Published online 27 August 2008 | Nature | doi:10.1038/news.2008.1067

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Quantum cryptography can go the distance

Proof-of-concept system could lead to ultra-secure international communication.

lightEntangled photons of light could help to create ultra-secure communication systems.Punhstock

Physicists have built a communication network, secured by quantum cryptography, that could one day work on a global scale.

Quantum cryptography scrambles data using the laws of quantum mechanics, relying on a concept known as entanglement to ensure absolutely security. Entanglement allows two particles to be quantum-mechanically connected even when they are physically separated. Although the specific condition of either particle cannot be precisely known, taking measurements of one will instantly tell you something about the other.

The trick can't be used to actually send information, because each particle's condition is random until it is measured. But entanglement can be used for encrypting data if a sender and a receiver make measurements on a number of entangled particles and then compare their results.

After performing the measurements, they use their data to generate a quantum mechanical 'key' that can be used to share top-secret information. Any eavesdropper will disrupt the entanglement, ruining the key and causing the sender and receiver to break off their communication.

Quantum cryptographic networks usually work by entangling two light particles, or photons. But individual photons can only travel so far down a fibre optic line before they are disrupted, says Yu-Ao Chen, a physicist at the University of Heidelberg in Germany. Although some experiments have managed to send entangled photons over distances of around 100 kilometres using bulky telescopes, networks based on commercial optic fibres have so far been limited to just a few kilometres in length. "It was impossible to go the distance," Chen says.

Long-distance runaround

Now Chen and his colleagues have found a way to extend the entangled photons' reach1. Instead of photons, communication begins with two clouds of rubidium atoms: one by the sender and the other acting as a staging post on the way to the receiver. Stimulating these atoms makes them each release a photon, which remains entangled with its parent cloud.

When the photons arrive at a central point, the physicists can measure them in a way that entangles their parent clouds together, a process known as entanglement swapping.

The clouds are relatively stable, so the experiment can be repeated with a third cloud that is further along the path towards the ultimate destination. This ultimately leaves the first cloud entangled with the third.

By creating a long chain of clouds and repeating the process, the group can, in theory, reach a situation in which the two clouds at each end of the chain are tied together. Sender and receiver can then measure their clouds and build their key in a manner similar to that of a normal quantum cryptographic system, but over a much longer range.

The experiment was conducted with just two clouds of atoms over a distance of 300 metres, much less than that of existing single-photon systems. With current technology, the scheme would require atom clouds every 10 kilometres or so, Chen says. But, in principal, there is no reason why it could not be extended to a global scale.

Despite the difficulties, quantum cryptographic networks are already on the cusp of commercialization. Private companies have been able to develop multi-node networks that can operate over city-scale distances, and, in 2007, the Swiss city of Geneva used quantum cryptography during its elections.

The latest findings are a "proof of principle experiment", says Marek Zukowski, a physicist at the University of Gdansk in Poland who did not work on the project. "But they are at the cutting edge of research in quantum information," he adds. "It shows that development of the technology may be very fruitful." 

  • References

    1. Yuan, Z.-S. et al. Nature 454, 1098–1101 (2008)
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