Speaker
Description
Optical frequency standards now surpass caesium frequency standards in accuracy and stability, forming the motivation for the future redefinition of the SI second [1]. Key requirements for the redefinition are the validation of uncertainty budgets at the level of ≲ 2 × 10⁻¹⁸ and agreement between independent optical frequency ratios at uncertainty ≲ 5 × 10⁻¹⁸. The ¹⁷¹Yb⁺ optical clock at the National Physical Laboratory (NPL), based on the electric octupole (E3) transition (NPL-E3Yb+3), has demonstrated the first of these requirements through systematic evaluations reported in [2], achieving a total fractional systematic uncertainty of 2.2 × 10⁻¹⁸. Recent improvements in NPL-E3Yb+3, including reduced systematic uncertainties, longer coherence times of the atom–laser interaction, and increased operational uptime have reduced the overall measurement uncertainties still further. Local comparisons with the strontium optical frequency standard at NPL (NPL-Sr1) and remote comparisons with European optical frequency standards have now demonstrated measurement uncertainties at the required level of 5 × 10⁻¹⁸. These results represent significant progress towards fulfilling the criteria for redefining the second.
To extend capability, a second ¹⁷¹Yb⁺ optical clock is under construction at NPL, and we report progress on its development. The new system is based on an updated end-cap ion trap [3] with a redesigned RF feed-through that reduces capacitance to the vacuum chamber, suppressing RF dielectric heating and the associated temperature rise. This will improve control of black-body radiation shifts and support trap-related fractional uncertainties below 10⁻¹⁸. The setup also incorporates a multi-layer magnetic shield with a shielding factor of ~1000 to suppress the ambient field fluctuations. This enhanced stability enables the operation of the E3 clock on Zeeman-sensitive transitions, cancelling the electric quadrupole shift and reducing the associated uncertainty in the second-order Zeeman shift. In addition, automated beam alignment will be implemented using piezo-actuated translation stages to increase operational robustness and unmanned operation. The second Yb+ system will enable a direct comparison with the first, providing a rigorous test of calculated systematic uncertainty budgets and supporting frequency ratio measurements at the low 10⁻¹⁸ level of uncertainty.
References
[1] N. Dimarcq et al., “Roadmap towards the redefinition of the second”, Metrologia, vol. 61, p. 012001, 2024.
[2] A. Tofful et al., “171Yb+ optical clock with 2.2 × 10-18 systematic uncertainty and absolute frequency measurements”, Metrologia, vol. 61, p. 045001, 2024.
[3] P. B. R. Nisbet-Jones et al., “A single-ion trap with minimized ion–environment interactions”, Appl. Phys. B vol. 122, 57, 2016.
| Academic level | Other |
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