Consisting of pairs of photons of light whose fundamental properties are inextricably linked, the technology has attracted increasing interest over the past 10 years. It has many possible uses in addition to encryption, including communications, quantum computing, medical imaging and chip production.
The device is important for two reasons, said Andrew Shields, head of the Quantum Information group at Toshiba Research Europe. First, it's made from ordinary semiconductors; second, it produces entangled photons on command.
"For the first time, we can produce pulses of photons that are regular and reliable enough to be used as a clock in quantum computing, for example, from something we can make almost as easily as any other semiconductor," Shields said in an interview.
Inside the device
Mostly made from gallium arsenide, a common semiconductor already widely used in fast logic and optoelectronics, the device's key components are quantum dots of indium arsenide 12 nanometers in diameter and 6 nanometers high. (In comparison, a human hair measures about 100,000 nanometers in width.)
"The indium arsenide self-organizes into the dots, like raindrops on a car bonnet," Shields said. "We found that the key was producing the dots with a high degree of symmetry, and the physics of the materials does that for us."
In use, the dot is excited by a laser pulse, which energizes two electrons in the indium arsenide. That energy is then converted into two entangled photons at slightly different frequencies, which can be split off and transported independently outside the device.
Currently, the light is in the near-infrared frequency range, with a wavelength of around 900 nanometers. The device itself has to be cooled to extremely low temperatures.
"There's no reason, in theory, why we can't replicate this effect at room temperature, and we've already seen emission at 1,300 nanometers where telecommunications lasers work," Shields said. "There are challenges still to be overcome, and I'd expect to see this in production in three to four years."
With pairs of entangled photons, the state of one can be deduced by measuring the state of the other. Combining this with statistical techniques, it's possible to send encryption keys to a remote location and to be sure they haven't been intercepted.
Another use is in chip production. By combining two entangled photons on a single focused spot, they can be made to behave as if they were one photon with half the wavelength and twice the energy. As the smallest possible feature that can be made on a chip depends on the wavelength, this technique could be used to halve the current theoretical minimum--doubling the number of devices on a silicon wafer.
The same technique can be used to produce light microscope images much finer than before.
"Analogy with developments after the invention of the semiconductor laser suggests there may be many more applications that we have not yet even imagined", Shields said in a statement.
Rupert Goodwins of ZDNet UK reported from London.