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arXiv:quant-ph/0402037 · Physical Review A

Proposed magnetoelectrostatic ring trap for neutral atoms

Asa Hopkins, Benjamin Lev, Hideo Mabuchi

Published Nov 22, 2004• 20 citations
arXivHTMLPDF

Abstract

We propose a trap for confining cold neutral atoms in a microscopic ring using a magnetoelectrostatic potential. The trapping potential is derived from a combination of a repulsive magnetic field from a hard drive atom mirror and the attractive potential produced by a charged disk patterned on the hard drive surface. We calculate a trap frequency of $[29.7,42.6,62.8]\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ and a depth of $[16.1,21.8,21.8]\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ for [$^{133}\mathrm{Cs}$, $^{87}\mathrm{Rb}$, $^{40}\mathrm{K}$], and discuss a simple loading scheme and a method for fabrication. This device provides a one-dimensional potential in a ring geometry that may be of interest to the study of trapped quantum degenerate one-dimensional gases.

DOI
10.1103/physreva.70.053616

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We propose a trap for confining cold neutral atoms in a microscopic ring using a magnetoelectrostatic potential. The trapping potential is derived from a combination of a repulsive magnetic field from a hard drive atom mirror and the attractive potential produced by a charged disk patterned on the hard drive surface. We calculate a trap frequency of [29.7,42.6,62.8]kHz[29.7,42.6,62.8]\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}[29.7,42.6,62.8]kHz and a depth of [16.1,21.8,21.8]MHz[16.1,21.8,21.8]\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}[16.1,21.8,21.8]MHz for [133Cs^{133}\mathrm{Cs}133Cs, 87Rb^{87}\mathrm{Rb}87Rb, 40K^{40}\mathrm{K}40K], and discuss a simple loading scheme and a method for fabrication. This device provides a one-dimensional potential in a ring geometry that may be of interest to the study of trapped quantum degenerate one-dimensional gases.
We propose and experimentally demonstrate a novel scheme to magneto-optically trap neutral atoms in a ring shaped trap that can be used to transfer atoms into a circular magnetic trap with high density. This inturn enables to evaporatively cool atoms and study the behaviour of ultra cold gases in a periodic 2-dimensional potential. The circular magneto-optical trap itself is also of interest to investigate the properties of magneto-optical trap of deformed shape, such as reduction of photon-reabsorption.
We have created a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate in a magnetic trapping potential produced by a hard disk platter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled to Bose-Einstein condensation on the surface with radio-frequency evaporation. Fragmentation of the atomic cloud due to imperfections in the magnetic structure was observed at distances closer than $40\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ from the surface. Attempts to use the disk as an atom mirror showed dispersive effects after reflection.
We describe planar and mostly planar current geometries for constructing microscopic magnetic traps for neutral atoms. These geometries are well-suited for fabrication from superconductors using standard microfabrication techniques. Magnetic-field gradients greater than 5\ifmmode\times\else\texttimes\fi{}105{10}^{5}105 G/cm, and field curvatures greater than 108{10}^{8}108 G/cm2{\mathrm{cm}}^{2}cm2 can be produced in microscopic traps. Trap loading could be accomplished by constructing a nested series of traps, and compressing atom clouds from larger traps into smaller ones. In the smallest magnetic microtraps the motional ground-state energy of atoms in the trap can exceed the recoil energy from resonant photons, which may allow direct laser cooling to the trap ground state. If a number of atoms can be simultaneously cooled to the ground state, the resulting ``Bose clusters'' should exhibit interesting optical behavior, since their spatial extent would be less than the resonant light wavelength.
Multiple bounces of cold rubidium atoms have been observed for times up to one second in a trap formed by gravity and a 2 cm-diameter spherical mirror made from a sinusoidally magnetized floppy disk. We have studied the dynamics of the atoms bouncing in this trap from several different heights up to 40.5 mm and we conclude that the atoms are reflected specularly and with reflectivity 1.01(3). Slight roughness of the mirror is caused by harmonics in the magnetization of the surface and by discontinuities at the boundaries between recorded tracks. As the next step in this atom optics program we propose using a magnetic mirror to create a 2D atomic gas. We discuss how cold atoms can be loaded into the ground state of a static magnetic potential well that exists above the surface of the mirror as a consequence of the intermediate-field Zeeman effect.
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.
Many proposals for quantum information processing require precise control over the motion of neutral atoms, as in the manipulation of coherent matter waves or the confinement and localization of individual atoms. Patterns of micron-sized wires, fabricated lithographically on a flat substrate, can conveniently produce large magnetic-field gradients and curvatures to trap cold atoms and to facilitate the production of Bose-Einstein condensates. The intent of this paper is to provide the researcher who has access to a standard clean-room enough information to design and fabricate such devices.