Nature 465, 594–597 (2010)

Optical quantum computers promise to solve problems that are intractable using conventional logic by manipulating photons whose properties are inextricably linked, or entangled. At present, entangled photons are generated by using lasers and optics to excite a luminescent material, a method that is relatively bulky and complicated. Now, Andrew Shields and colleagues at Toshiba Research Europe and Cambridge University have constructed an electrically driven source of entangled photon pairs.

The team's device consists of a single layer of quantum dots embedded inside a semiconductor light-emitting diode. Charges are injected from heavily doped regions of the device onto the quantum dots, where they form a bound state consisting of two electrons and two holes, called a biexciton. The biexciton then releases a pair of entangled photons when it decays. Entanglement is encouraged by spatially separating the quantum-dot layer from the heavily doped regions, and by careful tuning of the quantum-dot emission energy.

The device can emit entangled photons at a particular time when it is driven with pulsed current. Pulsed current also increases the degree of photon entanglement, or 'fidelity', as does limiting the time that the photon detectors are active. The demonstrated fidelity is high enough to be used for teleportation (useful for quantum computers) and quantum relays and keys (useful for communications).