  The nitrogen-vacancy (NV) center in diamond is of high importance in quantum
information processing applications which relies on the efficient optical
polarization of its electron spin. However, the full optical spinpolarization
process, in particular, the intersystem crossing between the shelving singlet
state and the ground state triplet, is not understood. Here we develop a
detailed theory on this process which involves strong electron-phonon couplings
and correlation of electronic states that can be described as a combination of
pseudo and dynamic Jahn-Teller interactions together with spin-orbit
interaction. Our theory provides an explanation for the asymmetry between the
observed emission and absorption spectra of the singlet states. We apply
density functional theory to calculate the intersystem crossing rates and the
optical spectra of the singlets and we obtain good agreement with the
experimental data. As NV center serves as a template for other
solid-state-defect quantum bit systems, our theory provides a toolkit to study
them that might help optimize their quantum bit operation.
