Antarctica Starts Here.

A team of researchers at MIT have figured out how to partially compromise quantum cryptography systems through a creative interpretation of the entanglement principle. In a system protected with quantum cryptographic principles, bits of information are encoded by assigning meaning to the polarisation of individual photons of light (up-down could mean a one, left-right could mean a zero) and thus exchange keying material. The very act of observing quantum particles changes their properties and thus destroys the data encoded in the particles, so in theory an eavesdropper Somewhere Out There listening in would corrupt the stream of data by damaging the key used for a portion of the message. If one end or the other of such a protected link detected a sudden spike in the error rate, they would assume that someone is monitoring and move to another communications medium to dodge the efforts of the eavesdropper.

The researchers were able to entangle (and thus correlate) the polarisation of photons with their momentum, and thus determine what about 40% of the bits used in the key were without disturbing the keying information or corrupting the message on the other end. That is not much of the key but it is enough to figure out nearly half of a message.. that 'nearly half' could be the most dangerous part of a military communique', for example.