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 classes of mathematical problems. Quantum cryptography represents a new paradigm for secure communications systems since its security is based not on computational complexity, but instead on the laws of quantum physics, the same fundamental laws that govern the behavior of the universe. This paper describes recent progress at BT Laboratories in the development of practical optical fiber-based quantum cryptography systems. These developments include interferometric systems operating in the 1.3-μm-wavelength fiber transparency window over point-to-point links up to ∼ 50 km in length and on multi-user passive optical networks. We describe how this technology performs on fiber links installed in BT's public network and discuss issues such as cross-talk with conventional data channels propagating at different wavelengths in the same fiber. The experimental results are used to make some conclusions about the likely performance parameters and application opportunities for this new technology.