Igor Aharonovich

Hexagonal boron nitride (hBN) is gaining interest as a wide bandgap van der Waals host of optically active spin defects for quantum technologies. Most studies of the spin‐photon interface in hBN focus on the negatively charged boron vacancy (V B − ) defect, which is typically fabricated by ion irradiation. However, the applicability and wide deployment of V B − defects is limited by V B − fabrication methods which lack robustness and reproducibility, particularly when applied to thin flakes (≲10 nm) of hBN. Here, two key factors are elucidated that underpin the formation and quenching of V B − centers by ion irradiation—density of defects generated in the hBN lattice and recoil‐implantation of foreign atoms into hBN. Critically, it is shown that the latter is extremely efficient at inhibiting the generation of optically‐active V B − centers. This is significant because foreign atoms such as carbon are commonplace on both the top and bottom surfaces of hBN during ion irradiation, in the form of hydrocarbon contaminants, polymer residues from hBN transfer methods, protective capping layers and substrates. Recoil implantation must be accounted for when selecting ion beam parameters such as ion mass, energy, fluence, incidence angle, and sputter/span yield, which are discussed in the context of a framework for V B − generation by high‐resolution focused ion beam (FIB) systems.

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of “flat” optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down‐conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field are highlighted.