Microcomb chips help pave the way for thousand times more accurate GPS systems

Today, our mobile phones, computers, and GPS systems can give us very accurate time indications and positioning thanks to the over 400 atomic clocks worldwide. All sorts of clocks — be it mechanical, atomic or a smartwatch — are made of two parts: an oscillator and a counter. The oscillator provides a periodic variation of some known frequency over time while the counter counts the number of cycles of the oscillator. Atomic clocks count the oscillations of vibrating atoms that switch between two energy states with very precise frequency.

Most atomic clocks use microwave frequencies to induce these energy oscillations in atoms. In recent years, researchers in the field have explored the possibility of using laser instead to induce oscillations optically. Just like a ruler with a great number of ticks per centimeter, optical atomic clocks make it possible to divide a second into even more time fractions, resulting in thousands of times more accurate time and position indications.

“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,” says Prof. Minghao Qi from Purdue University, co-author of a study recently published in Nature Photonics.

However, the optical atomic clocks that exist today are bulky and require complex laboratories with specific laser settings and optical components, making it difficult to use them outside lab environments, such as in satellites, remote research stations, or drones. Now, a research team at Purdue University, and Chalmers, has developed a technology that makes optical atomic clocks significantly smaller and accessible for more widespread use in society.

System miniaturised by microcombs

The core of the new technology, described in a recently published research article in Nature Photonics, are small, chip-based devices called microcombs. Like the teeth of a comb, microcombs can generate a spectrum of evenly distributed light frequencies.

“This allows one of the comb frequencies to be locked to a laser frequency that is in turn locked to the atomic clock oscillation,” says Minghao Qi.

While the optical atomic clocks offer much higher precision, the oscillation frequency is at hundreds of THz range — a frequency too high for any electronic circuits to “count” directly. But the researchers’ microcomb chips were able to solve the problem — while enabling the atomic clock system to shrink considerably.

“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,” says Victor Torres Company, Professor of Photonics at Chalmers and co-author of the study.

Solving the challenge of self-reference

Another major obstacle has been achieving simultaneously the “self-reference” needed for the stability of the overall system and aligning the microcomb’s frequencies exactly with the atomic clock’s signals.

“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, ” says Kaiyi Wu, the leading author of the study at Purdue University.

Chip-based laser optics paving way for accessible atomic optical clocks

The new system also includes integrated photonics, which uses chip-based components rather than bulky laser optics.

“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 micrometer to millimeter sizes, significantly reducing the size and weight of the system,” says Dr. Kaiyi Wu.

The innovation could pave the way for mass production, making optical atomic clocks more affordable and accessible for a range of applications in society and science. The system that is required to “count” the cycles of an optical frequency requires many components besides the microcombs, such as modulators, detectors and optical amplifiers. This study solves an important problem and shows a new architecture, but the next steps are to bring all the elements necessary to create a full system on a 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,” says Victor Torres Company.