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arXiv:cond-mat/0211345 · Physical Review Letters

Bose-Einstein Condensates near a Microfabricated Surface

A. E. Leanhardt, Yong-il Shin, A. P. Chikkatur, D. Kielpinski, Wolfgang Ketterle, David E. Pritchard

Published Mar 12, 2003• 138 citations
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Abstract

Magnetically and optically confined Bose-Einstein condensates were studied near a microfabricated surface. Condensate fragmentation observed in microfabricated magnetic traps was not observed in optical dipole traps at the same location. The measured condensate lifetime was >or=20 s and independent of the atom-surface separation under both magnetic and optical confinement. Radio-frequency spin-flip transitions driven by technical noise were directly observed for optically confined condensates and could limit the condensate lifetime in microfabricated magnetic traps.

DOI
10.1103/physrevlett.90.100404

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Magnetically and optically confined Bose-Einstein condensates were studied near a microfabricated surface. Condensate fragmentation observed in microfabricated magnetic traps was not observed in optical dipole traps at the same location. The measured condensate lifetime was >or=20 s and independent of the atom-surface separation under both magnetic and optical confinement. Radio-frequency spin-flip transitions driven by technical noise were directly observed for optically confined condensates and could limit the condensate lifetime in microfabricated magnetic traps.
We describe an experiment in which Bose-Einstein condensates and cold atom clouds are held by a microscopic magnetic trap near a room-temperature metal wire 500 microm in diameter. The lifetime for atoms to remain in the microtrap is measured over a range of distances down to 27 microm from the surface of the metal. We observe the loss of atoms from the microtrap due to spin flips. These are induced by radio-frequency thermal fluctuations of the magnetic field near the surface, as predicted but not previously observed.
We investigate the stability of magnetically trapped atomic Bose-Einstein condensates and thermal clouds near the transition temperature at small distances $0.5\text{ }\ensuremath{\mu}\mathrm{m}\ensuremath{\le}d\ensuremath{\le}10\text{ }\ensuremath{\mu}\mathrm{m}$ from a microfabricated silicon chip. For a $2\text{ }\ensuremath{\mu}\mathrm{m}$ thick copper film, the trap lifetime is limited by Johnson noise induced currents and falls below 1 s at a distance of $4\text{ }\ensuremath{\mu}\mathrm{m}$. A dielectric surface does not adversely affect the sample until the attractive Casimir-Polder potential significantly reduces the trap depth.
Atom chips are devices used to magnetically trap and manipulate ultracold atoms and Bose-Einstein condensates near a surface. In particular, permanent magnetic film atom chips can allow very tight confinement and intricate magnetic field designs while circumventing technical current noise. Research described in this thesis is focused on the development of a magnetic film atom chip, the production of Bose-Einstein condensates near the film surface, the characterisation of the associated magnetic potentials using rf spectroscopy of ultracold atoms and the realisation of a precision sensor based on splitting Bose-Einstein condensates in a double-well potential. The atom chip itself combines the edge of a perpendicularly magnetised GdTbFeCo film with a machined silver wire structure. A mirror magneto-optical trap collects up to 5 x 108 87Rb atoms beneath the chip surface. The current-carrying wires are then used to transfer the cloud of atoms to the magnetic film microtrap and radio frequency evaporative cooling is applied to produce Bose-Einstein condensates consisting of 1 x 105 atoms. We have identified small spatial magnetic field variations near the film surface that fragment the ultracold atom cloud. These variations originate from inhomogeneity in the film magnetisation and are characterised using a novel technique based on spatially resolved radio frequency spectroscopy of the atoms to map the magnetic field landscape over a large area. The observations agree with an analytic model for the spatial decay of random magnetic fields from the film surface. Bose-Einstein condensates in our unique potential landscape have been used as a precision sensor for potential gradients. We transfer the atoms to the central region of the chip which produces a double-well potential. A single BEC is formed far from the surface and is then dynamically split in two by moving the trap closer to the surface. After splitting, the population of atoms in each well is extremely sensitive to the asymmetry of the potential and can be used to sense tiny magnetic field gradients or changes in gravity on a small spatial scale.
Ultra-cold neutral atoms and Bose-Einstein condensates (BECs) offer fascinating possibilities for quantum information processing. Long decoherence times, easy qubit addressing and the potential for scalability are some advantages. Storing and manipulating atoms in magnetic microtraps or 'Atom Chips' provides a promising base on which quantum logic operations could be performed. A primary source of decoherence in atom chip experiments arises from the interaction of the micro-Kelvin atoms with the room temperature chip surface. We have experimentally investigated these interactions and observe two interesting effects. Firstly, ultra-cold clouds are seen to fragment into clumps when brought within 100 microns of the chip surface. Secondly, the lifetime of atom clouds is seen to drop off near the surface due to spin flip transitions. These are caused by radio-frequency thermal fluctuations of the magnetic field as predicted but not previously observed.
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.
Gaseous Bose-Einstein condensates of 2-3 x 10(6) 23Na atoms were loaded into a microfabricated magnetic trap using optical tweezers. Subsequently, the condensates were released into a magnetic waveguide and propagated 12 mm. Single-mode propagation was observed along homogeneous segments of the waveguide. Inhomogeneities in the guiding potential arose from geometric deformations of the microfabricated wires and caused strong transverse excitations. Such deformations may restrict the waveguide physics that can be explored with propagating condensates. Finer perturbations to the guiding potential fragmented the condensate when it was brought closer to the surface.