Spin relaxometry using solid-state spin defects, such as the diamond nitrogen-vacancy (NV) center, probes dynamical processes by measuring how environmental fluctuations enhance the spin relaxation rate. In the weak-coupling limit, relaxation rates sample the transverse magnetic-noise power spectral density through a sensor-specific filter function, turning the defect into a local, frequency-selective noise spectrometer. This review bridges theory and experiment, clarifying how measured relaxation rates map onto noise spectra and how near-field geometry shapes the response. We highlight representative applications across condensed-matter physics, chemical and biological sensing, and relaxometry-based magnetic-resonance spectroscopy. We conclude with emerging opportunities and key challenges.
@article{arxiv.2602.01521,
title = {Spin Relaxometry with Solid-State Defects: Theory, Platforms, and Applications},
author = {Ruotian Gong and Alex L. Melendez and Guanghui He and Zhongyuan Liu and Chong Zu and Huan Zhao},
journal= {arXiv preprint arXiv:2602.01521},
year = {2026}
}