We use frequency-comb-based optical two-way time-frequency transfer (O-TWTFT) to measure the optical frequency ratio of state-of-the-art ytterbium and strontium optical atomic clocks separated by a 1.5-km open-air link. Our free-space measurement is compared to a simultaneous measurement acquired via a noise-cancelled fiber link. Despite nonstationary, ps-level time-of-flight variations in the free-space link, ratio measurements obtained from the two links, averaged over 30.5 hours across six days, agree to $6\times {10}^{-19}$, showing that O-TWTFT can support free-space atomic clock comparisons below the ${10}^{-18}$ level.

中文翻译：

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通过湍流空气进行光学原子钟比较

我们使用基于频梳的光学双向时频传输（O-TWTFT）来测量由1.5公里的露天链路分隔的最先进的tter和锶光学原子钟的光学频率比。我们的自由空间测量值与通过消除噪声的光纤链路获得的同时测量值进行了比较。尽管在自由空间链路中存在非平稳的ps级飞行时间变化，但从这两个链路获得的比率测量值（在六天内平均30.5小时）是同意的$6\times {10}^{-19}$，表明O-TWTFT可以支持以下的自由空间原子钟比较 ${10}^{-\mathrm{18岁}}$ 水平。