NPL’s optical frequency metrology programme: our contributions towards a redefinition of the SI second

AUTHORS
Helen Margolis

Head of Science (Time and Frequency), National Physical Laboratory (NPL), United Kingdom

Ben Allen

National Physical Laboratory (NPL)

Rebecca Allen

National Physical Laboratory (NPL)

Filip Butuc-Mayer

National Physical Laboratory (NPL)

E Anne Curtis

National Physical Laboratory (NPL)

Thomas Easton

National Physical Laboratory (NPL)

Maxime Favier

National Physical Laboratory (NPL)

Chen-Hao Feng

National Physical Laboratory (NPL)

Zitong Feng

National Physical Laboratory (NPL)

Jacques-Olivier Gaudron

National Physical Laboratory (NPL)

Patrick Gill

National Physical Laboratory (NPL)

Rachel Godun

National Physical Laboratory (NPL)

Ian Hill

National Physical Laboratory (NPL)

Wei Huang

National Physical Laboratory (NPL)

Dan Jackson-Thomas
Reinhard Karembera

National Physical Laboratory (NPL)

Namneet Kaur

National Physical Laboratory (NPL)

Giuseppe Marra

National Physical Laboratory (NPL)

Adam Parke

National Physical Laboratory (NPL)

Patrick Regan

National Physical Laboratory (NPL)

Marco Schioppo

National Physical Laboratory (NPL)

Roxanne Siadat

National Physical Laboratory (NPL)

Max Tamussino

National Physical Laboratory (NPL)

Alexandra Tofful

National Physical Laboratory (NPL)

Dang-Bao-An Tran

National Physical Laboratory (NPL)

Jacob Tunesi

National Physical Laboratory (NPL)

Xi Zhang

National Physical Laboratory (NPL)

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DETAILS
DOI 10.59161/KAUH7093
Theme 1. SI units and fundamental metrology
Conference 150th anniversary of the Metre Convention — From Units to the Universe
Year 2025
Location Palais des congrès de Versailles, Versailles, France
Publisher Bureau International des Poids et Mesures (BIPM)
Language EN
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)
CITE THIS POSTER
Include:
Margolis, H., Allen, B., Allen, R., Butuc-Mayer, F., Curtis, E. A., Easton, T., Favier, M., Feng, C., Feng, Z., Gaudron, J., Gill, P., Godun, R., Hill, I., Huang, W., Jackson-Thomas, D., Karembera, R., Kaur, N., Marra, G., Parke, A., Regan, P., Schioppo, M., Siadat, R., Tamussino, M., Tofful, A., Tran, D., Tunesi, J., & Zhang, X.. (2025). NPL’s optical frequency metrology programme: our contributions towards a redefinition of the SI second. 150th anniversary of the Metre Convention — From Units to the Universe. Palais des congrès de Versailles, Versailles, France. Bureau International des Poids et Mesures (BIPM). https://doi.org/10.59161/KAUH7093
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