  Electronic defects in semiconductors form the basis for many emerging quantum
technologies. Understanding defect spin and charge dynamics in solid state
platforms is crucial to developing these building blocks, but many defect
centers are difficult to access at the single-particle level due to the lack of
sensitive readout techniques. A method for probing optically inactive spin
defects would reveal semiconductor physics at the atomic scale and advance the
study of new quantum systems. We exploit the intrinsic correlation between the
charge and spin states of defect centers to measure defect charge populations
and dynamics through the steady-state spin population, read-out at the
single-defect level with a nearby optically active qubit. We directly measure
ionization and charge relaxation of single dark defects in diamond, effects we
do not have access to with traditional coherence-based quantum sensing. These
spin resonance-based methods generalize to other solid state defect systems in
relevant materials.
