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Physical Review A

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

Phil A. Willems, Kenneth G. Libbrecht

Published Feb 1, 1995• 43 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.51.1403

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Physical Review A•Jan 1, 1995•54 citations
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\ensuremath−12{10}^{\mathrm{\ensuremath{-}}12}10\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.
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.
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.
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.
Spin-polarized neutral cesium atoms are confined in a magnetostatic trap in a cryogenic apparatus. The atoms are prepared in the F = 4, mF = 4 state and cooled, with rf-assisted evaporative cooling, to less than 10 microKelvin. The elastic scattering cross section is measured by disequilibrating magnetostatically trapped atoms and observing the rate of equilibration. A computer simulation is presented, the results of which are used to extract the elastic scattering cross section from the measured rates of equilibration. For cesium atoms in the F = 4, mF = 4 state, the elastic cross section is confirmed to have a resonant magnitude which varies inversely with the mean kinetic energy, implying a scattering resonance. A design for a new beam source of cold neutral atoms, based on an optical funnel with a pyramid mirror configuration, is also presented.
With a vapor-cell magneto-optical trap (VCMOT) atoms are trapped directly from the equilibrium vapor at room temperature without the complication of a precooling stage. The background vapor, however, can also be a source of energetic atoms hitting and ejecting those atoms already trapped. The background collision limits trap confinement time /spl tau/ and consequently the total number N of atoms trapped in a VCMOT. In order to have large /spl tau/ and N without losing the convenience and simplicity of a VCMOT, double MOT systems were built. In such a system there is a VCMOT with a usual background vapor pressure, and connected to it is another MOT without the vapor. The VCMOT captures atoms and transfers them to the second MOT through a long tube. The tube provides the constriction to maintain the pressure difference between the two MOT chambers. For the second MOT there is a steady supply of slow atoms without the complication of the background vapor, and it can hold a large number of atoms for a long time. Such systems were successfully used for a Bose Einstein condensation experiment and to study cold collisions.
We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1\ifmmode\times\else\texttimes\fi{}108{10}^{8}108 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3\ifmmode\times\else\texttimes\fi{}109{10}^{9}109 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.