Chirped Frequency Transfer with an accuracy of 10⁻¹⁸ and its Application to the Remote Synchronisation of Timescales

We demonstrate combined high-fidelity long-haul transfer of a linearly chirped, optical frequency and time transfer. In a proof-of-principle experiment we transfer an optical frequency with a linear chirp of around 238 kHz/s via a phase-stabilized underground fiber link of 150 km. We find a fractional frequency transfer instability (Allan deviation, 18000 s averaging time) and simultaneity of the chirped frequency between both ends on a level of around $2\times10^{-19}$, where the active phase stabilization suppresses cumulative, symmetrical effects. In a second step, we demonstrate the remote measurement of synchronisation taking advantage of chirped-frequency transfer. The uncertainty of time transfer here is around 500 ps.