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arXiv:physics/0007051 · Physical Review Letters

Quasicontinuous Atom Laser in the Presence of Gravity

Fabrice Gerbier, Philippe Bouyer, Alain Aspect

Published May 21, 2001• 55 citations
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Abstract

We analyze the extraction from a trapped Bose-Einstein condensate to a nontrapping state of a coherent atomic beam and its subsequent fall, at T = 0 K. Our treatment fully takes gravity into account but neglects the mean-field potential exerted on the free falling beam by the trapped atoms. We derive analytical expressions for the output rate and the output mode of the quasicontinuous "atom laser." Comparison with experimental data of Bloch et al. [Phys. Rev. Lett. 82, 3008 (1999)] is satisfactory.

DOI
10.1103/physrevlett.86.4729

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We analyze the extraction from a trapped Bose-Einstein condensate to a nontrapping state of a coherent atomic beam and its subsequent fall, at T = 0 K. Our treatment fully takes gravity into account but neglects the mean-field potential exerted on the free falling beam by the trapped atoms. We derive analytical expressions for the output rate and the output mode of the quasicontinuous "atom laser." Comparison with experimental data of Bloch et al. [Phys. Rev. Lett. 82, 3008 (1999)] is satisfactory.
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Atoms can be extracted from a trapped Bose-Einstein condensate (BEC) by driving spin-flips to untrapped states. The coherence properties of the BEC are transfered to the released atoms, creating a coherent beam of matter refered to as an atom laser. In this work, the extraction of atoms from a BEC is investigated numerically by solving a coupled set of Gross-Pitaevskii equations in up to three dimensions. The result is compared to experimental data and a semiclassical rate model. In the weak-coupling regime, quantitative agreement is reached between theory and experiment and a semiclassical rate model. In the strong-coupling regime, the atom laser enters a trapped state that manifests itself in a saturation of the rate of out-coupled atoms observed in new experimental data. The semiclassical rate model fails, but the numerical descriptions yield qualitative agreement with experimental data at the onset of saturation.
We study the coherence properties of an atom laser, which operates by extracting atoms from a gaseous Bose-Einstein condensate via a two-photon Raman process, by analysing a recent experiment (Hagley et al 1999 Phys. Rev. Lett. 83 3112). We obtain good agreement with the experimental data by solving the time-dependent Gross-Pitaevskii equation in three dimensions both numerically and with a Thomas-Fermi model. The coherence is strongly affected by the space-dependent phase developed by the condensate when the trapping potential is turned off.
We report the first realization of a guided quasicontinuous atom laser by rf outcoupling a Bose-Einstein condensate from a hybrid optomagnetic trap into a horizontal atomic waveguide. This configuration allows us to cancel the acceleration due to gravity and keep the de Broglie wavelength constant at 0.5 microm during 0.1 s of propagation. We also show that our configuration, equivalent to pigtailing an optical fiber to a (photon) semiconductor laser, ensures an intrinsically good transverse mode matching.
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.