Defects in solids for quantum technologies

Point defects introducing deep levels in the bandgap of semiconductors were long regarded merely as imperfections to be eliminated, degrading the performance of electronic devices. Over the past three decades, however, point defects in wide-bandgap semiconductors have become one of the most diverse resources for quantum technology, providing atomic-scale spin qubits, spin-photon interfaces, and quantum sensors that operate not just at cryogenic temperatures but also well above room temperature. By now, the field has passed the first stage, in which a handful of celebrated centers, such as the nitrogen-vacancy center in diamond and the silicon vacancy and divacancy in 4H-SiC, dominated developments, and has become a vibrant, rapidly evolving, application-driven research field. Today, the activity of many researchers is directed toward finding new promising color centers, their fabrication, characterization, and control. The central questions here are which of the host materials and which of the enormous number of conceivable defects can meet the requirements of a given application, how such defects can be identified unambiguously, how they can be created on demand in a reproducible manner, and how their properties can be controlled. This Guest Editorial introduces the special collection that samples the field along these lines. We place the collection in context, summarize the contributions grouped by the problem they address, and comment on where the community is heading.