Refroidissement d'atomes de césium confinés dans un piège dipolaire très désaccordé

Abstract : Several methods for cooling caesium atoms trapped in a non dissipative optical trap (crossed dipole trap) are presented in this thesis. The trap consists of two Nd:YAG focused beams. Atoms in one of the hyperfine ground states remain trapped at the crossing of the two beams for one to two seconds. Available densities are as high as 10^12 atomes/cm3. To manipulate these atoms, we use a two-photon stimulated Raman transition between the hyperfine ground states. We present a new Raman pulse very efficient, using rapid adiabatic passage between these states. This new pulse is used in all the cooling experiments presented here.

For a first kind of cooling experiment, we produce a one-dimensional lattice with a period of the order of the wavelength superposed to the trap. The resulting vibrational levels are resolved by the Raman transitions. Atoms are cooled in this lattice using resolved-sideband cooling, a method first used on ions and applied here on neutral atoms for the first time. 90% of the atoms are cooled to the zero-point of motion in the lattice.

This thesis also presents the results obtained by Raman cooling of trapped atoms. This technique, very efficient in one dimension on free atoms, is applied in three dimensions to trapped atoms, polarized or not. We have developed a method to polarize and cool the atoms at the same time using Raman transitions. We get temperatures in the 2 µK range at atomic densities of 10^12 atomes/cm3 which represents a gain of three to four orders of magnitude in phase-space density with respect to a magneto-optical trap. We show that the limit we reach is due to reabsorption of resonant photons produced by the repumping process. By lowering the atomic density during Raman cooling, the reabsorption is reduced and we get temperatures as low as 680 nK.
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Hélène Perrin. Refroidissement d'atomes de césium confinés dans un piège dipolaire très désaccordé. Physique Atomique [physics.atom-ph]. Université Pierre et Marie Curie - Paris VI, 1998. Français. ⟨tel-00003896⟩

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