Networks of optical clocks find applications in precise navigation1,2, in efforts to redefine the fundamental unit of the ‘second’3,4,5,6 and in gravitational tests7. As the frequency instability for state-of-the-art optical clocks has reached the 10−19 level8,9, the vision of a global-scale optical network that achieves comparable performances requires the dissemination of time and frequency over a long-distance free-space link with a similar instability of 10−19. However, previous attempts at free-space dissemination of time and frequency at high precision did not extend beyond dozens of kilometres10,11. Here we report time–frequency dissemination with an offset of 6.3 × 10−20 ± 3.4 × 10−19 and an instability of less than 4 × 10−19 at 10,000 s through a free-space link of 113 km. Key technologies essential to this achievement include the deployment of high-power frequency combs, high-stability and high-efficiency optical transceiver systems and efficient linear optical sampling. We observe that the stability we have reached is retained for channel losses up to 89 dB. The technique we report can not only be directly used in ground-based applications, but could also lay the groundwork for future satellite time–frequency dissemination.
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This work is supported by the National Key Research and Development Programme of China (grant nos. 2017YFA0303900, 2020YFA0309800 and 2020YFC2200103); the Strategic Priority Research Programme of Chinese Academy of Sciences (grant nos. XDB35030000 and XDA15020400); the National Natural Science Foundation of China (grant nos. T2125010 and 61825505); the Anhui Initiative in Quantum Information Technologies (grant no. AHY010100); the Key R&D Plan of Shandong Province (grant nos. 2020CXGC010105 and 2021ZDPT01); the Key Research and Development Programme of Guangdong Province (grant no. 2018B030325001); the Shanghai Municipal Science and Technology Major Project (grant no. 2019SHZDZX01); and the Innovation Programme for Quantum Science and Technology (grant no. 2021ZD0300100).
The authors declare no competing interests.
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Extended data figures and tables
USL, ultra-stable laser; EDFA, erbium-doped fiber amplifiers; Cir, circulator; BPD, balanced photodiode; Tele, Telescope.
USL, ultra-stable laser; SMC, single-mode coupler; PD, photon diode; RF, radio frequency source; FM, Faraday mirror; AOM, acoustic optical modulator; EPC, electric polarization controller; Bi-EDFA, bidirectional erbium doped fiber amplification.Underlying map from Google, DigitalGlobe.
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Shen, Q., Guan, JY., Ren, JG. et al. Free-space dissemination of time and frequency with 10−19 instability over 113 km. Nature 610, 661–666 (2022). https://doi.org/10.1038/s41586-022-05228-5