  The advancement of quantum optical science and technology with solid-state
emitters such as nitrogen-vacancy (NV) centers in diamond critically relies on
the coherence of the emitters' optical transitions. A widely employed strategy
to create NV centers at precisely controlled locations is nitrogen ion
implantation followed by a high-temperature annealing process. We report on
experimental data directly correlating the NV center optical coherence to the
origin of the nitrogen atom. These studies reveal low-strain,
narrow-optical-linewidth ($<500$ MHz) NV centers formed from
naturally-occurring $^{14}$N atoms. In contrast, NV centers formed from
implanted $^{15}$N atoms exhibit significantly broadened optical transitions
($>1$ GHz) and higher strain. The data show that the poor optical coherence of
the NV centers formed from implanted nitrogen is not due to an intrinsic effect
related to the diamond or isotope. These results have immediate implications
for the positioning accuracy of current NV center creation protocols and point
to the need to further investigate the influence of lattice damage on the
coherence of NV centers from implanted ions.
