Quantum dots lead to entanglement breakthrough

Researchers at Cambridge University and Toshiba have announced a new quantum device that produces entangled photons. Consisting of pairs of photons whose fundamental properties are inextricably linked, this light has been attracting increasing interest over the past ten years. It has many possible uses in encryption, communication, quantum computing, medical imaging and chip production.

"The device is important for two reasons. Firstly, it's fabricated in a similar way to an ordinary semiconductor light emitter, and secondly it produces entangled photons on command," Dr Andrew Shields, head of the Quantum Information group at Toshiba Research Europe, told ZDNet UK. "The latter means we can trigger the generation of entangled photons with an external clock signal, essential for many applications in quantum computing."

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 12nm in diameter and 6nm high. "The indium arsenide self-organises into the dots like raindrops on a car bonnet," said Shields. "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 energises 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 900nm and 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 1300nm where telecommunications lasers work," said Shields. "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", said Shields in a statement.

The research is due to be published in Nature on 12 January.