RING RESONATOR
A silicon ring resonator with its access waveguide is shown here. The green wave at the input represents the laser pump, the red and blue wavepackets at the output represent the generated photon pairs, and the infinity symbol linking the two outputs indicates the entanglement between the pair of photonsUniversità degli Studi di Pavia

Entangled photon emitters that can fit onto a silicon chip have been created, promising ultrafast computing, communications and cyber security.

By tailoring micro-ring resonator -- loops etched onto silicon wafers that capture and then reemit particles of light -- researchers have provided a source of 'entangled photons' that is small, efficient and silicon based.

"The diameter of the ring resonator is a mere 20 microns, which is about one-tenth of the width of a human hair. Previous sources were hundreds of times larger than the one we developed," said Daniele Bajoni, a researcher at the Università degli Studi di Pavia in Italy and co-author of the paper.

Entangled photons are a fall-out of a quantum world phenomenon where two particles, once associated, retain the flavours of that association even after separation, across vast space.

The two particles created at the same point and time share a single existence, which in quantum mechanics is defined by the wavefunction.

Spooky action

This property is popularly referred to as 'Einstein's spooky action at a distance', an experiment he designed to discredit quantum theory.

The mathematics said that a small tweak on one particle will produce instantaneously a corresponding change in the other, even if at the other end of the galaxy. Einstein's experiment 'disproved' it until it was resuscitated after his death by Cern scientists John Bell.

By virtue of their behaviour as one entity, the entangled photons offer possibilities in the world of security and spying. The instantaneous reaction property can help increase the power and speed of computations.

To date, entangled photon emitters made from specially designed crystals could be scaled down to a few millimeters in size, still too large for on-chip applications.

They also require vast amounts of power for operation.

The ring resonators only require a laser beam to enable the entangling process. Directed along an optical fibre to the input side of the sample, the laser beam then couples to the resonator where the photons race around the ring and become entangled.

"Our device is capable of emitting light with striking quantum mechanical properties never observed in an integrated source," said Bajoni. "The rate at which the entangled photons are generated is unprecedented for a silicon integrated source, and comparable with that available from bulk crystals that must be pumped by very strong lasers."

The study was published in The Optical Society's (OSA) new journal Optica.