  Optically accessible solid state defect spins are primary platform for
quantum information processing where tight control of the electron spin and
ancilla nuclear spins is pivotal for the operation. We demonstrate on the
exemplary nitrogen-vacancy (NV) color center in diamond by means of a combined
group theory and density functional theory study that spin-phonon relaxation
rate of the nitrogen nuclear spin is with several orders of magnitude enhanced
by the strong electron-phonon coupling in the optical excited state of the
defect. The mechanism is common to other solid state defect spins sharing
similar optical excited states with that of the NV center.
