Fig. 7.

NV level schemes for simulation. a Level scheme showing the relevant parts of negative and the neutral charge state of the NV center. In NV⁻, the two spin sub-levels show different fluorescence intensities, as well as a spin polarization by an inter system crossing (ISC) into the metastable singlet state (here drawn as a “black box”). Ionization occurs as an effective two photon process from the NV⁻ ground state³⁷, ending up in the uncharged NV⁰ with a high-spin dissipation rate. Recombination also occurs as a two photon process. For the simulation of the pure NV⁻ and NV⁰ case, only the respective ground states are modeled. b The reduced level scheme for the simulation of the continuous excitation case. The NV⁻ excited state is omitted due to its short lifetime. The spin-dependent ISC is realized by spin-dependent excitation rates. Ionization to the NV⁰ charge state is implemented by immediate target spin decoherence