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Another Look at Quantum Crypto
Mills said he's been aware of prototypes of such systems in research laboratories for about four years. "The idea is conceptually very simple," he wrote. "In order to steal information from a communications link, you have to steal energy."
Among other things, quantum mechanics describes the smallest amount of energy which can exist - one quantum - which in light equates to a single photon. Stealing information, in this context, means stealing at least one photon; and since there's only one there, the light goes out.
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