  Solid-state spin defects are attractive candidates for developing quantum
sensors and simulators. The spin and charge degrees of freedom in large defect
ensembles are a promising platform to explore complex many-body dynamics and
the emergence of quantum hydrodynamics. However, many interesting properties
can be revealed only upon changes in the density of defects, which instead is
usually fixed in material systems. Increasing the interaction strength by
creating denser defect ensembles also brings more decoherence. Ideally one
would like to control the spin concentration at will, while keeping fixed
decoherence effects. Here we show that by exploiting charge transport, we can
take some first steps in this direction, while at the same time characterizing
charge transport and its capture by defects. By exploiting the cycling process
of ionization and recombination of NV centers in diamonds, we pump electrons
from the valence band to the conduction band. These charges are then
transported to modulate the spin concentration by changing the charge state of
material defects. By developing a wide-field imaging setup integrated with a
fast single photon detector array, we achieve a direct and efficient
characterization of the charge redistribution process by measuring the complete
spectrum of the spin bath with micrometer-scale spatial resolution. We
demonstrate the concentration increase of the dominant spin defects by a factor
of 2 while keeping the $T_2$ of the NV center, which also provides a potential
experimental demonstration of the suppression of spin flip-flops via hyperfine
interactions. Our work paves the way to studying many-body dynamics with
temporally and spatially tunable interaction strengths in hybrid charge-spin
systems.
