Seven optical clocks in four countries linked by fibre: two independently built ytterbium-ion clocks agree within an uncertainty of 7.7 × 10⁻¹⁸
On 2 September 2026 Italy’s metrology institute INRiM reported the results of a two-month campaign comparing seven optical clocks at the metrology institutes of Italy, France, the United Kingdom and Germany, linked by the European optical fibre network. The paper was published on 1 September 2026 in Physical Review Research.
- The campaign involved INRIM, LNE-OP (formerly LNE-SYRTE), NPL and PTB. Optical frequency ratios were determined with uncertainties ranging from 7.7 × 10⁻¹⁸ to 6.1 × 10⁻¹⁷.
- The ¹⁷¹Yb⁺ (E3) clocks at NPL and PTB, developed independently, agreed within an uncertainty of 7.7 × 10⁻¹⁸ — according to the authors, the first international verification of two independently developed optical clocks below one part in 10¹⁷.
- Operation of the ¹⁹⁹Hg clock at LNE-OP yielded frequency ratios with improved uncertainties with the ¹⁷¹Yb⁺ (E3), ¹⁷¹Yb and ⁸⁷Sr clocks. INRiM’s ytterbium optical lattice clock gave results in good agreement with previous international measurements.
- Such comparisons are a prerequisite for redefining the second: before optical clocks can become the international reference, it has to be shown that independent clocks in different countries give consistent results. According to INRiM, the fibre network can also serve fundamental physics, geodesy and studies of general relativity.
Source: INRiM — L’Europa collega gli orologi che potrebbero ridefinire il secondo (02.09.2026)