<p>EMPIR project contributes to developing time scales to harness the superior performance of optical clocks</p>
Time and frequency are at the heart of many everyday applications that we take for granted, such as satellite navigation and telecommunications. They underpin some of the most precise measurements in many areas of research such as fundamental physics, molecular spectroscopy and geodesy.
One of the central challenges of time and frequency metrology has been the redefinition of the second, in the International System of Units (SI), based on optical frequency standards (also known as optical clocks). The roadmap adopted by the Consultative Committee for Time and Frequency (CCTF) made it clear that remote international optical clock comparisons at the highest level of accuracy would be essential for this process. For these, the required accuracy is only achievable using long fibre links.
Completed EMPIR project Advanced time/frequency comparison and dissemination through optical telecommunication networks (18SIB06, TiFOON) addressed key areas of development to transform fibre-based frequency transfer capabilities in Europe into a universal tool for time and frequency metrology and beyond. Accurate, reliable, and efficient solutions for optical time and frequency over fibre links were developed. By the end of the project, fibre networks supporting time and frequency dissemination had been or were being rolled out in several European states. The technologies and insights developed in this project have made a pan-European fibre network for time and frequency metrology both more feasible and more valuable.
Project achievements
Specific project achievements include:
1. World first
Ultra-stable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultra-stable laser, it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. In this project, two geographically separated lasers were compared, over the longest ever reported metrological optical fibre link network at the time, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10−17 for averaging times between 30 s and 200 s. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre.
This work was reported in the journal Nature Communications in the article Comparing ultrastable lasers at 7 × 10−17 fractional frequency instability through a 2220 km optical fibre network.
2. Best practice
Best practice regarding the compatibility of time and frequency services and data traffic were communicated to network operators and equipment manufacturers. This will enable new fibre links to be implemented. Cooperation with GÉANT, Europe's leading collaboration on e-infrastructure and services for research and education, is an example of such activity.
The Good practice guide for the coexistence of time and frequency signals with data traffic in DWDM fibre optic networks produced within this project was submitted to EURAMET and is expected to be published as a Technical Guide.
3. New instruments
An extensive toolbox of technical solutions for improving the reliability and performance of optical fibre links was developed.
Hardware and software developed within this project, such as the auto-locking tracking filter, the combined optical phase and polarisation tracker and the Optical-Electrical-Optical regeneration technique, will be incorporated into commercial products. Commercial systems, sub-systems and components resulting from outputs of this project will, in turn, facilitate the uptake of time and frequency services by the metrological and wider scientific community.
4. Space observatories
The benefits of fibre-based synchronisation identified in this project were made available to a number of space-geodetic facilities including Matera and Medicina observatories, where a first experimental demonstration was performed. This was described in the paper Common-clock very long baseline interferometry using a coherent optical fiber link.
5. Open-source data
To facilitate uptake, three specific outputs of this project have been designated “open hardware”:
The work of this project has fed into the international optical clock comparison in EMPIR project Robust Optical Clocks for International Timescales (18SIB05, ROCIT).
Project coordinator Jochen Kronjaeger from PTB said
‘The TiFOON project, together with its predecessors NEAT-FT and OFTEN, has laid the foundations of a unique, developing infrastructure, enabling optical clock comparisons at a scale and precision not possible anywhere else in the world. In future, this infrastructure will not only support leading-edge research in quantum technologies and fundamental physics, it may also help alleviate our critical reliance on GNSS by providing resilient timing at the highest level of accuracy.’
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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