Many-body entanglement lies at the heart of the current developement of quantum technologies. While the realization of large-scale entanglement remains an outstanding challenge, we propose within HighDy a new approach based on the manipulation of highly magnetic dysprosium atoms. We will first aim at generating entanglement among atomic spins in a single-mode Bose-Einstein condensate. We propose a new route based on the initial preparation of each atomic spin in a highly non-classical state. Interactions between spins then induce a collective spin magnification, leading to Dicke squeezing with entanglement-assisted metrological capabilities. Given the complexity of interactions between highly magnetic atoms, a joint experimental and theoretical effort will be required to design the optimal entangling protocol. Our second objective will be the study of Bose-Einstein condensates in a quantum Hall structure. An effective magnetic field will be produced via the light-induced coupling between the atomic motion and the spin, which acts as a synthetic dimension. By controlling the interaction range, we will create Bose-Einstein condensates with a regular lattice of quantized vortices. We will study quantum fluctuations of the vortex lattice, and generate quantum correlations between atoms by quenching the system to a flat-band regime. Lastly, we will integrate ideas from both objectives to develop a novel type of inertial quantum-enhanced sensor. This sensor will be based on spin/momentum locking resulting from light-induced spin-orbit coupling.

Quantum Optomechanics is a disruptive technology which is experiencing an unstoppable progress that could help face up to present metrology challenges. A particularly exciting new development is the possibility of nano-optomechanical systems to produce quantum primary standards that use fundamental aspects of quantum mechanics to gauge thermodynamical quantities. The aim of LaRaQrOfT is to demonstrate and validate innovative primary temperature sensors using quantum technologies that could either work at low temperature near the quantum regime of the device up to room temperature far from this regime. The monitoring of both thermal and quantum fluctuations of a mechanical resonator allow to scale the size level of thermal motion in term of quantum energies determined by Planck’s constant and can lead to a quantum primary determination of the temperature. Moreover optomechanical devices provide a small, reliable and cost effective primary temperature sensing method that could be used with exchange gazes or liquid without compromising the measurement. All this aspect addresses both a metrological and an industrial challenge, with a self-calibrated, integrated and easily thermalizable sensor. The project LaRaQrOfT brings together French leading players in the field of optomechanics, nanofabrication and temperature metrology. The Optomechanics and Quantum Measurements team of LKB is a pioneering group in the domain of cavity optomechanics and has widely studied thermal effects and quantum correlations in optomechanical devices necessary to the success of the project. Micro-fabrication will be supported by the state of the art clean-room facilities housed by C2N which has a thorough knowledge of the fabrication of photonics and phononics crystals. Finally CNAM and LNE with their unique expertise in temperature metrology, will develop methods for the metrological validation of the optomechanical thermometer, and its traceability to the International Temperature Scale.

Our project aims at proposing theoretically and demonstrating experimentally quantum computation protocols in the optical domain. Quantum information is a pluridisciplinary field of research whose goal is to benefit from the specific properties of quantum mechanics to provide original communication and calculation protocols. These protocols can provide for instance security advantages or computational speed-up. While quantum computation is very promising, it still lacks a clear roadmap to perform relevant calculations, that is calculations undoable on classical computers. We propose here to benefit from the skills of two communities, namely physicists and computer scientists, to explore the advantages of a specific computation protocol, Measurement-Based Quantum Computing (MBQC), using the frequency spectrum of light. MBQC is based on the availability of a large, multipartite entangled state on which a series of measurements are performed. For each operation to implement, a specific entangled state as well as a specific measurement order are used. The output of the computation is given by a set of one or more qubits which are to be measured in the end. The advantage of this protocol lies mainly on the absence of requirement of two qubits gates, which are probably the hardest to implement faithfully experimentally. The difficulty then lies mainly in producing a suitable entangled state. The solution which will be studied in this project uses the frequency spectrum of light beams produced by parametric down-conversion. Parametric down-conversion, a nonlinear optical process by which a pump field is split in two coherent fields, is well known to produce nonclassical states of light. In particular, within an optical cavity, entangled beams are produced. We will base our study on parametric down conversion occurring in an optical cavity pumped by a femtosecond frequency comb. We will not consider standard variables, like polarization or intensity, but rather field quadratures of different frequency component of the light spectrum. Indeed, it can be shown that, while the standard variables can be described by bipartite entanglement, multipartite entanglement can be produced in both regimes in the frequency spectrum. This system produces multipartite entanglement which is the key ingredient for MBQC. The advantage of using the frequency spectrum of short light pulses is that it can involve hundreds of thousands of frequency components that are mutually coherent at the classical level and that are likely to be entangled at the quantum level, which makes the scalability to many qubits an easier task than in other possible schemes that are presently under study. The goal of this project is to determine and demonstrate how such entangled states can be used in a way that puts in evidence an advantage of the MBQC approach over the standard quantum circuit model in terms of the number of operations for instance. This goal is relevant both in the physics community where this model is little explored for the moment and in the computer science community where the difference of the MBQC and quantum circuit models are not yet fully quantified and demonstrated. Reaching this ambitious objective requires several steps. Firstly, there is a need to devise proper measurements schemes which can detect the multipartite entanglement present in such states and in particular characterize its dimensionality. Indeed, one of the crucial aspects of this project lies in the number of modes which can be entangled. Then we will show that we are able to tailor at will such a multimode entangled state. Once these steps have been taken, we will design and then implement basic quantum operations such as Fourier transform. Finally, we will tackle the more ambitious part of the project that is demonstrating a protocol with a clear advantage of MBQC protocols over the standard circuit model.

Achieving strong light-matter interactions at the single photon level is an important milestone of the quantum technologies roadmap. Recently, the quantum optics and quantum information communities have witnessed the emergence of waveguide quantum electrodynamics (waveguide QED), where the atom-photon interaction is increased without cavity by using subwavelength waveguides that confine the electromagnetic field to deeply subwavelengths scales in the transverse directions. Different waveguide QED platforms coexist, each one having its own advantages and drawbacks: superconducting qubits coupled to transmission lines at microwave frequencies, quantum dots in nanophotonic structures, and cold atoms trapped along a nanofiber. However, currently, no system can provide both many emitters and a high coupling strength. This interaction regime is largely unexplored in waveguide QED. The eQUANDIS project has the ambition to realize for the first time non-linear quantum optics experiments in this regime. We target the realization of a single-photon switch. Moreover, because of the lack of atom-compatible structures with an engineered dispersion, the impact of the waveguide dispersion has been scarcely investigated in waveguide QED studies. Its role in the formation of the collective properties of an atomic ensemble is still unknown. The eQUANDIS project aims at filling this blank page by implementing a waveguide QED platform based on cold atoms trapped along slow-light waveguides whose dispersion can be engineered through symmetry breaking. The transverse symmetry breaking in this type of photonic-crystal waveguides allows us to engineer the dispersion beyond the usual parabolic shape. The eQUANDIS project brings together 5 teams with complementary competences: theoretical and computational nanophotonics, inverse design with advanced optimization algorithms, nanofabrication, quantum-optics theory, quantum-optics experiments with ensembles of cold atoms.

Light has been since the beginning of the XXth century described as a gas of photon, with limited interactions. Since the recent development in semiconductor fabrication, this picture tends to be overcome, to welcome a new analogy connecting light to fluid. Nowadays the interaction between photons can be engineered such as the light behaviour inside nano-structured devices mimic a fluid system. One of the most emblematic system described in this new paradigm is polaritonic devices. Exciton-polariton are quasi–particle rising form the strong coupling between cavity electromagnetic modes and a quantum well exciton transition. These particles half-way between light and matter were observed in different quantum state of matter such as superfluidity and Bose-Einstein condensate paving the way for correlated fluid of light studies. The dynamic acting appears to be dominated by an interplay of strong dissipation and non-linear properties leading to rich features such as optical bistability, cavity soliton or other typical properties of non-linear dynamics. The conjonction of these outstanding features leads to unprecedented physics such as out-of-equilibrium quantum fluids, which remains mostly unexplored. Simon Pigeon and the Quantum Optic Group at Laboratoire Kastler Brossel (Université Pierre et Marie Curie, École Normale Supérieure and CNRS) are world-recognised experts of this important field. In this project, we propose significant advances on the understanding of out-of-equilibrium quantum fluids. Moreover based on Simon Pigeon expertise, an insight of the corresponding physics will be given on through hydrodynamic and thermodynamic approaches. — The first research line is dedicated to the in-depth study of polariton fluids properties. Focusing on the spin properties and the topological excitations taking place in such fluid, we expect to provide genuine progresses in the understanding of polariton systems and the link to other non-linear optical devices presenting similar dynamics. ?— The second component of the C-FLigHT project is to use polariton systems to simulate quantum states of matter. Thanks to the great controllability of these systems, polariton quantum fluids can provide an efficient simulator of phenomena such as Anderson localisation or Mott-Superfluid transition. ?— The third axis of C-FLigHT relies on recent advances in the emerging field of Quantum Thermodynamics. By studying microcavity polaritons from an innovative thermodynamics of open-quantum-systems perspective we expect to gain a finer understanding of these system dynamics. The goal is to describe thermodynamic phenomenon and to explore universal mechanism such as Kibble-Zurek mechanism in the general context of quantum optics and solid-state physics using polariton devices as a benchmark. ? C-FLigHT is expected to simultaneously impact two separated field of physics building a bridge in between. Quantum gas physics will gain on this project through experimenting quantum state of matter in unexplored situation, while semiconductor optical devices sciences will profit for an original insight on there devices operating behaviour. Applied and fundamental outcomes are expected from C-FLigHT, paving the way to the exploration of correlated fluid of light.