Seven optical atomic clocks in four European countries have passed one of the most precise international timekeeping comparisons yet, strengthening the scientific case for eventually redefining the second. Nature highlighted the result on September 4, after the peer-reviewed study appeared in Physical Review Research on September 1.
The two-month campaign linked independently operated clocks at national metrology institutes in Italy, France, the United Kingdom and Germany. Researchers compared their frequencies through a stabilized optical-fibre network spanning thousands of kilometres, avoiding much of the uncertainty that satellite-based transfers can introduce over long distances.
The study reported uncertainty in measured optical-frequency ratios ranging from 7.7 times 10 to the power of minus 18 to 6.1 times 10 to the power of minus 17. Its most notable test involved single ytterbium-ion clocks at Britain's National Physical Laboratory and Germany's Physikalisch-Technische Bundesanstalt, which agreed within the campaign's tightest uncertainty.
According to the authors and Italy's Istituto Nazionale di Ricerca Metrologica, that result marks the first international verification of two independently developed optical clocks below one part in 10 to the power of 17. The Italian institute's ytterbium lattice clock also agreed with earlier international measurements, while a mercury clock in France produced several frequency ratios with improved uncertainty.
Optical clocks measure the rapid oscillations associated with transitions in atoms or ions and can outperform the microwave caesium clocks behind the current definition of the second. Before an optical transition can serve as a new global reference, laboratories must show that separate devices produce consistent results after accounting for systematic effects, fibre transfer and gravitational redshift.
The researchers said the measurements will provide evidence for international work on a future redefinition of the SI second. They also said reliable fibre-linked clock networks could support tests of fundamental physics and general relativity, improve geodesy by detecting tiny height-related changes in gravity, and let laboratories compare next-generation standards without moving the clocks.
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