NPL’s optical frequency metrology programme: our contributions towards a redefinition of the SI second
Year: 2025
Type: conference-paper
Abstract: Enormous progress has been made by time and frequency laboratories around the globe on a new generation of optical frequency standards based on narrow optical transitions in laser-cooled trapped atoms or ions. Some of these optical frequency standards, including those at NPL, have significantly surpassed the performance of the best caesium fountain primary frequency standards in both systematic frequency uncertainty and frequency instability. This situation has led to the international community developing a roadmap towards a future redefinition of the second [1], which gives a set of mandatory criteria to be met before redefinition, as well as some auxiliary conditions related to work that should be well advanced by that time. NPL has contributed to worldwide progress towards meeting the various mandatory criteria in the roadmap. Our optical frequency standard based on the electric octupole transition in an ion of ytterbium-171 recently reached an evaluated systematic uncertainty of 2.2E-18, very close to the target in the roadmap [2]. The uncertainty published for our strontium optical lattice clock in 2020 was 1E-17 [3], but this has since been improved to within a factor of two of the target through a series of technical improvements to reduce leading sources of systematic uncertainty. The absolute frequency of both clocks has been measured against caesium primary frequency standards. Our optical clocks have participated in various international comparisons, using satellite links, optical fibre links and transportable optical clocks. Notable recent examples include a large scale comparison of ten optical clocks in six countries, performed as part of the Robust Optical Clocks for International Timescales (ROCIT) project [4], and the first intercontinental comparison using transportable optical clocks as part of the International Clock Oscillator Networking (ICON) project [5]. We have also performed local optical frequency ratio measurements between the two clocks. Our strontium optical lattice clock was approved for steering of International Atomic Time (TAI) in March 2023 and has so far contributed data in seven months since then. A significant body of data has been gathered from our ytterbium ion optical clock and we are close to submitting this system to the BIPM for consideration for inclusion in TAI. Locally to NPL, we have also demonstrated a prototype real-time optically steered UTCx(k) time scale using our two optical clocks, and compared this against a similar prototype time scale generated simultaneously at LNE-SYRTE [6]. Looking further ahead, we are developing an ytterbium optical lattice clock targeted at routine contributions to TAI, and investigating the possibility of intercontinental comparisons via optical fibre by adapting techniques originally conceived for seismic detection [7]. [1] N. Dimarcq et al., Metrologia 61, 012001 (2024) [2] A. Tofful et al., Metrologia 61, 045001 (2024) [3] R. Hobson et al., Metrologia 57, 065026 (2020) [4] H. S. Margolis et al., Journal of Physics: Conference Series 2889, 012022 (2024) [5] International Clock Oscillator Networking (ICON) collaboration, arXiv:2410.22973 (2024) [6] M. Abgrall et al., Journal of Physics: Conference Series 2889, 012024 (2024) [7] G. Marra et al., Science 376, 874 (2022)
DOI: 10.59161/kauh7093
Authors: Helen Margolis, Ben Allen, Rebecca Allen, Filip Butuc-Mayer, E Anne Curtis, Thomas Easton, Maxime Favier, Chen-Hao Feng, Zitong Feng, Jacques-Olivier Gaudron, Patrick Gill, Rachel Godun, Ian Hill, Wei Huang, Dan Jackson-Thomas, Reinhard Karembera, Namneet Kaur, Giuseppe Marra, Adam Parke, Patrick Regan, Marco Schioppo, Roxanne Siadat, Max Tamussino, Alexandra Tofful, Dang-Bao-An Tran, Jacob Tunesi, Xi Zhang
Institution: National Physical Laboratory
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W7172328617