  The application of color centers in wide-bandgap semiconductors to nanoscale
sensing and quantum information processing largely rests on our knowledge of
the surrounding crystalline lattice, often obscured by the countless classes of
point defects the material can host. Here we monitor the fluorescence from a
negatively charged nitrogen-vacancy (NV-) center in diamond as we illuminate
its vicinity. Cyclic charge state conversion of neighboring point defects
sensitive to the excitation beam leads to a position-dependent stream of
photo-generated carriers whose capture by the probe NV- leads to a fluorescence
change. This "charge-to-photon" conversion scheme allows us to image other
individual point defects surrounding the probe NV, including non-fluorescent
"single-charge emitters" that would otherwise remain unnoticed. Given the
ubiquity of color center photo-chromism, this strategy may likely find
extensions to material systems other than diamond.
