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 $2times10^{-19}$, 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.

Optical frequency transfer via 146 km fiber link with 10^{-19} relative accuracy

We demonstrate the long-distance transmission of an ultra-stable optical frequency derived directly from a state-of-the-art optical frequency standard. Using an active stabilization system we deliver the frequency via a 146 km long underground fiber link with a fractional instability of 3*10^{-15} at 1 s, which is close to the theoretical limit for our transfer experiment. The relative uncertainty for the transfer is below 1*10^{-19} after 30 000 seconds. Tests with a very short fiber show that noise in our stabilization system contributes fluctuations which are two orders of magnitude lower, namely 3*10^{-17} at 1 s, reaching 10^{-20} after 4000 s.

The stability of an optical clock laser transferred to the interrogation oscillator for a Cs fountain

We stabilise a microwave oscillator at 9.6 GHz to an optical clock laser at 344 THz by using a fibre-based femtosecond laser frequency comb as a transfer oscillator. With a second frequency comb we measure independently the instability of the microwave source with respect to another optical clock laser frequency at 456 THz. The total fractional frequency instability of this optic-to-microwave and microwave-to-optic conversion resulted in an Allan deviation sigma_y, of sigma_y=1.2E-14 at 1 s averaging time (band width 50 kHz). The residual phase noise density is -97 dBc/Hz at 10 Hz offset from the 9.6 GHz carrier. Replacing the existing quartz-based interrogation oscillator of the PTB caesium fountain CSF1 with this optically stabilised microwave source will reduce the instability contribution due to the Dick effect from the 1E-13-level at 1s averaging time to an insignificant level at the current status of CSF1. Therefore this new microwave source can be an alternative to cryogenic sapphire-loaded cavity oscillators in order to overcome the limitations of state-of-the-art quartz oscillators.