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arXiv:cond-mat/0004015 · Journal of Physics B Atomic Molecular and Optical Physics

Bose-Einstein condensation in a stiff TOP trap with adjustable geometry

E Hodby, G Hechenblaikner, Onofrio M. Maragò, J. Arlt, S Hopkins, Christopher J. Foot

Published Sep 15, 2000• 7 citations
arXivHTMLPDF

Abstract

We report on the realization of a stiff magnetic trap with independently adjustable trap frequencies, ω z and ω r , in the axial and radial directions, respectively. This has been achieved by applying an axial modulation to a time-averaged orbiting potential (TOP) trap. The frequency ratio of the trap, ω z /ω r , can be decreased continuously from the original TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein condensate (BEC) into this trap and obtained very good agreement between its observed anisotropic expansion and the hydrodynamic predictions. Our method can be extended to obtain a spherical trapping potential, which has a geometry of particular theoretical interest.

DOI
10.1088/0953-4075/33/19/319

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We report on the realization of a stiff magnetic trap with independently adjustable trap frequencies, ω z and ω r , in the axial and radial directions, respectively. This has been achieved by applying an axial modulation to a time-averaged orbiting potential (TOP) trap. The frequency ratio of the trap, ω z /ω r , can be decreased continuously from the original TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein condensate (BEC) into this trap and obtained very good agreement between its observed anisotropic expansion and the hydrodynamic predictions. Our method can be extended to obtain a spherical trapping potential, which has a geometry of particular theoretical interest.
We have added an optical potential to a conventional time-averaged orbiting potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold atoms. Axial confinement is provided by the optical potential; the maximum frequency currently obtainable in this direction is 2.2 kHz for rubidium. The radial confinement is independently controlled by the magnetic trap and can be a factor of 700 times smaller than in the axial direction. This large anisotropy is more than sufficient to confine condensates with ∼10 5 atoms in a quasi-2D (Q2D) regime, and we have verified this by measuring a change in the free expansion of the condensate; our results agree with a variational model.
We show that topological vortex pumping can be implemented for a dilute Bose-Einstein condensate confined in a magnetic time-averaged orbiting potential trap with axial optical confinement. Contrary to earlier proposals for the vortex pump, we do not employ an additional optical potential to trap the condensate in the radial direction, but instead, the radial confinement is provided by the magnetic field throughout the pumping cycle. By performing numerical simulations based on the spin-1 Gross-Pitaevskii equation, we find that several pumping cycles can be carried out to produce a highly charged vortex before a majority of the particles escape from the trap or before the vortex splits into singly charged vortices. On the other hand, we observe that an additional, relatively weak optical plug potential is efficient in preventing splitting and reducing particle loss. With these results, we hope to bring the vortex pump closer to experimental realization.
We report on the manipulation of the center-of-mass motion (``sloshing'') of a Bose-Einstein condensate in a time-averaged orbiting potential (TOP) trap. We start with a condensate at rest in the center of a static trapping potential. When suddenly replacing the static trap with a TOP trap centered about the same position, the condensate starts to slosh with an amplitude much larger than the TOP micromotion. We show, both theoretically and experimentally, that the direction of sloshing is related to the initial phase of the rotating magnetic field of the TOP. We show further that the sloshing can be quenched by applying a carefully timed and sized jump in the phase of the rotating field.
We present the first experimental realization of Bose–Einstein condensation in a purely magnetic double-well potential. This has been achieved by combining a static Ioffe–Pritchard trap with a time orbiting potential. The double trap can be rapidly switched to a single-harmonic trap of identical oscillation frequencies, thus accelerating the two condensates towards each other. Furthermore, we show that time-averaged potentials can be used as a means to control the radial confinement of the atoms. Manipulation of the radial confinement allows vortices and radial quadrupole oscillations to be excited.
We have observed Bose-Einstein condensation of large numbers of 87Rb{}^{87}\mathrm{Rb}87Rb atoms in a magnetic time-averaged orbiting potential (TOP) trap. Over $1.5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms are captured in the trap and evaporatively cooled to produce condensates containing up to $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms. We discuss special considerations relating to the production of large Bose-Einstein condensates in a TOP trap, and present measurements of the condensate fraction for this case.