Entangled LED shines light on quantum computing

Researchers at the Cavendish Laboratory in Cambridge and Toshiba Research Europe have produced a new way to produce entangled photons, a key technique for quantum encryption and computing.

The electronic device, called an entangled light-emitting diode (ELED), is produced in a similar way to existing LEDs, but creates a special form of light that previously required complex and inefficient laser apparatus.

"This is a real breakthrough," lead researcher Andrew Shields told ZDNet UK on Wednesday. "This is a voltage-powered LED, not a large, cumbersome system. It's cheaper and much more robust. We can see many of these being built into a single chip for quantum computing."

The ELED works by combining LED technology with a quantum dot, a 10-nanometre-wide patch of indium arsenide. When two electrons combine with two holes — areas without electrons — within the dot, two infrared photons are produced that are entangled. Entangled photons are an indeterminate system where one photon assumes a particular condition the instant the other is measured, no matter how far apart they are. This can be used to encode data for quantum processing.

"Optical quantum computing seems very promising," Shields said. "Compared to electron spin quantum computing, photons keep their state very well, and we know how to manipulate quantum bits in a photon. We hope to see the simplest logic circuit within two to three years, and larger scale in five years or so. Our immediate target is improving performance, which is already very good, but we need to make the entanglement better."

Currently, the device only works at very low temperatures of about five degrees above absolute zero. It produces at most 10,000 pairs of photons per hour, compared with the hundreds of exa-photons a normal LED emits. It also produces significant amounts of non-entangled light — mostly from the material surrounding the quantum dot. The dot itself produces relatively pure entangled light.

"Entanglement fidelity is up around 82 percent," Shields said. "If it's less than 100 percent, it can cause problems. Most of our photons are entangled, some are not. The biggest component of the unentangled light comes from the surrounding diode."

Quantum computing has the potential to carry out some classes of calculation many times faster than conventional technology, as it can create huge numbers of answers simultaneously and then select the correct one. To date, only simple calculations have been done, with much work remaining to scale the technology up and make it practical.