Coherence of nitrogen-vacancy electronic spin ensembles in diamond

We present an experimental and theoretical study of electronic spin decoherence in ensembles of nitrogen-vacancy (NV) color centers in bulk high-purity diamond at room temperature. Under appropriate conditions, we find ensemble NV spin coherence times (T₂) comparable to that of single NV with T₂>600 for a sample with natural abundance of ^13C and paramagnetic impurity density 10^15 cm^-3. We also observe a sharp decrease in the coherence time with misalignment of the static magnetic field relative to the NV electronic spin axis, consistent with theoretical modeling of NV coupling to a ^13C nuclear-spin bath. The long coherence times and increased signal-to-noise provided by room-temperature NV ensembles will aid many applications of NV centers in precision magnetometry and quantum information.