A Coherence-Protection Scheme for Quantum Sensors Based on Ultra-Shallow Single Nitrogen-Vacancy Centers in Diamond

Recent advances in the engineering of diamond surfaces make it possible to stabilize the charge state of 7-30 nanometers deep nitrogen-vacancy (NV) quantum sensors in diamond and to remove the charge noise at the surface principally. However, it is still a challenge to simultaneously increase the action volume of the quantum sensor by placing NV centers 0.5-2 nanometers deep and to maintain their favorable spin coherence properties which are limited by the magnetic noise from the fluctuating nuclear spins of the surface termination of diamond. Here we show by means of first principles simulations that leveraging the interplay of the surface-induced strain and small constant magnetic fields, the spin coherence times of the ultra-shallow 1-nanometer deep NV center can be significantly enhanced near the spin-phonon limited regime at room temperature in $^{12}$C enriched diamonds. We demonstrate that our protocol is beneficial to $\sim$10-nanometers deep NV centers in natural diamond too where the variable coherence properties of the center to the direction of the small constant magnetic fields establish vector magnetometry at the nanoscale.