A SIMPLE semiconductor chip has been used to generate pairs of entangled photons, a vital step towards making quantum computers a reality.

Famously dubbed "spooky action at a distance" by Einstein, entanglement is the mysterious phenomenon of quantum particles whereby two particles such as photons behave as one regardless of how far apart they are. It is widely regarded as essential to the development of quantum computers and quantum cryptography.

To generate entangled photons, Andrew Shields at Toshiba Research Europe Limited (TREL) in Cambridge, UK, and colleagues from TREL and the University of Cambridge manufactured a silicon chip containing a nanometre-sized quantum dot. A quantum dot is a semiconductor crystal that has discrete energy states like an atom and can be optically triggered to generate photons. The team found that the precise shape of the dot dictates whether the emitted photons are entangled or not, and the shape can be controlled by how the quantum dot is grown or by applying an external magnetic field (Nature, vol 439, p 179).

Entangled photons have been previously generated using laser beams, but this is bulky and complex. "Semiconductor devices are attractive as they are compact and robust," says Shields. "Furthermore, since they can be produced using standard techniques, they could potentially be mass-manufactured for a fraction of the cost."