Seven of Europe’s most accurate optical atomic clocks have been compared across four countries using a highly stable optical fiber network. The campaign connected clocks developed and operated independently in Italy, France, the United Kingdom, and Germany, producing some of the most precise international clock comparisons to date.
INRiM also participated in the campaign with its ytterbium-based optical clock, as part of a research team led by INRiM Researcher - Dr. Marco Pizzocaro.
To compare clocks with such high precision, their frequencies must be transferred between laboratories, even when they are hundreds or thousands of kilometers apart, without introducing significant variations. The European optical fiber network made it possible to connect the four institutes across thousands of kilometers, enabling direct and highly precise comparisons.
The most significant result concerns two single ytterbium-ion optical clocks, independently developed at the National Physical Laboratory (NPL) in the United Kingdom and the Physikalisch-Technische Bundesanstalt (PTB) in Germany. The comparison showed agreement at the 18-digit level of precision. This represents the first international verification of the consistency of two independently developed optical clocks at this level of precision.
INRiM ytterbium optical lattice clock was also compared with the other clocks in the network, producing results in good agreement with previous international measurements and contributing to confirming the reproducibility of its measurements.
These comparisons are an important part of the path towards the redefinition of the second. Before optical clocks can be adopted as an international reference, it is necessary to demonstrate that clocks developed independently in different laboratories and countries produce consistent results.
The research also highlights the potential of the European optical fiber network, which can support not only increasingly precise time measurements, but also applications in fundamental physics, geodesy, and tests of general relativity.