  The spin dynamics of an interwell excitons gas has been investigated in n-i-n
GaAs/AlGaAs coupled quantum wells (CQWs). In these heterostructures the
electron and the hole are spatially separated in neighboring quantum wells by a
narrow AlAs barrier, when an electric field is applied. The time evolution
kinetics of the interwell exciton photoluminescence has been measured under
resonant excitation of the 1sHH intrawell exciton, using a pulsed tunable
laser. The formation of a collective exciton phase in time and the temperature
dependence of its spin relaxation rate have been studied. The spin relaxation
rate of the interwell excitons is strongly reduced in the collective phase.
This observation provides evidence for the coherence of the indirect excitons
collective phase at temperatures below a critical $T_c$.
