Coherent dynamics, relaxation and fragmentation of a spinor Bose-Einstein condensate

In this thesis, we present some experiments realized with ultracold gases of sodium atoms, trapped at the intersect between two laser beams. At very low temperature, the discretization of energy and the indistinguishability of particles, lead to a new state of matter, a Bose-Einstein condensate. This remarkable phenomenon was initially introduced to describe an ideal gas, that is to say with no interactions between its constituents. Here, we are interested in the efects of the interactions between the atoms. More precisely, our atoms carry a spin 1, and we focus on the collective spin state, in a regime where the spatial degrees of freedom are frozen. Two important results that we present were obtained by embedding the condensate in a nearly vanishing magnetic ‚€eld. In that regime, interactions dominate and favor the emergence of strongly correlated states. In a ‚€rst series of experiment, the magnetic ‚€eld is suddenly decreased to bring the system out-of-equilibrium. The ensuing relaxation dynamics leads to a stationnary state that can be well described by a Gibbs ensemble. In a second experiment, the ‚€eld is slowly reduced, in order to follow the ground state of the system.We thereby produce a fragmented condensate, which possesses the remarkable feature of being invariant upon spin rotations. The restoration of this symmetry, always broken by single (i.e. non-fragmented) condensates, is driven by the pairing of atoms in singlet states.