Putting a quantum dot together with an LED can produce entangled photons on demand. Credit: D. Brunner / iStockphoto

UK physicists have made a key step towards practical quantum computing: they have created a light source that fires entangled photons when triggered by an electric current.

Quantum computers exploit the inherent uncertainties of quantum physics to perform calculations much faster than computers currently in use. Whereas conventional 'bits' of information take only the values zero or one, quantum bits, or 'qubits', exist in a fuzzy superposition of both. In theory, this ambiguity allows any number of qubits to be lumped together or 'entangled' and processed in parallel, so that a huge number of calculations can be made at once.

One of the obstacles to practical quantum computing has been entangling particles reliably. Qubits are typically made from photons, and in the past physicists have achieved entanglement by sending a single photon through a certain type of crystal lattice. The transition through the crystal can split the photon into a pair of entangled photons, each with half the energy of the first — but the process is so sporadic that it cannot be relied on in practice.

Mark Stevenson and his colleagues at Toshiba Research Europe in Cambridge, UK, and the University of Cambridge now offer a way around this problem. They have combined semiconductor regions known as quantum dots, which can emit entangled photons when triggered by light, with light-emitting diodes (LEDs), which emit light in response to an electric current. The result is a device that emits single pairs of entangled photons on demand.

The device's quantum dot, which is two micrometres wide and made of indium arsenide, sits on a 360-micrometre gallium arsenide LED. When the researchers supply the LED with electric current, two electrons hop into two positively charged 'holes' in the quantum dot's lattice, releasing energy in the form of a photon pair. Crucially, the nature of this process means that the polarization of one generated photon is determined by the other, so the pair is entangled1.

High fidelity

"The work demonstrates that with relatively simple technology it is possible to obtain entangled photon pairs with sufficient fidelity for relevant applications," says Armando Rastelli, a quantum-dot physicist at the Leibniz Institute for Solid State and Materials Research in Dresden, Germany. "Although this result is not so surprising in view of previous work comparing optical and electrical injection, an experimental demonstration was still missing."

But Rastelli points out a couple of drawbacks with the present device. First, the process of making the quantum dots currently yields only about 1 in 100 that successfully entangles photons. And, like many of those based on quantum dots, the device only works when it is cooled to liquid-helium temperatures of around 5 kelvin.

Stevenson believes that the operating temperature of the device could rise once the team has tailored the types of semiconductor used and the way they are layered. Still, his group's first goal is to improve the device's fidelity from 82% towards 100%, which would represent total reliability.

"I believe that these can be important, maybe even essential, devices for several quantum information-processing applications," says Eli Kapon, an expert in the optical properties of quantum dots at the Swiss Federal Institute in Lausanne.