Over the past decade, the field of quantum computing has become a major driving force for both fundamental physics and engineering, with impressive milestones demonstrated on several competing platforms. But today, even for an expert in the field, it is still hazardous to predict which physical system will be the most successful implementation in 10 years’ time, because achieving large-scale universal operations on a quantum computer requires not only world-class engineering, but also major conceptual breakthroughs. Optical quantum computers, and in particular nanophotonic circuits, are among the leading candidates due to their high scalability and small footprint. However, implementing universal operations on scalable nanophotonic platforms requires deterministic nonlinearities at the single-photon level, and so far, no known approach has been able to implement it reliably. The CoCoON project proposes to solve this critical problem by using quantum nano-emitters efficiently coupled to a nanophotonic waveguide, a nanofiber, to mediate deterministic nonlinear interactions between photonic qubits. With its original approach, the project bridges the gap between nanophotonics (usually limited to the discrete-variable approach) and the continuous-variable regime, through the generation of non-gaussian states. In doing so, I will answer fundamental questions about the amount of nonlinearity and quantumness contained in the light-matter interaction described by the Jaynes Cummings model, and that is required to achieve scalable universal photonic quantum computing.

Quantum computers are perhaps the most anticipated of all potential quantum technologies. They provide a quantum advantage that makes it possible to solve problems that lie beyond the reach of classical devices. Yet, the fundamental question “Which physical property lies at the basis of the computation power of a quantum computer?” does not have a clear-cut answer. In NoRdiC we aim to answer this question for continuous variable platforms, where quantum information is encoded in continuous degrees of freedom. Based on results from computational complexity theory, we formulate the research hypothesis that non-Gaussian quantum correlations play a crucial role in achieving a quantum advantage in such platforms. To understand the role of this phenomenon in quantum computational protocols, it is essential to explore its fundamental physics. Therefore, the first goal of NoRdic is to develop a theoretical framework to study these quantum correlations and understand their properties in small-scale systems. These small-scale systems serve as building blocks to create the large systems required for quantum protocols. The next step in NoRdiC is to find coarse-grained signatures that provide the means to identify the presence of non-Gaussian quantum correlations in these large systems. To do so, we exploit the idea that, even though the full multimode quantum state is inaccessible, we can extract information of all the two-mode subsystems. This allows us to construct networks, where every node corresponds to a mode, and a connection in the network represents the correlation between these modes. Our aim is to find signatures of non-gaussian entanglement in these network. Once the fundamental physics of non-Gaussian quantum correlations is unveiled, we can investigate their role in quantum protocols. As a first step, we aim to formally prove that such quantum correlations are necessary to implement of protocol that is hard to simulate with classical resources. Then, we aim to develop protocols that use non-Gaussian quantum correlations to solve graph-theory problems. These problems are narrowly related with the network-based analysis that was carried out to find signature of non-Gaussian quantum correlations. Hence, the same techniques that unveil the fundamental physics of the large non-Gaussian states, will now be fine-tuned to implement computational protocols. Thus, NoRdiC should make it possible to demonstrate a practical quantum advantage on continuous variable platforms.

Quantum networks could revolutionize the way we traditionally communicate and process information by exploiting the quantum nature of the physical carrier of data. This endeavor however requires a double challenge to be met: on the one hand, realizing quantum gate operations with high fidelity by coupling individual quantum systems that are extremely well isolated from their environment, and on the other hand, being able to store and transfer individual bits of quantum information over long times and large distances. In spite of their difficulty, these challenges are about to be overcome: the quickly growing field of circuit cavity quantum electrodynamics has shown that superconducting circuits could be used to realize extremely fast and efficient quantum gate operations. These constitute the most promising candidates for the development of scalable quantum computing units. Besides, the task of transferring and storing quantum information is most easily addressed with optical photons that can travel tens of kilometers in optical fibers at room temperature and interact strongly with matter. In this regard, the availability of commercial fiber-based quantum communication systems testifies of the maturity of the field. Hence, bridging the gap between those technologies, that is, achieving quantum coherent transfer of information between microwave photons in a cryogenic apparatus and propagating optical fields, appears as one of the key challenges of the next decade. This project aims at achieving such a quantum link by coupling a nanomechanical resonator simultaneously to a microwave and an optical field. The mechanical effects of light on resonators have been the subject of an intense research effort during the last decade. The field of optomechanics, pioneered by a few groups, including the team “Mesure et bruits fondamentaux” at laboratory Kastler Brossel has literally broken up into a myriad of talented research teams trying to observe quantum effects of radiation pressure on various experimental platforms. All these systems exploit the effects of radiation pressure to control the dynamics of a mechanically compliant element by coupling it to a high-finesse cavity. This research effort has culminated recently in the observation of a resonator cooled in its quantum ground state via radiation pressure. Noticeably, this achievement was performed successively with resonators coupled to microwave and optical photons. Importantly, this breakthrough is an undisputable signature that the coupling between optical and mechanical degrees of freedom exceeds the thermal decoherence rate. In this regime, the optomechanical interaction can be used to swap optical and mechanical quantum states reversibly. This project builds on the latest insights in the field of quantum optomechanics to perform a photon-phonon-photon conversion from the microwave to the optical domain. To reach the required coupling strength between mechanical, optical and microwave degrees of freedom, we will adopt an all integrated design: a high-Q and very low mass nano-membrane will be suspended atop of a Bragg reflector grown by epitaxial techniques. We will take advantage of the universal capacitive coupling between the dielectric membrane and a coplanar waveguide superconductive cavity to achieve the microwave optomechanical system. The expertise developed at LKB in the realization of high-finesse microcavities and their cryogenic operation together with the realization of highly optimized nanomechanical membranes in close collaboration with the Laboratoire de Photonique et de Nanostructures will be a decisive asset for the success of this ambitious project. This project will likely have a large impact in various active fields of research ranging from nano-electromechanical systems to the quantum engineering community. Other outcomes might include the development of electromechanical sensors with direct optical readout reaching unprecented sensitivities.

Creating and characterizing multi-particle entanglement in systems of material particles is becoming a major focus of experimental quantum physics. Besides its fundamental importance, such entanglement is the key ingredient for new applications such as quantum metrology, quantum simulations, quantum information and perhaps others still to come. Yet, there are few methods to create and measure multi-particle entanglement while maintaining single-qubit resolution as the number of qubits is scaled up. The goal of this project is to extend the generation of multi-particle entanglement to “mesoscopic” ensembles of up to ~100 neutral atoms while maintaining analysis at the single-atom level. To achieve this goal, we will combine methods of optical Cavity Quantum Electro-Dynamics (CQED) with quantum gas microscope techniques developed in the field of optical lattices. We will realize a single-atom qubit register in a one-dimensional (1D) optical lattice, where each lattice site is strongly and identically coupled to the mode of a high-finesse optical cavity. The cavity allows entanglement creation by the collective interaction of the atoms with the cavity mode, while site-resolved detection with a high-resolution optical microscope will add the capability of full state tomography, similar to ion traps. At the heart of this ambitious experimental system is a new type of fiber-based Fabry-Pérot (FFP) cavity developed in our group. This experimental platform gives access to powerful methods for generation of symmetric entangled states, which notably includes Schrödinger cats, Dicke states and spin-squeezed states. Thanks to our recent progress in FFP cavity fabrication, combined with the 1D lattice, we will realize theoretically proposed entanglement schemes that could not be implemented in any experiment so far. Combined with the full state tomography, our project goes far beyond the current state of the art, both in state creation and analysis. In a more exploratory phase, we will use the high-resolution microscope to perform local operations on each atom of the register. This may open the way for the generation and analysis of entangled states beyond the symmetric ones. This system provides an ideal test-bed to investigate different methods for multi-particle entanglement generation and to study their fundamental limits. As a first approach, we will study “Quantum Zeno Dynamics”, a powerful, theoretically proposed mechanism which takes place when the evolution of a system is confined by repeated projective measurement to an eigenspace defined by the degenerate eigenvalues of a measured observable. Experimentally testing this very general method will be an important step in view of possible practical applications of entanglement. As a second approach, it will be possible to implement an experimentally adjustable and controllable realization of the Dicke model, where an ensemble of two-state atoms interacts with a single quantized mode of the electromagnetic field. It exhibits a quantum phase transition at a critical value of the coupling, in the vicinity of which multi-atom entanglement is predicted. Our tomography technique is well adapted to quantify the created entanglement, unraveling the links between quantum phase transition and entanglement. At the crossing point of cavity quantum electrodynamics, quantum information, and quantum gases in optical lattices, this project will allow us to explore the scaling-up of quantum entanglement in “mesoscopic” systems, essential for the emergence of new quantum technologies.

The goal of this project is to determine the fine structure constant alpha with an uncertainty below 10^{-10}. This measurement will be three times more accurate than the measurement of alpha that is obtained using the quantum electrodynamics theory (QED) with the measurement of the anomalous magnetic moment of the electron. It will also be seven times more accurate than measurements not involving directly QED. It will therefore strengthen the test of QED using the anomalous magnetic moment of the electron. This project involves the construction of an atom interferometer in order to precisely deduce the ratio h/m between the Planck constant and the mass of an atom from atom recoil measurement. This ratio is actually the limiting factor of the most precise determinations of alpha that are independent of QED. The quantum electrodynamics has been introduced in the 1940s in order to explain electromagnetism in the framework of quantum mechanics and special relativity. It succeed in describing two phenomena not described by the previous theory due to Dirac. Those phenomena, the Lamb shift and the anomalous magnetic moment of the electron are still used to precisely test this theory. Indeed, it is important for theoretical physics to test QED with a higher accuracy in order to confirm the Standard Model and possibly see effects beyond this model. In order to realise such a test, the fine structure constant which is the free parameter of QED need to be measured independently. The ratio between the kinetic energy of the electron and its mass energy in the ground state of the hydrogen atom is directly linked to the fine structure constant. However, it cannot be deduced directly from hydrogen spectroscopy as one need to compare an energy expressed in terms of frequency $h\nu$ with an energy expressed in terms of mass $mc^2$. The purpose of our interferometer is to precisely measure such a ratio h/m using rubidium atoms. This measurement will also have an impact in the atomic mass community. Indeed, the international committee in charge of the definition of the International System of units (SI) plan to redefine the unit of kilogram using the Planck constant. Our measurement will therefore provide the best link between the atomic mass units and the SI. We want to build an interferometer of the "next generation". The source of atom will use state-of-the art techniques developed for Bose-Einstein condensates (fast evaporative cooling in an optical dipole trap). We will also develop and implement new interferometric schemes in order to enhance the sensitivity of the interferometer. Such schemes based on "large momentum beamsplitter" are under development in many groups around the world as they promise significant improvement in atomic interferometer. This new project that follows more that 10 years of research in atom interferometry at the Laboratoire Kastler Brossel is part of this worldwide effort in atom interferometry. Our experiment will therefore not only affect the determination of alpha but also other applications of atom interferometry, especially inertial sensors for navigation or geophysics.