According to New Scientist, the first quantum-encrypted network is online. Only six servers strong as of yet, it will be a test bed of whether the technology is as unbreakable as it is theoretically predicted to be.

The data in Qnet flows through ordinary fibre optic cables and stretches the 10 kilometres from BBN to Harvard University. It is encrypted using keys determined by the exchange of a series of single, polarised photons.

The first money transfer encrypted by quantum keys was performed between two Austrian financial institutions in April 2004. But Qnet is the first network consisting of more than two nodes to use quantum cryptography - a more complex challenge. "Imagine making a phone call. If you just have one possible receiver, you wouldn't even need buttons," explains Elliott. "But with a network you need a system that will connect anyone on the network to anyone else." In Qnet, software-controlled optical switches made of lithium niobate crystals steer photons down the correct optical fibre...

...But Elliott points out that even quantum cryptography "does not give you 100 per cent security". Although quantum keys are theoretically impossible to intercept without detection, implementing them in the real world presents hackers with several potential ways to listen in unobserved. One example is if a laser inadvertently produces more than one photon, which happens occasionally. An eavesdroppper could potentially siphon off the extra photons and decrypt the key, although no one has actually done this.