Etude des propriétés de cohérence d'un condensat de Bose-Einstein à l'équilibre et hors équilibre

Mathilde Hugbart-Fouché 1
1 laboratoire Charles Fabry de l'Institut d'Optique / Optique atomique
LCFIO - Laboratoire Charles Fabry de l'Institut d'Optique
Abstract : The Bose-Einstein condensate, the atomic counterpart to laser in optics, is usually considered as a fully coherent object. However this is true for systems of moderate anisotropy only. For highly elongated condensates, phase fluctuations appear along the long axis, and these imply a reduction of the coherence length. Such a phase fluctuating condensate is called a quasi-condensate. In this thesis, we report on two experimental studies about the coherence of highly elongated condensates. The first study has been realized on condensates at equilibrium. The measurement of the coherence length has been done through the measurement of the correlation function using an interferometric method. Our results show the transition between a full phase coherent condensate and a quasi-condensate, the coherence length decreasing when the amplitude of the phase fluctuations is increased. We have also observed that the expected shape of the correlation function changes from a Gaussian-like shape to an exponential-like shape. The second study focuses on the formation of the condensate, and more particularly on the kinetic of the establishment of the phase coherence. Our results show that the growth of the condensed fraction proceeds qualitatively as for 3D non-phase-fluctuating condensates. Finally, we have measured the coherence length during the formation using Bragg spectroscopy. Our results show a fast establishment of the coherence compared to the time needed for the condensed fraction to reach equilibrium.
Document type :
Physique Atomique [physics.atom-ph]. Université Paris Sud - Paris XI, 2005. Français
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Mathilde Hugbart-Fouché. Etude des propriétés de cohérence d'un condensat de Bose-Einstein à l'équilibre et hors équilibre. Physique Atomique [physics.atom-ph]. Université Paris Sud - Paris XI, 2005. Français. 〈tel-00010825〉



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