Isotopic identification of engineered nitrogen-vacancy spin qubits in   ultrapure diamond
  Nitrogen impurities help to stabilize the negatively-charged-state of NV$^-$ in diamond, whereas magnetic fluctuations from nitrogen spins lead to decoherence of NV$^-$ qubits. It is not known what donor concentration optimizes these conflicting requirements. Here we used 10-MeV $^{15}$N$^{3+}$ ion implantation to create NV$^-$ in ultrapure diamond. Optically detected magnetic resonance of single centers revealed a high creation yield of $40\pm3$% from $^{15}$N$^{3+}$ ions and an additional yield of $56\pm3$% from $^{14}$N impurities. High-temperature anneal was used to reduce residual defects, and charge stable NV$^-$, even in a dilute $^{14}$N impurity concentration of 0.06 ppb were created with long coherence times. 
T. Yamamoto, S. Onoda, T. Ohshima, T. Teraji, K. Watanabe, S. Koizumi,   T. Umeda, L. P. McGuinness, C. M\"uller, B. Naydeno...
cond-mat.mes-hall cond-mat.mtrl-sci