The consortium brings together the main European National Metrology institutes (NMIs) active in the field of optical atomic clocks with highly qualified research groups belonging to universities and other research institutions, active in the broader field of atomic physics. It combines atomic physics and metrological expertise in atom and ion manipulation, ultra-stable laser realisation and coherent and quantum techniques applied to cold matter. The NMI based partners will bring state-of-the-art operational optical clocks and frequency metrology capabilities, while the external partners will bring complementary expertise in quantum techniques, such as theoretical quantum metrology, experimental quantum optics applied to atom optics and photonics, and novel techniques that will enhance the clock capabilities across the whole consortium. National Metrology Institutes Istituto Nazionale di Ricerca Metrologica Torino, Italy INRIM brings expertise in neutral atom (Yb) optical clocks. A clock system based on Sr is being developed and will be operated (WP2, WP4), focusing on strongly coupled atom-cavity system to enhance optical clock stability beyond the QPN and generating active optical clock (WP3). INRIM has also a world leading expertise in frequency and time dissemination toward radio astronomical facilities along optical fibres. INRIM’s Time & Frequency and Quantum Technology laboratories NPL Management Limited Teddington, United Kingdom NPL brings expertise in neutral atom (Sr) and single ion clocks (Sr+, Yb+), and laser stabilisation. The clocks are already operating at internationally competitive levels. The Sr clock system will be utilised in WP1. NPL’s experimental capability in monolithic three-dimensional microfabricated ion traps, coherent spectroscopy, resolved-sideband cooling and agile frequency, phase and amplitude control of stable laser light will be used in WP2 and WP4 to the pursuit of entanglement in a trapped ion system. NPL’s ion microtraps NPL’s optical clocks Observatoire de Paris Paris, France OBSPARIS brings expertise in neutral atom optical clocks and laser stabilisation. The clocks are already operating at internationally competitive levels. Two clock systems based on Sr will be operated (WP1, WP4), relying on current developments on ultra-stable lasers based on cavities and potentially cryogenic rare-earth doped crystals. One of the Sr clocks is equipped with a readout of the transition probability with a SNR in the quantum regime. SYRTE’s Optical Strontium Lattice Clocks at the Paris Observatory Physikalisch-Technische Bundesanstalt Braunschweig, Germany PTB brings expertise in neutral atom and single ion clocks, and laser stabilisation. The clocks are already operating at internationally competitive levels that benefit from available ultra-stable interrogation lasers with instabilities below 10-16. Further improvements of the oscillators’ instability will be investigated with systems based on Ca that are contained within a high-cooperativity optical cavity (WP3, WP4). PTB’s working group Unit of Length Academic Partners Consiglio Nazionale delle Ricerche Italy CNR brings expertise in theoretical protocols involving the creation, protection and optimal exploitation of useful entanglement for quantum metrology. These include methods to witness metrologically-useful entanglement in experiments and to Monte-Carlo simulated interferometric experiments – in an open-system approach – taking into account noise and decoherence. CNR will contribute to WP2 and WP4. CNR’s National Institute of Optics in Florence Fundacio Institut de Ciencies Fotoniques Barcelona, Spain ICFO brings a demonstrated expertise in quantum-enhanced sensing with atoms and non-classical states of light, with a focus on dispersively-probed atomic sensors, including DC and RF magnetometers with improved sensitivity through spin squeezing and optical squeezing. The group has developed theoretical and experimental methods to describe, quantify, and control quantum noise in these and similar atomic instruments. ICFO will contribute to WP1, WP2 and WP4. ICFO’s Atomic Quantum Optics group Kobenhavns Universitet Kopenhagen, Danmark KU brings expertise in the emerging field of combining neutral atoms with optical cavities to reduce the frequency noise of the laser. The focus of KU’s activities in this project will be in the exploration of novel techniques for generating an active atomic clock (WP3) in collaboration with INRIM, UMK and PTB, as well as demonstrating techniques for near-zero dead-time interrogation of a cold atomic sample. The Precision Measurements with Ultracold Atoms group at KU’s Niels Bohr Institute Gottfried Wilhelm Leibniz Universität Hannover Hannover, Germany LUH brings expertise in multi-ion traps for optical clocks and logic operations in trapped ion systems. LUH will implement entangling gates for two ions, optimise the choice of motional modes with the perspective of longer ion chains (WP2), and characterise the performance of the gates in terms of fidelity, systematic frequency shifts and superiority over uncorrelated ions (WP4). The Institute for Experimental Quantum Metrology at LUH and PTB University of Durham Durham, United Kingdom UDUR brings expertise in cold Sr Rydberg gases (experiment and theory). In this project they will work with NPL to implement their proposal for Rydberg-mediated spin squeezing in an optical lattice clock, building on recent experiments at UDUR and joint work with NPL on Rydberg electrometry for lattice clocks. UDUR will contribute to WP1 and WP4. 2-Electron Rydberg Systems at UDUR’s and Newcastle University’s Joint Quantum Center Uniwersytet Mikołaja Kopernika w Toruniu Torun, Poland UMK brings expertise in neutral atom optical clocks and atomic physics theory. Two existing clock systems based on Sr will be operated (WP3, WP4), along with an investigation of new design of an active Sr frequency standard. The Optical Atomic Clock group at UMK’s National Laboratory of Atomic, Molecular and Optical Physics Linked Parties Centre National de la Recherche Scientifique France, CNRS is a third party linked to partner OBSPARIS. OBSPARIS and CNRS have a strong collaboration over many years. In this role CNRS will contribute to WP1 and WP4. The coordinator Filippo Levi is the leader of the Time & Frequency group at INRIM. He has conducted pioneering research on Cs and Rb atomic frequency standards and has a long track record in coordination and participation in collaborative projects of national and international relevance.