Quantum Cryptography

Quantum cryptography uses phenomena from quantum mechanics in order to provide secure communcation over insecure channels. In a typical implementation, one sends very faint pulses of polarized light from sender to receiver, and the information is for example communicated in the direction in which the light is polarized. The pulses are in fact so faint that only a single photon (the smallest entity of light) is sent in each pulse.

It turns out that the information can be coded in such a way that an eavesdropper who does not know in advance what is being sent cannot measure the photons without facing some probability of not only getting useless results, but at the same time destroying completely the information sent.

This immediately means that we can make a communication channel where eavesdropping can be detected. While this is not good enough for most purposes, we can, by using techniques from computer science, build from this a system where eavesdropping is impossible. This involves some further communication between sender and receiver which may take place over a standard, classical network.

In this way, one can build communication systems that are secure, even against enemies with unlimited computing power, unlike almost all systems in use today. Experimental implementations of quantum cryptography have existed since 1990, and today quantum crypto is performed over distances of 30-40 Km of optical fibre.

Follows this link to access Gilles Brassard's bibliography of quantum cryptography.

Charles Bennett's introduction to quantum cryptography that appeared in Science, vol. 257, August 1992,pp.752-753 is available here in postcript format.