The recent observation of coherent backscattering (CBS) of light by atoms has
emphasized the key role of the velocity spread and of the quantum internal
structure of the atoms. Firstly, using highly resonant scatterers imposes very
low temperatures of the disordered medium in order to keep the full contrast of
the CBS interference. This criterion is usually achieved with standard laser
cooling techniques. Secondly, a non trivial internal atomic structure leads to
a dramatic decrease of the CBS contrast. Experiments with Rubidium atoms (with
a non trivial internal structure) and with Strontium (with the simplest
possible internal structure) show this behaviour and confirm theoretical
calculations