  We study the statistics of quantum interference for completely positive maps.
We calculate analytically the mean interference and its second moment for
finite dimensional quantum systems interacting with a simple environment
consisting of one or several spins (qudits). The joint propagation of the
entire system is taken as unitary with an evolution operator drawn from the
Circular Unitary Ensemble (CUE). We show that the mean interference decays with
a power law as function of the dimension of the Hilbert space of the
environment, with a power that depends on the temperature of the environment.
