Scientists have drawn up the blueprint for a new device that could make absolutely secret communications possible over huge distances within the next few years.

Quantum physics can provide a completely secure method of communication between two distant correspondents. Sending photons entangled in a quantum state makes it impossible for an eavesdropper to intercept a message.

But currently this form of communications only works over a limited distance. Optical absorption along fibre optics means that photons start to lose their quantum state beyond about 15 kilometres.

The new device promises to overcome this problem and has the advantage of being constructed from available technology. "The work shows that a quantum repeater can be built with tools that either exist today or are under construction," says one of the team, Mikhail Lukin at Harvard University.

Richard Hughes, an expert in quantum communications at Los Alamos National Laboratory says: "My first impression is that this is a very important development towards making quantum communications practical."

But Hughes cautions that there are still some technical issues to be overcome: "There will be many details to work out before experiments can be attempted."

Temporary storage

Quantum repeaters were first proposed a number of years ago and tackle the problem of signal loss by temporarily storing the state of each photon. This allows new photons with the same state to be generated at each repeater, meaning a long travel distance is achieved by a number of short steps.

Researchers have previously demonstrated that single atoms can be used to temporarily store photons in a quantum state. But the process has never been reliable enough to make a useful quantum repeater.

The new design uses a number of atoms per photon at each repeater, which the researchers say greatly improves reliability.

"This is not only experimentally simpler, but also works better - it improves the signal to noise ratio of the scheme," says team member Peter Zoller of the Institute for Theoretical Physics at the University of Innsbruck, Austria.

Journal reference: Nature (vol 414, p 413)