  Charge control of color centers in semiconductors promises opportunities for
novel forms of sensing and quantum information processing. Here, we articulate
confocal fluorescence microscopy and magnetic resonance protocols to induce and
probe charge transport between discrete sets of engineered nitrogen-vacancy
(NV) centers in diamond, down to the level of individual defects. In our
experiments, a "source" NV undergoes optically-driven cycles of ionization and
recombination to produce a stream of photo-generated carriers, one of which we
subsequently capture via a "target" NV several micrometers away. We use a
spin-to-charge conversion scheme to encode the spin state of the source color
center into the charge state of the target, in the process allowing us to set
an upper bound to carrier injection from other background defects. We attribute
our observations to the action of unscreened Coulomb potentials producing giant
carrier capture cross-sections, orders of magnitude greater than those
typically attained in ensemble measurements. Besides their fundamental
interest, these results open intriguing prospects in the use of free carriers
as a quantum bus to mediate effective interactions between paramagnetic defects
in a solid-state chip.
