Surface Coupling of NV Centers over Nanoscale Lengths

Shallow nitrogen-vacancy (NV^-) centers in diamond are among the most promising quantum sensors, offering high sensitivity and nanoscale spatial resolution. These systems are, however, prone to decoherence due to coupling with surface states. Here, we study subsurface NV^- centers embedded into large diamond slabs (8 nm) using various surface orientations (100 and 111) and terminations (hydrogen and nitrogen terminators). Our results show how dynamical charge fluctuations near the surface influence defect stability. We find that the (100) N-terminated surface introduces strong surface-state instabilities, while the (111) N-terminated surface provides a more favorable configuration. Many-body calculations (within the GW approximation) reveal that defects placed shallower than ∼4 nm are prone to surface-induced ionization. These findings establish an accurate theoretical limit on the minimum depth required for stable NV^- centers, guiding the design of NV^- based quantum sensors.