Abstract : We report the results of few experiments on ultra-cold gases of metastable helium which tackle two topics : quantum atom optics and collisional properties between different spin-states. The first bunch of studies presented in this thesis have been carried out on a set of correlated atom pairs created by collision between two Bose-Einstein condensates which might be seen as an analogous of parametric down conversion in quantum photon optics. Contrary to an ideal s-wave scattering distribution, however, we find that the scattered halo is not uniform and spherical, but instead has an angle dependent thickness and radius due to the interactions between atoms. In addition, by considering opposing regions of the halo, we observe relative number squeezing whereas relative atom number distributions between nonopposing regions show a poissonnian behaviour. This leads us to believe that the quantum states of the correlated pairs are not only inherently fascinating , but they also have the potential to improve sensitivity in atom interferometers. Moreover, these pairs should be entangled and are thus well suited to investigations of (nonlocal) EPR entanglement and Bell's inequalities using atoms. Then, we give a detailed description of our new optical trap which permits us to best match the resolution characteristics of our delay-line anode microchannel plate detector capable of registering single atoms in the three dimensions of the space. We also confirm the stability of certain spin-state combinations of metastable helium to two-body inelastic processes, which gives the scope of future experiments using optically trapped spin mixtures.