The nitrogen-vacancy (NV) center is a point defect in diamond — a nitrogen atom next to a missing carbon atom (a vacancy). Its ground state is an electron-spin triplet (S = 1) with a zero-field splitting between the ms = 0 and ms = ±1 sublevels:
D ≈ 2.870 GHz (zero-field splitting)
A green (532 nm) laser optically pumps the NV center; spin-dependent intersystem crossing makes the ms = 0 state fluoresce brighter (red, ~637–800 nm) than ms = ±1. Sweeping a microwave field across D drives the 0 → ±1 transition and dips the fluorescence — this is Optically Detected Magnetic Resonance (ODMR).
An external magnetic field B along the NV axis Zeeman-splits the ±1 sublevels into two separate dips:
f± = D ± γB
γ/2π ≈ 28.0 MHz/mT (electron gyromagnetic ratio)
Δf = f+ − f− = 2γB → B = Δf / (2γ)
Reading the two dip positions therefore measures the local magnetic field with nanoscale spatial resolution and no cryogenics required. This is the working principle of diamond quantum magnetometers used in biomedicine: detecting magnetically-labelled biomarkers, imaging free-radical concentrations in living cells, and sensing the tiny magnetic fields produced by firing neurons.
- B field — sets the true splitting; the sensor "discovers" it from where the dips land.
- Sweep speed — how fast the microwave frequency scans across the resonance each cycle.
- Laser power — raises fluorescence brightness and dip contrast (with a realistic saturation limit).
- Temperature — higher phonon occupation shortens spin coherence (T₂), broadening the dips.