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Quantum Information and Computation

Fabrication of micro-magnetic traps for cold neutral atoms

Benjamin Lev

Published Jan 1, 2003• 21 citations
Publisher

Abstract

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.

DOI
10.26421/qic3.5-5

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•Sep 22, 2005•6 citations
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.
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. This paper describes tools and techniques for the construction of such devices.
Micro-electromagnets for atom manipulation have been constructed, including magnetic mirrors (serpentine patterns) and traps (circular patterns). They consist of planar micron-scale Au wires on sapphire substrates fabricated using lithography and electroplating. At liquid nitrogen or helium temperatures in vacuum the wires support currents of several amperes with current density ∼108 A/cm2 and power dissipation ∼10 kW/cm2, and they produce magnetic fields to 0.3 T and gradients to 103 T/cm. The micro-electromagnet mirror was used to deflect a beam of metastable helium atoms at grazing angles ∼0.5 mrad.
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.
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.
The system comprised of an atom strongly coupled to photons, known as cavity quantum electrodynamics (QED), provides a rich experimental setting for quantum information processing, both in the implementation of quantum logic gates and in the development of quantum networks. Moreover, studies of cavity QED will help elucidate the dynamics of continuously observed open quantum systems with quantum-limited feedback. To achieve these goals in cavity QED, a neutral atom must be tightly confined inside a high-finesse cavity with small mode volume for long periods of time. Microfabricated wires on a substrate---known as an atom chip---can create a sufficiently high-curvature magnetic potential to trap atoms in the Lamb-Dicke regime. We have recently integrated an optical fiber Fabry-Perot cavity with such a device. The microwires allow the on-chip collection and laser cooling of neutral atoms, and allow the magnetic waveguiding of these atoms to an Ioffe trap inside the cavity mode. Magnetically trapped intracavity atoms have been detected with this cavity QED system. A similar experiment employing microdisks and photonic bandgap cavities is nearing completion. With these more exotic cavities, a robust and scalable atom-cavity chip system will deeply probe the strong coupling regime of cavity QED with magnetically trapped atoms. Atom chips have found great success in producing and manipulating Bose-Einstein condensates and in creating novel atom optical elements. An on-chip BEC has been attained in a miniaturized system incorporating an atom chip designed for atom interferometry and for studies of Josephson effects of a BEC in a double-well potential. Using similar microfabrication techniques, we created and demonstrated a specular magnetic atom mirror formed from a standard computer hard drive. This device, in conjunction with micron-sized charged circular pads, can produce a 1-D ring trap which may prove useful for studying Tonks gases in a ring geometry and for creating devices such as a SQUID-like system for neutral atoms. This thesis describes the fabrication and employment of these atoms chips in experiments at both Caltech and Munich, the latter in collaboration with Professors Theodore Haensch and Jakob Reichel at the Max Planck Institute for Quantum Optics.