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A Cryogenic Trap for Neutral Atoms

Richard Alan Boyd

Published Apr 21, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.7907/dzwd-sc80

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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.
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.
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 the design, construction and performance of three realizations of a high-field superconducting magnetic trap for neutral atoms and molecules. Each of these traps utilizes a pair of coaxial coils in the anti-Helmholtz geometry and achieves depths greater than 4 T, allowing it to capture magnetic atoms and molecules cooled in a cryogenic buffer gas. Achieving this depth requires that the repulsive force between the coils (which can exceed 30 metric tons) be contained. We also describe additional features of the traps, including the elimination of trapped fluxes from the coils and the integration of the coils into a cryogenic vacuum environment suitable for producing cold atoms and molecules.
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.
We propose two double-well magnetic traps for cold neutral atoms that use current-carrying wires. These traps can be used to trap two different atomic species. The spatial distributions of the magnetic fields, gradients, and curvatures from these two wire configurations were calculated and analyzed. Our study shows that the double-well traps proposed here can be continuously changed into single-well traps by reduction of the current in a wire (or a coil) and vice versa and that a maximum field gradient greater than 5×104 G/cm and a maximum field curvature (at each trap center) greater than 2.5×107 G/cm2 can be generated in our double-well traps and used to achieve two-species Bose–Einstein condensations and study the properties of double-well Bose–Einstein condensations or to achieve sympathetic cooling between two atomic samples or even to achieve two-species magneto-optical traps and study cold collisions between two atomic samples.
We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, 1012{10}^{12}1012 chromium atoms are trapped at an initial temperature of \ensuremath{\sim}1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as \ensuremath{\sim}10 mK. Elastic and inelastic 52Cr{}^{52}\mathrm{Cr}52Cr collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a 52Cr{}^{52}\mathrm{Cr}52Cr Bose-Einstein condensate and 53Cr{}^{53}\mathrm{Cr}53Cr Fermi degenerate gas will be discussed.