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arXiv:cond-mat/0002311

Rotating Atomic Traps for Bosons and the Centrifuge Effect

T. S. Majmudar, A. Widom

Published Feb 19, 2000• 0 citations
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Abstract

A rigorous time independent Hamiltonian for rotating atomic traps is discussed. The steady states carry a mass current and thereby an angular momentum. It is shown that the rotation positions the atoms away from the rotation axis (after taking both the time and quantum mechanical averages) as in a conventional centrifuge. Some assert that the rotation for Bose condensates cause the atoms to move towards the rotation axis; i.e. act oppositely to fluids in a centrifuge. The opposing physical pictures are reminiscent of the difference between the rotational motion views of Newton and Cassini.

DOI
10.48550/arxiv.cond-mat/0002311

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A rigorous time independent Hamiltonian for rotating atomic traps is discussed. The steady states carry a mass current and thereby an angular momentum. It is shown that the rotation positions the atoms away from the rotation axis (after taking both the time and quantum mechanical averages) as in a conventional centrifuge. Some assert that the rotation for Bose condensates cause the atoms to move towards the rotation axis; i.e. act oppositely to fluids in a centrifuge. The opposing physical pictures are reminiscent of the difference between the rotational motion views of Newton and Cassini.
Single particle states in the atomic trap employing the rotating magnetic field are found using the full time-dependent instantaneous trapping potential. These states are compared with those of the effective time-averaged potential. We show that the trapping is possible when the frequency of the rotations exceeds some threshold. Slightly above this threshold the weakly interacting gas of the trapped atoms acquires the properties of a quasi-1D system in the frame rotating together with the field. The role of the atom-atom interaction in changing the ideal gas solution is discussed. We show that in the limit of large numbers of particles the rotating field whose angular frequency is appropriately modulated can be utilized as a driving force principally for the center of mass motion as well as for the angular momentum L=2L = 2L=2 normal modes of the Bose condensate. A mechanism of quantum evaporation forced by the rotating field is analyzed.
Single-particle states in the atomic trap employing the rotating magnetic field are found using the full time-dependent instantaneous trapping potential. These states are compared with those of the effective time-averaged potential. We show that the trapping is possible when the frequency of the rotations exceeds some threshold. Slightly above this threshold the weakly interacting gas of the trapped atoms acquires the properties of a quasi-one-dimensional system in the frame rotating together with the field. The role of the atom-atom interaction in changing the ideal gas solution is discussed. We show that in the limit of large numbers of particles the rotating field whose frequency is appropriately modulated can be utilized as a driving force principally for the center-of-mass motion as well as for the angular momentum L=2 normal modes of the Bose condensate. A mechanism of quantum evaporation forced by the rotating field is analyzed.
We study the rotational properties of a Bose-Einstein condensate confined in a rotating harmonic trap for different trap anisotropies. With simple arguments, we obtain the velocity field of the quantum fluid for condensates with or without a centered vortex. While the bosons describe open spiraling trajectories, on the frame of reference of the rotating trap, the fluid moves against the trap's rotation. We also find expressions for the angular momentum of linear and Thomas-Fermi solutions for a vortexless state. In these two limits we find the same analytic relation between the shape of the cloud and the rotation speed. Our predictions are supported by numerical simulations of the mean-field Gross-Pitaevskii model and by current experiments with rotating Bose-Einstein condensates (K. W. Madison, F. Chevy, V. Bretin, and J. Dalibard, e-print cond-mat/0101051).
We investigate classical motions of cold atoms in three types (harmonic, linear, and Gaussian) of off-axis rotating traps in the presence of damping forces. When the radius of rotation of the trap is within the trap volume, the atoms are always attracted towards the rotation’s origin if the damping rate Γ is larger than the trap frequency ω ( i.e. , Γ > ω ). This is opposed to being repelled by the centrifugal force in a rotating reference frame. Contrarily, when the trap’s rotation radius is larger than the trap size, the atoms experience a time-averaged ring-shape trap at a high rotational frequency. These anomalous properties can be used to realize high-speed rotation sensors and effective static ring-trap potentials for manipulating cold atoms.
We study the stability of a rotating repulsive-atom Bose-Einstein condensate in a toroidal trap. The resulting macroscopic angular-momentum states with integer vorticity l spread radially, lowering rotational energies. These states are robust against vorticity-lowering decays, with estimated metastability barriers capable of sustaining large angular momenta ( l ≲10) for typical parameters. We identify the centrifugally squashed l -dependent density profile as a possible signature of condensate rotation and superfluidity.
We have observed the expansion of vortex-free, rotating Bose condensates after their sudden release from a slowly rotating anisotropic trap. Conservation of angular momentum, combined with the constraint of irrotational flow, cause the rotating condensate to expand in a distinctively different way to one released from a static (nonrotating) trap. This difference provides clear experimental evidence of the purely irrotational velocity field associated with a superfluid. We observed this behavior in absorption images taken along the rotation axis.