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arXiv:cond-mat/0308457 · Physical Review Letters

Impact of the Casimir-Polder Potential and Johnson Noise on Bose-Einstein Condensate Stability Near Surfaces

Y.-J. Lin, Igor Teper, Cheng Chin, Vladan Vuletić

Published Feb 6, 2004• 285 citations
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Abstract

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.

DOI
10.1103/physrevlett.92.050404

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We investigate the stability of magnetically trapped atomic Bose-Einstein condensates and thermal clouds near the transition temperature at small distances 0.5 \ensuremathμm\ensuremath≤d\ensuremath≤10 \ensuremathμm0.5\text{ }\ensuremath{\mu}\mathrm{m}\ensuremath{\le}d\ensuremath{\le}10\text{ }\ensuremath{\mu}\mathrm{m}0.5 \ensuremathμm\ensuremath≤d\ensuremath≤10 \ensuremathμm from a microfabricated silicon chip. For a 2 \ensuremathμm2\text{ }\ensuremath{\mu}\mathrm{m}2 \ensuremathμm thick copper film, the trap lifetime is limited by Johnson noise induced currents and falls below 1 s at a distance of 4 \ensuremathμm4\text{ }\ensuremath{\mu}\mathrm{m}4 \ensuremathμm. A dielectric surface does not adversely affect the sample until the attractive Casimir-Polder potential significantly reduces the trap depth.
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.
Cold atoms can be trapped and guided using nanofabricated wires on microengineered surfaces, achieving the scales required by quantum information proposals on the consequences of Casimir-Polder (CP) force to its stability and lifetime. The atoms are found to be less attractive to the thin films compared to dielectric half plate, where the attractive potential drops even further when the thickness of the surface is lower than both thermal wavelength and the spacing length. Amiably, for the spherical surface, the CP potential decays even further as radial thickness decreases. We approximate the CP potential using the analytical expression obtained from the pairwise summation (PWS) calculated by taking a sum of single atom with the jumbled atoms inside the surface, treating them as two-bodies system as a whole. The means of PWS are to calculate the possibilities of geometrical effect: planar and curved surface on the CP potential.
We have measured magnetic trap lifetimes of ultra-cold Rb87 atoms at distances of 5-1000 microns from surfaces of conducting metals with varying resistivity. Good agreement is found with a theoretical model for losses arising from near-field magnetic thermal noise, confirming the complications associated with holding trapped atoms close to conducting surfaces. A dielectric surface (silicon) was found in contrast to be so benign that we are able to evaporatively cool atoms to a Bose-Einstein condensate by using the surface to selectively adsorb higher energy atoms.
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We report on the first measurement of a temperature dependence of the Casimir-Polder force. This measurement was obtained by positioning a nearly pure 87Rb Bose-Einstein condensate a few microns from a dielectric substrate and exciting its dipole oscillation. Changes in the collective oscillation frequency of the magnetically trapped atoms result from spatial variations in the surface-atom force. In our experiment, the dielectric substrate is heated up to 605 K, while the surrounding environment is kept near room temperature (310 K). The effect of the Casimir-Polder force is measured to be nearly 3 times larger for a 605 K substrate than for a room-temperature substrate, showing a clear temperature dependence in agreement with theory.
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.