  Inofrmation-theoretical restrictions on information transferred in the
measurement of object S by information system O are studied. It is shown that
such constraints, induced by Heisenberg commutation relations, result in the
loss of information about the purity of S state. Consequently, it becomes
impossible for O to discriminate pure and mixed S states. In individual events
this effect is manifested by the stochastic outcomes of pure S state
measurement, i.e. the collapse of pure S state.
