Researchers at Stanford University have observed the quantum properties of soliton frequency combs, opening the door to confirm quantum theories. According to the research team, the demonstration is one of the first to show how a miniaturized frequency comb can generate quantum light on a chip. The work supports broader explorations for quantum light using frequency comb and PICs for large-scale experiments, the researchers said. “Many groups have demonstrated on-chip frequency combs in a variety of materials, including recently in silicon carbide by our team. However, until now, the quantum optical properties of frequency combs have been elusive,” said Jelena Vuckovic, the Jensen Huang Professor of Global Leadership in the School of Engineering and professor of electrical engineering at Stanford. “We wanted to leverage the quantum optics background of our group to study the quantum properties of the soliton microcomb.” The silicon carbide microrings developed by the Vuckovic Lab, as seen through a scanning electron microscope at the Stanford Nano Shared Facilities. The team used a miniaturized frequency comb to generate quantum light on a chip. The demonstration opens a pathway for large-scale quantum experiments. Courtesy of the Vuckovic Lab, Stanford University. When created in pairs, microcomb solitons are thought to exhibit entanglement — a relationship between particles that allows them to influence one another even at extreme distances. Proving the utility of their tool, the researchers also provided convincing evidence of quantum entanglement within the soliton microcomb, which had been theorized and assumed but has yet to be proven by any existing studies. “I would really like to see solitons become useful for quantum computing because it’s a highly studied system,” said Melissa Guidry, a graduate student in the Nanoscale and Quantum Photonics Lab and co-author of the paper. “We have a lot of technology at this point for generating solitons on chips at low power, so it would be exciting to be able to take that and show that you have entanglement.” To create their comb, the researchers pump laser light through a microscopic ring of silicon carbide. Traveling around the ring, the laser builds in intensity, and if all goes well, a soliton is born. Conceptual diagram of the frequency comb and the microring, with solitons, that produce it. The frequency comb diagram shows both the coherent light teeth and the quantum light between those teeth. Courtesy of the Vuckovic Lab, Stanford University. “It’s fascinating that, instead of having this fancy, complicated machine, you can just take a laser pump and a really tiny circle and produce the same sort of specialized light,” said Daniil Lukin, a graduate student in the Nanoscale and Quantum Photonics Lab and co-author of the paper. He added that generating the microcomb on a chip enabled a wide spacing between the teeth, which was one step toward being able to look at the comb’s finer details. The next steps involved equipment capable of detecting single particles of the light and packing the microring with several solitons, creating a soliton crystal. “With the soliton crystal, you can see there are actually smaller pulses of light in between the teeth, which is what we measure to infer the entanglement structure,” Guidry said. “If you park your detectors there, you can get a good look at the interesting quantum behavior without drowning it out with the coherent light that makes up the teeth.” The researchers also examined a theoretical model called the linearized model, which is often used as a shortcut to describe complex quantum systems. When they ran the comparison, the researchers found that the experiment matched the theory well. Though they have yet to directly measure that their microcomb has quantum entanglement, they have shown that its performance matches a theory that implies entanglement. “The take-home message is that this opens the door for theorists to do more theory because now, with this system, it’s possible to experimentally verify that work,” Lukin said. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00901-z).