Quartet state of the neutral nitrogen-vacancy center in diamond and optimized spin readout under near-infrared two-photon excitation

Photoionization of the excited nitrogen-vacancy center (NV-) in diamond leaves the neutral defect in a metastable quartet state of unknown energy. Using two-photon optically detected magnetic resonance with a tunable femtosecond laser, we resolve its onset in the spin contrast at 1.23 +/- 0.02 eV and, with the ground-state threshold from the same setup, place the quartet 0.54 +/- 0.04 eV above the neutral ground state. The threshold sets the optimum excitation for spin readout at 1060-1075 nm, in the near-infrared tissue window, with 26-37% higher shot-noise-limited sensitivity than at 1030-1040 nm.