Quantum sensing has emerged as a prominent field in quantum technology, with the Nitrogen-Vacancy (NV) center in diamond standing out as an exceptionally versatile platform. This atomic-scale defect combines remarkable optical and spin properties, enabling highly sensitive magnetic-field detection and nanoscale imaging. We investigated how external fields influence the photophysical properties of individual Nitrogen-Vacancy (NV) centers by measuring their photoluminescence as a function of magnetic field across a wide temperature range. These measurements revealed strong dependencies on both magnetic field and strain, which lead to significant changes in optical spin contrast, a key factor for sensing performance. Combined with an extended theoretical model, the results offer new insight into the excited-state structure of the NV center and the role of phonon-induced orbital averaging at elevated temperatures. Our approach provides an alternative to conventional optical spectroscopy and can be applied to other optically active quantum systems. It also opens avenues for new sensing modalities, such as highly sensitive electric-field measurements at cryogenic temperatures. These findings deepen the understanding of NV centers at low temperatures and support their continued development in quantum sensing and condensed-matter research.

Dr. Jodok Happacher obtained his bachelor's and master's degrees in physics at the Ludwig-Maximilian-University Munich, where he worked with Prof. Lode Pollet and Prof. Alexander Högele. He then moved to the University of Basel for his doctoral studies in the group of Prof. Patrick Maletinsky. His thesis, "Low Temperature Quantum Sensing with Single Nitrogen-Vacancy Centers in Diamond", advanced the understanding of NV center photophysics from cryogenic to room temperature and supported progress in quantum sensing and imaging at low temperatures. For this work, he received one of this year's Swiss Physical Society Awards for young physicists, recognizing his contributions in the field of sensing, detection, and monitoring. He is now a postdoctoral researcher in the Department of Physics at Basel, developing quantum sensing approaches based on color centers in diamond and in two dimensional materials at low temperatures.