We report direct measurements of the frequency ratio of the 642 THz electric octupole transition in ¹⁷¹Yb⁺ and the 429 THz transition in ⁸⁷Sr. A series of 107 measurements has been performed at the Physikalisch-Technische Bundesanstalt between December 2012 and October 2019. Long-term variations of the ratio are larger than expected from the individual measurement uncertainties of few 10⁻¹⁷. The cause of these variations remains unknown. Even taking these into account, we find a fractional uncertainty of the frequency ratio of 2.5 10⁻¹⁷, which improves upon previous knowledge by one order of magnitude. The average frequency ratio is . This represents one of the most accurate measurements between two different atomic species to date.

我们报告了¹⁷¹ Yb ^{+ 中}642 THz电八极跃迁和⁸⁷ Sr 中429 THz跃迁的频率比的直接测量。在2012 年 12 月至 2019 年 10 月期间，在德国联邦物理技术研究所进行了一系列 107 次测量。该比率的 - 项变化大于预期的几个 10 ^-17的个别测量不确定性。这些变化的原因仍然未知。即使考虑到这些，我们发现频率比的分数不确定性为 2.5 10 ^-17，这比先前的知识提高了一个数量级。平均频率比为. 这是迄今为止两种不同原子种类之间最准确的测量之一。