Light gun fires photons one by one [from New Scientist]

[Jim Cheesman]

    Light gun fires photons one by one

    * 16:19 24 May 2005
    * NewScientist.com news service
    * Justin Mullins


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 
allows a data transfer rate of 1 megabit 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.



[From: http://www.newscientist.com/article.ns?id=dn7420]

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