Research shows microcombs' potential for atomic clocks

Devices like our mobile phones, computers and GPS systems are able to provide accurate timings and positioning due to 400 plus atomic clocks operating wolrdwide.

All clocks are made up of two parts: an oscillator and a counter. The oscillator provides a periodic variation of some 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 extremely precise frequency.

Most atomic clocks utilise microwave frequencies to induce these energy oscillations in atoms. Researchers have explored the potential of using laser instead to induce oscillations optically. Similar to a ruler with a great number of ticks per centimetre, optical atomic clocksmake it possible to divide a second into even more time fractions, causing thousands of times more accurate time and position indications.

"Today's atomic clocks enable GPS systems with a positional accuracy of a few metres. With an optical atomic clock, you may achieve a precision of just a few centimetres. 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," explained Professor Minghao Qi from Purdue University.

But owing to the bulky nature of optical atomic clocks and the associated complexity that requires specific laser settings and optical components - which means they are difficult to use outside of lab environments such as in satellites, remote research stations, or drones - but thanks to the work of the researchers, they have developed a technology with the potential to make use of optical atomic clocks more widespread.

At the core of the new technology is small, chip-based devices called microcombs. 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,” said Minghao Qi.

Althoug optical atomic clocks offer much higher precision, the oscillation frequency is at hundreds of THz range, which is a frequency too high for electronic circuits to count directly. The researchers' microcomb chips were able to reolve this problem, while facilitating the size of the atomic clock system to shrink significantly.

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

Another major obstacle has been achieving the self-reference needed for the stability of the 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. 20GHz. This 20GHz 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, " said Dr. Kaiyi Wu, the leading author of the study at Purdue University.

Integrated photonics are also partof the new technology, using chip-based components rather than 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 micrometre to millimetre sizes, significantly reducing the size and weight of the system,” added Kaiyi Wu.

The hope is that the research will pave the way for mass production. The system required to count the cycles of an optical frequency calls for many components in addition to the microcombs, such as modulators, detectors, and optical amplifiers. The new technology solves an important problem and demonstrates a new architecture, with the next steps being to bring all the required elements for a system-on-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," concluded Victor Torres Company.