  We utilize nonlinear absorption to design all-optical protocols that improve
both charge state initialization and spin readout for the nitrogen-vacancy (NV)
center in diamond. Non-monotonic variations in the equilibrium charge state as
a function of visible and near-infrared (NIR) optical power are attributed to
competing multiphoton absorption processes. In certain regimes, multicolor
illumination enhances the steady-state population of the NV's negative charge
state above 90%. At higher NIR intensities, selective ionization of the singlet
manifold facilitates a protocol for spin-to-charge conversion that dramatically
enhances the spin readout fidelity. We demonstrate a 6-fold increase in the
signal-to-noise ratio for single-shot spin measurements and predict an
orders-of-magnitude experimental speedup over traditional methods for emerging
applications in magnetometry and quantum information science using NV spins.
