'Unbreakable' encryption unveiled

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.

"All quantum security schemes are based on the Heisenberg Uncertainty Principle, on the fact that you cannot measure quantum information without disturbing it," he explained.

"Because of that, one can have a communications channel between two users on which it's impossible to eavesdrop without creating a disturbance. An eavesdropper would create a mark on it. That was the key idea."

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.

Numerical key

From the detected photons, a totally secret numerical key can be distilled, which encodes the users' data much like the keys used in normal computer networks do.

The advantage is that no-one else can know the key without revealing themselves.

As we saw in the demonstration, when an intruder did try to listen in on the quantum exchange, photons became scrambled, and a rise in the error rate at the node detectors signalled the attack. The system automatically shut down without being compromised.

More importantly, the demonstration also showed that the network is robust.

If one quantum link breaks down, the connections can be re-routed via other nodes, much as phone calls get re-routed automatically through a telecoms network, so that any two users on the network can remain in continuous secure contact. ... - BBC