A nitrogen-vacancy (NV) center is a point defect in diamond — a missing carbon atom next to a substituted nitrogen atom — whose electron spin can be initialised and read out with light at room temperature. A green laser pumps the defect into a bright, spin-polarised state; the red fluorescence it emits depends on the spin's energy sublevels.
With no magnetic field the two spin sublevels (ms=±1) are degenerate at the zero-field splitting D≈2870 MHz, so sweeping a microwave field across D drives the spin out of its bright state and produces one fluorescence dip — optically detected magnetic resonance (ODMR).
f± = D ± γ·B
γ ≈ 28 MHz/mT (NV electron gyromagnetic ratio)
Δf = f+ − f− = 2·γ·B → B = Δf / (2γ)
- External field B — an applied magnetic field Zeeman-splits the resonance into two dips, f± = D ± γB, symmetric about D.
- MW frequency — scans the drive across the resonance; the defect dims each time the drive matches a spin transition.
- MW power — stronger driving deepens and broadens each dip (power broadening), trading resolution for signal.
- Measuring — drag the two dashed cursors onto the two dips in the live spectrum; their frequency separation gives Δf, and B = Δf/(2γ) recovers the field without ever touching the B slider directly — the same principle an NV magnetometer uses on an unknown sample.
Real-world relevance: NV magnetometers read nanoscale magnetic fields — from single spins to current-carrying wires and rock magnetism — at room temperature, unlike SQUIDs which need cryogenic cooling.