The use of solid-state spin defects as qubits for quantum applications requires long spin coherence times (defined as the maximum duration a system can remain in a prepared quantum state). This coherence time is limited by the interaction of spin qubits with the magnetic noise resulting from the surrounding electron and nuclear spin bath. The quality of the host material is thus an essential factor for the development of high performance solid-state qubits.

Based on our recognized expertise in the field of diamond synthesis by plasma enhanced chemical vapour deposition (PECVD), we work on the development of quantum-grade diamond, presenting ultra-high isotopical and structural purity compared to commercially available materials. This work involves the optimization of PECVD reactors and epitaxial growth parameters for the preparation of high-quality single crystal diamond layers.

Another objective is the formation of spin defects with controlled density, depth, charge state and coherence time by in-situ doping during PECVD. The properties and location of color centres in the diamond crystal can be engineered, for example through co-doping with different atoms or by etching and selective CVD overgrowth of diamond. Our research is particularly focused on the fabrication of doped single-crystal materials presenting optimal properties for the photoelectric readout of spin states. In parallel, we also investigate the seedless PECVD growth of doped nanodiamonds presenting high crystal quality, controlled size and long T₁ spin coherence times, which are required for nanoscale quantum sensing especially in biological environment.

The optimization of the host material quality, as well as the study of the spin defects formation mechanism and the assessment of their quantum properties is based on the feedback of in-house characterization of structural, optical, photoelectric and spin properties by various advanced techniques including SEM, AFM, confocal microscopy, photoluminescence and photocurrent spectroscopy, time of flight and spin coherence measurements.

Quantum sensors, in which the influence of external parameters on quantum superposition states is detected, have the potential to reach ultra-low sensitivities. Solid-state magnetometers based on ensembles of NV centres spins in diamond are among the most developed quantum sensors, with the first integrated devices being expected to become commercially available within the next ten years. They enable the detection of sub-picoTesla magnetic fields with high dynamic range and bandwidth, and could find applications in the fields of automotive, space, failure analysis or magnetic field sensing in biological environment.

The QST division at Imo-Imomec works on the development of photoelectrically readout NV diamond sensors, for magnetic field sensing and microwave detection. Compared to optically readout systems, these sensors could present improved sensitivities and higher compactness, as well as an easier integration with electronics. One of our objectives is the development of architectures and protocols enabling the parallel addressing of adjacent micrometric pixels, to form spatially-resolved photoelectric quantum sensors. In parallel, we develop fully integrated compact magnetometers, containing all required components, from the control and readout system to magnetic, microwave, radio-frequency, and optical manipulation tools. This type of miniaturized device is of a high interest for scientific space missions, Earth magnetic field measurements for geological exploration, satellite navigations or monitoring of space weather.

Our research aims in particular at improving the sensitivity of photoelectrically readout diamond quantum sensors, by maximizing the NV centres charge state purity and the spin initialization fidelity, minimizing the background photocurrent and improving the charge carriers collection efficiency. Achieving these goals requires to better control the NV centres environment, to improve the diamond-electrode interface and to optimize experimental parameters. Protocols are also developed to make the photoelectric readout compatible with high-frequency AC quantum sensing.

In parallel, we investigate the use of nanodiamonds containing color centres for nanoscale temperature sensing in biological environment.

An example of quantum sensor applications is project OSCAR-QUBE, which was a demonstration of NV based quantum magnetometer sensor tested aboard the International Space Station.

Current research on quantum communication is motivated by the need for encryption methods which cannot be broken by quantum computers, contrary to classical encryption. The development of quantum networks could lead to applications such as enhanced sensor networks or distributed quantum computing. To enable communications over large distances, quantum networks require the presence of quantum repeaters, allowing the distribution of quantum states and information between entities without direct point-to-point connections.

The QST division at imo-imomec studies the development of quantum memories that could be used as outer nodes for quantum communication networks. We investigate in particular memories based on NV centres in diamond, presenting the longest memory coherence properties at room temperature and for which we already established the photoelectric readout of a single nuclear spin coupled to an NV electron spin. However, other point defects in diamond - in particular group IV-vacancy defects, presenting better optical properties  - are also considered for this application.

Another axis of our work deals with ultra-low-noise light detection methods, for application in the field of gravitational waves detection. Theoretical studies aiming at the mathematical description of light quantum detection (in particular squeezing and entanglement-based method) are performed, and an experimental setup enabling the production of squeezed light and the heterodyne detection of light is developed. We investigate in particular the suppression of quantum noise by using spin-optomechanical hybrid systems based on ensembles of nuclear spins in diamond.