Per Olof Hedekvist, RISE Research Institute of Technology; Josef Vojtech, CESNET; Jochen Kronjäger, Physikalisch-Technische Bundesanstalt; Peter Whibberley, National Physical Laboratory; Davide Calonico, Istituto Nazionale di Ricerca Metrologica; Jacques Morel, Eidgenössisches Institut für Metrologie (METAS); Erik Dierikx, VSL – Nederlands Metrologisch Instituut; Krzysztof Turza, Poznan Supercomputing and Networking Center; Paul-Eric Pottie, LNE-SYRTE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC; Ondejíp, Institute of Scientific Instruments of the CAS, v.v.i. (ISI); Ulrich Schreiber, Technical University of Munich; Przemysaw Krehlik, AGH University of Science and Technology; Anne Amy-Klein, LPL, Université Sorbonne Paris Nord, CNRS; Jean Lautier-Gaud, iXblue; Vittori Curri, Politechnico di Torino

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After two successful EU Metrology projects aimed at developing and enhance techniques for time and frequency transfer over optical fiber, this third project was planned to find robust implementation for 24/7 connectivity, as well as applications of robust traceable time and frequency dissemination to non-NMI users. The four scientific objectives were to refine the optical time transfer, specifically in combination with optical frequency transfer, to increase reliability of frequency transfer, to identify potential for sharing telecom infrastructure, and to identify and support applications beyond metrology. The performance requirements have covered metrology level suitable for comparing optical clocks, high performance to academic institutions, and dissemination of time to industry and society. Paramount for all performance levels is the traceability to UTC through a national UTC(k) realization, or an optical frequency realized according to BIPM mises en pratique. The consortium of 15 partners had representation from National Metrology Institutes, National Research and Education Networks, Academic Institutions, and Industrial partners, and with influence of stakeholders representing the users of traceable time. In addition to producing scientific results, the project have arranged workshops for presentation of results and getting feedback from the external participants. The project did also manage to handle the restrictions of travel and researcher exchange induced by the pandemic and was able to move many of the tasks to be solved through online collaboration. Key results from the project includes an enhanced reliability of the actively phase stabilized fiber links necessary for optical clock comparison and paramount for a future all-optical timescale. The intervention-free coherent fiber link is almost solved and the technological outcomes from this project decreases the gap to a reliable service. Any deterioration from coexistence of time and frequency transfer signal and coherent communication channels exceeding 100 Gbit/s has also been investigated, both theoretical through simulations and empirical through the unavoidable need to utilize the cost efficiency of sharing the fiber with active communication channels. Time dissemination to users outside the NMI and metrology applications was made by fiber links to sites radio telescopes, actively participating in VLBI campaigns and geodetic analyses. While time synchronization is of high importance there, the fiber must perform better than the presently used GNSS techniques. While fiber optic solutions may achieve this, it requires customizations of the link not always possible unless a dedicated fiber is rented. The practical experiments include buried fiber as well as aerial along the power grid, and bidirectional as well as unidirectional duplex. In conclusion, the project has pushed the limits on optimizing the performance of fiber optic communication networks with respect to transfer ultra-stable optical frequencies and comparing optical clocks, but even more important are the findings of practical and physical limitations. It has also generated 12 peer-reviewed papers, 32 conference presentations, and a substantial amount of dissemination activities and collaborations. Being an EU funded project, all published results are in Open Access, and data is published in Open Science platform Zenodo under CC-BY or CC.BY-SA licenses. The project TiFOON 18SIB06 Project was supported through the EMPIR Programme co-financed by the Participating States and through the European Union’s Horizon 2020 Research and Innovation Program.