Direct comparison of a Ca+ single ion clock against a Sr optical lattice clock

TL;DR

This paper reports a precise optical frequency comparison between a calcium ion clock and a strontium optical lattice clock, achieving high accuracy and demonstrating the advantages of optical over microwave frequency measurements.

Contribution

It provides the first direct optical frequency ratio measurement between a Ca+ ion clock and a Sr lattice clock with unprecedented precision.

Findings

Frequency ratio .9576312023580499(23)

Achieved statistical uncertainty of 1-15 in 1000 seconds

Absolute frequency of 40Ca+ measured with 14 times smaller systematic uncertainty

Abstract

Optical frequency comparison of the 40Ca+ clock transition \nu_{Ca} (2S1/2-2D5/2, 729nm) against the 87Sr optical lattice clock transition \nu_{Sr}(1S0-3P0, 698nm) has resulted in a frequency ratio \nu_{Ca} / \nu_{Sr} = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio \nu_{Ca} / \nu_{Sr} to reach 1x10-15 in only 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of \nu_{Ca} using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We report the absolute frequency of ^{40}Ca+ with a systematic uncertainty 14 times smaller than our previous measurement [1].