One of the first things any grade school science student learns is that white light is a composite of many photons, those little droplets of energy that makeup light, from every color of the rainbow—red, orange, yellow, green, blue, indigo, and violet.
Stanford University researchers 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 color mixture they desire. The result is a new photonic architecture that could transform fields ranging from digital communications and artificial intelligence to cutting-edge quantum computing, according to a paper published in Nature Communication.
“This powerful new tool places a level of control in the hands of the engineer that was not previously possible,” said Shanhui Fan, a Stanford professor of electrical engineering and senior author of the paper.
Researchers 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 clover-leaf effect
The structure is made up of a low-loss wire for light that transports a stream of photons that pass by like cars on a busy highway. The photons are then directed into a series of rings, similar to the off-ramps on a highway cloverleaf. Each ring contains a modulator that alters the frequency of passing photons, which our eyes perceive as color. Engineers can fine-tune the modulators to achieve the desired frequency transformation by using as many rings as necessary.
Among the applications envisaged by the researchers are optical neural networks for artificial intelligence, which perform neural computations using light rather than electrons. Existing methods for achieving optical neural networks do not change the frequencies of photons, but rather reroute photons of a single frequency. According to the researchers, performing such neural computations through frequency manipulation could lead to much smaller devices.
“With a small footprint and yet offering tremendous new engineering flexibility, our device is a significant departure from existing methods,” said Avik Dutt, a postdoctoral scholar in Fan’s lab and the paper’s second author.
Seeing the light
A photon’s color is determined by the frequency at which it resonates, which is a factor of its wavelength. A red photon has a low frequency and a wavelength of approximately 650 nanometers. Blue light, on the other hand, has a much faster frequency and a wavelength of about 450 nanometers.
A simple transformation might involve shifting a photon from 500 nanometers to, say, 510 nanometers—or, as the human eye would perceive it, from cyan to green. The Stanford team’s architecture has the ability to perform not only these simple transformations, but also much more sophisticated ones with fine control.
To illustrate, Fan uses the example of an incoming light stream that contains 20% photons in the 500-nanometer range and 80% photons in the 510-nanometer range. An engineer could use this new device to fine-tune that ratio to 73 percent at 500 nanometers and 27 percent at 510 nanometers, all while preserving the total number of photons. Alternatively, the ratio could be between 37 and 63 percent. The ability to adjust the ratio is what distinguishes this device as novel and promising. Furthermore, a single photon in the quantum world can have multiple colors. In that case, the new device allows for a change in 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, a graduate student in Fan’s lab at the time of the research and the paper’s first author, who now works at Facebook Reality Labs. “Instead, we mean that we can perform any linear transformation required by the engineer. There’s a lot of engineering control going on here.”
“It’s extremely adaptable. The engineer can precisely control the frequencies and proportions, and a wide range of transformations are possible “Fan chimed in. “It bestows new authority on the engineer. It is up to them how they will use it.”