Precisely synchronized networks of clocks are key not only to modern everyday services such as GPS networks, but also to scientific endeavors such as giant telescope networks and tests of gravitation. Scientists now have devised a way to use laser pulses to synchronize two different clocks separated by kilometers of turbulent open air to within a few millionths of a billionth of a second, a more than thousand-fold improvement over previous strategies. The scientists detailed their findings online May 11 in the journal Physical Review X.

The best clocks nowadays are accurate down to nearly an attosecond (10-18 of a second). However, techniques for synchronizing clocks over large distances are less accurate—conventional methods for this synchronization, which depend on radio signals exchanged between clocks, are limited to uncertainties on the level of picoseconds (10-12 of a second).

Better clock networks could lead to better maps of all kinds, because more precise measures of time help improve measures of space as well. This not only can help enhance GPS, but also help scientists analyze gravitational fields to test and evaluate the general theory of relativity, allowing for a more complete theory of space-time. And clock networks can help orchestrate networks of telescopes, says study lead author Jean-Daniel Deschênes, an electrical engineer at Laval University in Canada.

One way to improve clock synchronization is to use optical laser pulses, which are higher in frequency than radio signals and can potentially enable attosecond levels of accuracy. However, such pulses typically travel via fiber optics, making them unsuitable for applications where clocks have no optical fiber connection, such as GPS networks.

Now the researchers have demonstrated synchronization over free space with unprecedented accuracy, on the scale of femtoseconds (10-15 seconds). Using a pair of clocks linked by optical laser pulses across four kilometers of turbulent air, the researchers showed the devices could keep the same time to within 20 femtoseconds over the course of 50 hours. The scientists also found no evidence that increasing the effective distance between the clocks degraded synchronization, which suggests it may be possible to apply this technique to very long distances on the order of hundreds of kilometers.

The scientists placed the clocks beside each other on top of the National Institute of Standards and Technology (NIST)’s Boulder, Colo., facility. The signals that each clock beamed at the other were bounced off a half-meter mirror on nearby foothills. The more trips this reflected light took, the greater the effective distance between the clocks.

“All the pieces had to perform at a state-of-the-art level and be coupled just right—at no point could we have anything disturb the timing,” says study co-author Laura Sinclair, a physicist at NIST. “Length variation of just a few microns could matter, which could be caused by just a few degrees change in temperature.”

Air turbulence can distort laser pulses, says study senior author Nathan Newbury, a physicist at NIST. To help mitigate air turbulence interference, researchers had both clocks beam signals at each other to synchronize rather than having just one clock beam signals at the other clock. By measuring the differences in the arrival times of the signals at both clocks, they could account for potential synchronization errors.

“With this method, the amazing timing of clocks does not have to be limited to the lab or a dedicated fiber network,” Newbury says. “We can imagine building a network of clocks connected only by free-space laser links.

“This technology potentially offers a means for implementing two-way time transfer at femtosecond accuracy even between continents,” says applied physicist Michael Dennis at Johns Hopkins University, who did not take part in this research. “Such a precision would make it possible to bring the extraordinary advances in optical clocks to bear on problems in fundamental physics, geophysics, and astrophysics.”

Future research will seek to use this strategy to synchronize moving clocks, Sinclair says.