  We calculate the decoherence time of the ground state wave function of a
nucleus in a high energy heavy ion collision. We define this time as the decay
time of the ratio Tr D^2 / (Tr D)^2 of traces of the density matrix D. We find
that this time is smaller or equal to 1/Q_s, where the saturation scale Q_s is
defined within the color glass condensate model of parton saturation. Our
result supports the notion that the extremely rapid entropy production deduced
for the early stage of heavy ion collisions at collider energies is to a large
extent caused by the decoherence of the initial-state wave functions.
