begood Posted June 10, 2010 Report Posted June 10, 2010 SPOTTING a lone messenger in a crowd of decoys is tricky - a concept that might make it possible to improve the security of quantum cryptography. Quantum links are said to be unhackable because the "key" used to establish a secure channel is encoded into the spin of a photon. If the photon is intercepted, it becomes altered in a detectable way. However, hackers have discovered loopholes that allow them to escape detection, for instance, by intercepting the photons and replacing them with copies. Now Steve Harris's team at Stanford University in California has developed a photon-hiding system to make the key harder to intercept. They fired a laser at rubidium atoms, causing them to release infrared "signal" photons, each with an average frequency of 377 terahertz. The actual values are scattered 3.5 megahertz on each vside of the average. These photons are fed into a modulator, which uses a random number generator to increase the variation in their frequencies by another three orders of magnitude, meaning each photon could be anywhere within a 20 gigahertz region of the spectrum. An eavesdropper would then have to scan that entire region to locate all of the key photons. Harris's team then made an eavesdropper's task harder by flooding the region with a sea of decoys, all with the same frequencies as the original key. "A spy will have to pick out the photons from noise of the same frequency, which is very difficult," says Chinmay Belthangady, a member of Harris's team. A spy will have to pick out the photons from noise of the same frequency, which is very difficult The intended receiver uses a second modulator connected to the same random number generator to reverse the work of the first modulator. The signal photons are restored to their original narrow band of frequencies, but because the decoys did not pass through the first modulator, they are randomly scattered at the second. When the photons then pass through a filter tuned to the narrow region surrounding 377 terahertz, the decoys are weeded out (Physical Review Letters, DOI: 10.1103/PhysRevLett.104.223601). Altering the frequency of the signal in this way is known as spread-spectrum technology, and this is the first time it has been applied to single photons, says the team. The technique could also be used to improve the robustness of the signal, because any noise is removed at the final filter, says Dan Browne at University College London. "It is certainly possible that it could be used to make quantum cryptography systems more noise-tolerant."Deter quantum hackers by hiding the photon keys - tech - 09 June 2010 - New Scientist Quote
