Two separate groups of physicists have proven the real-world efficacy of a protocol introduced last year by University of Toronto physicists that theoretically closed a loophole in quantum cryptography. In 2010, researchers proved that hackers could breach a quantum cryptography system by exploiting a weakness in the avalanche photodiodes (APDs) that detect individual photons. Because APDs react differently to intense pulses of light than to single photons, the energy of the pulse must exceed a threshold to register a hit. As a result, a hacker could intercept single photons, estimate measurements of their polarizations, and send answers to the intended recipient as new, brighter pulses. Last year, University of Toronto physicist Hoi-Kwong and colleagues said the problem could be resolved if two parties engaging in quantum cryptography began the quantum key process by sending randomly polarized signals to a third party. The third party would measure the signals to determine whether the polarizations were at right angles, but not the actual polarization. With a third party comparing photon polarizations without determining what they are, no photon splitting or half-strength signals could occur and no tampering could go unnoticed. Scientists at the University of Calgary and the University of Science and Technology of China have carried out independent experiments that prove Lo's theory.
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