Refining the clock’s second takes time—and lasers

A Chinese research team set a new distance record for syncing two timepieces thanks to some very precise lasers.
Gold pocket watch and hourglass
Metrologists hope optical clocks will provide a redefined 'second' by 2030. Deposit Photos

As technology progresses, so does our ability to more precisely delineate the passage of time. Since their introduction in 1967, the standard bearers of accurate timekeeping have been atomic clocks, whose caesium-33 atoms’ oscillations serve as a reference point for a single “second.” But as a new paper published with Nature earlier in this month reminds us, atomic clocks are so literally and metaphorically yesterday’s news.

According to the publication’s writeup yesterday, a group of researchers at the University of Science and Technology of China in Hefei recently synced optical clocks located 113 kilometers (about 70.2 miles) apart using precise pulses of laser light—roughly seven times farther than that of the previous record. The milestone represents a significant step forward for metrologists, scientists who study measurement, to “redefine” the second by the end of the decade. Once successful, their studies could be an estimated 100 times more accurate than those using existing atomic clock readings.

[Related: What the heck is a time crystal?]

Unlike atomic clocks’ caesium microwaves, optical clocks are reliant on the movement of higher-frequency elements such as strontium and ytterbium to measure time. To measure this, metrologists need to transmit and compare clocks’ readings on different continents, but since satellites are required to accomplish this, our atmosphere’s occluding effects need to be addressed to ensure as accurate a measurement as possible. These latest advances using lasers offer a major step forward towards bypassing these hurdles.

There are a bunch of potential optical clock benefits apart from simply getting an even more nitty-gritty second. According to Nature, researchers will be able to more accurately test the general theory of relativity, which states that time passes slower in regions with higher gravitational pull, i.e. lower altitudes. Optical clocks’ ticking “could even reveal subtle changes in gravitational fields caused by the movement of masses — for example by shifting tectonic plates,” explains the writeup.

[Related: Best sunrise clocks of 2022.]

Researchers still have a lot of work ahead of them before they can confidently reboot the second. In particular, sending signals to orbiting satellites—while roughly the same distance as what the Chinese team just pulled off—must take other factors into consideration, particularly their orbital speeds. For this, metrologists will turn to their recent advances in a whole other field—quantum-communications satellites.