A 920-kilometer optical fiber link for frequency metrology at the 19th decimal place

Optical clocks show unprecedented accuracy, surpassing that of previously available clock systems by more than one order of magnitude. Precise intercomparisons will enable a variety of experiments, including tests of fundamental quantum physics and cosmology and applications in geodesy and navigation. Well-established, satellite-based techniques for microwave dissemination are not adequate to compare optical clocks. Here, we present phase-stabilized distribution of an optical frequency over 920 kilometers of telecommunication fiber. We used two antiparallel fiber links to determine their fractional frequency instability (modified Allan deviation) to 5 × 10⁻¹⁵ in a 1-second integration time, reaching 10⁻¹⁸ in less than 1000 seconds. For long integration times τ, the deviation from the expected frequency value has been constrained to within 4 × 10⁻¹⁹. The link may serve as part of a Europe-wide optical frequency dissemination network.