NPL clock watchers hit the big time
6 Aug 2023
Britain’s National Physical Laboratory is celebrating a major achievement by its scientists with the inclusion of a UK optical clock in the determination of International Atomic Time (TAI).
NPL’s ultra-precise clock joins a select group of optical clocks from around the world that contribute to global timekeeping by helping to define TAI. And it will contribute to improving the accuracy and precision of the second as a unit of time.
International Atomic Time is calculated monthly by the International Bureau of Weights and Measures (BIPM) which relies upon the weighted average of more than 400 atomic clocks located at some 80 metrology laboratories around the world, including NPL.
Calculating TAI is a major contributor for determining Coordinated Universal Time (UTC), the shared international reference time scale that countries worldwide use to synchronise clocks. Global navigation, financial and telecoms systems and applications all depend for proper functioning upon the effectiveness of UTC as a globally-agreed and consistent time-base.
Optical atomic clocks are expected to play a key role in defining the second as a unit of time, on account of their extreme accuracy.
A target date of 2030 has been set for redefining the SI second and among the mandatory criteria that must first be met is the regular contribution to International Atomic Time by optical secondary frequency standards.
NPL's team submitted historic optical frequency data that compared NPL-Sr1 with the UK’s own UTC(k) time scale over several periods, with one lasting a month. Following assessment by the CIPM’s Consultative for Time and Frequency Working Group, NPL-Sr1 became the first UK optical clock to be accepted to contribute to International Atomic Time.
On-time data submissions then followed, starting in late April, when NPL-Sr1 ranked as the third highest contributor (9.63%) of the 14 primary and secondary frequency standards that month. A second submission followed at the end of May. These and subsquent contributions will enable NPL-Sr1 to have a much greater significance in steering TAI, said the laboratory.
How significant is this achievement?
Dr Ian Hill, Principal Scientist and lead for the optical lattice clock project in NPL’s Optical Frequency Metrology group: "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: “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.”
Professor Helen Margolis, Head of Science (Time and Frequency) and Senior NPL Fellowd: “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.”
Stefan Weyers, Chair of the CCTF Working Group on Primary and Secondary Frequency Standards: “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.”
Director, BIPM Time Department Patrizia Tavella: “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."
