states. We
show that a dilute cloud of cold atoms is an ideal system to look for
subradiance in free space and study various mechanisms to control this
subradiance.Comment: 5 pages, 4 figure
Effets coopératifs dans les nuages d'atomes froids
In this thesis, we investigate experimentally as well as theoretically collective effects in dilute clouds of cold atoms. In order to study the competition between cooperative effects and strong localization, we then implement a dipole trap that allows us to compress the cloud to dense regimes. The first chapter shows how a system of N atoms interacting via the electromagnetic field gives rise to cooperative effects: superradiance, subradiance, collective Lamb shift. Considering the situation where atoms are driven by an external laser field, we point out how collective effects occur and compute the cooperative radiation pressure force acting on the center of mass of the cloud. Subradiance is then studied by considering the system relaxation after switching off the driving laser field. The second chapter describes the experimental investigation of cooperative effects through measurements of the collective radiation pressure force. These measurements show good agreement with the theoretical model we have previously developed. The last chapter explores the experimental realization of a blue detuned crossed dipole trap whose size can be dynamically adjusted enabling us to compress the cloud to reach high density regimes.Ce travail de thèse concerne d'une part l'étude théorique et expérimentale des effets coopératifs dans les nuages d'atomes froids dilués et, d'autre part, le développement d'un piège dipolaire pour comprimer le nuage vers les régimes denses afin d'étudier la compétition entre les effets coopératifs et la localisation forte. Le premier chapitre montre comment un système de N atomes interagissant via le champ électromagnétique donne naissance aux effets coopératifs : superradiance, sousradiance, déplacement de Lamb collectif. En considérant la situation où les atomes sont pilotés par un champ laser extérieur, nous montrons comment les effets coopératifs se manifestent et calculons la force de pression de radiation collective s'exerçant sur le centre de masse du nuage. Le phénomène de sousradiance est ensuite étudié en considérant la relaxation du système après avoir coupé le laser. Le deuxième chapitre traite l'étude expérimentale des effets coopératifs en mesurant la force de pression de radiation coopérative. Les mesures sont en bon accord avec le modèle théorique développé précédemment. Enfin, le dernier chapitre décrit la réalisation d'un piège dipolaire croisé, désaccordé dans le bleu, dont la taille peut être ajustée dynamiquement pour comprimer le nuage dans les régimes de forte densité