The design of superconducting materials in order to achieve higher transition temperatures (Tc) to the zero-resistance state has been recognised by recent international and national reviews as at the extreme forefront of current challenges in condensed matter science with potential for transforming existing and enabling new technologies of tremendous economic and societal benefits in energy and healthcare. Achieving the zero-resistance state requires close control of the interactions of electrons with each other (known as electron correlation) and with lattice vibrations (phonons). This project addresses these challenges by building on EPSRC-supported collaborative work by the project team, which has shown that high Tc superconductivity, defined both in terms of transition temperature and the key role played by electronic correlations, is accessible in molecular systems. In the fullerene-based molecular superconductors, superconductivity occurs in competition with electronic ground states resulting from a fine balance between electron correlations and electron-phonon coupling in an electronic phase diagram strikingly similar to that of the atom-based copper oxide high Tc superconductors, where correlation plays a key role. A second molecular superconductor family with transition temperatures over 30 K, based on metal intercalation into aromatic hydrocarbons, has also been reported. It is therefore timely to optimise and understand superconducting materials made from molecules arranged in regular solid structures. The scope of synthetic chemistry to tailor molecular electronic and geometric structure makes the development of molecular superconducting systems important, because this chemical control of the fundamental building units of a superconductor is not possible in atom-based systems. The molecular systems are the only current candidates for the important target of isotropic correlated electron superconductivity. We will exploit these opportunities by integration of new chemistry with new physical understanding, exemplified by revealing how changes in molecular-level orbital degeneracy driven by chemical control of molecular charge and overlap direct the electronic structure of an extended solid. We will develop the new chemistry of the molecular solid state that will be needed for this level of electronic structure control, in particular mastering the chemistry of metal intercalation into hydrocarbons. This new materials chemistry will include the use of new building blocks (such as endohedral metallofullerenes) and will harness the assimilation of defects to access new molecular packings, motivated by our discovery that different packings of the same molecular unit give different Tc and distinct electronic properties. Further structural control will be exercised by binding small molecules to the cations intercalated into the molecular lattices. The synthesis of metal-intercalated solids based on multiple molecular components will be undertaken to permit detailed optimisation of the electronic structure. We will thus specifically exploit the molecular system advantages of isotropy, packing and molecular-level electronic structure control by developing the new chemistry of the molecular solid state needed to establish the new electronic ground states. Physical understanding of the structural and chemical origins of the new electronic states is essential to identify the factors controlling the electron pairing in the superconductors. This understanding will emerge from an integrated investigation of the insulator-metal-superconductor competition, spanning thermodynamic, spectroscopic and electronic property measurements closely linked to comprehensive structural work in order to produce the structure-composition-property relationships required for the design of next generation systems. The project benefits from an international multidisciplinary collaborative team to ensure all relevant techniques are deployed.

Currently world-wide efforts are concentrated along research and industrial developments of quantum computers based on large cryogenic or vacuum installations, having attracted massive investments. Nevertheless, an integrable portable quantum processor, operating at ambient conditions, would bring massive benefits for developing quantum algorithms, software and novel applications, widely spreading the quantum technologies. The research on the realisation of integrable devices is also subject to several research activities within the Quantum Flagship. However, these concepts represent chief technical challenges in terms of scalability of qubit gates that are still limited to operation of only a very few qubits and requiring temperatures of a few milliKelvin. An integrable, small to medium scale and portable quantum processor, operating at ambient conditions, represents thus chief technical challenges in terms of developing of concepts and technologies. Photonic technologies represent the major development trends towards integrable devices, offering the potential of operating a large number of qubits, still the current photonic technologies are probabilistic. The recent breakthrough results, established within this consortium, discovered possibility of near-deterministic generation of solid-state-qubits, NV centres in diamond, placed at a distance of about 10-40 nm allowing for high fidelity two-qubit gates based on magnetic dipole-dipole coupling. The method of electrical readout of the spin, also discovered last year by the consortium members, allows for individual spin qubit readout, dealing with the major constrain of the diamond quantum hardware scalability. MAESTRO proposes a novel approach to develop solid-state qubit architectures for working at ambient conditions, which become available based on the aforementioned ground-breaking work, that will be used as a basis for scaling up, providing a compelling path towards industrially interesting medium scale quantum systems. The central aim of the proposal is to tackle the bottle neck issues of our technology for diamond processor hardware scalability, e.g., the deterministic engineering and fabrication of NV qubits with a high fabrication yield that will allow then to scale our technology to industrially relevant processes. Together with the selective electric qubit readout, applied to an entangled qubit network in a nanoscale device, MAESTRO will provide a quantum processor platform that can be further developed, in future, towards marketable industrial applications.

Over the past two decades the nitrogen-vacancy (NV) center in diamond has been used to demonstrate and develop a variety of sensing protocols for static magnetic and electric fields, pressure, temperature, fluctuating fields, etc. Large corporations and start-ups are currently bringing NV sensors to the next technology-readiness level. However, it has also become apparent that such devices suffer from certain shortcomings, in particular for sensing at very high magnetic fields (>1 Tesla) and high stresses (> 100 GPa). These drawbacks could be overcome using group-IV-vacancy centers in diamond, in particular SiV, GeV, and SnV complexes. The goal of the project is the development of diamond-based quantum sensors for sensing at high magnetic fields and high stresses, which can be generally called “sensing at extreme conditions”. At the core of the project is the fabrication of shallow group-IV-vacancy centers with superior optical and spin coherence properties. Preliminary estimates demonstrate that, depending on exact experimental conditions, with appropriate defect engineering, coherence times could be increased by two orders of magnitude beyond what is currently achievable. Advances in engineering will be supported by theoretical work that will provide guidance and insights into the fundamental limits of optical and spin coherence times of group-IV-vacancy complexes. These centers will then be used to demonstrate two proof-of-concept sensing protocols beyond the limitations of NV-center-based technologies: (i) quantum magnetometry at Tesla-range magnetic fields; (ii) quantum sensing at stresses >100 GPa. For the latter, we aim at the measurement of the magnetic field, as well as the entire stress tensor and its distribution in the diamond crystal at “extreme” pressures. Along with the experimental demonstrations of these protocols, the work in the project will yield new knowledge about fundamental properties of point defects in extreme conditions, expanding the general knowledge of diamond as a quantum material. Fundamental aspects of defect physics will be investigated via a very close collaboration between theory and experiment.

The goal of this project is to pave the way for long-distance quantum communications between superconducting quantum (sub)processors with optical photons. We shall develop integrated chips for the conversion between microwave and optical photons using ultracold atomic ensembles. The hybrid chip developed in this project, with atoms simultaneously interacting with microwave and optical cavity fields, could be connected to superconducting quantum processors and fiber optical communication networks for realizing coherent links between distant computational nodes. Our project will demonstrate experimental techniques for microwave to optical conversion that are integrable on chips. We will fabricate planar superconducting cavities as well as integrated optical waveguides and cavities. After separate evaluation and benchmarking of the optical and microwave components in dedicated cold atom experiments, we will fabricate hybrid chips that combine both. The final goal of the project is to evaluate the operation and optimal design of a superconducting atom chip with integrated microwave and optical cavities for the coherent transduction of photons. This project combines the expertise of five groups (Tübingen - UT, Bordeaux – LP2N, Budapest – WRCP, Turin – INRIM, Heraklion - FORTH) of experimental and theoretical physics, from five european countries: Germany, France, Hungary, Italy and Greece. The project will contribute to scientific excellence, competitiveness and leadership in the broad field of Quantum technologies at European level. The project will explore several new functional units of a chip-based coherent interface between microwave and optical photons, paving the way towards the realization of quantum links between distant quantum (sub)registers implemented with SC circuits. The research has direct implications to the fields of quantum communication, quantum computation, and quantum metrology and sensing.

The design of superconducting materials in order to achieve higher transition temperatures (Tc) to the zero-resistance state has been recognised by recent international and national reviews as at the extreme forefront of current challenges in condensed matter science with potential for transforming existing and enabling new technologies of tremendous economic and societal benefits in energy and healthcare. Achieving the zero-resistance state requires close control of the interactions of electrons with each other (known as electron correlation) and with lattice vibrations (phonons). This project addresses these challenges by building on EPSRC-supported collaborative work by the project team, which has shown that high Tc superconductivity, defined both in terms of transition temperature and the key role played by electronic correlations, is accessible in molecular systems. In the fullerene-based molecular superconductors, superconductivity occurs in competition with electronic ground states resulting from a fine balance between electron correlations and electron-phonon coupling in an electronic phase diagram strikingly similar to that of the atom-based copper oxide high Tc superconductors, where correlation plays a key role. A second molecular superconductor family with transition temperatures over 30 K, based on metal intercalation into aromatic hydrocarbons, has also been reported. It is therefore timely to optimise and understand superconducting materials made from molecules arranged in regular solid structures. The scope of synthetic chemistry to tailor molecular electronic and geometric structure makes the development of molecular superconducting systems important, because this chemical control of the fundamental building units of a superconductor is not possible in atom-based systems. The molecular systems are the only current candidates for the important target of isotropic correlated electron superconductivity. We will exploit these opportunities by integration of new chemistry with new physical understanding, exemplified by revealing how changes in molecular-level orbital degeneracy driven by chemical control of molecular charge and overlap direct the electronic structure of an extended solid. We will develop the new chemistry of the molecular solid state that will be needed for this level of electronic structure control, in particular mastering the chemistry of metal intercalation into hydrocarbons. This new materials chemistry will include the use of new building blocks (such as endohedral metallofullerenes) and will harness the assimilation of defects to access new molecular packings, motivated by our discovery that different packings of the same molecular unit give different Tc and distinct electronic properties. Further structural control will be exercised by binding small molecules to the cations intercalated into the molecular lattices. The synthesis of metal-intercalated solids based on multiple molecular components will be undertaken to permit detailed optimisation of the electronic structure. We will thus specifically exploit the molecular system advantages of isotropy, packing and molecular-level electronic structure control by developing the new chemistry of the molecular solid state needed to establish the new electronic ground states. Physical understanding of the structural and chemical origins of the new electronic states is essential to identify the factors controlling the electron pairing in the superconductors. This understanding will emerge from an integrated investigation of the insulator-metal-superconductor competition, spanning thermodynamic, spectroscopic and electronic property measurements closely linked to comprehensive structural work in order to produce the structure-composition-property relationships required for the design of next generation systems. The project benefits from an international multidisciplinary collaborative team to ensure all relevant techniques are deployed.