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arXiv:0808.3931 · Physical Review A

Radio-frequency-induced ground-state degeneracy in a Bose-Einstein condensate of chromium atoms

Quentin Beaufils, T. Zanon-Willette, Radu Chicireanu, B. Laburthe-Tolra, É. Maréchal, L. Vernac, J. Keller, O. Gorceix

Published Nov 20, 2008• 17 citations
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Abstract

We study the effect of strong radio-frequency (rf) fields on a Bose-Einstein condensate (BEC) of chromium atoms, in a regime where the rf frequency is much larger than the Larmor frequency. We use the modification of the Land\'e factor by the rf field to bring all Zeeman states to degeneracy, despite the presence of a static magnetic field of up to $100\phantom{\rule{0.3em}{0ex}}\mathrm{mG}$. This is demonstrated by analyzing the trajectories of the atoms under the influence of dressed magnetic potentials in the strong-field regime. We investigate the problem of adiabaticity of the rf dressing process and relate it to how close the dressed states are to degeneracy. Finally, we measure the lifetime of the rf dressed BECs and identify a rf-assisted two-body loss process induced by dipole-dipole interactions.

DOI
10.1103/physreva.78.051603

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We study the effect of strong radio-frequency (rf) fields on a chromium Bose-Einstein condensate, in a regime where the rf frequency is much larger than the Larmor frequency. We show that the Lande factor of the atoms is lowered by the presence of the rf, and can be even set to zero for a proper combination of the rf power and frequency. The trajectories of the atoms under the influence of magnetic potentials are thus greatly modified. We study the criteria for adiabaticity of the rf dressing process leading to such a modification of the atom magnetic susceptibility. We also measure the lifetime of the rf dressed BECs.
Radio-frequency-induced evaporative cooling of 87Rb{}^{87}\mathrm{Rb}87Rb atoms in the F=2,F=2,F=2, m=2m=2m=2 ground state in a magnetic trap with a high bias field is found to be hampered, and eventually interrupted, by the presence of nonlinear Zeeman shifts. An experiment studying evaporation at a fixed adjustable rf frequency was used to illustrate this effect. These nonlinear shifts prevent us from reaching quantum degeneracy in the F=2,F=2,F=2, m=2m=2m=2 ground state; however, evaporation in the F=1,F=1,F=1, $m=\ensuremath{-}1$ ground state is not adversely affected, and we can produce a Bose-Einstein condensate (BEC). Our analysis shows that this effect can play a role in many BEC experiments.
We study dressed Bose-Einstein condensates in an atom chip radio-frequency trap. We show that in this system sufficiently strong dressing can be achieved to cause the widely used rotating-wave approximation (RWA) to break down. We present a full calculation of the atom-field coupling which shows that the non-RWA contributions quantitatively alter the shape of the emerging dressed adiabatic potentials. The non-RWA contributions furthermore lead to additional allowed transitions between dressed levels. We use rf spectroscopy of Bose-Einstein condensates trapped in the dressed state potentials to directly observe the transition from the RWA to the beyond-RWA regime.
Bose-Einstein condensates of 87Rb^{87}\mathrm{Rb}87Rb atoms are transferred into radio-frequency induced adiabatic potentials and the properties of the corresponding dressed states are explored. We report on measurements of the spin composition of dressed condensates. We also show that adiabatic potentials can be used to trap atom gases in novel geometries, including suspending a cigar-shaped cloud above a curved sheet of atoms.
We evaluate the impact that nonlinear Zeeman shifts have on resonant radio-frequency (RF) dressed traps in an atom-chip configuration. The degeneracy of the resonance between Zeeman levels is lifted at large intensities of a static field, modifying the spatial dependence of the atomic adiabatic potential. In this context, we find effects that are important for the next generation of atom chips with tight trapping: in particular, that the vibrational frequency of the atom trap is sensitive to the RF frequency and, depending on the sign of the Landé factor, can produce significantly weaker, or tighter trapping when compared to the linear regime of the Zeeman effect. We take 87 Rb as an example and find that it is possible for the trapping frequency on F = 1 to exceed that of the F = 2 hyperfine manifold.