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Comment: Re:decryption (Score 1) 48

by ibillin (#21710578) Attached to: Toward On-Chip Quantum Computing
Quantum encryption works a little differently - basically the whole point is that you can create one time keys with almost 100% safety, because a quantum system is sensitive to measurement. With classical keys someone can copy it and then attempt to break it. A quantum system, on the other hand, cannot be copied, because to make a copy you have to first measure what it is. Typically the way the algorithm works is that I send say photons that are linearly polarized. I can randomly send them vertically or horizontally polarized, or polartized at +45 degrees and -45 degrees. If you are listening in, and attempt to copy, sometimes you don't get the right polarization - i send vertical, and you turn your polarizer 45 degrees and -45, you're equally likely to get a detection event. In this case you have to guess whether the original photon was vertical or horizontal and send one back so that the guy on the other end doesn't get suspicious. However, you'll be wrong sometimes, and by sending some test photons I can quickly figure this out. The way the actual transmission works out is that the other guy randomly measures as well. He also turns his polarizer randomly 45/-45 or 0/90 degrees. At the end I publicly send him all the polarizer settings that I used. He then compares these to his, and uses the ones where we agree to establish a key, since if I sent the photon at 45 and he measured at 45 we will have the same information. What you could do is take advantage of losses in the fiber optic network. You could replace my bad optical fiber with a perfect one, and just split off some photons and measure them with no problems. Then I wouldn't know. Or you could make a device that does non-demolition measurements on photons, but that's pretty hard ...

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