Microwave-stimulated Raman adiabatic passage in a Bose-Einstein condensate on an atom chip

We report the achievement of stimulated Raman adiabatic passage (STIRAP) in the microwave frequency range between internal states of a Bose-Einstein condensate (BEC) magnetically trapped in the vicinity of an atom chip. The STIRAP protocol used in this experiment is robust to external perturbations as it is an adiabatic transfer, and power-efficient as it involves only resonant (or quasi-resonant) processes. Taking into account the effect of losses and collisions in a non-linear Bloch equations model, we show that the maximum transfer efficiency is obtained for non-zero values of the one- and two-photon detunings, which is confirmed quantitatively by our experimental measurements

Estimating random close packing in polydisperse and bidisperse hard spheres via an equilibrium model of crowding

We show that an analogy between crowding in fluid and jammed phases of hard spheres captures the density dependence of the kissing number for a family of numerically generated jammed states. We extend this analogy to jams of mixtures of hard spheres in $d=3$ dimensions, and thus obtain an estimate of the random close packing (RCP) volume fraction, $\phi_{\textrm{RCP}}$, as a function of size polydispersity. We first consider mixtures of particle sizes with discrete distributions. For binary systems, we show agreement between our predictions and simulations, using both our own and results reported in previous works, as well as agreement with recent experiments from the literature. We then apply our approach to systems with continuous polydispersity, using three different particle size distributions, namely the log-normal, Gamma, and truncated power-law distributions. In all cases, we observe agreement between our theoretical findings and numerical results up to rather large polydispersities for all particle size distributions, when using as reference our own simulations and results from the literature. In particular, we find $\phi_{\textrm{RCP}}$ to increase monotonically with the relative standard deviation, $s_{\sigma}$, of the distribution, and to saturate at a value that always remains below 1. A perturbative expansion yields a closed-form expression for $\phi_{\textrm{RCP}}$ that quantitatively captures a distribution-independent regime for $s_{\sigma} < 0.5$. Beyond that regime, we show that the gradual loss in agreement is tied to the growth of the skewness of size distributions

Étude d'un modèle Hamiltonien de liquide non-Galiléen : du mouvement collectif sans activité

Collective motion, the spontaneous ordering of the velocities across a macroscopic system, is a hallmark of living systems like flocks of birds.It is captured by models of self-propelled particles, that are usually active: they do not conserve energy nor momentum. In my thesis, using notions from the theory of liquids, magnetism, and statistical mechanics, I study a conservative model of collective motion, composed of particles that carry spins, which are coupled to their velocities. I show that the alignment of spins creates an effective attraction, that is responsible for a phase separation between an isotropic gas and a ferroliquid. This phase separation ends in a tricritical point, from which stems the Curie line. I then establish the full phase diagram of the model with a spin-velocity coupling, varying its amplitude, the number of particles, the density, and the temperature.The conservation of momentum imposes that all polar phases move collectively. At low temperatures and densities, I show that the system spontaneously generates alignment defects so as to stop moving, and thus escapes a high kinetic energy cost. I also show that the system can go from an apolar state to a polar one as the temperature increases, betraying an order-by-disorder phenomenon. Finally, I show that the dynamics of the system is well described by an effective model of self-propelled particles, with a rotational inertia that soars at the rigidity transition. At high inertia, the system moves with spontaneous turns and rotations caused by the conservation of angular momentum.Le mouvement collectif, l'ordre spontané des vitesses dans un système macroscopique, est une propriété marquante des systèmes vivants tels que les vols d'oiseaux. Il est prédit par des modèles de particules auto-propulsées, qui sont actives : elles ne conservent ni énergie, ni impulsion. Dans ma thèse, j'étudie un modèle conservatif de mouvement collectif, composé de particules portant des spins couplés à leur vitesse, en m'aidant de notions de physique des liquides, de magnétisme, et de mécanique statistique. Je montre que l'alignement des spins génère une attraction effective, qui est responsable d'une séparation de phase entre un gaz isotrope et un ferroliquide se terminant en un point triple, d'où émerge la ligne de Curie. Je dresse ensuite le diagramme des phases du modèle en présence d'un couplage spin-vitesse, en faisant varier son intensité, le nombre de particules, la densité, et la température. La conservation de l'impulsion impose que les phases polaire soient en mouvement collectif. A basse température et basse densité, je montre que le système peut créer spontanément des défauts d'alignement pour ne pas avoir à se mouvoir et ainsi échapper à un coût élevé en énergie cinétique. Je montre que le système peut transiter d'un état apolaire vers un état polaire lorsque la température augmente, trahissant un phénomène d'ordre par le désordre. Enfin, je montre que le mouvement du système est bien décrit par un modèle effectif de particules auto-propulsées avec de l'inertie de rotation, qui augmente fortement à la transition de rigidité. A haute inertie, le système présente des virages et des rotations spontanées dus à la conservation du moment cinétique

Study of a non-Galilean Hamiltonian liquid : collective motion without activity

Le mouvement collectif, l'ordre spontané des vitesses dans un système macroscopique, est une propriété marquante des systèmes vivants tels que les vols d'oiseaux. Il est prédit par des modèles de particules auto-propulsées, qui sont actives : elles ne conservent ni énergie, ni impulsion. Dans ma thèse, j'étudie un modèle conservatif de mouvement collectif, composé de particules portant des spins couplés à leur vitesse, en m'aidant de notions de physique des liquides, de magnétisme, et de mécanique statistique. Je montre que l'alignement des spins génère une attraction effective, qui est responsable d'une séparation de phase entre un gaz isotrope et un ferroliquide se terminant en un point triple, d'où émerge la ligne de Curie. Je dresse ensuite le diagramme des phases du modèle en présence d'un couplage spin-vitesse, en faisant varier son intensité, le nombre de particules, la densité, et la température. La conservation de l'impulsion impose que les phases polaire soient en mouvement collectif. A basse température et basse densité, je montre que le système peut créer spontanément des défauts d'alignement pour ne pas avoir à se mouvoir et ainsi échapper à un coût élevé en énergie cinétique. Je montre que le système peut transiter d'un état apolaire vers un état polaire lorsque la température augmente, trahissant un phénomène d'ordre par le désordre. Enfin, je montre que le mouvement du système est bien décrit par un modèle effectif de particules auto-propulsées avec de l'inertie de rotation, qui augmente fortement à la transition de rigidité. A haute inertie, le système présente des virages et des rotations spontanées dus à la conservation du moment cinétique.Collective motion, the spontaneous ordering of the velocities across a macroscopic system, is a hallmark of living systems like flocks of birds.It is captured by models of self-propelled particles, that are usually active: they do not conserve energy nor momentum. In my thesis, using notions from the theory of liquids, magnetism, and statistical mechanics, I study a conservative model of collective motion, composed of particles that carry spins, which are coupled to their velocities. I show that the alignment of spins creates an effective attraction, that is responsible for a phase separation between an isotropic gas and a ferroliquid. This phase separation ends in a tricritical point, from which stems the Curie line. I then establish the full phase diagram of the model with a spin-velocity coupling, varying its amplitude, the number of particles, the density, and the temperature.The conservation of momentum imposes that all polar phases move collectively. At low temperatures and densities, I show that the system spontaneously generates alignment defects so as to stop moving, and thus escapes a high kinetic energy cost. I also show that the system can go from an apolar state to a polar one as the temperature increases, betraying an order-by-disorder phenomenon. Finally, I show that the dynamics of the system is well described by an effective model of self-propelled particles, with a rotational inertia that soars at the rigidity transition. At high inertia, the system moves with spontaneous turns and rotations caused by the conservation of angular momentum

Ferromagnetism-induced Phase Separation in a Two-dimensional Spin Fluid

25 pages, 17 figures, 1 appendixInternational audienceWe study the liquid-gas phase separation observed in a system of repulsive particles dressed with ferromagnetically aligning spins, a so-called `spin fluid'. Microcanonical ensemble numerical simulations of finite-size systems reveal that magnetization sets in and induces a liquid-gas phase separation between a disordered gas and a ferromagnetic dense phase at low enough energies and large enough densities. The dynamics after a quench into the coexistence region show that the order parameter associated to the liquid-vapour phase separation follows an algebraic law with an unusual exponent, as it is forced to synchronize with the growth of the magnetization: this suggests that for finite size systems the magnetization sets in along a Curie line, which is also the gas-side spinodal line, and that the coexistence region ends at a tricritical point. This picture is confirmed at the mean-field level with different approximation schemes, namely a Bethe lattice resolution and a virial expansion complemented by the introduction of a self-consistent Weiss-like molecular field. However, a detailed finite-size scaling analysis shows that in two dimensions the ferromagnetic phase escapes the Berezinskii-Kosterlitz-Thouless scenario, and that the long-range order is not destroyed by the unbinding of topological defects. The Curie line becomes thus a magnetic crossover in the thermodynamic limit. Finally, the effects of the magnetic interaction range and those of the interaction softness are characterized within a mean-field semi-analytic low-density approach

Ferromagnetism-induced Phase Separation in a Two-dimensional Spin Fluid

25 pages, 17 figures, 1 appendixInternational audienceWe study the liquid-gas phase separation observed in a system of repulsive particles dressed with ferromagnetically aligning spins, a so-called `spin fluid'. Microcanonical ensemble numerical simulations of finite-size systems reveal that magnetization sets in and induces a liquid-gas phase separation between a disordered gas and a ferromagnetic dense phase at low enough energies and large enough densities. The dynamics after a quench into the coexistence region show that the order parameter associated to the liquid-vapour phase separation follows an algebraic law with an unusual exponent, as it is forced to synchronize with the growth of the magnetization: this suggests that for finite size systems the magnetization sets in along a Curie line, which is also the gas-side spinodal line, and that the coexistence region ends at a tricritical point. This picture is confirmed at the mean-field level with different approximation schemes, namely a Bethe lattice resolution and a virial expansion complemented by the introduction of a self-consistent Weiss-like molecular field. However, a detailed finite-size scaling analysis shows that in two dimensions the ferromagnetic phase escapes the Berezinskii-Kosterlitz-Thouless scenario, and that the long-range order is not destroyed by the unbinding of topological defects. The Curie line becomes thus a magnetic crossover in the thermodynamic limit. Finally, the effects of the magnetic interaction range and those of the interaction softness are characterized within a mean-field semi-analytic low-density approach

Droplets impaling on a cone

International audienceThis paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.V0013