  Fluorescent nanodiamonds (NDs) are crystal defect-based light-emitting
nanoparticles that can be applied to quantum information science and quantum
sensing. Of particular interest are nitrogen vacancy (NV) centers that allow
optical access to their coherently controllable electron spin systems, leading
to spin-photon quantum devices and nanoscale sensing of various physical
parameters. However, the NV spin coherence time (T2) in NDs has been limited to
one or two orders of magnitude shorter than those in bulk diamonds owing to the
complicated surface effect that decoheres the NV spin systems. Here, we study
the relation between the surface properties and T2 of single NV centers in NDs
by systematically analyzing the effect of surface oxidation. We apply aerobic
and acidic oxidation methods with various heating temperatures and processing
times and find that aerobic oxidation most effectively oxidizes the surface and
extends T2 by a factor of 1.6 to the original NDs. The ND-size dependence of T2
clearly shows that the surface oxidation removes a constant decoherence
contribution irrespective of the ND size and that a new surface derived
decoherence source emerges when the ND size reduces below 50 nm. The present
results provide quantitative information on the decoherence sources of NV spin
systems of NDs and will enable a strategic surface modification for better spin
manipulations of NV centers in the context of quantum information science and
nanoscale quantum sensing.
