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Transportable optical clocks for key comparisons

Short Name: TOCK, Project Number: 22IEM01
Image showing an analogue clock
An analogue clock

Developing travelling standards for high-performance optical clock comparisons

The definition of the second underpins all precision timekeeping, such as that used in banking, Global Navigation Satellite Systems, and communications. It is also fundamental to metrology, with six base SI units using time in their own definitions.

Currently, the second is defined using Caesium atomic clocks, which use transitions between energy levels in Caesium to accurately measure microwave frequencies (in s-1) with measurement uncertainties around 2-3 x10-16. Optical clocks work similarly but use optical light, allowing for measurements with uncertainties as low as 1x10-18.

Agreement between atomic clocks held at National Measurement Institutes (NMIs) and Designated Institutes (DIs) has previously been achieved through satellite communication. However, this is not precise enough for optical clocks. Optical fibres are being investigated as an alternative but are not available on a wide enough scale. Therefore, travelling standards for optical clocks must be developed to provide stable, traceable standards outside of NMI/DIs.


This project builds upon the achievements of previous EMRP and EMPIR projects, most recently ROCIT and TSCA to create a new metrology infrastructure for time and frequency by developing transportable optical clock (TOC) systems. It will design new components, allowing for faster set-up times, and evaluate TOCs’ performance compared to stationary systems. It will also demonstrate the feasibility of TOCs as alternatives for time comparisons and their use in a larger European metrology infrastructure.


The outcomes of the project will facilitate comparisons between high-performance clocks, open a path to redefining the second, and extend the provision of precision timekeeping to the industries that rely on it.



Project website
Other Participants
Gottfried Wilhelm Leibniz Universität Hannover (Germany)
Heinrich-Heine-Universitaet Duesseldorf (Germany)
The University of Birmingham (United Kingdom)
Universiteit van Amsterdam (Netherlands)
Uniwersytet Mikolaja Kopernika w Toruniu (Poland)
Ústav přístrojové techniky AV ČR, v.v.i. (Czechia)


Metrology Partnership
Integrated European Metrology
Total EU contribution (in M €)