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arXiv:2003.10919 · Physical Review A

Instability threshold in a large balanced magneto-optical trap

Marius Gaudesius, Robin Kaiser, G. Labeyrie, Yong-Chang Zhang, Thomas Pohl

Published May 18, 2020• 14 citations
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Abstract

Large clouds of cold atoms prepared in a magneto-optical trap can develop spatiotemporal instabilities when the frequency of the trapping lasers is brought close to the atomic resonance. This system bears close similarities with trapped plasmas, whereby effective Coulomb interactions are induced by the exchange of scattered photons and lead to collective nonlinear dynamics of the trapped atoms. We report in this paper a detailed experimental study of the instability threshold, and comparisons with three-dimensional simulations of the interacting, laser-driven cloud.

DOI
10.1103/physreva.101.053626

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Large clouds of cold atoms prepared in a magneto-optical trap can develop spatiotemporal instabilities when the frequency of the trapping lasers is brought close to the atomic resonance. This system bears close similarities with trapped plasmas, whereby effective Coulomb interactions are induced by the exchange of scattered photons and lead to collective nonlinear dynamics of the trapped atoms. We report in this paper a detailed experimental study of the instability threshold, and comparisons with three-dimensional simulations of the interacting, laser-driven cloud.
Large clouds of cold atoms prepared in a magneto-optical trap are known to present spatiotemporal instabilities when the frequency of the trapping lasers is brought close to the atomic resonance. This system bears similarities with trapped plasmas where the role of the Coulomb interaction is played by the exchange of scattered photons, and astrophysical objects, such as stars whose size is dependent on radiative forces. We present in this paper a study of the phase space of such instabilities and reveal different dynamical regimes. Three-dimensional simulations of the highly nonlinear atomic dynamics permit a detailed analysis of the experimental observations.
Large clouds of atoms in a magneto-optical trap (MOT) are known to exhibit spatiotemporal instabilities when the frequency of the trapping lasers comes close to the atomic resonance. Such instabilities possess similarities with stars and confined plasmas, where corresponding nonlinearities may give rise to spontaneous oscillations. In this paper, we describe the kinetic model that has recently been employed in three-dimensional (3D) simulations of spatiotemporal instabilities in a MOT, yielding qualitative agreements with experimentally observed instability thresholds and regimes. Details surrounding its implementation are included, and the impact of its physical effects on the instabilities is investigated to improve the understanding of the complex mechanism at work.
In this Thesis, we study the fundamental aspects of self-oscillating atomic clouds in a large, balanced Magneto-Optical Trap (MOT). These instabilities bear analogies with astrophysical systems such as Cepheid variable stars, whose self-oscillations depend on radiation pressure and gravity, as well as technological systems such as confined plasmas, where self-oscillations may arise due to interactions between the plasma particles and the confining magnetic field. We begin with an experimental study of self-oscillating balanced MOT instabilities, investigating first the instability threshold behavior versus various MOT parameters, including magnetic field gradient, atom number and laser intensity. We proceed then investigating the spatio-temporal properties of the unstable regime and identify distinct instability regimes. Different analysis techniques are employed that allow us to learn about the oscillation modes of clouds of these regimes as well as the size distribution of the structures developing in clouds of these regimes. In a theoretical approach, we develop a kinetic 3D model for the MOT with many-atom effects included and successfully use it in numerical simulations of our instabilities. We find qualitative agreements with the instability thresholds from the experiments as well as predict experimentally obtained instability regimes. We investigate the impact of our model’s physical ingredients on the instabilities, to aid the understanding of the complex physics at work. Finally, we show a preliminary simulated result for a self-oscillating instability in the misaligned MOT, known to experimentally exist, in that way demonstrating that our developed numerical tool can be employed beyond the balanced MOT.
We have observed self-sustained radial oscillations in a large magneto-optical trap, containing up to 10(10) Rb85 atoms. This instability is due to the competition between the confining force of the magneto-optical trap and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms N>10(9) are required to observe this unstable behavior.
Considering light-mediated long-range interactions between cold atoms in a magneto-optical trap (MOT), we present numerical evidence of a nonequilibrium steady state (NESS) for sufficiently large number of atoms (>108). This state manifests itself as the appearance of an anisotropic distribution of velocity when a MOT approaches the threshold beyond which self-oscillating instabilities occur. Our three-dimensional (3D) spatiotemporal model with nonlocal spatial dependencies stemming from the interatomic interactions has recently been compared successfully to predict different instability thresholds and regimes in experiments with rubidium atoms. The behavior of the NESS is studied as a function of the main MOT parameters, including its spatiotemporal characteristics.
The instabilities observed in the atomic cloud of a magneto-optical trap are experimentally studied through the dynamics of the center of mass location and the cloud population. Two dynamical components are identified: a slow, stochastic one affects both variables, and a fast, deterministic one affects only the center of mass location. A one-dimensional stochastic model taking into account the shadow effect is developed from these observations and reproduces the experimental behavior. It is shown that instabilities are driven by noise and present stochastic resonancelike characteristics.