Light gun fires photons one by one

New Scientist

The first photon gun capable of firing single particles of light over optical fibres was unveiled on Tuesday. The breakthrough may remove one of the final obstacles keeping perfectly secure messages from being sent over standard telephone fibres. Encryption techniques change each character in a message in a way that can be reversed by a receiver who possesses the relevant key. But sending the key to the receiver is just as troublesome as sending the message as it too can be intercepted - a problem known as key distribution. Twenty years ago, North American physicists Giles Brassard and Charles Bennett outlined a way to send a key without anyone being able to eavesdrop. Their idea rests on the notion that a message sent using quantum particles -such as photons - is so fragile that measuring the photons changes their properties. So anybody listening in to a transmission would destroy it - which the sender and receiver would easily notice. But so-called quantum encryption works only if the key is sent using individual photons, rather than the pulses of many photons that are used for communication today. But sending single photons is tricky.