Quantum two-level systems offer attractive opportunities for sensing and imaging, especially at the nanoscale. In the almost twenty years since its inception, this idea [1] has advanced from proof of concept [2] to a mature quantum technology [3], with broad fields of applications in physics, materials engineering, life sciences, and beyond. In this talk, I will present the field’s founding principles and key engineering challenges and highlight especially rewarding applications of single spin-based quantum sensors. A particular focus will lie on new insights these sensors bring to mesoscopic condensed-matter physics, such as super-conductors [4] or novel magnetic materials. Specifically, I will discuss the use of single-spin quantum
sensors to study and engineer atomically thin “van der Waals” magnets [5-7] – an emerging class of magnetically ordered systems that combine fundamental and practical interests and which so far were notoriously hard to address due to their weak magnetization and nanoscale spin-textures. I will conclude with an outlook on future developments of quantum sensors, such as their use in studying dynamical phenomena in quantum materials [8] or their applications under extreme conditions, such asTesla-range magnetic fields, or millikelvin temperatures, where new exciting applications wait to beexplored.