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Bulletin of the American Physical Society

A long-lived neutral atom trap

Kenneth G. Libbrecht, Phil A. Willems

Published May 1, 1993• 0 citations
OpenAlex

Abstract

We have constructed a Zeemanshift Optical Trap (ZOT) for neutral cesium atoms within a sealed cryostat cooled to 4.2K. The low residual background pressure of these temperatures, plus low velocities and masses of the background molecules (mostly He and H{sub 2}), leads to an exceptionally long holding time for trapped Cs atoms in the ZOT. In our initial attempts we measured trap holding times {approximately}5-10 minutes, but increasing this to several hours should be possible. Our latest experimental results will be presented. Long trap holding times may be useful for achieving a Bose condensed gas via evaporative cooling, since thermal relaxation times are fairly long and background gas collisions are therefore important. We are also beginning experiments to trap atoms with superconducting coils inside the cryostat. Our initial aim is to construct microscopic neutral atom traps and other structures with large magnetic field gradients (as high as 10{sup 8} G/cm), where the {triangledown}({rvec {mu}}{center_dot}{rvec B}) force is much greater than has been realized in previous trapping geometries.

Related papers

Creating long-lived neutral-atom traps in a cryogenic environment

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We describe techniques for creating long-lived magneto-optical and magnetostatic traps for neutral atoms. These traps exist in a sealed cryogenic environment with a temperature near 4 K, where the background gas pressure can be extremely low. To date we have observed cesium magneto-optical traps with background-limited lifetimes in excess of 1 h, and magnetostatic traps with lifetimes of nearly 10 min. From these observations we use the known He-Cs van der Waals collision cross section to infer typical background gas pressures in our apparatus below 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ Torr. With hardware improvements we expect this pressure can be made much lower, extending the magnetostatic-trap lifetimes one to two orders of magnitude. Furthermore, with a cryogenic system one can use superconducting magnets and SQUID detectors to trap and to nondestructively sense spin-polarized atoms. With superconducting microstructures one can achieve very large magnetic-field gradients and curvatures, as high as \ensuremath{\sim}${10}^{6}$ G/cm and \ensuremath{\sim}${10}^{9}$ G/${\mathrm{cm}}^{2}$, respectively, for use in magnetic and magneto-optical traps.

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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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Summary form only given. Bose-Einstein condensates (BEC) are generally produced by evaporative cooling of laser cooled atomic samples in magnetic traps. Though magnetic traps are indispensable for the production of a BEC, they pose restrictions on the possible studies with a BEC. Previous experiments of our group have demonstrated that it is possible to trap a BEC in the circular focus of a far-detuned infrared laser beam. This first realization of an optical dipole trap (ODT) for BEC was used to prepare and investigate spinor condensates of Na atoms in the F=1 hyperfine manifold. However, the initial peak density in this ODT after loading from the magnetic trap reached 10/sup 15/ atoms/cm/sup 3/, leading to rapid losses due to three-body collisions. To realize an ODT capable of storing large condensates for a long time, we have used a laser beam with an elliptical beam profile. The optical trap is loaded by superimposing the light field onto the magnetic trap and then slowly turning off the magnetic fields. The transfer efficiency is close to 100%, and thus more than 10/sup 7/ Bose-condensed atoms can be loaded into the ODT.

Quantum Electronics and Laser Science Conference•Nov 13, 2002•0 citations
We have constructed a Zeemanshift Optical Trap (ZOT) for neutral cesium atoms within a sealed cryostat cooled to 4.2K. The low residual background pressure of these temperatures, plus low velocities and masses of the background molecules (mostly He and H{sub 2}), leads to an exceptionally long holding time for trapped Cs atoms in the ZOT. In our initial attempts we measured trap holding times {approximately}5-10 minutes, but increasing this to several hours should be possible. Our latest experimental results will be presented. Long trap holding times may be useful for achieving a Bose condensed gas via evaporative cooling, since thermal relaxation times are fairly long and background gas collisions are therefore important. We are also beginning experiments to trap atoms with superconducting coils inside the cryostat. Our initial aim is to construct microscopic neutral atom traps and other structures with large magnetic field gradients (as high as 10{sup 8} G/cm), where the {triangledown}({rvec {mu}}{center_dot}{rvec B}) force is much greater than has been realized in previous trapping geometries.
We describe techniques for creating long-lived magneto-optical and magnetostatic traps for neutral atoms. These traps exist in a sealed cryogenic environment with a temperature near 4 K, where the background gas pressure can be extremely low. To date we have observed cesium magneto-optical traps with background-limited lifetimes in excess of 1 h, and magnetostatic traps with lifetimes of nearly 10 min. From these observations we use the known He-Cs van der Waals collision cross section to infer typical background gas pressures in our apparatus below 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ Torr. With hardware improvements we expect this pressure can be made much lower, extending the magnetostatic-trap lifetimes one to two orders of magnitude. Furthermore, with a cryogenic system one can use superconducting magnets and SQUID detectors to trap and to nondestructively sense spin-polarized atoms. With superconducting microstructures one can achieve very large magnetic-field gradients and curvatures, as high as \ensuremath{\sim}106{10}^{6}106 G/cm and \ensuremath{\sim}109{10}^{9}109 G/cm2{\mathrm{cm}}^{2}cm2, respectively, for use in magnetic and magneto-optical traps.
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].
A cryogenic system for the trapping and cooling of neutral atoms is described. The system uses superconducting coils inside a trapping chamber cooled to liquid helium temperatures. The cryogenic nature of the system allows the production of extremely long-lived traps at high field gradients and curvatures. At the highest field gradients, evidence for discrete jumps in the trap fluorescence is presented; these jumps are shown to be due to single atoms entering and leaving the trap. The stability of high field gradient traps is studied, and a newly identified loss mechanism, stochastic diffusion, is shown to be the dominant loss mechanism for such traps. Finally, evidence for evaporative cooling of cesium is presented, and an upper bound to a cross section critical to the evaporation process is obtained.
The use of a cryogenic apparatus has several advantages for the magneto-optical and magnetostatic trapping of atoms. We have measured the lifetimes of magneto-optical and magnetostatic traps for cesium to be greatly increased in a cryogenic vacuum environment relative to a room-temperature vacuum environment, due to reduced background pressure and reduced mass and kinetic energy of background gas atoms. Another advantage of a cryogenic apparatus is that superconducting magnetic field coils can easily be used to generate very high magnetic field gradients for stronger confinement of the trapped atoms. We report the first measurements of the stability of MOT's at high magnetic field gradients and laser intensities, and compare the results to a model based on evaporation of atoms from the MOT using a Fokker-Planck equation approach. This model is more consistent with the data than an alternative model based on ejection of atoms by magnetic dipole forces. We also measure the dynamical parameters for magneto-optically trapped atoms at both high and low magnetic field gradients and with trapping laser intensities above saturation and compare the data to a simple physical model based on polarization gradient cooling. Consistent with measurements by other groups, the measured friction is consistent with the polarization gradient model, but the measured spring constant is drastically reduced and more consistent with the predictions of the Doppler theory.
Summary form only given. Bose-Einstein condensates (BEC) are generally produced by evaporative cooling of laser cooled atomic samples in magnetic traps. Though magnetic traps are indispensable for the production of a BEC, they pose restrictions on the possible studies with a BEC. Previous experiments of our group have demonstrated that it is possible to trap a BEC in the circular focus of a far-detuned infrared laser beam. This first realization of an optical dipole trap (ODT) for BEC was used to prepare and investigate spinor condensates of Na atoms in the F=1 hyperfine manifold. However, the initial peak density in this ODT after loading from the magnetic trap reached 10/sup 15/ atoms/cm/sup 3/, leading to rapid losses due to three-body collisions. To realize an ODT capable of storing large condensates for a long time, we have used a laser beam with an elliptical beam profile. The optical trap is loaded by superimposing the light field onto the magnetic trap and then slowly turning off the magnetic fields. The transfer efficiency is close to 100%, and thus more than 10/sup 7/ Bose-condensed atoms can be loaded into the ODT.