A six-nation experiment achieves world-first precision in syncing optical clocks, edging closer to a new time standard.
Time has always been defined by the vibrations of cesium atoms. But as the limits of precision are pushed further by modern physics, scientists are now preparing to rewrite the rules.Last month, a consortium of 69 scientists from across Europe and Japan completed the largest and most coordinated comparison of optical clocks ever undertaken, bringing the world closer to replacing the definition of a second with a far more accurate standard.
Conducted over 45 days and spanning six countries, including Finland, France, Germany, Italy, the United Kingdom, and Japan, the effort linked 10 optical clocks using both satellite signals and ultra-stable optical fiber connections.Ticking toward new timeTogether, they performed 38 frequency ratio measurements with unprecedented consistency. Four of these comparisons were carried out directly for the first time, demonstrating the feasibility of a future global optical time scale.“These measurements provide critical information about what work is still needed for optical clocks to achieve the precision and reliability required for use in international timekeeping,” said Marco Pizzocaro, senior researcher at the Instituto Nazionale Di Ricerca Metrologica in Italy.“Our experiment also showed how optical clocks across Europe can be linked to measure frequency ratios with state-of-the-art precision. This creates a distributed lab, which could also be used for carrying out tests of fundamental physics, such as searching for dark matter or testing the basic rules of physics.” Optical clocks are advanced atomic clocks that use lasers to trigger precise energy shifts in atoms. These shifts occur at exact frequencies, acting as the ultra-stable “ticks” that mark time.These ticks are more precise than the cesium-based microwave atomic clocks currently used to define Coordinated Universal Time .While cesium clocks can drift by a second every 100 million years, today’s best optical clocks wouldn’t lose or gain a second in billions of years.Since different optical clocks use different atoms, tapping into their full potential requires comparing them across vast distances to ensure consistency and accuracy. “Comparing multiple clocks at the same time and using more than one type of link technology provides far more information than the mostly pairwise clock comparisons that have been carried out to date,” said Thomas Lindvall, senior scientist at VTT MIKES in Finland.“With a coordinated set of measurements, it becomes possible to check consistency while also providing more trusted results. These results can help determine which optical clock should be used in the new definition of the second.” Science syncs the secondThe project relied on a combination of technologies to link clocks across countries. GPS satellite signals offered global connectivity but were limited in precision due to factors such as signal noise, atmospheric interference, and instrument constraints.To achieve higher accuracy, researchers also employed custom optical fiber links, enabling measurements with up to 100 times greater precision than satellite-based methods. These ultra-stable connections were used to link clocks in France, Germany, and Italy.Additionally, in-country comparisons within the UK and Germany, where multiple clocks were housed at the same institute, used short-range optical fibers, further minimizing uncertainty and boosting measurement stability.Researchers said the experiment demanded meticulous planning and presented significant challenges during data analysis.“Not all the results confirmed what we expected, and we observed some inconsistencies in the measurements,” said Rachel Godun, principal scientist at NPL.“However, comparing so many clocks at once and using more than one technique for linking the clocks made it easier to identify the source of the problem.”The upcoming redefinition of the second in the International System of Units is expected by 2030.The findings have been published in Optica.
Frequency Ratio Global Synchronization Laser Clocks Metrology Optical Clocks Precision Measurement Redefine Second SI Unit Timekeeping
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