Arcsecond

The international metrology community aims to redefine the second of the International System of Units (SI) using optical atomic clocks by 2030. A roadmap has been established with criteria that must be met to achieve this goal, including the development of high-accuracy optical clocks, their regular contribution to international time scales, and the sustainability of clock comparisons. Arcsecond will address these challenges at the European level. The project will assess optical-clock uncertainties through a coordinated European programme of clock comparisons and will develop and improve optical clocks with uncertainties down to the 10⁻¹⁸ level. It will also expand and strengthen the European optical fibre-link network used for clock comparisons, supporting reliable long-term operation. Finally, Arcsecond will enable more regular contributions of European optical clocks to Coordinated Universal Time (UTC), the globally used reference time scale. With these activities, Arcsecond will provide key evidence for optical-clock-based time-and- frequency references. These underpin both tests of fundamental physics and everyday technologies (satellite navigation, telecoms, the internet).