Observing bulk diamond spin coherence in high-purity nanodiamonds.

Nitrogen-vacancy (NV) centres in diamond are attractive for research straddling quantum information science, nanoscale magnetometry and thermometry. Whereas ultrapure bulk diamond NVs sustain the longest spin coherence times among optically accessible spins, nanodiamond NVs exhibit persistently poor spin coherence. Here we introduce high-purity nanodiamonds accommodating record-long NV coherence times, >60 μs, observed through universal dynamical decoupling. We show that the main contribution to decoherence comes from nearby nitrogen impurities rather than surface states. We protect the NV spin free precession, essential to d.c. magnetometry, by driving solely these impurities into the motional narrowing regime. This extends the NV free induction decay time from 440 ns, longer than that in type Ib bulk diamond, to 1.27 μs, which is comparable to that in type IIa (impurity-free) diamond. These properties allow the simultaneous exploitation of both high sensitivity and nanometre resolution in diamond-based emergent quantum technologies.