International timescales with optical clocks

The SI unit of time, the second, underpins international timekeeping and is relied upon by power grids, the internet, financial transactions and navigation systems. Improving the accuracy and stability of international timescales will bring more precision to many applications.

The second is currently kept constant by comparing a microwave source to a fundamental property - an absorption frequency of caesium atoms. However, a new generation of optical clocks, using higher frequency laser light, can now achieve greater accuracy and stability. For any future redefinition of the SI second based on optical clocks, the accuracy of different types of clock must be confirmed by a coordinated programme of comparisons, and their frequencies robustly linked to existing caesium standards.

This project tackled these and other important considerations for a redefinition of the second using optical frequency clocks.

The Project:

• Performed a comparison of different types of optical clocks, within and between institutes, establishing ratios between their operating frequencies and relating them to the caesium standard
• Investigated enhanced satellite techniques for comparing the frequencies of clocks in remote locations, and found that advanced processing of comparison data from GNSS satellites was comparable in performance to using dedicated satellite transponders, but far cheaper
• Measured the Earth’s gravity potential with improved precision at four European NMIs, which will enable the operating frequencies of the optical clocks to be corrected for gravitational effects when contributing to international timescales
• Developed a transportable optical clock and used it to measure the gravity potential in the middle of a mountain, demonstrating a future application of the new technology.

Robust procedures, developed by this project, for analysing the self-consistency of clock comparison data and deriving optimised values for the frequencies of the optical clocks will allow them to be integrated into international timescales, immediately improving their stability. The work will also enable better informed decisions to be taken on any future redefinition of the second.

The international scientific community will also benefit from validated optical clock performance when conducting tests of fundamental physical theories. Early beneficiaries will include the European Space Agency (ESA) and the European Very Long Baseline Interferometry Network (EVN), who have facilities requiring accurate time and frequency signals. Measurements of gravity potential differences using optical clocks will be important in geodesy, for example, in monitoring the effects of climate change on sea levels.

EMPIR project 15SIB03 OC18 builds on this work.