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Spectroscopie en impulsion des fluctuations de phase dans des condensats de Bose-Einstein très allongés

Simon Richard 1
1 laboratoire Charles Fabry de l'Institut d'Optique / Optique atomique
LCFIO - Laboratoire Charles Fabry de l'Institut d'Optique
Abstract : In this thesis, we present an experiment that measures phase fluctuations in very elongated Bose-Einstein condensates. Bose-Einstein condensates are usually described as fully coherent objects. This is true for condensates with a moderate anisotropy. It is not correct, however, for very elongated condensates, whose low energy excited states are 1D, and thus may be strongly thermally populated even at low temperature. The large population of low energy excitations results in fluctuations of the phase and thus in a reduced coherence length of the condensate along its long direction, while its density profile is not affected. Such a phase fluctuating condensate is called a quasi-condensate. We use Bragg spectroscopy to measure phase fluctuations. This method allows us to measure momentum distribution of atoms in the quasi-condensate. The Fourier transform of the momentum distribution then gives the correlation function whose characteristic decay length is the coherence length of the quasi-condensate. Our measurements show a lorentzian momentum distribution, whose width increases with temperatures. This is evidence for a loss of coherence as temperature increases, in quantitative agreement with theoretical predictions. We also study the density profile of quasi-condensates. No deviation from a true condensate density profile could be measured, confirming the suppression of density fluctuations in quasi-condensates.
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https://tel.archives-ouvertes.fr/tel-00004396
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Submitted on : Tuesday, July 3, 2012 - 12:23:16 PM
Last modification on : Wednesday, September 16, 2020 - 5:42:34 PM
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  • HAL Id : tel-00004396, version 2

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Simon Richard. Spectroscopie en impulsion des fluctuations de phase dans des condensats de Bose-Einstein très allongés. Physique Atomique [physics.atom-ph]. Université Paris Sud - Paris XI, 2003. Français. ⟨tel-00004396v2⟩

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