Humble LEDs power quantum computing breakthrough

Cambridge team cuts out need for lasers

Cambridge scientists have come up with a simple way to generate the entangled photons needed by quantum computers using a cheap light emitting diode (LED) powered by electrical voltage.

The development, a project between Toshiba's Research Lab and the Cavendish Laboratory in Cambridge, sounds like the sort of incremental discovery made regularly by scientific teams the world over every day, but this one solves a critical engineering challenge in its field.

The team has named their invention the 'Entangled Light Emitting Diode' because it is based around feeding an electrical current into what is called a 'quantum dot', a miniscule crystal semiconductor made up of clusters of atoms, to stimulate the production of entangled pairs of photons via LED instead of using complex lasers.

The innovation is not only to produce these photons at high levels of efficiency - the team managed 82 percent - but to feed the quantum dot with ordinary voltage. This is similar to how conventional semiconductors work except that the dots stand in for the much more massive transistors and their output is photons instead of a binary on/off electrical state in a logic gate.

The photons matter because entanglement lies at the heart of what a quantum computer is, and the higher the rate of efficiency the lower the error rate of the quantum device they are being output from.

Entanglement itself matters because without it the massive computing potential of a quantum computer will remain just an interesting theory. Unlike a convention bit, a quantum bit, or 'quibit', can be in an on and off state at the same time as well as a superposition of states between those two states. Such a calculating design would increase the useful output of a computer by almost unimaginable orders of magnitude compared to today's semiconductor systems.

The challenge has always been how to access the data from a quantum device without forcing the collapse of the Heisenberg's famous probability wave function, a technical way of saying that such an act of direct access would turn a quantum computer back into a conventional digital device.

The answer is to leave the photons doing the work inside the device alone and instead deduce those particle's states of being from a special entangled partner photon to which each has a defined relationship.

What the Toshiba and Cavendish teams have managed to do is simply to make the production of those all-important photons simple enough that the process could happen outside a physics lab.

"Although entangled light has been produced previously by shining an intense laser beam on crystals, the new simple device is the first voltage-powered source," said Toshiba Research head, Dr Andrew Shields.

"The discovery is significant because it will allow electrical addressing of many entangled light emitters on a single chip, opening the path to ultra-powerful semiconductor processors based on quantum computation."

Shields' team is also a pioneer in the overlapping field of quantum cryptography, where Toshiba is credited with being a private-sector world leader.

With vast computing power on tap, what might quantum computers be useful for? Possibly, they could help us understand what the quantum state of matter actually is or simply design other computers more capable of grappling with this mystery than the human mind.

The team's efforts will be written up in the next issue of Nature.

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