The NV center has a ground-state triplet (A), an excited-state triplet (E) and two intermediate-state singlets (A and E). Both A and E contain m = ±1 spin states, in which the two electron spins are aligned (either up, such that m = +1 or down, such that m = -1), and an m = 0 spin state where the electron spins are antiparallel. Due to the magnetic interaction, the energy of the m = ±1 states is higher than that of the m = 0 state. A and E only contain a spin state singlet each with m = 0.
If an external magnetic field is applied along the defect axis (the axis which aligns with the nitrogen atom and the vacancy) of the NV center, it does not affect the m = 0 states, but it splits the m = ±1 levels (Zeeman effect). Similarly the following other properties of the environment influence the energy level diagram :
# Amplitude and orientation of a static magnetic field splits the m = ±1 levels in the ground and excited states.
# Amplitude and orientation of elastic (strain) or electric fields have a much smaller but also more complex effects on the different levels.
# Continuous-wave microwave radiation (applied in resonance with the transition between m = 0 and (one of the) m = ±1 states) changes the population of the sublevels within the ground and excited state.
# A tunable laser can selectively excite certain sublevels of the ground and excited states.
# Surrounding spins and spin–orbit interaction will modulate the magnetic field experienced by the NV center.
# Temperature and pressure affect different parts of the spectrum including the shift between ground and excited states.
The above-described energy structure is by no means exceptional for a defect in diamond or other semiconductor.  It was not this structure alone, but a combination of several favorable factors (previous knowledge, easy production, biocompatibility, simple initialisation, use at room temperature etc.) which suggested the use of the NV center as a qubit and quantum sensor.