Fig. 5.

Optomagnetic properties of the NV^- center. a One carbon atom is replaced by a nitrogen next to a vacancy in the diamond lattice. b Simplified energy diagram of an NV^- center. After excitation with a green laser, the NV^- center emits red photons. If the electron is in the m _s = ±1 state, there is also an alternative way to the ground state over a dark state. As a result, fewer red photons are emitted and decreased fluorescence is observed. If a microwave is applied whose energy equals the difference between the energies of the two states (2.88 GHz at zero field), the spins flip into the m _s = ±1 state. This effect can be observed as a drop in fluorescence (bottom curve in c). In presence of an external magnetic field the m _s = ±1 states are no longer equal in energy and thus split into two lines (top three curves in c). The difference is proportional to the field (Zeemann splitting), and the magnetic field can be determined. (a Reprinted with permission from [27]; c reprinted with permission from [164])