A photon pair source that can bridge the visible and telecom bands could be useful for transporting quantum communications over optical fibers. However, the optical components that store and process quantum information typically require visible-light photons to operate — and only near-infrared (NIR) photons have wavelengths that are long enough to transport quantum information over several kilometers of optical fibers. Researchers at the National Institute of Standards and Technology (NIST) and the University of Maryland Nanocenter have developed a novel chip-based device to address this issue. They created entangled pairs made up of one visible photon and one NIR photon using chip-based optical components that can be mass-produced. The visible-light photon from the pair can interact with trapped atoms, ions, or other systems that serve as quantum versions of computer memory, while the NIR photon from the pair can propagate over long distances through optical fiber. By carefully engineering the geometry of a micrometer-scale, ring-shaped resonator, researchers at NIST produced pairs of entangled photons (particles of light) that have two very different colors or wavelengths. Light from a pump laser (purple regions in the resonator) generates one photon in each pair at a visible-light wavelength (red patches in and around resonator); the other photon has a wavelength in the telecommunications (near-infrared) part of the spectrum (blue patches). From the perspective of quantum communication, these pairings combine the best of both worlds in an optical circuit: The visible-light partner can interact with trapped atoms, ions, or other systems that serve as quantum versions of computer memory, while the telecommunications wavelength member of each couple is free to propagate over long distances through an optical fiber network. Courtesy of S. Kelley/NIST. To create the entangled pairs, the team constructed an optical whispering gallery in the form of a nano-size silicon nitride resonator. When a selected wavelength of laser light was directed into the resonator, entangled pairs of visible-light photons and NIR photons emerged. The type of entanglement used by the team, known as time-energy entanglement, linked the energy of the photon pairs with the time at which they were generated. “We figured out how to engineer these whispering gallery resonators to produce large numbers of the pairs we wanted, with very little background noise and other extraneous light,” researcher Xiyuan Lu said. The researchers confirmed that the entanglement persisted even after the photons traveled through 20 kilometers of optical fiber. Typically, entangled photons have similar wavelengths. In this case, the researchers deliberately set out to create “odd couples” — that is, entanglement between photons of very different wavelengths. To make the photons suitable for interacting with most quantum information storage systems, the team also needed the light to be sharply peaked at a particular wavelength rather than broad and diffuse. “We wanted to link together visible light photons, which are good for storing information in atomic systems, and telecommunication photons, which are in the near infrared and good at traveling through optical fibers with low signal loss,” said researcher Kartik Srinivasan. The researchers believe that their design methods could be used to create other visible-light/NIR pairs tailored to specific systems. Their achievement promises to boost the ability of light-based circuits to more securely transmit information to faraway locations. In the future, the researchers said that by combining two of the entangled pairs with two quantum memories, the entanglement inherent in the photon pairs could be transferred to the quantum memories. This technique, known as entanglement swapping, would allow the memories to be entangled with each other over a much longer distance than would normally be possible. “Our contribution was to figure out how to make a quantum light source with the right properties that could enable such long-distance entanglement,” Srinivasan said. The research was published in Nature Physics (https://doi.org/10.1038/s41567-018-0394-3).