In order to further improve timing accuracy, researchers from six European countries teamed up to compare ten ultra-precision optical clocks at the same time - this has never been done on such a scale before. These optical clocks use laser light to measure the transitions of atoms between energy levels, much more accurately than traditional cesium atomic clocks. In fact, optical clocks may be off by no more than a second over billions of years.
To check the consistency between these clocks, the team made 38 measurements, a so-called frequency ratio. Four of the measurements have never been made directly, and many are more precise than ever before. This experiment helps us get closer to updating the world's definition of a second, perhaps switching from cesium clocks to optical clocks.
Helen Margolis of the UK's National Physical Laboratory said: "Atomic clocks provide precise time and frequency signals that are vital to many everyday technologies such as GPS, power grid management and keeping financial transactions in sync."
Connecting these clocks over long distances is tricky. The scientists used two connection methods: satellite GPS signals and custom fiber optic cables. All clocks can use GPS, but its accuracy is not ideal due to noise and signal issues. Fiber optic links used in France, Germany and Italy are 100 times more precise but can only cover short distances. For clocks in the same laboratory, such as those in Germany and the UK, short fiber optic cables help to further reduce uncertainty.
The findings were published in Optica, a journal dedicated to optical science. The research team also compared differences in various frequency ratios in different systems to find any mismatches or patterns.
"These measurements provide key information on what further work is needed for optical clocks to achieve the precision and reliability required for international timekeeping," said Marco Pizzocaro of Italy's National Institute of Atomic Energy (INRiM). He added that the device is like a distributed laboratory that could be used for deeper physics research, such as the search for dark matter or testing the foundations of physics.
Coordinating all ten clocks and keeping them running in sync across six countries required a lot of preparation. Some results didn't match predictions, but having so many clocks running simultaneously helped to spot problems.
"Not all results were what we expected, and we observed some inconsistencies in our measurements," said NPL's Rachel Godun. "However, comparing so many clocks simultaneously, and using multiple techniques to correlate the clocks, makes it easier to identify the source of the problem."
The researchers say more work is needed to reduce measurement uncertainties and ensure these optical clocks are reliable over the long term. If this can be done, these clocks may soon become the clocks we use to define time around the world. As Thomas Lindvall from VTT MIKES in Finland said: “With a harmonized set of measurement methods, we can both check consistency and provide more reliable results.”
Source: Optica (Link 1, Link 2)