Light holds the key [quantum cryptography]

Cryptography used to be of interest only to spies and the security services. However, the rise of e-commerce means that everyday actions, such as withdrawing cash from ATM machines or buying goods on the Internet, are mediated by cryptographic techniques that would be the wonder of WWII code breakers. The widespread deployment of such techniques shows that they are basically sound. However, all these cryptographic techniques rely on keys and ensuring the security of these keys is a problem. Quantum cryptography (QC) is a technique for distributing keys in a way that would provide, in theory at least, total protection against hackers and eavesdroppers. The basic principle is to encode the 0s and 1s of a classical cryptography key on the polarisation state of individual photons. An eavesdropper attempting to 'read' one of these photons risks changing its polarisation state, thus revealing his/her presence. As this idea depends on creating an optical link operating at the single photon level, barriers to its implementation were considerable. Progress has been steady and transmission distances over fibre now exceed 100 km. The article looks at the latest developments in QC.