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Quantum Electronics and Laser Science Conference

Atomic collisions in an optical trap

Carl Wieman, David W. Sesko, Thad G Walker, Christopher R. Monroe

Published Apr 24, 1989• 0 citations
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Abstract

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.

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Curious behavior of optically trapped neutral atoms

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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.

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Since its invention the magneto-optical trap (MOT) has offered a relatively simple way to trap and study cold neutral atoms.1 However, the maximum density achieved in such a trap is much lower than is desirable for the study of cold collisions and the realization of evaporative cooling schemes. The density is limited by two processes; (1) trap loss due to excited-state collisions2 and (2) the repulsive force generated by the rescattering of photons.3

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In order to confine any object, it is necessary to exchange kinetic for potential energy in the trapping field, and in neutral atom traps, the potential energy must be stored as internal atomic energy. Thus practical traps for ground-state neutral atoms are necessarily very shallow compared with thermal energy because the energy level shifts that result from conveniently sized fields are typically considerably smaller than k B T , even for T = 1 K. Neutral atom trapping therefore depends on substantial cooling of a thermal atomic sample, and is often connected with the cooling process. In most practical cases, atoms are loaded from magneto-optical traps, where they have been efficiently accumulated and cooled to mK temperatures (see Sec. 20.3), or from optical molasses, where they have been optically cooled to μK temperatures (see Sec. 19.4). The small depth of typical neutral atom traps dictates stringent vacuum requirements, because an atom cannot remain trapped after a collision with a thermal energy background gas molecule. Since these atoms are vulnerable targets for thermal energy background gas, the mean free time between collisions must exceed the desired trapping time. The cross-section for destructive collisions is quite large because even a gentle collision (i.e. large impact parameter) can impart enough energy to eject an atom from a trap. At pressure P sufficiently low to be of practical interest, the trapping time is ~(10 -8 / P ) s, where P is in torr. This chapter begins with a discussion of ordinary, single-center traps, but also considers arrays of traps. Such “optical lattices” are playing an increasingly important role in research in atomic physics, so discussion of their properties is an appropriate part of the education of a student of the subject. Dipole force optical traps Single-beam optical traps for two-level atoms The simplest imaginable optical trap consists of a single, strongly focused Gaussian laser beam (see Fig. 20.1) [214, 215] whose intensity at the focus varies transversely with r as where is the beam waist size. Such a trap has a well-studied and important macroscopic classical analog in a phenomenon called optical tweezers (see App. 1.C) [216–218].

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Light-induced collisions of ultracold rubidium atoms

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In this dissertation I present a variety of experiments on collisions between ultra-cold rubidium atoms. These collisions take place in light-force atom traps, where temperatures below 1 mK are reached. At these low temperatures weak, long-range interactions between the atoms are important, and collision times are long. Consequently, the absorption and spontaneous emission of photons plays an important role in determining the collision dynamics. The collisions studied in this dissertation involve excitation of the atoms to a state where the relatively strong dipole-dipole interaction with another atom results in attraction (or repulsion) after which spontaneous emission may take place. The resulting energy transfer in such a collision can eject the atoms from the trap. We have observed such collisions in both stable isotopes of rubidium. If the excitedstate is involved, the two isotopes behave quite di ff erently. This makes clear the importance of hyperfine interactions in the collision dynamics. In contrast, for the excited-state such di ff erences disappeare, because the hyperfine interaction is strong enough to eliminate hyperfine mixing of the potentials. Another collision process involved collisions with a repulsive dipole-dipole interaction. This experiment gives strong support for a Landau-Zener treatment of the excitation process. These collisions enabled the measurement of the trap depth. Finally, collisions between rubidium atoms that have undergone a two-step excitation by two near-infrared photons produce violet fluorescence as a signature of the collision. We have observed this e ff ect. The data shows unexpected features, which warrant further investigation. P3 2 ⁄

•Jan 1, 1996•0 citations
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.
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.
Ultracold collisions of neutral atoms can now be studied in the laboratory in the temperature range below 0.001 K. Rate coefficients for ultracold collisions of excited states are decreased by spontaneous emission during the long duration of the collision, but can be dramatically modified by intense laser effects on collision dynamics. Generalized MCQDT is used to develop criteria for the onset of quantum threshold behavior, which generally begins at temperatures above the Doppler cooling limit of light traps. Threshold Penning ionization of He metastable atoms in a light trap is predicted to occur with a rate coefficient larger than 5×10 (−11) cm3/sec.
In order to confine any object, it is necessary to exchange kinetic for potential energy in the trapping field, and in neutral atom traps, the potential energy must be stored as internal atomic energy. Thus practical traps for ground-state neutral atoms are necessarily very shallow compared with thermal energy because the energy level shifts that result from conveniently sized fields are typically considerably smaller than k B T , even for T = 1 K. Neutral atom trapping therefore depends on substantial cooling of a thermal atomic sample, and is often connected with the cooling process. In most practical cases, atoms are loaded from magneto-optical traps, where they have been efficiently accumulated and cooled to mK temperatures (see Sec. 20.3), or from optical molasses, where they have been optically cooled to μK temperatures (see Sec. 19.4). The small depth of typical neutral atom traps dictates stringent vacuum requirements, because an atom cannot remain trapped after a collision with a thermal energy background gas molecule. Since these atoms are vulnerable targets for thermal energy background gas, the mean free time between collisions must exceed the desired trapping time. The cross-section for destructive collisions is quite large because even a gentle collision (i.e. large impact parameter) can impart enough energy to eject an atom from a trap. At pressure P sufficiently low to be of practical interest, the trapping time is ~(10 -8 / P ) s, where P is in torr. This chapter begins with a discussion of ordinary, single-center traps, but also considers arrays of traps. Such “optical lattices” are playing an increasingly important role in research in atomic physics, so discussion of their properties is an appropriate part of the education of a student of the subject. Dipole force optical traps Single-beam optical traps for two-level atoms The simplest imaginable optical trap consists of a single, strongly focused Gaussian laser beam (see Fig. 20.1) [214, 215] whose intensity at the focus varies transversely with r as where is the beam waist size. Such a trap has a well-studied and important macroscopic classical analog in a phenomenon called optical tweezers (see App. 1.C) [216–218].
We confine 85Rb^{85}\mathrm{Rb}85Rb atoms in an optical dipole force trap with a very large detuning from resonance of up to 65 nm. Confinement times of 200 ms, limited only by background-gas collisions, are obtained without additional cooling. A typical trap contains 1300 atoms at a temperature of 0.4 mK and a peak density of 8\ifmmode\times\else\texttimes\fi{}1011{10}^{11}1011 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. We measure spontaneous photon scatter rates of the trapped atoms to be less than 1.6\ifmmode\times\else\texttimes\fi{}103{10}^{3}103 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, corresponding to recoil heating rates below 0.6 mK/s. The far-detuned trap confines atoms with a strong, nearly conservative optical force and negligible atomic excitation.
In this dissertation I present a variety of experiments on collisions between ultra-cold rubidium atoms. These collisions take place in light-force atom traps, where temperatures below 1 mK are reached. At these low temperatures weak, long-range interactions between the atoms are important, and collision times are long. Consequently, the absorption and spontaneous emission of photons plays an important role in determining the collision dynamics. The collisions studied in this dissertation involve excitation of the atoms to a state where the relatively strong dipole-dipole interaction with another atom results in attraction (or repulsion) after which spontaneous emission may take place. The resulting energy transfer in such a collision can eject the atoms from the trap. We have observed such collisions in both stable isotopes of rubidium. If the excitedstate is involved, the two isotopes behave quite di ff erently. This makes clear the importance of hyperfine interactions in the collision dynamics. In contrast, for the excited-state such di ff erences disappeare, because the hyperfine interaction is strong enough to eliminate hyperfine mixing of the potentials. Another collision process involved collisions with a repulsive dipole-dipole interaction. This experiment gives strong support for a Landau-Zener treatment of the excitation process. These collisions enabled the measurement of the trap depth. Finally, collisions between rubidium atoms that have undergone a two-step excitation by two near-infrared photons produce violet fluorescence as a signature of the collision. We have observed this e ff ect. The data shows unexpected features, which warrant further investigation. P3 2 ⁄