QUANTUM CRYPTOGRAPHY ON OPTICAL-FIBER NETWORKS

The security of conventional or classical cryptography systems relies upon the supposed (but often unproven) difficulty of solving certain c lasses of mathematical problems. Quantum cryptography represents a new paradigm for secure communications systems since its security is base d not on computational complexity, but instead on the laws of quantum physics, the same fundamental laws that govern the behavior of the uni verse. This paper describes recent progress at BT Laboratories in the development of practical optical fiber-based quantum cryptography syst ems. These developments include interferometric systems operating in t he 1.3-mu m-wavelength fiber transparency window over point-to-point l inks up to similar to 50 km in length and on multi-user passive optica l networks. We describe how this technology performs on fiber links in stalled in BT's public network and discuss issues such as cross-talk w ith conventional data channels propagating at different wavelengths in the same fiber. The experimental results are used to make some conclu sions about the likely performance parameters and application opportun ities for this new technology. (C) 1998 Academic Press.