A satellite flies above planet Earth, and red dotted lines indicate communication between the satellite and both North America and Europe.
New experimental results pave the way for synchronizing ground- and space-based clocks. Credit: B. Hayes/NIST

From the pendulum clocks of the 17th century to cutting-edge chronometers based on atomic transitions, humans’ ability to mark time has evolved dramatically. Researchers have now gone one step further and shown that two clocks can be exquisitely synchronized over a distance of several hundred kilometers using significantly less power than previously required. This advance paves the way for creating networks of synchronized ground- and space-based clocks, which could facilitate geodetic investigations of Earth’s structure and searches for dark matter and gravitational waves.

Keeping Track of Digits

“It’s pretty funny when you have this many digits.”

Most people, save for perhaps elite athletes, don’t think about periods of time shorter than a second. But Laura Sinclair, a physicist at the National Institute of Standards and Technology in Boulder, Colo., regularly concerns herself with an entirely different realm of time: femtoseconds. One femtosecond is a millionth of a billionth (quadrillionth) of a second, or 15 digits past the decimal point. Even computer programs often have trouble handling that level of precision, Sinclair said. “It’s pretty funny when you have this many digits.”

Sinclair and her colleagues have now conducted an experiment in Hawaii using pulses of laser light to precisely and efficiently synchronize two clocks down to the level of attoseconds—the even-shorter brethren of femtoseconds.

Another team of researchers recently demonstrated similar linking over a distance of 113 kilometers, but their experiment required much, much larger amounts of power and specialized instrumentation that compensated for atmospheric turbulence.

In the new experiment, Sinclair and her collaborators worked out of Mauna Loa Observatory, an atmospheric research station on the north flank of Mauna Loa on the Big Island of Hawaii. The team fired two lasers, each producing pulses of light 200 million times per second, toward a small telescope located on the summit of Haleakalā on Maui. The telescope’s modified optics reflected the light back to Mauna Loa, a roughly 300-kilometer round trip, and Sinclair and her colleagues recorded the arrival times of the individual pulses, which function like the “ticks” of a very precise clock.

Like astronomers working on nearby Mauna Kea, the researchers primarily collected their data at night. But sunlight wasn’t what Sinclair and her colleagues were hoping to avoid; instead, the team sought to evade clouds. Every morning, clouds tend to rise to the elevation of Mauna Loa Observatory (roughly 3,400 meters) before finally sinking back down below the observatory somewhere between 5 and 10 p.m. A feeble laser beam is no match for clouds, said Sinclair. “One hundred and fifty kilometers of clouds is enough to stop your near-infrared laser beam.”

The researchers synchronized the output of their two lasers with a precision of 0.32 femtosecond (320 attoseconds). That’s comparable to the precision attained with a different approach, but the lasers that Sinclair and her collaborators used were powered by as little as 270 femtowatts, or 270 millionths of a billionth of a watt. Tens of nanowatts of power were required previously, so these new results represent a roughly 10,000-fold improvement in power use.

“The really impressive thing here is how little power they used.”

That efficiency—which approaches the so-called quantum limit—makes it possible to sync time using smaller hardware, an advance critical for one day linking ground-based clocks and those located in orbit far above Earth.

The advance was made possible by an instrument known as a time-programmable frequency comb, which Sinclair and her colleagues developed. The device modifies the rate at which a laser pulses, Sinclair said. “We can dynamically adjust the output time and phase with attosecond-level precision.” That in turn allowed the researchers to use all of the light received to synchronize the pulsed signals, rather than throwing away the majority of it, as was done previously. When an experiment wastes light, it’s necessary to send more photons, which requires power. Sinclair and her team reported their results in Nature.

“The really impressive thing here is how little power they used,” said David Gozzard, a physicist at the University of Western Australia in Perth not involved in the research. Drawing less power is important because all the equipment associated with synchronizing clocks can be made more streamlined, he said. “Your systems can be smaller and lighter.”

Clocks for Earth Science

In the future, Sinclair and her colleagues hope to synchronize clocks located on the ground and in geosynchronous orbit roughly 36,000 kilometers above Earth’s surface. That would open up all sorts of interesting research studies spanning fundamental physics and Earth and space sciences, the team suggested.

For instance, one of the tenets of the theory of general relativity is that clocks in different gravitational fields accumulate time at different rates. By synchronizing clocks on different parts of the globe—using an intermediate clock on a satellite with sight lines to all of the ground-based clocks—and then watching how those clocks desynchronize, it’s possible to accurately probe minute changes in Earth’s gravitational field. Such changes are often due to geophysical processes like the movement of water and magma. “You’ve got mass moving around,” Gozzard said. Precisely timing experiments could help reveal those environmental changes.

—Katherine Kornei (@KatherineKornei), Contributing Writer

Citation: Kornei, K. (2023), Precisely synced clocks pave the way for new science, Eos, 104, https://doi.org/10.1029/2023EO230296. Published on 4 August 2023.
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