White light is not white at all but rather a composite of many photons — droplets of energy that make up light from every color of the rainbow. 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 mixture of colors.
The structure consists of a low-loss wire for light carrying a stream of photons that passes by like 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 that 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 are 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.
The color of a photon is determined by the frequency at which the photon resonates, which in turn is a factor of its wavelength. A red photon has a relatively slow frequency and a wavelength of about 650 nanometers. At the other end of the spectrum, blue light has a much faster frequency with a wavelength of about 450 nanometers.
A simple transformation may involve shifting a photon from a frequency of 500 nanometers to 510 nanometers or, as the human eye would register it, a change from cyan to green. The new architecture can perform these simple transformations with fine control.
For an example, an incoming light stream is comprised of 20 percent photons in the 500-namometer range and 80 percent at 510 nanometers. Using the new device, an engineer could fine-tune that ratio to 73 percent at 500 nanometers and 27 percent at 510 nanometers, all while preserving the total number of photons. 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.
The device allows for “arbitrary” transformation but that does not mean random. Instead, it means that it can achieve any linear transformation that the engineer requires. The engineer can control the frequencies and proportions very accurately and a wide variety of transformations are possible.