Nonlinear optical pumping to a diamond NV center dark state

The photodynamics of diamond nitrogen-vacancy (NV) centers limits their performance in many quantum technologies. Quenching of photoluminescence, which degrades NV readout, is commonly ascribed to a dark state that is not fully understood. Using a nanoscale cavity to generate intense infrared fields that quench NV emission nonlinearly with field intensity, we show that the dark state is accessed by two-photon pumping into the \(^4A_2\) quartet state of the neutrally charged NV (NV\(^0\)). We constrain this state's energy relative to the NV\(^0\) ground-state (\(^2E\)) to \(>0.58\)\,eV and the recombination energy threshold to the NV\(^-\) ground state (\(^3A_2\)) to \(