  Relativistic quantum field theory imposes additional fundamental restrictions
on the distinguishability of quantum states. Because of the unavoidable
delocalization of the quantum field states in the Minkowski space-time, the
reliable (with unit probability) distinguishability of orthogonal states
formally requires infinite time. For the cryptographic protocols which are
finite in time the latter means that the effective ``noise'' is present even in
the ideal communication channel because of the non-localizability of the
quantum field states.
