  The sensitivity of magnetic and electric field sensors based on
nitrogen-vacancy (NV) center in diamond strongly depends on the available
concentration of NV and their coherence properties. Achieving high coherence
times simultaneously with high concentration is a challenging experimental
task. Here, we demonstrate that by using a high-pressure, high-temperature
growing technique, one can achieve nearly maximally possible effective
coherence T2* times, limited only by carbon nuclear spins at low nitrogen
concentrations or nitrogen electron spin at high nitrogen concentrations.
Hahn-echo T2 coherence times were also investigated and found to demonstrate
reasonable values. Thus, the high-pressure, high-temperature technique is a
strong contender to the popular chemical vapor deposition method in the
development of high-sensitivity, diamond-based sensors.
