To create their miniature comb, the researchers pump laser light through a microscopic ring of silicon carbide (which was painstakingly designed and fabricated using the resources of the Stanford Nano Shared Facilities and Stanford Nanofabrication Facilities). Traveling around the ring, the laser builds up intensity and, if all goes well, a soliton is born.

“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 micro-ring 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,” explained Guidry. “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.”

Seeing as they were performing some of the first experimental studies of the quantum aspects of this system, the researchers decided to try to confirm a theoretical model, called the linearized model, which is commonly used as a shortcut to describe complex quantum systems. When they ran the comparison, they were astonished to find that the experiment matched the theory very well. So, while they have not yet directly measured 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,” said Lukin.