  Prototypes of quantum impurities (QI), such as NV and SiV centers in diamond,
have been recently growing in popularity due to their minimally invasive and
high-resolution magnetic field sensing. Here, we focus on quantum-impurity
relaxometry as a method to probe collective excitations in magnetic insulators.
We develop a general framework that relates the experimentally-measurable
quantum-impurity relaxation rates to the properties of a magnetic system via
the noise emitted by the latter. We suggest that, when the quantum-impurity
frequency lies within the spin-wave gap, quantum-impurity relaxometry can be
effectively deployed to detect signatures of the coherent spin dynamics, such
as magnon condensation, both in ferromagnetic and antiferromagnetic systems, as
well as open prospects to nonintrusively probe spin-wave transport regimes in
magnetic insulators.
