The transfer of high-quality time–frequency signals between remote locations underpins many applications, including precision navigation and timing, clock-based geodesy, long-baseline interferometry, coherent radar arrays, tests of general relativity and fundamental constants, and future redefinition of the second1,2,3,4,5,6,7. However, present microwave-based time–frequency transfer8,9,10 is inadequate for state-of-the-art optical clocks and oscillators1,11,12,13,14,15,16 that have femtosecond-level timing jitter and accuracies below 1 × 10−17. Commensurate optically based transfer methods are therefore needed. Here we demonstrate optical time–frequency transfer over free space via two-way exchange between coherent frequency combs, each phase-locked to the local optical oscillator. We achieve 1 fs timing deviation, residual instability below 1 × 10−18 at 1,000 s and systematic offsets below 4 × 10−19, despite frequent signal fading due to atmospheric turbulence or obstructions across the 2 km link. This free-space transfer can enable terrestrial links to support clock-based geodesy. Combined with satellite-based optical communications, it provides a path towards global-scale geodesy, high-accuracy time–frequency distribution and satellite-based relativity experiments.
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This work was funded by the Defense Advanced Research Projects Agency (DARPA) QuASAR program and by the National Institute of Standards and Technology (NIST). The authors acknowledge helpful discussions with S. Diddams, J.-D. Deschênes, S. Kaushik, S. Michael, R. Parenti, T. Parker, F. Quinlan and T. Rosenband, and assistance from E. Williams and A. Zolot.
The authors declare no competing financial interests.
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Giorgetta, F., Swann, W., Sinclair, L. et al. Optical two-way time and frequency transfer over free space. Nature Photon 7, 434–438 (2013) doi:10.1038/nphoton.2013.69
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