Mar 17, 2016
In slightly less technical terms, researchers at the Toshiba Research Europe facility in Cambridge, England have figured out how to make it harder for eavesdroppers to steal keying information from a quantum cryptosystem (registration required, Bugmenot has login credentials for this site). For an attacker to have a chance at breaking a quantum cryptosystem, he or she would have to splice a tap into the optical fibre which connects the two crypto units and record the pulses of light that encode the key used to encrypt the data. There are ways to use the principles of quantum mechanics to detect the presence of an eavesdropper, however, because the act of observing the photons in the fibre changes their meaning. The two parties would discover that they had different keys, and the data could not be decrypted.
It is now possible to drop decoy photons into the optical fibre, bits that aren't part of the key and don't code for anything. An eavesdropper could just as easily record those bits and leave the actual keying information intact. Also, the act of eavesdropping would lower the number of decoy photons in the optical channel, and if the other side of the conversation detected a smaller number of decoys they would be alerted to the presence of an eavesdropper. This sounds like a logical countermeasure, but it wasn't possible before due to how laser diodes were manufactured. A new generation of laser diodes that are capable of emitting single photons of light compatible with optical fibres has been created which allow this to take place.