Bose-Einstein condensation of metastable helium

We present in this thesis the experimental achievement of Bose-Einstein condensation of polarized metastable 2³S₁ helium atoms. First, we summarize the theoretical predictions on elastic and inelastic collision rates between polarized metastable helium atoms. Our hope of reaching Bose-Einstein condensation of metastable helium relied on those promising predictions. We then describe the experimental set-up we have built. The technique we use consists in pre-cooling a sample of gar in a magneto-optical trap which is loaded from an intense and Zeeman slowed atomic beam. The density in the magneto-optical trap is limited by very high inelastic collision rates, which we studied for a wide range of trapping parameters. The cold gas is then transferred in a magnetostatic trap, where it is further cooled by evaporative cooling, down to the threshold of quantum degeneracy. We measured lifetimes of the atoms in the magnetostatic trap of the order of one minute, which shows that inelastic collisions are suppressed by two orders of magnitude when the atoms are polarized. Using a purely optical detection scheme, based on the absorption of a resonant laser beam which travels through the cloud, we have been able to demonstrate Bose-Einstein condensation. Finally, from the measurements of the number of atoms and the size of the condensate, we have deduced an estimate for the scattering length, a=(16±8) nm. (author)