Chip-Sized Frequency Comb Matches Tabletop Systems, Raising Hopes for Portable Atomic Clocks

Physicists who work with light have long lacked the equivalent of a pocket tape measure for one of the basic measurements of their craft, the frequency, or color, of a light wave. The standard tool is the optical frequency comb, a device that produces a rainbow of frequencies spaced at regular intervals like tick marks on a ruler. Until now the best ones have been cumbersome lab setups. Researchers at the Joint Quantum Institute, a partnership of the University of Maryland and the National Institute of Standards and Technology, and an international team now report a chip-based comb that performs as well as the older tabletop versions in a fraction of the space.

The work is published in Nature. Frequency combs let researchers measure differences between distinct frequencies and are crucial to many experiments that use light. In large lab setups they enabled the most precise clocks in the world, atomic clocks, among many other applications. Smaller combs could make portable atomic clocks that help map underground variations in mineral deposits and enable navigation that does not rely on GPS satellite signals.

"Though we have been working on chip-integrated optical frequency combs for many years, their control and stabilization, essential for many applications, have often been complicated and difficult," said JQI research scientist Grégory Moille, the paper's first author and a NIST associate. "With this new approach, we finally see a viable path for their use in deployable atomic timekeeping, which is one of their most demanding and important applications."

The design relies on a phenomenon called parametrically driven cavity solitons, first predicted in 2023 by a team led by Miro Erkintalo of the University of Auckland and the Dodd-Walls Center for Photonic and Quantum Technologies. Light from two lasers circulates around a tiny ring called a microresonator. If the ring has the right shape and the light is injected in the right way, the circulating light interacts with itself through the ring's material and generates a train of pulses that serves as a comb.

The collaboration began after Moille and JQI fellow Kartik Srinivasan, also a NIST fellow, learned of the prediction and teamed up with Erkintalo's group. Their 2024 experiments produced the solitons and unlocked a way to measure a range of previously inaccessible frequencies. But the first comb contained multiple overlapping sets of frequency lines, which made it almost unusable, like a ruler misprinted with several sets of tick marks.

In the new paper, the team combined the soliton approach with a synchronization technique that Srinivasan, Moille and colleagues had earlier shown can stabilize combs. The combination locked all the frequency lines into alignment, a process called self-alignment, producing a pristine ruler with a single set of tick marks. The effort also included colleagues at the University of Maryland, Baltimore County, the University of California, Santa Barbara, AV Incorporated and the Air Force Research Laboratory.

To show the device is adaptable, the researchers used it with a variety of light sources on the routine tasks that are the bread and butter of frequency combs. It builds on a decade of work by Moille and Srinivasan on miniature combs that fit on a portable chip, and prior chip-based approaches had not managed to free the technique from bulky equipment and deploy it reliably outside labs.