The satellites that make the GPS in your car and smartphone work consist of many atomic clocks. About 400 such atomic clocks are currently operational, providing real-time locations that are accurate to a few meters.
However, scientists aim to make GPS much more precise, as this would enable autonomous cars, navigation systems, and geological monitoring to be far more accurate than they are today.
This could be done by replacing the current atomic clocks in GPS satellites with optical atomic clocks, which are far more advanced and precise timekeeping devices. For instance, while a regular atomic clock is accurate to one second in millions of years, an optical atomic clock is even more precise, with an error of one second in billions of years.
“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,” Minghao Qi, professor of electrical and computer engineering at Purdue University, said.
However, current versions of optical atomic clocks are quite large, occupying several square meters of space in a lab and preventing scientists from installing them into GPS satellites — but this could soon change.
Researchers from Purdue University and Chalmers University of Technology have developed a new technology that could reduce the size of optical atomic clocks to a great extent.
A microcomb chip to shrink optical atomic clocks
Traditional optical atomic clocks are not only big but complex, requiring specialized laser setups and optical components. The size of this setup makes the optical atomic clock impractical for widespread use in applications like smartphones, satellites, or drones.
To solve this challenge, the researchers created tiny, chip-based devices known as microcombs. These microcombs generate a spectrum of evenly spaced light frequencies, similar to the teeth of a comb.
Optical atomic clocks operate at a frequency of hundreds of terahertz (THz). An electronic circuit isn’t meant to measure such high-level frequency. However, the evenly spaced light frequencies help divide this ultra-fast oscillation into lower, more manageable frequencies.
Plus, it allows one of the microcomb’s frequencies to be locked to a laser, which is then locked to the atomic clock’s oscillation. This creates a stable reference that can be used for precise timekeeping. Therefore, by integrating these microcombs into optical atomic clocks, it is possible to miniaturize the clocks’ components without compromising their precision.
“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,” Victor Torres-Company, one of the study authors and a professor at Chalmers.
“Moreover, the minimal size of the microcomb makes it possible to shrink the atomic clock system significantly while maintaining its extraordinary precision,” Victor added.
One microcomb isn’t enough
Creating a tiny microcomb is just one part of the problem. The next challenge is to align it with the atomic clock’s signal and conduct stable operations. When the researchers tested their microcomb chip, they noticed that it wasn’t enough to achieve the desirable results.
With one microcomb, they were not able to use the ultra-fast time signals from the atomic clock into usable radio frequencies that were necessary to measure time and position.
“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,” Kaiyi Wu, lead study author and a postdoc researcher at Purdue, said.
This 20 GHz difference acts as a clock signal that can be detected electronically, allowing the system to measure the readings from the optical atomic clock.
However, apart from microcombs, an optical clock also requires components such as optical amplifiers and modulators. Future research will focus on bringing such elements to a chip so that we can soon mass produce and use this ultra-precise timekeeping device in our satellites and everyday GPS devices.
The study is published in the journal Nature Photonics.
