Evaluation of a $^{88}$Sr$^+$ optical clock with a direct measurement of the blackbody radiation shift and determination of the clock frequency

TL;DR

This paper evaluates a $^{88}$Sr$^+$ optical clock, directly measures the blackbody radiation shift, and determines its frequency with high precision, improving accuracy and consistency over previous measurements.

Contribution

It provides a direct blackbody radiation shift measurement during operation and achieves a more precise absolute frequency determination of the $^{88}$Sr$^+$ clock transition.

Findings

Fractional uncertainty of $2.3 imes 10^{-17}$ in frequency ratio

Absolute frequency of $^{88}$Sr$^+$ clock transition: 444779044095485.271(59) Hz

Increased ion number improves short-term stability without systematic degradation

Abstract

We report on an evaluation of an optical clock that uses the $\phantom{}^2S_{1/2} \rightarrow \phantom{}^2D_{5/2}$ transition of a single $^{88}$Sr$^+$ ion as the reference. In contrast to previous work, we estimate the effective temperature of the blackbody radiation that shifts the reference transition directly during operation from the corresponding frequency shift and the well-characterized sensitivity to thermal radiation. We measure the clock output frequency against an independent $^{171}$Yb$^+$ ion clock, based on the $\phantom{}^2S_{1/2} (F=0) \rightarrow \phantom{}^2F_{7/2} (F=3)$ electric octupole (E3) transition, and determine the frequency ratio with a total fractional uncertainty of $2.3\times 10^{-17}$. Relying on a previous measurement of the $^{171}$Yb$^+$ (E3) clock frequency, we find the absolute frequency of the $^{88}$Sr$^+$ clock transition to be $444779044095485.271(59)\,\text{Hz}$. Our result reduces the uncertainty by a factor of $3$ compared to the previously most accurate measurement and may help to resolve so far inconsistent determinations of this value. We also show that for three simultaneously interrogated $^{88}$Sr$^+$ ions, the increased number causes the expected improvement of the short-term frequency instability of the optical clock without degrading its systematic uncertainty.