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AIP conference proceedings

Curious behavior of optically trapped neutral atoms

Carl Wieman, Thad G Walker, David W. Sesko, Chris Monroe

Published Jan 1, 1991• 0 citations
Publisher

Abstract

We have studied the behavior of clouds of neutral atoms contained in a spontaneous force optical trap. Because of the low temperatures of the atoms (<1 mK) we have observed a variety of new phenomena which involve both short and long range interactions between the atoms. The short range interactions are manifested in two body collisions which are notable because of the very long collision times. This time scale causes the dynamics to be profoundly affected by absorption and spontaneous emission during the collision. An interesting assortment of collective behaviors are observed when the trapped atom cloud contains more than 105 atoms. These include the expansion of the cloud as the number is increased and dramatic changes in the distribution of the atoms at higher numbers. We can explain much of the collective behavior using a simple model that includes a 1/r2 force between the atoms arising from the multiple scattering of photons. Finally, we discuss the optical trapping of atoms directly from a low pressure vapor in a small glass cell. We have used these optically trapped atoms to load a magnetostatic trap in the same cell. This provided a high density sample of atoms with a temperature of less than 2 μK.

DOI
10.1063/1.41002

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We have studied the behavior of clouds of neutral atoms contained in a spontaneous force optical trap. Because of the low temperatures of the atoms (<1 mK) we have observed a variety of new phenomena which involve both short and long range interactions between the atoms. The short range interactions are manifested in two body collisions which are notable because of the very long collision times. This time scale causes the dynamics to be profoundly affected by absorption and spontaneous emission during the collision. An interesting assortment of collective behaviors are observed when the trapped atom cloud contains more than 105 atoms. These include the expansion of the cloud as the number is increased and dramatic changes in the distribution of the atoms at higher numbers. We can explain much of the collective behavior using a simple model that includes a 1/r2 force between the atoms arising from the multiple scattering of photons. Finally, we discuss the optical trapping of atoms directly from a low pressure vapor in a small glass cell. We have used these optically trapped atoms to load a magnetostatic trap in the same cell. This provided a high density sample of atoms with a temperature of less than 2 μK.
We describe experiments that show collective behavior in clouds of optically trapped neutral atoms. This collective behavior is demonstrated in a variety of observed spatial distributions with abrupt bistable transitions between them. These distributions include stable rings of atoms around a small core and clumps of atoms rotating about the core. The size of the cloud grows rapidly as more atoms are loaded into it, implying a strong long-range repulsive force between the atoms. We show that a force arising from radiation trapping can explain much of this behavior.
A classical collective behavior is observed in the spatial distributions of a cloud of optically trapped neutral atoms. They include extended uniform-density ellipsoids, rings of atoms around a small central ball, and clumps of atoms orbiting a central core. The distributions depend sensitively on the number of atoms and the alignment of the laser beams. Abrupt bistable transitions between different distributions are seen. This system is studied in detail, and much of this behavior can be explained by the incorporation of long-range interactions between the atoms in the equation of equilibrium. It is shown how attenuation and multiple scattering of the incident photons lead to these interactions.
We describe experiments that show collective behavior in clouds of optically trapped neutral atoms.This collective behavior is demonstrated in a variety of observed spatial distributions with abrupt bistable transitions between them.These distributions include stable rings of atoms around a small core and clumps of atoms rotating about the core.The size of the cloud grows rapidly as more atoms are loaded into it, implying a strong long-range repulsive force between the atoms.We show that a force arising from radiation trapping can explain much of this behavior.
Spontaneous force light traps1,2 have provided a way to obtain relatively deep static traps for neutral atoms. However, the usefulness of any such trap will depend strongly on the density of the trapped atoms. We have studied the collisions within such a trap which limit the atomic density. These collisions are of considerable interest in their own right because the temperatures of the trapped atoms (10−4 K) are far lower than in usual atomic collision experiments. As a result the collision times are very long, and the collision dynamics are dominated by long-range interactions and spontaneous emission.
Collective motions are observed in optically thick clouds of optically trapped atoms.1,2 Manifestations of the collective behavior include multiple bistable transitions between spherical and ring-shaped spatial distributions of the atoms and clumps of atoms rotating about a core. Detailed studies of the spherical spatial distributions are consistent with predictions of a simple model of long-range interatomic forces that arise from the radiation fields of the atoms. In particular, radiation trapping produces a long-range repulsion between the atoms. Again, a simple model that includes this repulsion as well as a vortex force produced by appropriate alignment of the laser beams reproduces many features of the rings, including approximate sizes and orbital frequencies. The model does not, however, explain the bistable transitions or the orbiting clumps of atoms. Supported by the National Science Foundation and the Office of Naval Research.
We have experimentally studied the dynamics of rubidium atoms in a magneto-optical trap employing optical σ+ σ--molasses with controlled time phase differences in a vapour cell. We find that the action of radiation pressure vortices gives rise to a novel long-range interaction between the atoms which may explain the irregular atomic-density distributions possible in such traps even at perfect alignment. We have also recorded absorption spectra of trapped low-density atomic samples which include time-phase-dependent narrow Raman and Rayleigh resonances.