ua nt - p h / 96 05 00 2 v 1 3 M ay 1 99 6 Quantum mechanics of measurement

We show that the recent discovery of negative (conditional) quantum entropy reveals that measurement in quantum mechanics is not accompanied by the collapse of a wavefunction or a quantum jump. Rather, quantum measurement appears as a sequence of unitary operations which are reversible in principle , although ususally not in practice. The probabilistic nature of quantum measurement emerges from the positive entropy of the observed subsystem, which however is exactly cancelled by the negative entropy of the remaining (unobserved) part. Thence, the entropy of the combined system is unchanged while measurement itself is probabilistic. In this framework, uncertainty relations which characterize the measurement of incompatible variables emerge naturally, as do all well-known relations of conventional quantum mechanics. Yet, quantum measurement is unitary, causal, and free of any ad hoc assumptions. We apply this theory to standard quantum measurement situations such as the Stern-Gerlach and double-slit experiments to illustrate how randomness, inherent in the conventional quantum probabilities, arises in a unitary framework. Finally, the present view clarifies the relationship beween classical and quantum concepts.