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Journal of Physics B Atomic Molecular and Optical Physics

A microchip ring trap for cold atoms

M. B. Crookston, Paul M. A. Baker, Michael Robinson

Published Sep 5, 2005• 24 citations
Publisher

Abstract

We describe a method to create a circular magnetic waveguide for deBroglie waves on a microchip. The guide is a two-dimensional magnetic minimum for trapping weak-field seeking states of atoms or molecules with a magnetic dipole moment. It is created completely by electric currents in wires that are lithographically patterned on a multi-level chip. We describe the geometry and time-dependent currents of the wires and show that it is possible to wrap the waveguide in a complete circle with minimal perturbations due to the leads or wire crossings. This maximal area geometry is suited for rotation sensing with atom interferometry via the Sagnac effect using either cold thermal atoms and molecules or Bose-condensed systems.

DOI
10.1088/0953-4075/38/18/001

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We describe a method to create a circular magnetic waveguide for deBroglie waves on a microchip. The guide is a two-dimensional magnetic minimum for trapping weak-field seeking states of atoms or molecules with a magnetic dipole moment. It is created completely by electric currents in wires that are lithographically patterned on a multi-level chip. We describe the geometry and time-dependent currents of the wires and show that it is possible to wrap the waveguide in a complete circle with minimal perturbations due to the leads or wire crossings. This maximal area geometry is suited for rotation sensing with atom interferometry via the Sagnac effect using either cold thermal atoms and molecules or Bose-condensed systems.
We describe the design and function of a circular magnetic waveguide produced from wires on a microchip for atom interferometry using de Broglie waves. The guide is a two-dimensional magnetic minimum for trapping weak-field seeking states of atoms or molecules with a magnetic dipole moment. The design consists of seven circular wires sharing a common radius. We describe the design, the time-dependent currents of the wires and show that it is possible to form a circular waveguide with adjustable height and gradient while minimizing perturbation resulting from leads or wire crossings. This maximal area geometry is suited for rotation sensing with atom interferometry via the Sagnac effect using either cold atoms, molecules and Bose-condensed systems.
We present a scheme for generating a ring magnetic waveguide on a single-layer atom chip. The wire layout consists of two interleaved Archimedean spirals of the same size. The waveguide avoids the trapping perturbation caused by the input and output ports, resulting in an enclosed guiding loop for neutral atoms in weak-field seeking states. Such a configuration can create a tight and deep trap potential with a small current. Taking the |F = 2, m F = 2〉 state of 87 Rb as an example, the trap frequency and depth are estimated to be 18 kHz and 335 μK, respectively, with a dc current of 2 A.
A new scheme to create a closed toroidal magnetic waveguide for deBroglie wave on a single layer atom chip is proposed and there is no zero magnetic field along the guide center. The guide is a two-dimensional magnetic trap for trapping weak-field seeking states of atoms with a magnetic dipole moment. The designed wire structure on the atom chip consists of three concentric and isometric ring wires, and six vertical current leads of the three ring wires. By using the through silicon via technology, the current leads can be made perpendicular to the atom chip surface instead of being generally arranged side by side on the chip surface. Compared with the general wiring way, the vertical lead way has two advantages. One is that each ring wire gap caused by the current leads is substantially smaller than the distance between the ring wires, which permits the generation of a closed toroidal magnetic guide near the atom chip surface when dc currents are supplied to the three ring wires. The other is that the distance between two leads of each ring wire is considerably reduced, resulting in the fact that the magnetic perturbation of the leads to the whole toroidal magnetic guide is negligible. We numerically calculate the magnetic field distribution generated by our wire layout when dc currents are applied, and it is shown that a closed and tight toroidal magnetic guide is formed near the atom chip surface. However, there are zero magnetic fields existing along the center of the toroidal guide, which leads to Majorana spin flips from trapped magnetic substate to an un-trapped magnetic sub-state. According to the time-orbiting-potential principle, we propose an ac current modulation method, which is simple and stable, to reduce the atom losses and suppress the atomic decoherence in the toroidal magnetic guide. We deduce the ac current expressions for the case of three isometric infinite straight wires and apply the ac modulation current expressions directly to our three ring wire structure. The numerical calculation results show that the closed toroidal guide does no longer have zero magnetic fields near the magnetic field minimum, and that the magnetic field fluctuation of the guide is smaller. Based on the vertical leads and ac current modulation, the closed toroidal wave guide with no zero magnetic field along the guide center can be generated near the atom chip surface. This scheme has important scientific significance and engineering value for developing the cold atomic chip gyroscope.
Abstract : During 2005, a new basic research effort was begun to better understand confined cold atom systems and determine if atom interferometry for the detection of rotation was possible utilizing spatially and temporally modulated magnetic fields generated using currents running through traces on microchips. During the next seven years, significant progress has been made in evaluating the constraints posed by both theory and practical experimental considerations. This report encompasses three of these studies: collisional decoherence in trapped interferometers, adjustable microchip ring-traps, and atom chips on direct bonded copper substrates. The first of these describes an analytical model developed to describe the effects of collisions between atoms in a harmonic magnetically confined atom interferometer. The second describes a method to create an agile circular magnetic waveguide for deBroglie waves on a microchip, while the third presents the use of direct bonded copper (DBC) for the straightforward fabrication of high power atom chips. Each of these efforts has incrementally added to our cold atom interferometry knowledge base and has enabled the future construction of a working atom chip based interferometer for the ultrasensitive detection of rotation.
We propose and numerically investigate a scalable ring trap for cold atoms that surmounts problems of roughness of the potential and end effects of trap wires. A stable trapping potential is formed about an electrically isolated, conducting loop in an ac magnetic field by time averaging the superposition of the external and induced magnetic fields. We investigate the use of additional fields to eliminate Majorana spin-flip losses and to create a stable trapping geometry. The possibility of microfabrication of these ring traps offers the prospect of developing Sagnac atom interferometry in atom-chip devices.
We demonstrate efficient loading of neutral atoms into a magnetic waveguide produced by the magnetic fields of microfabricated current-carrying conductors. The lithographically patterned conductors on this ``atom chip'' can be used to make a variety of guiding and trapping structures for manipulating cold atoms and Bose-Einstein condensates. A three-chamber vacuum apparatus collects atoms in a magneto-optical trap, precools them via evaporative cooling, and delivers them to the final chamber containing the atom chip. We describe in detail how the precooled atomic cloud is transferred from a macroscopic magnetic Ioffe-Pritchard trap to the microscopic magnetic waveguide on the atom chip 21cm21\phantom{\rule{0.3em}{0ex}}\mathrm{cm}21cm away. Permanent magnets provide a confining two-dimensional quadrupole field to guide the atoms between the two chambers while longitudinally the cloud is allowed to freely expand during the transfer. Strategically placed coils are used to control the longitudinal size and speed of the atomic cloud as it is loaded on the atom chip.