The negatively charged N-V state (NV-) forms a spin triplet in the ground state. The intrinsic lattice structure causes a zero field split of the spin sublevels in the triplet ground state (3A), such that the ms = 0 is the lowest and ms = ±1 are 2.87 GHz above it.
If an external magnetic field is applied along the N-V symmetry axis (the axis on which both the nitrogen and the vacancy lie), this will result in the splitting of the ±1 sublevels. In effect, what we have is an isolated spin-1/2 system, which, fortunately, can be initialized to a spin-polarized state ms = 0 optically by a green (532 nm) laser and read-out via spin-dependent fluorescence in the red and near-infrared (640 - 800 nm) range. Microwave manipulation of the spin state offers us a complete control over the N-V center's spin state in the following way: for a given spin population of the ground state we observe a certain level of fluorescence. When the microwave frequency is at resonance with the |ms = 0> → |ms = ± 1> transition in the ground state, together with optical pumping of the ground state to the excited state, a dip in that fluorescence will be observed, which is due to a spin-selective intersystem crossing decay of the excited |ms = ± 1> states to the |ms = 0> through a metastable singlet state. In terms of state lifetime, the singlet state's lifetime is one order of magnitude larger than that of the excited triplet state, which also makes it easier to observe the dip in fluorescence. In other words, microwave on-resonance destroys the optically pumped |ms = 0> state and results in a decrease in fluorescence.