Quantum key distribution can be leaky
Researchers at the University of Tsukuba and the Japan Science and Technology Agency have come up with a way to eavesdrop on quantum key distribution.
Although quantum theory precludes eavesdropping on a complete key, as the encoded information is destroyed by observing it, a partial key may be copied without exceeding a threshold of disturbance on the channel that would be detected by the legitimate parties to the exchange. The proposed technique uses linear optics – an array of polarizing beam splitters – to snoop on the communication path via a phenomenon known as optical cloning.
So far the research is theoretical only (PDF) – no practical device has apparently yet been built. Nevertheless, the work casts a further shadow on the supposed impregnability of quantum cryptography. However, as in previous discoveries, the weaknesses are in the implementation rather than the fundamental theory.