Conditional entropy and information in quantum systems

The concepts of conditional entropy of a physical system given the state of another system and of information in a physical system about another one are generalized for quantum systems. The fundamental difference between the classical case and the quantum one is that the entropy and information in quantum systems depend on the choice of measurements performed over the systems. It is shown that some equalities of the classical information theory turn into inequalities for the generalized quantities. Specific quantum phenomena such as EPR pairs and 'superdense coding' are described and explained in terms of the generalized conditional entropy and information. (C) 1999 Elsevier Science Ltd. All rights reserved.