A loophole in quantum cryptography that could allow a hacker to determine a secret key transmitted using the technology has been closed by new research.

Working at Toshiba Research Europe in Cambridge, scientists found that laser diodes used to transmit keys used to encrypt data, known as Quantum Key Distribution (QKD), sometimes transmitted more than one photon at a time. Quantum encryption works by transmitting key data as a stream of single photons.

Should an eavesdropper try to intercept the transmission, monitoring a single photon would change the state of that photon, and this would make both ends of the transmission aware that the data had been eavesdropped. However, the laser diodes can sometimes transmit more than one photon and so a hacker could monitor the second photon, leaving the first photon unchanged and this would not alert anyone that the key transmission had been compromised.

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