  Single charge detection with nanoscale spatial resolution in ambient
conditions is a current frontier in metrology that has diverse
interdisciplinary applications. Here, such single charge detection is
demonstrated using two nitrogen-vacancy (NV) centers in diamond. One NV center
is employed as a sensitive electrometer to detect the change in electric field
created by the displacement of a single electron resulting from the optical
switching of the other NV center between its neutral (NV$^0$) and negative
(NV$^-$) charge states. As a consequence, our measurements also provide direct
insight into the charge dynamics inside the material.
