Wojbor Bogacki, Artur Binczewski, Krzysztof Turza, Poznan Supercomputing and Networking Center; Vincenzo Capone, Tryfon Chiotis, Guy Roberts, Domenico Vicinanza, GÉANT VERENIGING; Harald Schnatz, Tara Cubel Liebisch, Physikalisch-Technische Bundesanstalt; Paul-Eric Pottie, Philip Tuckey, LNE-SYRTE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC; Davide Calonico, Cecilia Clivati, Istituto Nazionale di Ricerca Metrologica; Lada Altmannova, Vladimir Smotlacha, Radek Velc, Rudolf Vohnout, Josef Vojtech, CESNET; Ronald Holzwarth, Benjamin Sprenger, Menlo Systems; Ondrej Cíp, Lenka Pravdová, Simon Rerucha, Institute of Scientific Instruments of the CAS, v.v.i. (ISI); Javier Díaz Alonso, Eduardo Ros Vidal, University of Granada; Benoit Rat, Orolia; Jan Kodet, Ulrich Schreiber, Technical University of Munich; Jürgen Kusche, Dieter Meschede, Stefan Schröder, Simon Stellmer, Rheinische Friedrich-Wilhelms-Universität Bonn; Pawel Nogas, Robert Urbaniak, Piktime Systems sp. z o.o.; Przemyslaw Krehlik, Lukasz Sliwczynski, AGH University of Science and Technology; Anne Amy-Klein, Christian Chardonnet, LPL, Université Sorbonne Paris Nord, CNRS; Nicolas Quintin, Réseau National de Télécommunications pour la Technologie, l'Enseignement et la Recherche; Alwyn Seeds, University College London; Bruno Desruelle, Jean Lautier-Gaud, Vincent Ménoret, Martin Rabault, iXblue

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Long-distance time and frequency transfer methods based on optical fibre links have evolved rapidly in recent years, demonstrating excellent performance for frequency transfer and considerable promise for accurate time transfer. Optical fibre links are attractive both for very high-performance applications such as optical clock comparisons, basic research or relativistic geodesy, but also for many industrial and societal applications, and they complement and offer an alternative to radio- and satellite-based methods. Currently, national time and frequency dissemination networks are being developed in many countries. International connections are also being established. However, there is no coherent Pan-European time and frequency dissemination network, which would connect national networks and provide different services based on time and frequency for a wide range of users. The aim of the CLONETS-DS project is to fill this gap. The CLONETS-DS is a research and innovation action funded by the European Union and intended to facilitate the vision of a sustainable, pan-European optical fibre network for precise time and frequency reference dissemination, by bringing together expertise from national metrology institutes (NMI), academic groups and research infrastructures (RI), research and education networks at the national and European level (NREN, GÉANT) and innovative high-tech small and medium enterprises (SME). The proposed project aims to establish a pan-European time and frequency reference system as a European Research Infrastructure, which serves the European science community. It is based on transmitting ultra precise time and frequency information via optical fiber. Based on this information, it will be possible to deliver a range of services to end users. The CLONETS-DS project has identified the five main sciense cases: Science Case 1: Fundamental Science • Improvement of optical clocks • Precision spectroscopy to search for beyond standard model (BSM) physics • Re-definition of the SI unit second Science Case 2: Quantum Technologies • Improvement of real-world QKD • Development of new protocols • Entanglement distribution beyond QKD Science Case 3: Earth Observation / Geodesy • Height system unification • Satellite gravity mission validation • Geodetic network consistency Science Case 4: Astronomy • Radio interferometry and VLBI in astronomy • Laser ranging • Pulsar timing Science Case 5: Telecommunication and Networks / Position, Navigation, Synchronization, and Timing • Optical timescales • Position, navigation & timing, PNT • Resilience for GNSS • Supervision of telecommunication networks and synchronization (5G or 6G) The proposed project builds on several projects of the European metrology research programmes such as NEAT-FT, OFTEN and TiFOON, the former GN3+ project ICOF, the novel work package 6, Optical Time and Frequency Network (OTFN), of the GN4-3 project of GÉANT and its direct precursor project CLONETS. We now go far beyond previous efforts by designing a sustainable, pan-European, ultra-precise time-and-frequency reference-system available to the European research community. This newly designed European Research Infrastructure will consider user needs, architecture requirements, engineering models and roadmaps, and will develop a sustainability model for the future service, thus strengthening the European research area. The CLONETS-DS project envisions a ring-based topology of a T&F dissemination network. The first ring will connect Germany, the UK, France, Austria, the Czech Republic and Poland. The Second Ring will share routes with the first ring by France, Germany and Austria, additionally connecting Croatia, Slovenia, Italy and Switzerland. The Third Ring will share routes with the first ring by Poland and Germany, additionally connecting Denmark, Sweden, Finland and Lithuania. Each ring will have links to at least two optical clocks operated by National Metrology Institutes (NMIs), will allow future local extensions, and will allow for linear extensions for other countries. The network will make use of already existing fiber-based national T&F infrastructure, where available, but will also need to acquire new connections. The network will be equipped with specialized equipment for the transmission of time and frequency signals. In a parallel effort we are planning to contribute to the implementation of the ESFRI (European Strategy Forum on Research Infrastructures) Roadmap. This research infrastructure will enable first class research previously not even conceivable, foster the collaboration between time and frequency stakeholders across Europe and will thereby put Europe’s research community into a leading position. In this presentation we will discuss our current understanding of the needs of European science and society for accurate time and frequency reference delivery over fibre, the currently existing infrastructure as well as different scenarios for the implementation of such a service using existing fibre infrastructure. ACKNOWLEDGEMENTS This project is running since October 2020 and receives funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 951886 (CLONETS-DS).