The negatively-charged boron vacancy center in hBN: toward an atomically-thin quantum sensor

Tristan CLUA PROVOST

Quantum sensors based on optically active spin defects, most notably the nitrogen–vacancy (NV) center in diamond, have already demonstrated remarkable potential. These systems have found applications in diverse fields, from biology to geology, but their most striking impact has been in condensed-matter physics, where they have enabled magnetic imaging of microscopic textures such as domain walls and superconducting vortices. In recent years, these sensors have found a new class of preferred samples: van der Waals (vdW) materials. These lamellar crystals, composed of atomic layers held together by weak van der Waals forces, can be exfoliated down to a single monolayer, forming truly two-dimensional systems. Following the discovery of graphene, this family of materials rapidly expanded to include superconductors, ferro- and antiferromagnets, and topological insulators, covering almost the entire bestiary of condensed-matter physics. When stacked, vdW layers form heterostructures with even richer physics, giving rise, for instance, to topological superconductivity or unconventional electronic states in twisted bilayer graphene. Amid this enthusiasm, an intriguing question emerged: what if the quantum sensor itself could be made from a van der Waals material? Transitioning from a three-dimensional (3D) to a two-dimensional (2D) sensor would offer several major advantages, the foremost being an ultimate proximity between the sensor and the sample. Such atomic-scale proximity would not only enhance sensitivity, since magnetic fields decay rapidly with distance, but also drastically improve spatial resolution. A 2D, atomically thin sensor directly deposited on a sample could thus detect nanoscale magnetic features with unprecedented precision. Moreover, as a flexible atomic membrane, it could be integrated into complex vdW heterostructures, allowing in situ probing of their rich emergent physics. The first crucial step in developing such a sensor lies in the choice of the host material. It must possess a wide bandgap to accommodate optically active defects and remain chemically stable under ambient conditions. Hexagonal boron nitride (hBN) fulfills these criteria perfectly. With a bandgap of about 6 eV, exceptional chemical robustness up to 1000 °C, and resistance to oxidation, hBN has become a standard protective layer in the vdW materials community. In 2020, Gottscholl et al. identified a particularly promising defect within hBN: the negatively charged boron vacancy (VB). This defect carries an electronic spin and exhibits spin-dependent optical properties closely resembling those of the NV center in diamond, marking the birth of a new line of research, within which this present work finds its place. The main objective of this work is to evaluate the potential and viability of the VB defect in hBN as a two-dimensional quantum sensor. This includes demonstrating its sensing capabilities, identifying the main factors limiting its performance, engineering its environment, optimizing its creation process, and ultimately testing whether the defect remains operational down to the few-layer limit. This manuscript is organized into five chapters. The first introduces the electronic, optical, and spin properties of the defect and demonstrates its potential for magnetic imaging. The second provides a comprehensive study of its spin-dependent photodynamics at room and cryogenic temperature. The third focuses on engineering the nuclear-spin bath via isotopic purification and dynamical nuclear polarization. The fourth explores neutron irradiation as a method to generate boron vacancies and confirms that the defect remains functional even in atomically thin flakes. Finally, the fifth chapter investigates the physics of these ultrathin systems, showing that spin coherence is preserved despite surface proximity, a key result for the future development of 2D quantum sensors.