An international group of scientists led by Loughborough University’s Alessia Pasquazi, a professor of nonlinear optics, recently figured out how to switch optical clocks on and keep them running reliably.
Portable optical clocks are poised to replace satellite navigation systems because they’re incredibly precise measuring devices. But first, microcombs, the time-counting element within optical clocks, needed to be capable of starting and staying in a running state.
“A microcomb produces a set of equally spaced laser lines—an optical frequency comb—with a miniature resonator,” says Pasquazi. “Scientists, including 2005 Nobel Prize in Physics Laureates John Hall and Theodor Hänsch, use these combs to count the ultraprecise frequency oscillation of an optical atomic reference. A microcomb is a fundamental component of a portable atomic clock, which usually includes a laser to provide optical energy and a microcavity for nonlinear mixing.”
The most interesting pulse you can get from a microcomb is a soliton pulse, she says, because it’s well-behaved and has the large spectrum necessary for metrological applications.
Unlike other research groups, Pasquazi’s group inserted the microcavity inside the laser. “This did the trick,” she says. “We have efficient generation of special pulses (laser cavity soliton), thanks to the fast nonlinear responses of the laser, but also very robust operation, thanks to the slow nonlinearities of the system. This configuration allows us to engineer them and to make a soliton the dominant attractor of the system so we are sure it always comes back to that state once obtained.”
Laser cavity solitons
The group’s advance is the result of solid research work, which started at the University of Sussex. “We’ve been observing laser cavity solitons recovering for some time, and I’ve often shown this at conferences,” says Pasquazi. “But the reason for such a fantastic effect was a big puzzle.”
The credit goes to Max Rowley, who devised an experimental approach to study this as the last task of his Ph.D. Supported by Pierre-Henry Hanzard, a research fellow at the University of Sussex, Rowley spent a lot of time collecting data on data.
“We ended up with terabytes of measurements to analyze just in time for the pandemic lockdown, and the help of Antonio Cutrona, who had just started his Ph.D. then, was very critical, especially for simulations,” Pasquazi says.
Exceptional repeatability of the data put the group on the right track, and their understanding of the physics came from lots of collective brainstorming work.
“I love the ‘map of states’ we observed, because it shows the behavior of the soliton as an attractor of the system,” Pasquazi says. “It also shows typical ‘thermodynamical’ behavior. If you look at the map, it could well be the water phase diagram (liquid, solid, gas) against temperature and pressure, the global variables of the system. We mapped different types of laser states current and laser cavity length, but we still have similar behavior.”
The most interesting part of this work for Pasquazi is actually seeing the effect of the optical nonlinearities at two distinct and independent levels because they act on different timescales.
“Fast optical nonlinearities are responsible for generating the frequencies and locking optical phases of the soliton; this is the most common phenomenon observed in lasers,” she explains. “Surprisingly, the slow nonlinearities affect the soliton but don’t destroy it. They are, however, responsible for creating the attractor and maintaining the soliton in place, no matter happens to the laser.”
Pathway forward
There is still work to be done, but Pasquazi stresses the deep understanding of the temporal dynamic will be important for using self-emergence on different types of lasers and microcavities, as well as for putting everything onto a single chip.
“This work is an important piece of the puzzle for the scientific community to reach this goal,” she adds. “The self-emergence will need to be demonstrated with the octave-spanning oscillation other authors observed in microcavities to link the microcomb to the atomic reference.”
Her general impression is that microcomb science, thanks to the efforts of so many outstanding groups worldwide, is very close to providing a workable solution—likely on-chip. But a portable atomic clock is an interdisciplinary effort. “We also need to look at how atomic physicists are working toward fully portable atomic references, which are exceptionally challenging but close to reaching end-users in a rack-size format,” Pasquazi says.
Ultraprecise time-keeping in a portable format will ease the current dependence on the GPS system and its security criticalities. It can also provide the necessary accuracy to high-demanding sectors like aerospace or 6G communications and, in conjunction with future quantum sensing, Pasquazi envisions GPS-free navigation devices.
“Microcombs are especially critical for fiber networks,” she adds. “The combination of efficient, high-density data transfer with ultraprecise time-keeping distribution is a powerful combination with fallout to still be explored.”
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M. Rowley et al., Nature, 608, 303–309 (2022); https://doi.org/10.1038/s41586-022-04957-x.
