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.
At a glance
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