Light gun fires photons one by one

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.
Too many photons
In the last year, a number of companies have begun selling quantum encryption kits that create single photons by reducing the intensity of a laser beam so that it produces pulses each containing less than one photon, on average. But there always remains a small probability that any pulse will contain two or more photons.
This is a potentially serious weakness because a hacker could intercept the extra photons without the sender and receiver being any the wiser.
Now Andrew Shields and colleagues at Toshiba ‘s Cambridge Research Laboratory in the UK have developed a light-emitting diode (LED) that produces up to 1000 single photons per second - allowing a data transfer rate of 1 megabits per second.
And crucially, the photon gun works at the same light wavelength as commercial optical fibres - at 1.3 micrometers. “It could be commercially available within two to three years,” says Shields.
Exotic clusters
The device is essentially a standard LED made of gallium arsenide but containing a layer of quantum dots - exotic clusters of indium arsenide each containing just a few thousand atoms. In a conventional LED, electrons in the central layer combine with ‘holes’ - or absences of electrons - releasing a photon in the process.
In the new device, this recombination takes place only inside the quantum dots which emit photons of a wavelength similar to their size. So the size of the dots determines the wavelength at which the device operates. A masking layer then allows only the light from a single dot to escape, ensuring that the device emits only one photon at a time.
This device should finally close the security loophole in the current quantum encryption techniques. “We are in the process of building our own quantum encryption equipment,” says Shields.
“It will make the process of communicating using the quantum properties of light much more efficient,” notes Will Stewart, chairman of Innos, a silicon research and development company in the UK.
The Toshiba team unveiled the device at the Quantum Electronics and Laser Science Conference in Baltimore, US.