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Quantum encryption gets nearer

Eric Doyle
Two staging posts on the road to unbreakable encryption have been reached in the world of quantum encryption.

Researchers from Cambridge University and Toshiba have developed a light-emitting diode (LED) that emits a single photon - a discrete "particle" of light - at regular intervals. Meanwhile, physicists at Harvard University in the US have developed a repeater to extend the distances that these photons can be transmitted.

Quantum cryptography, which relies on the principles of quantum physics, is seen as the unbreakable encryption of the future. Anyone trying to intercept the photon trail of signals would change the properties of each photon. Such tampering would be immediately obvious.

The single-photon-emitting LED overcomes problems experienced with previous light sources, which sometimes emitted two photons at a time. If a second photon is available, it could be examined without affecting its twin and therefore the encrypted message could theoretically be intercepted by picking off one photon and letting the other pass without being touched. To the receiver this would appear to be a "clean" transmission.

The delicate nature of the photon is its strength for cryptographic purposes but is also its greatest weakness because changes can be introduced just by passing it along the fibre. This has kept maximum transmission distances down to about 15km.

The Harvard team claims to have solved the problem of reading the photon's orientation, generating an exact twin of that photon and passing it into the next stretch of fibre.

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