Quantum Cryptography Is Safe Again

In theory, so-called quantum cryptography provides a totally secure way of sending information. In practice, maybe not. But now physicists have demonstrated how to close a technological loophole that could have left secrets open to eavesdroppers.

[I]n 2010, an international team of researchers showed that [an attacker] could hack the system by exploiting a weakness in the so-called avalanche photodiodes (APDs) used to detect the individual photons. The problem is that APDs react differently to intense pulses of light than they do to single photons, so that the energy of the pulse must exceed a threshold to register a hit. As a result, all [the attacker] has to do is intercept the single photons, make her best-guess measurements of their polarizations, and send her answers off to Bob as new, brighter pulses. …

Last year, physicist Hoi-Kwong Lo at the University of Toronto and colleagues claimed to find a way around the problem. In the new protocol, Alice and Bob would begin the creation of a quantum key by sending randomly polarized signals to Charlie, a third party. Charlie would measure the signals to determine not their actual polarization, but only whether the polarizations were at right angles. … Now, in papers in press at Physical Review Letters, two independent groups of physicists have shown that the new protocol works.’

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