Abstract : Thanks to laser cooling techniques, very low temperatures are now attainable. Quantum collective effects may consequently be observed. Since the densities involved in laser cooling experiments are rather low, such collective effects become relevant only when the average atomic velocity is smaller than the one photon recoil velocity. A subrecoil laser cooling method (Raman cooling) has been performed experimentally in one dimension, and is presented in the first part of this thesis. In order to generalise this technique to three dimensions with trapped atoms, we have realised a new kind of trap (the opto-electrical trap). In the second part of this thesis, we study theoretically an optical method for detecting collective quantum effects (measurements of the refractive index and of the scattering cross section). Our treatement is valid for the case of a homogeneous and weakly dense atomic cloud. We find that collective effects are responsible for a small but detectable perturbation of the signal.