While the SI second is currently defined in terms of a microwave transition frequency in
caesium, atomic clocks based on an optical transition are currently outperforming caesium
clocks by up to two orders of magnitude. In order to fully exploit the potential accuracy
achievable by optical clocks, the SI second needs to be redefined in terms of an optical
frequency standard. The ¹⁷¹Yb⁺ ion is an excellent candidate thanks to the extremely
narrow linewidth of its electric octupole (E3) transition and its particular insensitivity to
external perturbations.
This thesis is focused on the ytterbium ion optical clock at the National Physical
Laboratory (NPL), consisting of a single ¹⁷¹Yb⁺ ion trapped in a radio frequency (RF)
Paul trap and probed by ultrastable 467-nm light to excite the E3 transition. Improved
measurement methods were developed for the evaluation of several systematic frequency
shifts. In particular, the electric quadrupole shift, which used to be the leading source of
uncertainty, can now be directly measured with an accuracy in the low parts in 10¹⁸.
A great focus was put on the automation of several aspects of the experiment.
Because all optical clocks generally require a lot of maintenance and attention during
their operation, many experimental routines were automated in order to minimise the
requirement for human intervention. Furthermore, the analysis of almost all systematic shifts was automated, requiring minimal manual input so that shifts could be evaluated on the fly. Finally, a generalised framework was developed for the automatic evaluation of the absolute frequency of the optical clock via the International Atomic Time (TAI).
In order to increase the confidence in the level of performance of the ytterbium ion optical clock, international clock comparison campaigns are regularly carried out. Between
2019 and 2022, several results were produced: two absolute frequency measurements via
TAI with an uncertainty at the 1 × 10⁻¹⁵ level; two local frequency ratio measurements between ¹⁷¹Yb⁺ (E3) and ⁸⁷Sr with an uncertainty in the low parts in 10¹⁷; three uncertainty budgets at the parts in 10¹⁸ level; and one measurement of the ratio of the octupole and quadrupole optical clock transitions in ¹⁷¹Yb⁺ with an uncertainty of 1.5 × 10⁻¹⁶. All of these results are shown to be consistent with each other and in good agreement with the literature. Furthermore, a prototype optically-steered time scale was successfully demonstrated for the first time at NPL with the contribution of both the ¹⁷¹Yb⁺ and ⁸⁷Sr optical clocks.