Long spin coherence and relaxation times in nanodiamonds milled from polycrystalline 12 C diamond

The negatively charged nitrogen-vacancy center (NVC) in diamond has been utilized in a wide variety of sensing applications. The long spin coherence and relaxation times (${T}_{2}^{\ensuremath{\ast}}$, ${T}_{2}$ and ${T}_{1}$) of the center at room temperature are crucial to this, as they often limit sensitivity. Using NVCs in nanodiamonds allows for operations in environments inaccessible to bulk diamond, such as intracellular sensing. We report long spin coherence and relaxation times at room temperature for single NVCs in isotopically purified polycrystalline ball-milled nanodiamonds. Using a spin-locking pulse sequence, we observe spin coherence times, ${T}_{2}$, up to $786\ifmmode\pm\else\textpm\fi{}200\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$. We also measure ${T}_{2}^{\ensuremath{\ast}}$ times up to $2.06\ifmmode\pm\else\textpm\fi{}0.24\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$ and ${T}_{1}$ times up to $4.32\ifmmode\pm\else\textpm\fi{}0.60\phantom{\rule{0.2em}{0ex}}\mathrm{ms}$. These results represent improvements on the leading literature values for ${T}_{2}$ and ${T}_{1}$ times in nano- and microdiamond and for ${T}_{2}^{\ensuremath{\ast}}$ times in nanodiamond. Scanning-electron-microscopy and atomic force microscopy measurements show that the diamond containing the NVC with the longest ${T}_{1}$ time is smaller than 100 nm. EPR measurements give an $\mathrm{N}$${}_{s}^{0}$ concentration of $0.15\ifmmode\pm\else\textpm\fi{}0.02$ parts per million for the nanodiamond sample.