Ab initio theory of intersystem crossings in diamond nitrogen-vacancy qubit

Dopants in solids are promising candidates for implementations of quantum bits for quantum computing. In particular, the high-spin negatively charged nitrogen-vacancy defect (NV) in diamond has become a leading contender in solid-state quantum information processing. The initialization and readout of the spin is based on the spin-selective decay of the photo-excited electron to the ground state which is mediated by spin-orbit coupling between excited states states and phonons, i.e. intersystem crossing (ISC).
Recently, we have shown [1] that strong coupling between electrons and phonons in the excited state can naturally explain the observed multiple ISC rates in the excited state branch. On the other hand, the mechanism of the spinpolarization at the ground state branch is not understood as group theory implies forbidden ISC between the lowest energy singlet and the ground state triplet. We present a theory including electron-phonon coupling and correlated electron states from ab intio calculations that accounts for the optical spinpolarization of NV center and also explains the absorption and photoluminescence spectra of the singlets. Our finding completes the theory of the loop of the optical spinpolarization cycle.
[1] G. Thiering and A. Gali, Phys. Rev. B 96, 081115(R) (2017).