Researchers at Stanford Univer­sity have developed an optical device that allows engineers to change and fine-tune the frequencies of each indi­vidual photon in a stream of light to virtually any mixture of colors they want. The result is a new photonic architecture that could transform fields ranging from digital communi­cations and arti­ficial intelli­gence to cutting-edge quantum computing. “This powerful new tool puts a degree of control in the engineer’s hands not previously possible,” said Shanhui Fan, a professor of electrical engi­neering at Stanford.

The structure consists of a low-loss wire for light carrying a stream of photons that pass by like so many cars on a busy throughway. The photons then enter a series of rings, like the off-ramps in a highway cloverleaf. Each ring has a modu­lator that transforms the frequency of the passing photons – frequencies which our eyes see as color. There can be as many rings as necessary, and engineers can finely control the modulators to dial in the desired frequency trans­formation.

Among the appli­cations that the researchers envision include optical neural networks for arti­ficial intelli­gence that perform neural compu­tations using light instead of electrons. Existing methods that accomplish optical neural networks do not actually change the frequencies of the photons, but simply reroute photons of a single frequency. Performing such neural compu­tations through frequency mani­pulation could lead to much more compact devices, say the researchers. “Our device is a significant departure from existing methods with a small footprint and yet offering tremendous new engi­neering flexi­bility,” said Avik Dutt, a post-doctoral scholar in Fan’s lab.

The color of a photon is determined by the frequency at which the photon resonates. A simple trans­formation might involve shifting a photon from a frequency of 500 nanometers to, say, 510 nanometers – or, as the human eye would register it, a change from cyan to green. The power of the Stanford team’s archi­tecture is that it can perform these simple trans­formations, but also much more sophis­ticated ones with fine control.

To further explain, Fan offers an example of an incoming light stream comprised of 20 percent photons in the 500-nanometer range and 80 percent at 510 nanometers. Using this new device, an engineer could fine-tune that ratio to 73 percent at 500 nanometers and 27 percent at 510 nanometers, if so desired, all while preserving the total number of photons. Or the ratio could 37 and 63 percent, for that matter. This ability to set the ratio is what makes this device new and promising. Moreover, in the quantum world, a single photon can have multiple colors. In that circumstance, the new device actually allows changing of the ratio of different colors for a single photon.

“We say this device allows for arbi­trary trans­formation but that does not mean random,” said Siddharth Buddhiraju, who was a graduate student in Fan’s lab during the research. “Instead, we mean that we can achieve any linear trans­formation that the engineer requires. There is a great amount of engi­neering control here.” “It’s very versatile. The engineer can control the frequencies and proportions very accurately and a wide variety of trans­formations are possible,” Fan added. “It puts new power in the engineer’s hands. How they will use it is up to them.” (Source: Stanford U.)

Reference: S. Buddhiraju et al.: Arbitrary linear transformations for photons in the frequency synthetic dimension, Nat. Commun. 12, 2401 (2021); DOI: 10.1038/s41467-021-22670-7

Link: Fan Group, Ginzton Laboratory, Dept. of Electrical Engineering, Stanford University, Stanford, USA