171 Yb + optical clock with 2.2 × 10 − 18 systematic uncertainty and absolute frequency measurements

A full evaluation of the uncertainty budget for the ytterbium ion optical clock at the National Physical Laboratory (NPL) was performed on the electric octupole (E3) 2 S 1 / 2 → 2 F 7 / 2 transition. The total systematic frequency shift was measured with a fractional standard systematic uncertainty of 2.2 × 10 − 18 . Furthermore, the absolute frequency of the E3 transition of the 171 Yb + ion was measured between 2019 and 2023 via a link to International Atomic Time (TAI) and against the local caesium fountain NPL-CsF2. The absolute frequencies were measured with fractional standard uncertainties between 3.7 × 10 − 16 and 1.1 × 10 − 15 , and all were in agreement with the 2021 BIPM recommended frequency.