Figure 1 - Level anticrossing and spin–spin interaction.

a, The energy levels E₀ and E₁ cross at a particular value of magnetic field B. A weak interaction connects the two states, forming new states E₊ and E_- that do not cross. Light emission is sensitive to the spin states and reveals the weak interaction. LACs occur for both the ground and optically excited states of the N–V centre. b, The spin (red) of the optically active N–V centre interacts with a nearby nitrogen spin (green) through a second, weak dipolar coupling, which 'reveals' the dark spin.