(New Scientist) The first computer network in which communication is secured with quantum cryptography is up and running in Cambridge, Massachusetts. Chip Elliott, leader of the quantum engineering team at BBN Technologies in Cambridge, sent the first packets of data across the Quantum Net (Qnet) on Thursday. The project is funded by the Pentagon’s Defense Advanced Research Projects Agency.

Currently the network only consists of six servers, but they can be integrated with regular servers and clients on the internet. Qnet’s creators say the implementation of more nodes in banks and credit card companies could make exchanging sensitive data over the internet more secure than it is with current cryptography systems.

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.

Large and expensive

At the moment computers capable of quantum cryptography are large and expensive, because they are custom-made. Elliott imagines a Qnet-like system may first appear in banks, for whom these factors might be less of a problem.

Another limitation is that, for distances over 50 kilometres, the photon signal is degraded by noise, and it is unclear as yet how this problem will be overcome.

However, quantum keys can potentially be exchanged over much larger distances through the air. Tiny, aligned telescopes can send and detect single photons sent through the air.

The distance record for this form of transmission is currently about 20 kilometres. But calculations suggest that photons transmitted through the air could be detected by a satellite, which would enable data to be sent between continents.