Photons link arms on chip to hasten march of quantum crypto

Cheap, fast, entangled photons for fun and profit

A multinational collaboration of boffins reckons it's come up with a chippable solution to one of the practical problems of quantum communications: getting a good source of entangled photons.

While commercial quantum key distribution (QKD) devices already exist, getting as much of the process onto silicon is the foundation of making such services widespread and affordable.

To be published in The Optical Society's (OSA's) journal Optica, the paper – also available as an Arxiv pre-print here – describes the use of a micro-ring resonator as a continuous on-chip source of bright, entangled photons.

The OSA's media announcement explains that the resonator is a 20 micron loop that traps and re-emits photons.

The paper notes that currently, non-linear crystals are the favourite source of entangled photons, but “these sources are very difficult to integrate”.

As the researchers note while the micro-ring resonator is an efficient source of photon pairs, “entangled state emissions have never been demonstrated”.

Not only did the tiny CMOS structure generate entangled photons, it did so reliably (with the researchers claiming Bell's inequality violations to as much as 11 standard deviations).

The pairs are generated at more than 107 per nanometre wavelength, operate – quite conveniently – at telecommunications wavelengths, and the structure's output couples well with an optical fibre.

The research team included scientists from Italy, the UK and Canada.

The particular kind of entanglement used in the experiment is called “time-energy entanglement”. In this, entanglement is expressed as a function of the wavelength and emission times of the two photons.

This type of entanglement has been a focus of research for many years, particularly because it can be preserved over distances practical for telecommunications applications. In 2008, Stanford researchers showed that the entanglement could be preserved for up to 100 km. ®

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