Engineers create device for finetuning individual photon colour

White light contains all the frequencies (perceived by human eyes as colours) of visible light: a mixture of photons – the quanta of light – of different frequencies.

Now, engineers have demonstrated it is possible to fine-tune the frequency of light down to individual photons, using an optical device with a unique architecture which could transform fields from quantum computing to digital communications.

“This powerful new tool puts a degree of control in the engineer’s hands not previously possible,” said Professor Shanhui Fan, senior author of the Nature Communications paper describing the development of the device. The structure consists of a low-loss wire for light carrying a stream of photons, which then enter a series of rings (as many rings as necessary can be added). Each ring has a modulator which can be finely controlled to select the frequency of the passing photons.

A simple frequency transformation may involve shifting a photon from a frequency of 500nm to 510nm: i.e. cyan to green. However, this architecture can perform not only these transformations, but also more sophisticated ones with fine control. For instance, if an incoming stream of light comprised of 20 per cent photons in the 500nm range and 80 per cent at 510nm, an engineer could fine-tune that ratio to 73 per cent at 500nm and 27 per cent at 510nm while preserving the overall number of photons.

In the quantum world, a single photon can exist in a superposition of multiple frequencies; in these circumstances, the device allows changing of the ratio of different colours for a single photon.

“We say this device allows for 'arbitrary' transformation but that does not mean random,” said Siddharth Buddhiraju, first author of the paper and now a researcher at Facebook Reality Labs. “Instead, we mean that we can achieve any linear transformation that the engineer requires. There is a great amount of engineering control here.”

The researchers envision such applications as optical neural networks for AI applications which perform neural computations using light instead of elections; existing methods for optical neural networks do not change the frequency of photons but reroute photons of a single frequency. Performing computations through frequency manipulation instead could lead to far more compact devices.

“Our device is a significant departure from existing methods with a small footprint and yet offering tremendous new engineering flexibility,” said Dr Avik Dutt, second author of the paper.

Fan added: “It’s very versatile. The engineer can control the frequencies and proportions very accurately and a wide variety of transformations are possible. It puts new power in the engineer’s hands. How they will use it is up to them.”