Chronometric levelling [1-3] is a technique of deriving gravitational potential differences from clock frequency comparisons allowing the determination of height differences. Interferometric optical fibre links (IFL) [4] enable chronometric levelling between distant labs housing optical clocks [5]. An IFL is a phase stabilized telecom fibre link connection establishing a fixed optical phase relationship between two locations up to ~1000 km apart [6]. To perform a chronometric levelling campaign between the stationary optical clocks of the Physikalisch-Technische Bundesanstalt (PTB) and PTB’s transportable Strontium clock [7] placed at the Max Planck Institute of Quantum Optics (MPQ) we have established a new 940 km long IFL between the two locations. At both link ends the frequencies of the clock lasers are compared to the 1,5 μm link transfer laser via fs-combs. To enable chronometric levelling experiments at the level of 10 cm height resolution, we aim a fractional frequency uncertainty of the frequency transfer of < 1×10−18 for averaging times of ~40 ks. To monitor the frequency transfer over the complete 940 km long fibre link the signal is sent back from the receiving end to the sender on a second fibre, which allows an out-of-loop characterization using data of the 1880 km loop. We study two different link setups, cf. Fig. 1a: i) a single-span loop configuration with the option to tap off the signal at MPQ with a passive frequency extraction unit [8]. This setup requires monitoring at the sending end only and allows us to investigate an ultralong IFL, even longer than the previous record [9]. And ii) an antiparallel configuration of two concatenated, individually stabilized interferometric fibre links. This setup offers lower instabilities due to a decreased delay-limit [10] but requires setting up and maintaining a stabilization system at both link ends. To compensate the attenuation of the fibres we used fibre Brillouin amplifier modules (FBAM) developed inhouse providing a typical gain of > 40 dB.

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