  Working in the framework of the Color Glass Condensate effective theory of
high energy QCD, we revisit the momentum space entanglement entropy of the soft
gluons produced in high energy dilute-dense collisions. We extend the work
of~\cite{Kovner:2015hga} by considering entropy produced in a single event.
This entropy arises due to decoherence of eigenstates with different energies
during the time evolution after the collisions with the target. We define it
rigorously as the entanglement entropy of the produced system with the
experimental apparatus. We compute the time dependent single event entropy in
the limit of weak projectile field. Further we compute the entropy for the
ensemble of events defined by the McLerran-Venugopalan model for the projectile
wave function. Interestingly the entropy of the ensemble has a much weaker time
dependence than the entropy in any single event. We attribute this feature to
the so called monogamy of entanglement.
