Time and Frequency Pathfinder Study: Helping redefine time for the 21st Century

“I’ll be a second”, “Can I borrow you for a second?”, “Have you got a second?”. Phrases we hear many times each day but what exactly is a second? Since, well time immemorial, we have been attempting to measure time to greater and greater degrees of precision and accuracy because so much of fundamental science relies on being able to measure everything and virtually all these measures rely on time.

Since 1968, the second has been defined using atomic clocks – these used to be room-sized delicate machines but have been refined and shrunk down to fit into the GPS satellites that we all use every day for our sat-nav systems. These atomic clocks are amazingly accurate – if one had started at the beginning of the universe then its time could have drifted by around 100 seconds – but current science needs even more accurate measurements and so there is an international effort underway to redefine the SI unit of time, the second, by 2030 using the latest generation of optical atomic clocks.

These clocks promise to be orders of magnitude better than the microwave atomic clocks currently used to maintain the global time scale Coordinated Universal Time (UTC). State-of-the-art optical clocks have an estimated uncertainty of 10⁻¹⁸, or 18 digits of precision, compared to 10⁻¹⁶ achievable with the best Caesium fountain clocks today. These optical clocks would have an error of less than a second over the lifetime of the universe.

However, this precision brings with it new and different problems. At present, ground-based atomic clocks can collaborate with each other using for example satellite systems such as GPS to provide a “consensus” of time, but because the new optical clocks are so much more precise this satellite system simply cannot cope. Using them would be like asking your co-worker what the time was and them replying “Tuesday-ish”.

Therefore in 2022, when the General Conference on Weights and Measures (CGPM) – the body ultimately in charge of the international system of units, the SI – approved Resolution 5 towards the redefinition of the second using optical clocks, it was agreed that, at the time, “only comparisons mediated by optical fibre links provide the required [low and predictable] instability and accuracy for comparing optical clocks”

And of course, this is where GÉANT, NRENs and the European science community are experts!

As of today, several European National Metrological Institutions (NMI) and universities are developing optical clocks. Four of them which are at the forefront of the development – in London, Paris, Turin and Braunschweig – are already connected through a network of international fibre links having emerged from national and bi-national initiatives. In addition, there are a series of national projects to interconnect sites either providing or benefitting from ultrastable and accurate frequency references. However much of this work is limited by national boundaries with very limited cross-border collaboration, and the future of existing cross-border links is always in the balance due to shifting national priorities.

This is why the next iteration of the GÉANT (GN5-1) project (starting in 2025) will be working with NRENs and NMIs to develop a pan-European fibre-based network to support the interconnection of these optical clocks, building on GÉANT’s expertise in providing cross-border connectivity for NRENs. The aim of this work is to support the redefinition of the SI second, enable fundamental physics research, and ultimately improve the resilience of critical infrastructures across Europe, by providing a higher-accuracy alternative to satellite-based time and frequency distribution.

Now, a pathfinder project to connect PSNC in Poznan, the hub of the Polish fibre network for the National Laboratory for Photonics and Quantum Technologies (NLPQT), to the PTB, the German NMI in Braunschweig has been successfully completed, demonstrating the ability of GÉANT and the NRENs to support this work. The link, which utilises dedicated fibre specifically procured for this project and equipment developed by PSNC, was completed on 25 September 2024.

This project is both a demonstration of the power of national and international research and education networks and also the ability of GÉANT, NRENs and the NMIs to work together to help advance science.

We would like to thank the invaluable work from PSNC, Pioneer, PTB and the team from GÉANT in the creation of this pathfinder connection and look forward to the continuing this success story in the next few years. Hopefully the next time you’re asked if you “have a second” you’ll be able to confidently answer “Yes”.

If you would like to find out more about the Time and Frequency work in the GÉANT community, a dedicated Special Interest Group (SIG) has been created and information can be found at community.geant.org/sig-tfn

The first meeting of this group took place in Amsterdam on 16-17 October 2024, read more about it here: https://connect.geant.org/2024/10/30/sig-tfn-meet-for-the-first-time

Details of this meeting and future meetings can also be found on the SIG-TFN wiki.