Abstract

One of the most intriguing and exciting recent developments in the applications of quantum mechanics has been the prediction and demonstration of a cryptographic key-distribution scheme, the security of which is guaranteed by the laws of quantum mechanics.¹ The security of these cryptographic schemes rests on the quantum property of complementarity. This technique, known as quantum cryptography, has forced us to rethink some of the fundamental information-theoretic descriptions of communication channels and to explore new ways in which some of these novel quantum properties can be exploited. We shall be primarily concerned here with applications to communications, and specifically with aspects of communication channels that require a quantum description for their explanation. Perhaps the two most important ways in which a quantum system differs from its classical counterpart can be summarized as complementarity and correlation. Complementarity ensures that observables represented by noncommuting operators cannot be simultaneously measured with arbitrary accuracy. This, in turn, implies that information cannot be encoded on incompatible properties. For two-state systems, such as spin-1/2 particles, this means that up to one bit of information can be conveyed per transmitted state. The novel properties of quantum communication channels become more apparent when we allow such channels to possess some degree of quantum correlation. By this we mean that the quantum channels are in a quantum state which violates the celebrated Bell inequalities. The use of such channels for secure key-distribution² is one important new application of quantum correlations. Although the security of such schemes does not strictly reside in a nonlocal correlation between quantum states,³ the exploitation of these correlations can give important new features, such as secure key storage. Recent work on quantum-correlated channels has also shown how two bits can be apparently transmitted by a single two-state particle.⁴

© 1994 IEEE