Abstract

I will describe the principles behind quantum cryptography and key shring systems, which can be either based on sources of pairs of entangled particles (typically photons), or based on the preparation and measurement of a single particle in one of two incompatible basis states. The first experimental demonstration of these techniques was based on polarization states and was limited to communications over distances of a few meters. More recent experiments use interferometric measurements on long wavelength photons transmitted through standard communications optical fibers and have demonstrated quantum cryptography at distances up to 10 kilometers. The information transmitted via quantum cryptography is completely secure—no eavesdropper can acquire any of the information without being detected. However, the complexity of the equipment required, and the relatively low rate at which data can be transmitted, means that the most likely application of these techniques is in the generation and distribution of keys for use with conventional cryptography systems. The sources of entangled pairs of particles can also in principle be used to demonstrate quantum teleportation, in which an unknown quantum state is destroyed at one place, and recreated at a remote location. Unfortunately, it is not yet possible to perform this experiment in practice, but I will discuss the fundamental quantum mechanical implications of this interesting thought experiment.

© 1995 Optical Society of America