The fine structure of the neutral nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is a widely utilized system due to its useful quantum properties. Almost all research focuses on the negative charge state (NV^-) and comparatively little is understood about the neutral charge state (NV⁰). This is surprising as the charge state often fluctuates between NV⁰ and NV^- during measurements. There are potentially under-utilized technical applications that could take advantage of NV⁰, either by improving the performance of NV⁰ or utilizing NV^- directly. However, the fine structure of NV⁰ has not been observed. Here, we rectify this lack of knowledge by performing magnetic circular dichroism measurements that quantitatively determine the fine structure of NV⁰. The observed behavior is accurately described by spin-Hamiltonians in the ground and excited states with the ground state yielding a spin-orbit coupling of λ = 2.24 ± 0.05 GHz and a orbital g-factor of 0.0186 ± 0.0005. The reasons why this fine structure has not been previously measured are discussed and strain-broadening is concluded to be the likely reason.