<p>EMPIR project influences approach to updating recommended values of standard frequencies, supporting the redefinition of the SI second</p>
EMPIR project influences approach to updating recommended values of standard frequencies, supporting the redefinition of the SI second
The project
Optical atomic clocks have now reached levels of performance that clearly surpass the stability and accuracy achievable with caesium microwave primary frequency standards, with the result that a future optical redefinition of the International System of Units, the SI, second is anticipated.
Completed EMPIR project Robust Optical Clocks for International Timescales (18SIB05, ROCIT) tackled key steps that must be taken prior to the redefinition:
- The robustness of optical clocks was improved so that they could run unattended for long periods,
- methods were developed for automatic data validation enabling prototype demonstrations of optically steered timescales,
- and the long-term reliability and reproducibility of the clocks were assessed through a coordinated programme of international comparisons.
By the end of the project several European optical clocks were contributing on a regular basis to International Atomic Time (TAI) as secondary representations of the second, improving its stability and accuracy.
These steps represent important progress towards the criteria set out in the international roadmap towards a redefinition of the second, and since international timescales underpin modern-day technology such as telecommunications and navigation systems, the work will benefit end users across a wide range of sectors in the longer term.
Good practice guide
The recommended frequency values and uncertainties of secondary representations of the second are derived from the worldwide body of clock frequency comparison data, including both absolute frequency measurements and direct frequency ratio measurements that do not involve a caesium primary standard.
To ensure that the recommended frequency values are unbiassed, and their uncertainties are properly estimated, account must be taken of any correlations between the measurements.
The project consortium has published a good practice guide entitled Guidelines on the evaluation and reporting of correlation coefficients between frequency ratio measurements. The guide includes a recommended reporting template and worked examples.
Project coordinator Helen Margolis from NPL said
‘These guidelines were shared with the CCL-CCTF Frequency Standards Working Group, and strongly influenced the 2021 update to the list of recommended frequency values. As a result of that update, six secondary representations of the second now have fractional uncertainties of 1.9 – 2.0 parts in 1016, meaning that they can contribute to International Atomic Time with a similar weight to caesium primary frequency standards, if they achieve similar uptimes.’
This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.
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