Quantum-Impurity Relaxometry of Magnetization Dynamics.

Prototypes of quantum impurities, such as NV and SiV color centers in diamond, have garnered much attention due to their minimally invasive and high-resolution magnetic field and thermal sensing. Here, we investigate quantum-impurity relaxometry as a method for probing collective excitations in magnetic insulators. We develop a general framework to relate the measurable quantum-impurity relaxation rates to the intrinsic dynamic properties of a magnetic system via the noise emitted by the latter. We suggest, in particular, that the quantum-impurity relaxometry is sensitive to dynamic phase transitions, such as magnon condensation, and can be deployed to detect signatures of the associated coherent spin dynamics, both in ferromagnetic and antiferromagnetic systems. Finally, we discuss prospects to nonintrusively probe spin-transport regimes and measure the associated transport coefficients in magnetic insulators.