  The negatively charged Nitrogen-Vacancy (NV-) center in diamond is one of the
most versatile and robust quantum sensors suitable for quantum technologies,
including magnetic field and temperature sensors. For precision sensing
applications, densely packed NV- centers within a small volume are preferable
due to benefiting from 1/N^1/2 sensitivity enhancement (N is the number of
sensing NV centers) and efficient excitation of NV centers. However, methods
for quickly and efficiently forming high concentrations of NV- centers are in
development stage. We report an efficient, low-cost method for creating
high-density NV- centers production from a relatively low nitrogen
concentration based on high-energy photons from Ar+ plasma. This study was done
on type-IIa, single crystal, CVD-grown diamond substrates with an as-grown
nitrogen concentration of 1 ppm. We estimate an NV- density of ~ 0.57 ppm (57%)
distributed homogeneously over 200 um deep from the diamond surface facing the
plasma source based on optically detected magnetic resonance and fluorescence
confocal microscopy measurements. The created NV-s have a spin-lattice
relaxation time (T1) of 5 ms and a spin-spin coherence time (T2) of 4 us. We
measure a DC magnetic field sensitivity of ~ 104 nT Hz^-1/2, an AC magnetic
field sensitivity of ~ 0.12 pT Hz^-1/2, and demonstrate real-time magnetic
field sensing at a rate over 10 mT s-1 using an active sample volume of 0.2
um3.
