Perfect secrecy has come a step closer with the launch of the world's first computer network protected by unbreakable quantum encryption at a scientific conference in Vienna. The network connects six locations across Vienna and in the nearby town of St Poelten, using 200 km of standard commercial fibre optic cables. Quantum cryptography is completely different from the kinds of security schemes used on computer networks today.  These are typically based on complex mathematical procedures which are extremely hard for outsiders to crack, but not impossible given sufficient computing resources or time. But quantum systems use the laws of quantum theory, which have been shown to be inherently unbreakable.

The basic idea of quantum cryptography was worked out 25 years ago by Charles Bennett of IBM and Gilles Brassard of Montreal University, who was in Vienna to see the network in action. In practice this means using the ultimate quantum objects: photons, the atoms of light. Incredibly faint beams of light equating to single photons fired a million times a second raced between the nodes in the Vienna network. Each node, housed in a different Siemens office (Siemens has provided the fibre links), contains a small rack of electronics - boxes about the size of a PC, and a handful of sensitive light detectors.