A compact, transportable optical clock with 1 x 10-17 uncertainty and its absolute frequency measurement

We report a robust, compact, and transportable optical clock (TOC-729-2) based on a trapped single Ca-40(+) ion with a systematic uncertainty of 1.1 x 10(-17), which is limited by the black-body radiation shift uncertainty at room temperature. By comparing it with the previous transportable optical clock (TOC-729-1) similar but completely independent, the instability was measured to be better than 1.2 x 10(-14)/root tau. Benefiting from the modular and integrated design, this TOC was constructed in a volume of & SIM;0.33 m(3) excluding the controlling electronics in 19-in. racks. After being moved & SIM;1200 km away by express delivery, the single-ion signal was restored within 24 h. With the TOC uptime of 92% in 35-day period, the absolute frequency of the 729 nm transition of Ca-40(+) was measured using a satellite link to International Atomic Time (TAI) to provide traceability to the SI second, and the result is 411 042 129 776 400.15(22) Hz, corresponding to a relative uncertainty of 5.3 x 10(-16). (C) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).