  The long spin coherence times in ambient conditions of color centers in
solids, such as nitrogen-vacancy (NV$^{-}$) centers in diamond, make these
systems attractive candidates for quantum sensing. Quantum sensing provides
remarkable sensitivity at room temperature to very small external
perturbations, including magnetic fields, electric fields, and temperature
changes. A photoreceptive molecule, such as those involved in vision, changes
its charge state or conformation in response to the absorption of a single
photon. We show the resulting change in local electric field modifies the
properties of a nearby quantum coherent spin center in a detectable fashion.
Using the formalism of positive operator values measurements (POVMs), we
analyze the photo-excited electric dipole field and, by extension, the arrival
of a photon based on a measured readout, using a fluorescence cycle, from the
spin center. We determine the jitter time of photon arrival and the probability
of measurement errors. We predict that configuring multiple independent spin
sensors around the photoreceptive molecule would dramatically suppresses the
measurement error.
