Atom interferometry is a technology that allows measurements to be performed with extreme precision and accuracy. It has been applied to the measurement of several physical quantities, covering inertial quantities, atomic polarizations and fundamental physical constants, such as the Newtonian gravitational constant G and the fine structure constant α. For the next decade, research in atomic interferometry has set itself major challenges such as the detection of gravitational waves in a frequency range inaccessible to optical interferometers. For that a gain in sensitivity of 3 to 4 orders of magnitude compared to the state of the art is required. The TONICS project aims at bringing together the competences of two teams from LKB and SYRTE to overcome the limits to the accuracy and sensitivity of atomic interferometers. In particular, the effects related to the distortion of the intensity profile of the laser beams. We will work jointly to implement reliable and robust methods to improve the accuracy of our state-of-the-art gravity and atomic recoil measurements. This step is essential to exploit and validate the benefit of novel methods advocated to realize ultra sensitive interferometers, such as large momentum atomic beam splitters (LMT) at high orders and quantum engineering protocols such as spin compression to surpass the standard quantum limit. The TONICS project is based on three parts. 1/ The development of new robust experimental tools to reduce and control the effect of optical aberrations. For this, the SYRTE team will work on designing collimators with high optical quality and the LKB team will study different laser cooling schemes to provide an experimental protocol for producing a dense source of ultra-cold atoms in a cycle time of the order of a second. In parallel, we will work on the improvement of our two experimental set-ups to improve their sensitivity to exploit fully the benefits of these two ingredients. 2/ The development of a common library for the numerical modeling and simulation of the physical mechanisms at the origin of the systematic effects that limit the accuracy of our measurements. This will require cross-testing of both experiments and will allow a reliable evaluation of the systematic biases. This sequence of work will require very frequent exchanges which will probably be facilitated by the recruitment of a common PhD student and the creation of a common website. 3/ Optimization of the coherence of the large momentum beam splitters based on a combination of the double Raman diffraction technique developed by the SYRTE team and the Bloch oscillation technique in an accelerated optical lattice, which remain the area of expertise of the LKB team. The study of a symmetric atomic interferometer using these LMTs will enable us to evaluate its performance. Our objective is to demonstrate a contrast higher than 30% with a separation of 200 ħk. We are aiming at performances that surpass the state of the art. The goal of the SYRTE team is an absolute gravity measurement with an accuracy of less than 10-8 m.s-2 and a long-term stability better than 10-10 m.s-2. A continuous, absolute gravity meter with long-term stability of this level will meet the needs of the geophysical community that are not covered by existing technologies. The objective of the LKB team is to measure the recoil velocity of the two rubidium isotopes with a relative uncertainty of a few 10-11. This should validate the recent determination of the fine-structure constant and/or will explain the significant discrepancy with the value deduced from the Cs recoil measurement. This level of uncertainty is also required to be able to observe on the electron, a possible effect that could be behind the persistent discrepancy between the theoretical and experimental values of the muon's magnetic moment anomalous.

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.

The integration of reliable quantum sources on a photonic microchip is at heart of intense research in today quantum photonics. The conventional pathways to achieve single photon sources rely on either III-V quantum dot technology or enhanced optical nonlinarities (four wave mixing) in silicon photonics. The former approach requires cryogenic temperatures, and its integration on a silicon chip at the telecom band remains challenging. The latter, on the other hand, operate at room temperature, but even if the efficiency of such integrated sources can be improved by spatial multiplexing, compactness and scalability remain open issues. UNIQ proposes a new paradigm to tackle single and quantum correlated photon generation using nonlinear III-V semiconductor materials (both passive and active) in optical nanocavities. In essence, our project is devoted to the realization of unconventional quantum correlated photonic sources based on nonlinear interactions in coupled nanocavities with few photons. Unlike conventional semiconductor quantum sources that require deterministic coupling of cavity modes to single nanoemitters (i.e. quantum dots) and operate at ultralow temperatures, UNIQ sources will achieve quantum correlations with few photons using optical nonlinearities at room temperature from uniformly grown materials (bulk, quantum wells) in coupled nanocavities. Such capabilities will rely on two recent paradigms in nonlinear coupled cavity systems: i) the generation of photon antibunching by means of the so-called unconventional photon blockade (UPB) mechanism; ii) nonlinear transitions with low photon numbers, such as spontaneous symmetry breaking (SSB) in coupled nanolasers. These two mechanisms result from the interplay between third order optical nonlinearities and photon dynamics in optical cavities. In UPB, weak nonlinearities are combined with photon tunneling between adjacent cavities to produce destructive interference and hence suppression of multi-photon states. As a result, strong photon antibunching has been predicted in the transmission of a resonant coherent beam. On the other hand, nonlinear optical transitions such as SSB give rise to strong photon localization, as recently demonstrated in coupled nanolasers. In this case, in contrast with UPB, nonlinearities are strong: the nonlinear shift of the laser frequency is large enough to overcome mode splitting. This has been shown to take place with only 100 intracavity photons, which can be further reduced for increased spontaneous emission factor of the nanocavities. Within SSB conditions, strong photonic correlations are theoretically predicted between output photons from both cavities. The photonic platform that will used to realize the coupled cavity systems are photonic crystal nanocavities, which enable a large parameter space to tailor both strong and tunable evanescent coupling, high quality factors, ultra-small mode volumes, efficient input/output light coupling and large spontaneous emission factor nanolasers. Such a platform is compatible with device integration on a photonic microchip, small foot print and scalability. Specifically, building blocks will be hybrid III-V semiconductor photonic crystal nanocavities and nanolasers on silicon, fully compatible with dense photonic integration on a CMOS microchip. Fabrication will be undertaken at LPN clean room, using the mature III-V photonic crystal nanotechnology and heterogeneous integration of III-V semiconductors on silicon. To sum up, UNIQ seeks advanced quantum functionalities of III-V semiconductor nanosources at the interface between quantum optics and nanophotonics. It brings together worldwide-major actors in the quantum optics domain (LKB and MPQ partners) and nanophotonics (LPN). UNIQ proposes innovative technological developments that will ultimately enable room-temperature, single photon sources in the telecommunication C-band, with a high potential for integration.

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.

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.