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arXiv:cond-mat/0309536 · Physical Review Letters

Two-Dimensional Bose-Einstein Condensate in an Optical Surface Trap

D. Rychtarik, B. Engeser, Hanns‐Christoph Nägerl, Rudolf Grimm

Published Apr 27, 2004• 194 citations
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Abstract

We report on the creation of a two-dimensional Bose-Einstein condensate of cesium atoms in a gravito-optical surface trap. The condensate is produced a few microm above a dielectric surface on an evanescent-wave atom mirror. After evaporative cooling by all-optical means, expansion measurements for the tightly confined vertical motion show energies well below the vibrational energy quantum. The presence of a condensate is observed in two independent ways by a magnetically induced collapse at negative scattering length and by measurements of the horizontal expansion.

DOI
10.1103/physrevlett.92.173003

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We report on the creation of a two-dimensional Bose-Einstein condensate of cesium atoms in a gravito-optical surface trap. The condensate is produced a few microm above a dielectric surface on an evanescent-wave atom mirror. After evaporative cooling by all-optical means, expansion measurements for the tightly confined vertical motion show energies well below the vibrational energy quantum. The presence of a condensate is observed in two independent ways by a magnetically induced collapse at negative scattering length and by measurements of the horizontal expansion.
Summary form only given. Optical dipole traps facilitate storage of atoms independent of their magnetic sub-state. Trapping at high densities is thus not limited by two-body collisions changing the spin state of the atom, which in the particular case of cesium have so far prevented the attainment of Bose-Einstein condensation in magnetic traps. For evaporative cooling of Cs we use the gravito-optical surface trap (GOST) which consists of a horizontal evanescent-wave (EW) atom mirror in combination with a blue-detuned hollow beam (HE) for transverse confinement. Starting conditions for evaporative cooling are prepared by optical cooling, based on inelastic reflections from the EW.
Abstract We study both static and dynamic properties of a weakly interacting Bose–Einstein condensate (BEC) in a quasi one-dimensional gravito-optical surface trap, where the downward pull of gravity is compensated by the exponentially decaying potential of an evanescent wave. First, we work out approximate solutions of the Gross–Pitaevskii equation for both a small number of atoms using a Gaussian ansatz and a larger number of atoms using the Thomas–Fermi limit. Then we confirm the accuracy of these analytical solutions by comparing them to numerical results. From there, we numerically analyze how the BEC cloud expands non-ballistically, when the confining evanescent laser beam is shut off, showing agreement between our theoretical and previous experimental results. Furthermore, we analyze how the BEC cloud expands non-ballistically due to gravity after switching off the evanescent laser field in the presence of a hard-wall mirror which we model by using a blue-detuned far-off-resonant sheet of light. There we find that the BEC shows significant self-interference patterns for a large number of atoms, whereas for a small number of atoms, a revival of the BEC wave packet with few matter-wave interference patterns is observed.
A dense gas of cesium atoms at the crossover to two dimensions is prepared in a highly anisotropic surface trap that is realized with two evanescent light waves. Temperatures as low as 100 nK are reached with 20,000 atoms at a phase-space density close to 0.1. The lowest quantum state in the tightly confined direction is populated by more than 60%. The system provides atoms at a mean distance from the surface as low as 1 microm, and offers intriguing prospects for future experiments on degenerate quantum gases in two dimensions.
The achievement of a Bose-Einstein condensate of cesium in the lowest internal state (F=3, m/sub F/=3) by evaporative cooling in an optical trap is reported. The corresponding scattering length shows a strong dependence on magnetic field, varying from large negative to large positive values in a conveniently accessible magnetic field range due to a unusual combination of broad and narrow Feshbach resonances. The inelastic processes resulting from this complex scattering behavior in thermal ensembles is studied. These measurements have been of crucial importance for identifying a suitable path in phase space towards condensation. In first experiments, the strong dependence of the condensate mean-field energy on the magnetic field is demonstrated. By briefly switching to different scattering lengths just before releasing the sample from the trap, imploding, exploding, and non-interacting 'frozen' condensates are produced. We will employ the tunable interaction in further experiments by looking at molecule formation on Feshbach resonances and exploring condensate dynamics at extreme values of the scattering length.
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 demonstrate a gravito-optical surface trap for Cs atoms which exploits cooling in an evanescent light wave. About 105{0}^{5}05 atoms were cooled down to 3 \ensuremath{\mu}K and formed a sample with a mean height of $\ensuremath{\sim}20\ensuremath{\mu}\mathrm{m}$ above the surface of a dielectric prism. The trap does not use a magnetic field and leads to very small atomic level perturbations. The excited-state population of the stored atoms is $\ensuremath{\sim}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and collisional losses are strongly suppressed.