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Physical Review A

Permanent magnet trap for cold atoms

J. J. Tollett, C. C. Bradley, Charles A. Sackett, Randall G. Hulet

Published Jan 1, 1995• 54 citations
Publisher

Abstract

We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1\ifmmode\times\else\texttimes\fi{}${10}^{8}$ 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{}${10}^{9}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. 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.

DOI
10.1103/physreva.51.r22

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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 cm\ensuremath−3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}cm\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.
Approximately4times10sp84\\times10\\sp84times10sp8 ground state lithium atoms have been confined in a non-zero 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 8times10sp98\\times10\\sp98times10sp9 cmsp−3\\sp{-3}sp−3 The trapped atom lifetime is 240 seconds, limited by collisions with background gas. The tightly confining fields generated through the use of permanent magnets create a trap which is an excellent environment for the study of quantum statistical effects, atomic collisions, and other ultra-low temperature phenomena.
We have constructed a permanent-magnet trap that provides a tightly confining potential useful for producing dense ultracold atomic vapors.1 This trap will be used to investigate quantum-statistical effects of 6Li and 7Li. Permanent magnets can produce field strengths and gradients comparable to those of superconducting-magnet traps but with reduced complexity and greater optical access. Our trap uses an arrangement of magnets in an Ioffe configuration,2 which features a nonzero magnetic-field minimum. Atoms are trapped about this minimum through the interaction of their total magnetic moment with the trap's magnetic field. The nonzero minimum prevents trap- loss-producing spin-flip (Majorana) transitions.
Achieving precise control over an array of ultracold molecules would provide a unique tool-set for carrying out quantum simulations and quantum computations, as a result of the molecules' rich internal structure. To realise this aim, the molecules have to be cooled and trapped. This is much more difficult for molecules than for atoms due to their complex internal structure. This thesis presents preliminary work towards realising a versatile, permanent magnet trap for buffer gas cooled molecules. Atoms are used throughout to test the feasibility of the trap, as they are easier to produce and detect. Two novel methods for trapping buffer gas cooled atoms in a permanent magnet trap are investigated. The first of these involves trapping the atoms directly from a cryogenic buffer gas cooled ablation plume. Dy atoms, with a magnetic moment of 10µB, are trapped with a lifetime of 800±30 μs, thought to be limited by collisions with a high density of background buffer gas atoms remaining in the trap region. Information gained from the direct trapping experiments motivated the design of a second trapping set-up. Here, a beam of Dy atoms is first extracted from a cryogenic buffer gas source, and when this beam reaches the trapping region, a fraction of the atoms are stopped through collisions with cold helium gas present in the trapping region. This second method reduces the density of buffer gas required in the trap region. The trap lifetime achieved in this arrangement of 810±40 μs is no longer than in the direct trapping experiments, but this arrangement is much more stable and repeatable. The lifetime here is also thought to be limited by collisions with background buffer gas atoms.
A microscopic magnetic quadrupole trap for neutral atoms has been realized with a combination of permanent magnets, coils, and ferromagnetic pole pieces. The attainable magnetic field gradients of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0ex}{0ex}}\mathrm{G}/\mathrm{cm}$ infer a spatial extension of the ground state much smaller than the wavelength of optical transitions. The field gradient can be varied over a wide range which allows for efficient loading of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium atoms from a shallow potential by adiabatic transport and compression. During compression the observed 275-fold density increase implies a collision rate increase by a factor of 1700.
Magnetic trapping of laser-cooled neutral atoms has been demonstrated at NBS and is progressing in other laboratories. Confinement of any object requires exchanging kinetic for potential energy, and for neutral atoms, this necessarily derives from shifts of internal energy levels. This is implemented through the force experienced by the atomic magnetic moment in a carefully designed, highly inhomogeneous field. Since magnetic fields of convenient strength can shift atomic energy levels by only a few gigahertz (temperature ≌ 0.1 K), neutral atom traps are very shallow and must be loaded with very cold atoms. We present some motivations for using magnetic traps and then discuss the constraints and optimal configurations of various arrangements. For example, no trap can be isotropic. Next we discuss both the classical and quantum mechanical motions of trapped atoms. These motions are important for at least two reasons: First, several schemes under study for further cooling depend on knowing the position and velocity of the atoms, as well as the vector field at each point along the orbit. Second, magnetic traps depend on the atomic moment remaining aligned with the field as the atom orbits in the trap, and this precludes rapid motion through a low-field region.
Lately there has been rapid progress in techniques for trapping samples of neutral atoms at elevated densities and extremely low temperatures. Now a group at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, has used a new type of magnetic trap to confine rubidium atoms at the lowest kinetic temperature yet recorded for an atomic gas. Compelling motivations for this effort include the use of such ultra-cold gases in high-precision spectroscopy, low-energy tests of the "standard model" of particle physics, and studies of atomic collisions at very low relative velocities. Particularly exciting is the prospect of studying quantum collective phenomena such as Bose-Einstein condensation (p37).