If a magnetic field is oriented along the defect axis it leads to Zeeman splitting separating the m = +1 from the m = -1 states. This technique is used to lift the degeneracy and use only two of the spin states (usually the ground states with m = -1 and m = 0) as a qubit. Population can then be transferred between them using a microwave field. In the specific instance that the magnetic field reaches 1027 G (or 508 G) then the m = –1 and m = 0 states in the ground (or excited) state become equal in energy (Ground/Excited State Level Anticrossing). The following strong interaction results in so-called spin polarization, which strongly affects the intensity of optical absorption and luminescence transitions involving those states.
Importantly, this splitting can be modulated by applying an external electric field, in a similar fashion to the magnetic field mechanism outlined above, though the physics of the splitting is somewhat more complex. Nevertheless, an important practical outcome is that the intensity and position of the luminescence lines is modulated. Strain has a similar effect on the NV center as electric fields.
There is an additional splitting of the m = ±1 energy levels, which originates from the hyperfine interaction between surrounding nuclear spins and the NV center. These nuclear spins create magnetic and electric fields of their own leading to further distortions of the NV spectrum (see nuclear Zeeman and quadrupole interaction). Also the NV center's own spin–orbit interaction and orbital degeneracy leads to additional level splitting in the excited E state.
Temperature and pressure directly influence the zero-field term of the NV center leading to a shift between the ground and excited state levels.
The Hamiltonian, a quantum mechanical equation describing the dynamics of a system, which shows the influence of different factors on the NV center can be found below.
Although it can be challenging, all of these effects are measurable, making the NV center a perfect candidate for a quantum sensor.