Here we share the relevant data of the manuscript “Comparing ultrastable lasers at 7×10-17 fractional frequency instability through a 2,220 km optical fibre network”. Raw data was acquired using multiple synchronised, dead-time free frequency counters in Lambda-mode [1]. The integration time for each data point was 1 s. The data provided here have been processed to reflect the fractional frequency difference between the ultrastable lasers at NPL and PTB, scaled to 1542 nm. Specifically, \(y=(\nu_{\text{NPL(ULE)}}\frac{777327}{1126090}-\frac{767233}{767235}\nu_{\text{PTB(Si)}})/194.4 \ \text{THz}\) where \(y\) is the value recorded in the data files, \(\nu_{\text{NPL(ULE)}}\) and \(\nu_{\text{PTB(Si)}}\) are the optical frequencies of the ultrastable lasers at NPL (referenced to a ULE cavity) and PTB (referenced to Si cavity), respectively. The numerators and the denominators of the scaling factors correspond to mode numbers of the optical frequency comb at NPL and PTB, respectively. The expression for \(y\) corresponds to the fractional transfer beat [2] between the NPL and PTB ultrastable lasers. The file “833000_s_874000_s_data_for_fig_2.txt” contains the timeseries data used to compute the modified Allan deviation reported in Fig. 2a. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. A linear drift of 40 mHz s-1 has been removed in these data. The file “432000_s_912077_s_data_for_fig_3.txt” contains the timeseries data used in Fig. 3. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. These data have additionally been high pass filtered with a cut off frequency of 1 mHz to decouple the short-term instability of the optical fibre link from the drift of the ultrastable lasers (with a characteristic time >1000 s), as described in the manuscript. The files “222000_s_232000_s_data_for_supp_fig_1.txt”, “270000_s_288000_s_data_for_supp_fig_1.txt”, “754000_s_765000_s_data_for_supp_fig_1.txt”, “832000_s_890000_s_data_for_supp_fig_1.txt”, contain the timeseries data used to compute the modified Allan deviation reported in Supplementary Fig. 1. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. A linear drift of 40 mHz s-1 has been removed in these data. The temporal starting point is displayed in seconds in the title of the files relative to 00:00 UTC of 2019/07/06. References [1] Dawkins, S. T., McFerran, J. J. & Luiten, A. N. Considerations on the Measurement of the Stability of Oscillators with Frequency Counters. IEEE Transactions on ultrasonics, ferroelectrics, and frequency control 54, 918-925 (2007). [2] Telle, H.R., Lipphardt, B. & Stenger, J. Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements. Appl. Phys. B 74, 1-6 (2002).

NPL: This work was financially supported by the UK Department for Business, Energy and Industrial Strategy as part of the National Measurement System Programme; the European Metrology Programme for Innovation and Research (EMPIR) projects 15SIB03 OC18, 15SIB05 OFTEN, 18SIB05 ROCIT, 18SIB06 TiFOON. These projects have received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. A.V. acknowledges funding from the Engineering and Physical Sciences Research Council (EPSRC UK) through the Controlled Quantum Dynamics Centre for Doctoral Training (EP/L016524/1) for the core duration of this work. LPL, LNE-SYRTE, RENATER: This work has received support under the program "Investissements d'Avenir" launched by the French Government and implemented by ANR with the references ANR-10-LABX-48-01 (Labex First-TF), ANR-11-EQPX-0039 (Equipex REFIMEVE+) and ANR-10-IDEX-0001-002 PSL (PSL). This work was also financially supported by Conseil Régional Ile de-France (DIM IFRAF-NanoK and DIM SIRTEQ) and the European Metrology Programme for Innovation and Research (EMPIR) in project 15SIB05 OFTEN, 18SIB05 ROCIT. These projects have received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. We acknowledge unfailing and continuing support of the network and engineering team of RENATER. PTB: This work was financially supported by the European Metrology Programme for Innovation and Research (EMPIR) projects 15SIB03 OC18, 15SIB05 OFTEN, 18SIB05 ROCIT, 18SIB06 TiFOON. These projects have received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. PTB acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy EXC-2123 Quantum Frontiers (Project-ID 390837967) and CRC 1227 DQ-mat (Project-ID 274200144) and CRC 1464 Terra-Q (Project-ID 434617780).