Long coherence time of spin qubits in $^{12}$C enriched polycrystalline   CVD diamond
  Single defects in diamond and especially negatively charged nitrogen vacancy (NV) centers are very promising quantum systems with wide applications in physics and biology. It was shown that their coherence properties can be strongly improved by growing ultrapure diamond with low concentration of parasitic spins associated with nitrogen electron spins and nuclear spins related to $^{13}$C carbon isotope. Here we report a high quality $^{12}$C-enriched polycrystalline CVD diamond material with properties comparable with single crystals. We find single NVs in the grains of this material, which show extremely long electron spin coherence time $T_2 > 2\,ms$. 
K. D. Jahnke, B. Naydenov, T. Teraji, S.Koizumi, T. Umeda, J. Isoya,   and F. Jelezko
quant-ph cond-mat.mes-hall