25 research outputs found
Two-temperature Brownian dynamics of a particle in a confining potential
We consider the two dimensional motion of a particle into a confining
potential, subjected to Brownian forces, associated with two different
temperatures on the orthogonal directions. Exact solutions are obtained for an
asymmetric harmonic potential in the overdamped and underdamped regimes,
whereas perturbative approaches are used for more general potentials. The
resulting non equilibrium stationary state is characterized with a nonzero
orthoradial mean current, corresponding to a global rotation of the particle
around the center. The rotation is due to two symmetry breaking: two different
temperatures and a mismatch between the principal axes of the confining
asymmetric potential and the temperature axes. We confirm our predictions by
performing Brownian dynamics simulation. Finally, we propose to observe this
effect on a laser cooled atomic system.Comment: 11 pages, 9 Figures, submitted to PR
Anisotropic long-range interaction investigated with cold atoms
In two dimensions, a system of self-gravitating particles collapses and forms
a singularity in finite time below a critical temperature . We investigate
experimentally a quasi two-dimensional cloud of cold neutral atoms in
interaction with two pairs of perpendicular counter-propagating quasi-resonant
laser beams, in order to look for a signature of this ideal phase transition:
indeed, the radiation pressure forces exerted by the laser beams can be viewed
as an anisotropic, and non-potential, generalization of two-dimensional
self-gravity. We first show that our experiment operates in a parameter range
which should be suitable to observe the collapse transition. However, the
experiment unveils only a moderate compression instead of a phase transition
between the two phases. A three-dimensional numerical simulation shows that
both the finite small thickness of the cloud, which induces a competition
between the effective gravity force and the repulsive force due to multiple
scattering, and the atomic losses due to heating in the third dimension,
contribute to smearing the transition.Comment: 11 pages, 8 figures, accepted in Physical Review
Effective two-level approximation of a multi-level system driven by coherent and incoherent fields
The numerical simulation of multiple scattering in dense ensembles is the
mostly adopted solution to predict their complex optical response. While the
scalar and vectorial light mediated interactions are accurately taken into
account, the computational complexity still limits current simulations to the
low saturation regime and ignores the internal structure of atoms. Here, we
propose to go beyond these restrictions, at constant computational cost, by
describing a multi-level system (MLS) by an effective two-level system (TLS)
that best reproduces the coherent and total scattering properties in any
saturation regime. The correspondence of our model is evaluated for different
experimentally realistic conditions such as the modification of the driving
field polarization, the presence of stray magnetic fields or an incoherent
resonant electromagnetic field background. The trust interval of the model is
quantified for the D2-line of 87Rb atoms but it could be generalized to any
closed transition of a multi-level quantum system.Comment: 11 pages, 6 figure
Non-equilibrium long-range phase transition in cold atoms : theory and experiment
We study the long-range force arising from the absorption of non-saturating
laser beams in a two-dimensional cloud of cold atoms. The force created by the lasers is attractive
and similar to the usual Newtonian gravity along the beam. The cloud is composed
of bosonic strontium 88 cooled and trapped on the intercombination line. Transferring the
atoms in a two-dimensional optical dipole trap in a magical wavelength configuration, a
canonical non-equilibrium phase transition is expected. Below a critical temperature, selfgravitating
particles in two dimensions can collapse, nearly by the same mechanism stars
are forming. We observed experimentally transient compressions, a halfway satisfactory
result originating from the power limitation of our dipole trap. The second part of the
thesis focuses on the theoretical realization a minimal Brownian motor within a system of
trapped particles in 2D, similarly to our experimental situation. The phenomenon characterizing
the Brownian motor is the appearance of a macroscopic current of particles.
We have shown that this direct transport of particles is independent of the details of the
trapping potential and obtained if and only if two symmetries are jointly broken: By the
presence of two heat baths along orthogonal directions together and an anisotropic trap
misaligned from the temperature axes.Doctor of Philosoph
Transition de phase hors d'équilibre d'un gaz d'atomes froids en interaction à longue portée : théorie et expérience
We study the long-range force arising from the absorption of non-saturating laser beams in a two-dimensional cloud of cold atoms. The force created by the lasers is at- tractive and similar to the usual Newtonian gravity along the beam. The cloud is composed of bosonic strontium 88 cooled and trapped on the intercombination line. Transferring the atoms in a two-dimensional optical dipole trap in a magical wavelength configuration, a canonical non-equilibrium phase transition is expected. Below a critical temperature, self- gravitating particles in two dimensions can collapse, nearly by the same mechanism stars are forming. We observed experimentally transient compressions, a halfway satisfactory result originating from the power limitation of our dipole trap. The second part of the thesis focuses on the theoretical realization a minimal Brownian motor within a system of trapped particles in 2D, similarly to our experimental situation. The phenomenon char- acterizing the Brownian motor is the appearance of a macroscopic current of particles. We have shown that this direct transport of particles is independent of the details of the trapping potential and obtained if and only if two symmetries are jointly broken: By the presence of two heat baths along orthogonal directions together and an anisotropic trap misaligned from the temperature axes.Nous avons Ă©tudiĂ© les effets de la force longue portĂ©e rĂ©sultante de lâabsorption de faisceaux lasers dans un nuage dâatomes froids Ă deux dimensions. La force crĂ©Ă©e est attractive et reproduit la gravitĂ© Newtonienne le long du faisceau. Le nuage dâatomes, composĂ© de strontium 88 (bosons), est refroidi et piĂ©gĂ© sur la transition dâintercombinaison. En transfĂ©rant les atomes dans un piĂšge dipolaire Ă deux dimensions Ă la longueur dâonde magique, une transition de phase hors dâĂ©quilibre se produit dans lâensemble canonique. En dessous dâune tempĂ©rature critique, un systĂšme de particules autogravitantes peut collapser, phĂ©nomĂšne rappelant le processus de formation des Ă©toiles. Nous avons observĂ© expĂ©rimentalement, des rĂ©gimes de compressions transitoires, un rĂ©sultat en partie satisfaisant qui pourrait ĂȘtre amĂ©liorĂ© avec lâutilisation de piĂšges dipolaires plus puissants. La seconde partie de la thĂšse se penche sur la rĂ©alisation thĂ©orique dâun moteur Brownien Ă partir dâun systĂšme de particules piĂ©gĂ©es Ă deux dimensions, une situation similaire Ă notre expĂ©rience de collapse Ă deux dimensions. Lâobtention dâun moteur Brownien est caractĂ©risĂ©e par lâapparition dâun courant macroscopique de particules. Nous avons montrĂ© que ce transport est indĂ©pendant des dĂ©tails du piĂšge et dĂ©pend seulement dâune double brisure de symĂ©trie : RĂ©sultant de la prĂ©sence de deux bains thermiques suivant deux axes orthogonaux et dâun piĂšge anisotrope dĂ©salignĂ© des axes thermiques
Towards integrated microwave-to-optical conversion by atoms on a superconducting chip
The coherent conversion of microwave to optical signals is for now a missing hardware for long-distance quantum communication between superconducting quantum (sub)processors that could form the nodes of a future quantum network. Various architectures for quantum simulations and information processing are being currently explored with different TRL levels. Among these, circuits of superconducting qubits have already moved from the fundamental research environment to the R&D units of companies. Recent developments have shown extraordinary abilities for performing fast and high-fidelity quantum logic operations. Their limitations are short coherence times precluding long-term storage of quantum information, and the difficulty of coupling distant quantum registers using microwave photons that are (near)resonant with qubit transitions. There is thus a need for (i) a quantum memory compatible with superconducting qubits, and (ii) a microwave to optical transducer to demonstrate the complete communication protocol between distant sub-registers consisting of a moderate number of superconducting qubits. Cold alkaline atoms which have long coherence time and possess transitions both in the MW and optical domain could certainly help improve on these limitations. In this presentation, I will describe how an integrated atom chip compatible with superconducting quantum processors and optical communication networks will be realized within the newly funded MOCA consortium that received the support from the QuantERA ERA-NET Cofund. I will also specifically discuss a newly developed solution to transport atoms in the near field of surfaces were the atom-light coupling strength will be enhanced.Atomes Ultra-Froids piégés dans des Réseaux Optiques Nano-StructurésConversion micro-onde - optique intégrée sur puce à atomes supraconductric
Ultra-cold atoms in a nano-structured optical lattice.
International audienc
Hollow core fibers : Improving laser architecture and cold atom loading
International audienc
Quantitative absorption imaging of optically dense effective two-level systems
Absorption imaging is a commonly adopted method to acquire, with high temporal resolution, spatial information on a partially transparent object. It relies on the interference between a probe beam and the coherent response of the object. In the low saturation regime, it is well described by a Beer Lambert attenuation. In this paper we theoretically derive the absorption of a polarized laser probe by an ensemble of two-level systems in any saturation regime. We experimentally demonstrate that the absorption cross section in dense Rb cold atom ensembles is reduced, with respect to the single particle response, by a factor proportional to the optical density b of the medium. To explain this reduction, we developed a model that incorporates, in the single particle response, the incoherent electromagnetic background emitted by the surrounding ensemble. We show that it qualitatively reproduces the experimental results. Our calibration factor that has a universal dependence on optical density for polarized light : = 1.17(9) + 0.255(2)b allows to obtain quantitative and absolute, in situ, images of dense quantum systems.Atomes Ultra-Froids piégés dans des Réseaux Optiques Nano-StructurésInitiative d'excellence de l'Université de Bordeau