Reviewed by Lexie CornerFeb 24 2025
A research team from Purdue University in the United States and Chalmers University of Technology in Sweden has developed a technology that utilizes on-chip microcombs to enable the miniaturization of ultra-precise optical atomic clock systems. This advancement could improve accessibility and has potential applications in geo-data monitoring, autonomous vehicles, and navigation. The study was published in Nature Photonics.
Like comb teeth, a microcomb consists of a spectrum of evenly distributed light frequencies. Optical atomic clocks can be built by locking a microcomb tooth to a ultranarrow-linewidth laser, which in turn locks to an atomic transition with extremely high frequency stability. That way, frequency combs act like a bridge between the atomic transition at an optical frequency and the clock signal at an electronically detectable radio frequency for counting the oscillations – enabling extraordinary precision. The researchers’ photonic chip, on the right-hand side of the image, contains 40 microcomb generators and is only five millimeters wide. Image Credit: Kaiyi Wu
Optical atomic clocks can improve time and location accuracy by a factor of ten, enhancing applications such as GPS systems, personal computers, and mobile phones. However, their current size and complexity limit their widespread use.
Over 400 atomic clocks worldwide provide precise timekeeping and positioning data for modern devices, including mobile phones, computers, and GPS systems. All clocks, whether mechanical, atomic, or digital, consist of two components: an oscillator and a counter. The oscillator produces periodic variations at a specific frequency, while the counter tracks the oscillation cycles. Atomic clocks measure the oscillations of atoms as they transition between two energy levels with high precision.
Most atomic clocks operate at microwave frequencies to induce energy transitions in atoms. In recent years, researchers have explored using lasers to generate optical-frequency oscillations. Optical atomic clocks function similarly to a ruler with more markings per unit length, enabling finer division of time. This results in significantly improved accuracy in timekeeping and positioning data.
Today's atomic clocks enable GPS systems with a positional accuracy of a few meters. With an optical atomic clock, you may achieve a precision of just a few centimeters. This improves the autonomy of vehicles, and all electronic systems based on positioning. An optical atomic clock can also detect minimal changes in latitude on the Earth's surface and can be used for monitoring, for example, volcanic activity.
Minghao Qi, Study Co-Author and Professor, Purdue University
Current optical atomic clocks are large and require specialized laboratory environments with precise laser configurations and optical components, limiting their use outside controlled settings. This makes deployment in satellites, remote research stations, or drones challenging.
A research team from Purdue University and Chalmers University of Technology has developed a method to miniaturize optical atomic clocks, improving their accessibility and potential for broader applications.
System Miniaturized By Micro Combs
The core of the new technology, described in a recently published study in Nature Photonics, is a set of small, chip-based devices known as microcombs. Microcombs generate a spectrum of evenly spaced light frequencies, similar to the teeth of a comb.
“This allows one of the comb frequencies to be locked to a laser frequency that is in turn locked to the atomic clock oscillation,” added Minghao Qi.
While optical atomic clocks offer significantly higher precision, their oscillation frequency is in the hundreds of terahertz (THz) range, which is too high for direct counting by electronic circuits. The researchers' microcomb circuits address this limitation, enabling a reduction in the overall size of the atomic clock system.
Fortunately, our microcomb chips can act as a bridge between the optical signals of the atomic clock and the radio frequencies used to count the atomic clock’s oscillations. Moreover, the minimal size of the microcomb makes it possible to shrink the atomic clock system significantly while maintaining its extraordinary precision.
Victor Torres Company, Study Co-Author and Professor, Chalmers University of Technology
Solving the Challenge of Self-Reference
Achieving both the “self-reference” necessary for system stability and precise alignment of the microcomb’s frequencies with atomic clock signals has been a significant challenge.
It turns out that one microcomb is not sufficient, and we managed to solve the problem by pairing two microcombs, whose comb spacings, i.e. frequency interval between adjacent teeth, are close but with a small offset, e.g. 20 GHz. This 20 GHz offset frequency will serve as the clock signal that is electronically detectable. In this way, we could get the system to transfer the exact time signal from an atomic clock to a more accessible radio frequency.
Kaiyi Wu, Study Leading Author, Purdue University
The new system incorporates integrated photonics, which replaces larger laser optics with chip-based components.
“Photonic integration technology makes it possible to integrate the optical components of optical atomic clocks, such as frequency combs, atomic sources, and lasers, on tiny photonic chips in the micrometer to millimeter sizes, significantly reducing the size and weight of the system,” stated Dr. Kaiyi Wu.
This innovation could enable mass production, making optical atomic clocks more cost-effective and accessible for various scientific and societal applications. In addition to microcombs, the system for counting optical frequency cycles includes modulators, detectors, and optical amplifiers. While this study demonstrates a new design, the next step is integrating all necessary components into a fully functional system on a single chip.
“We hope that future advances in materials and manufacturing techniques can further streamline the technology, bringing us closer to a world where ultra-precise timekeeping is a standard feature in our mobile phones and computers,” added Victor Torres Company.
The authors dedicated the study to the late Professor Andrew Weiner, who led the research but passed away before its publication.
Minghao Qi concluded, “Professor Weiner was a mentor for all of the authors in this publication, and his dedication to scientific discovery, his vision for research direction, and his high standards for research results and ethics will be forever remembered.”
Journal Reference:
Wu, K., et al. (2025) Vernier microcombs for integrated optical atomic clocks. Nature Photonics. doi.org/10.1038/s41566-025-01617-0.