Abstract
For the N-V centre in type Ib diamond the optical detection of spin coherence in the 3A state is reported. The 3A-state lifetime is studied as a function of the light intensity used for the optical excitation of the N-V centre by means of spin-locking experiments. The shortening of the lifetime at higher excitation intensities provides evidence for the previously proposed idea that the 3A state of the N-V centre is the ground state.