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arXiv:2006.14848 · Physical Review A

Anisotropic long-range interaction investigated with cold atoms

Vincent Mançois, Julien Barré, Chang Chi Kwong, Alain Olivetti, Pascal Viot, David Wilkowski

Published Jul 14, 2020• 2 citations
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Abstract

In two dimensions, a system of self-gravitating particles collapses and forms a singularity in finite time below a critical temperature ${T}_{c}$. We investigate experimentally a quasi-two-dimensional cloud of cold neutral atoms in interaction with two pairs of perpendicular counterpropagating quasiresonant 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 nonpotential, 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.

DOI
10.1103/physreva.102.013311

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In two dimensions, a system of self-gravitating particles collapses and forms a singularity in finite time below a critical temperature Tc{T}_{c}Tc​. We investigate experimentally a quasi-two-dimensional cloud of cold neutral atoms in interaction with two pairs of perpendicular counterpropagating quasiresonant 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 nonpotential, 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.
We propose to use a cloud of laser-cooled atoms in a quasi-two-dimensional trap to investigate a nonequilibrium collapse phase transition in the presence of a gravitational-like interaction. Using theoretical arguments and numerical simulations, we show that, like in two-dimensional gravity, a transition to a collapsed state occurs below a critical temperature. In addition and as a signature of the nonequilibrium nature of the system, persistent particle currents, dramatically increasing close to the phase transition, are observed.
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.
A quasi-resonant laser induces a long-range attractive force within a cloud of cold atoms. We take advantage of this force to build in the laboratory a system of particles with a one-dimensional gravitational-like interaction, at a fluid level of modeling. We give experimental evidences of such an interaction in a cold Strontium gas, studying the density profile of the cloud, its size as a function of the number of atoms, and its breathing oscillations. PACS numbers: 37.10.De, 05.20.Jj, 04.80.Cc, 37.10.Gh,04.40.-b
We analyze the light-induced atom–atom interactions in optically thick atomic clouds and show that, when the laser frequency is on-resonance with the atomic transition, they become attractive. On the basis of this analysis we propose and demonstrate a novel scheme to compress a cold and dense atomic cloud with a short on-resonance laser pulse. The compression force arises from attenuation of the laser light by the atomic cloud. The following free propagation of the atoms shows a lenslike behavior that yields a transient density increase at the focal time, where neither laser nor magnetic field perturbations exist. A cooling pulse, which is applied at the focal time of this lens, restores the initial temperature of atoms, and hence the phase space density is increased. Finally, we adopt our compression scheme to a quasi-steady-state mode by temporally chopping it with the cooling and trapping beams of a magnet-optical trap.