Quantum cryptography keys encoded in polarized photons of light have been transmitted more than 23 kilometers through air, British researchers have announced. They say the breakthrough is an important step towards a satellite-based global communications system that is completely secure and expect to have a system design by March 2003.

Quantum cryptography guarantees that keys cannot be intercepted without the sender and receiver knowing by using the quantum properties of individual photons (or particles) to encode the key&#8212any measurement of a photon will alter its quantum properties, betraying an interceptor.

Nature (vol 419, p 450)