  The widely considered assertion is that the unitarity of quantum mechanical
evolution assures the preservation of information. It is even promoted in
popular literature as an established fact. (Susskind, 2008) Yet, a simple chain
of reasoning demonstrates that: 1) almost any evolutionary operator can be well
approximated by a degenerate (finite-rank) operator and 2) one needs an
eternity to retrieve information exactly from a nonstationary quantum state and
to distinguish between arbitrary unitary operator and its finite-dimensional
approximations.
