Optical two-way time and frequency transfer over free space

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 second(1-7). However, present microwave-based time-frequency transfer(8-10) is inadequate for state-of-the-art optical clocks and oscillators(1,11-16) that have femtosecond-level timing jitter and accuracies below 1 x 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 x 10(-18) at 1,000 s and systematic offsets below 4 x 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.