A major milestone towards the redefinition of the second has been achieved by scientists at the National Physical Laboratory (NPL) with the first inclusion of a UK optical frequency standard (NPL-Sr1) in the determination of International Atomic Time (TAI).

Calculating TAI is a major resource for determining Coordinated Universal Time (UTC), the shared international reference time scale that countries worldwide use to synchronise clocks. Without UTC in place as the world’s agreed consistent time-base, global navigation, financial and telecommunications systems would not function properly.

The definition of the SI second is the foundation upon which all timing applications are built, and any improvement in its precision directly translates to knock-on improvements along the measurement chain. Currently, the best optical atomic clocks are approximately 100 times more precise than the best caesium atomic clocks – however, without redefining the second, this improved precision cannot as yet be harnessed. 

International Atomic Time is calculated monthly by the International Bureau of Weights and Measures (BIPM) by first taking the weighted average of more than 400 atomic clocks located at approximately 80 metrology laboratories around the world – including NPL in the UK. This average is then ‘steered’ by frequency standards from a pre-approved list. Most are caesium microwave standards, but recent years have seen optical standards contributing more and more. 

This regular contribution to International Atomic Time by optical secondary frequency standards is one of the mandatory criteria that must be met before a redefinition of the second can take place. The target date for redefining the SI second is set for 2030. 

The NPL team submitted their historic optical frequency data comparing NPL-Sr1 with the UK’s own UTC(k) time scale over several periods, the longest of which was one month. After a rigorous assessment by the CIPM’s Consultative for Time and Frequency Working Group, NPL-Sr1 was accepted as the first UK optical clock to contribute to International Atomic Time.

The next steps are to make on-time data submissions which enable NPL-Sr1 to have a much greater significance in steering TAI. The first of these was made at the end of April 2023 in which NPL-Sr1 was the third highest contributor (9.63%) of the 14 primary and secondary frequency standards that contributed that month. A second submission was made at the end of May 2023 benefitting from the momentum of the first. Expect to continue to see more contributions from NPL-Sr1 in the years leading up to redefinition.

Dr Ian Hill, Principal Scientist and lead for the optical lattice clock project in NPL’s Optical Frequency Metrology group, said: "This is a significant achievement for NPL and the wider time & frequency community with well over a decade of invested hard work coming to bear fruit. We now join a select group of optical clocks from around the world that have a say on TAI and support the transition to a possible new SI second."

Dr Jacob Tunesi, Senior Scientist at NPL, said: “An optical representation of the SI second requires that we demonstrate that optical clocks can be a benefit to the real world and there's no better way than by contributions to the international time scale which underpins modern life. It has been a pleasure to have played a part in bringing NPL’s Sr lattice optical clock onto this stage and I look forward to contributing more in the future.”

Prof. Helen Margolis, Head of Science (Time and Frequency) and Senior NPL Fellow said: “Reaching this milestone represents a significant achievement for the team and stems from a huge effort from many people over many years. I am delighted that we are now able to use our strontium optical lattice clock to contribute to global timekeeping and make progress towards a redefinition of the second.”

The chair of the CCTF Working Group on Primary and Secondary Frequency Standards, Stefan Weyers said, “Ensuring correct world time is accomplished using a relatively small ensemble of a good dozen caesium atomic clocks and optical atomic clocks. Since, in the context of a future redefinition of the second, optical atomic clocks will have a much greater importance in this process, the contributions of the new clock NPL-Sr1 are an important step into the future of national and international timekeeping.”

The Director of the BIPM Time Department, Patrizia Tavella said: “The BIPM welcomes all the activities leading to new optical frequency standards that can contribute to UTC and the recent achievements of NPL are greatly appreciated. The development of the NPL Sr1 frequency standard and its capacity to measure the accuracy of Coordinated Universal Time provide an added value to the entire UTC community. The NPL Sr1 measurement has been taken into account by the BIPM in the most recent UTC computations, together with all the other measurements of primary and secondary standards operated all over the world, including the cesium fountain NPL-CsF2. The availability and the great accuracy of the measurements are allowing a relative accuracy of UTC at the level of 2 parts in 1016 which means a possible error of half a nanosecond (half of a billionth of a second) over one month.

About NPL

NPL is the UK’s National Metrology Institute, providing the measurement capability that underpins the UK's prosperity and quality of life.

From new antibiotics to tackle resistance and more effective cancer treatments, to secure quantum communications and superfast 5G, technological advances must be built on a foundation of reliable measurement to succeed. Building on over a century’s worth of expertise, our science, engineering and technology provides this foundation. We save lives, protect the environment and enable citizens to feel safe and secure, as well as support international trade and commercial innovation. As a national laboratory, our advice is always impartial and independent, meaning consumers, investors, policymakers and entrepreneurs can always rely on the work we do.

Based in Teddington, south-west London, NPL employs over 600 scientists. NPL also has regional bases across the UK, including at the University of Surrey, the University of Strathclyde, the University of Cambridge and the University of Huddersfield's 3M Buckley Innovation Centre.