Long spin coherence times of nitrogen vacancy centers in milled nanodiamonds

Nanodiamonds containing negatively charged nitrogen vacancy centers (NV-) have applications as localized sensors in biological materials and have been proposed as a platform to probe the macroscopic limits of spatial superposition and the quantum nature of gravity. A key requirement for these applications is to obtain nanodiamonds containing NV- with long spin coherence times. Using milling to fabricate nanodiamonds processes the full 3D volume of the bulk material at once, unlike etching pillars, but has, up to now, limited NV- spin coherence times. Here, we use natural isotopic abundance nanodiamonds produced by Si3N4 ball milling of chemical vapor deposition grown bulk diamond with an average single substitutional nitrogen concentration of 121 ppb. We show that the electron spin coherence times of NV- centers in these nanodiamonds can exceed 400 mu s at room temperature with dynamical decoupling. Scanning electron microscopy provides images of the specific nanodiamonds containing NV- for which a spin coherence time was measured.