  We demonstrate optical readout of ensembles of nitrogen-vacancy(NV) center
spins in a bulk diamond sample via spin-to-charge conversion. A high power 594
nm laser is utilized to selectively ionize these paramagnetic defects in the
spin state with a contrast of up to 12%. In comparison with the conventional
520 nm spin readout, spin-to-charge-conversion-based readout provides higher
signal-to-noise ratio, with tenfold sensing measurement speedup for millisecond
long pulse sequences. This level of performance was achieved for an NV-
ionization of only 25%, limited by the ionization and readout laser powers.
These observations pave the way to a range of high-sensitivity metrology
applications where the use of NV- ensembles in bulk diamond has proven useful,
including sensing and imaging of target materials overlaid on the diamond
surface.
