Thèse de doctorat en Physique. Physique Quantique
Sous la direction de Jean Dalibard.
Soutenue en 2012
à Paris 6 .
This thesis reports on the magnetic properties of a sodium Bose-Einstein condensate in an optical dipole trap. In the first part, we present the apparatus and the experimental sequence for the production of an all-optical Bose-Einstein condensate. In a first step, we load a red-detuned crossed optical dipole trap from laser-cooled atoms. In a second step, atoms are evaporatively cooled in a combined trap formed by the crossed dipole trap and a tightly focused trap. We observe an efficient ``evaporative filling'' of the auxiliary trap, which maintains a high collision rate during the cooling stage. As a result, we cross the condensation threshold and are able to obtain almost pure Bose-Einstein condensates containing approximately 3000 atoms. In the second part, we concentrate on the study of the magnetic properties of the spinor gases. We present methods to control precisely the magnetization of the samples and describe the diagnostic of the spin composition. We use these sample to explore the low temperature phase diagram, as function of the magnetization and of the magnetic field. We find a good agreement between the experimental results and a mean-field theory assuming the single-mode approximation. Finally, we discuss the results of experiments where anomalous fluctuations of the relative populations are observed, at low field and at low magnetization. They are due to collective fluctuations that tend to restore spin symmetry. At the thermodynamic limit, they are expected to disappear, but are present in our finite size samples.
Spin 1 Bose-Einstein condensates : experimental study with sodium atoms in an optical trap
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