  The nitrogen vacancy (NV) center in diamond exhibits spin-dependent
fluorescence and long spin coherence times under ambient conditions, enabling
applications in quantum information processing and sensing. NV centers near the
surface can have strong interactions with external materials and spins,
enabling new forms of nanoscale spectroscopy. However, NV spin coherence
degrades within 100 nanometers of the surface, suggesting that diamond surfaces
are plagued with ubiquitous defects. Prior work on characterizing near-surface
noise has primarily relied on using NV centers themselves as probes; while this
has the advantage of exquisite sensitivity, it provides only indirect
information about the origin of the noise. Here we demonstrate that surface
spectroscopy methods and single spin measurements can be used as complementary
diagnostics to understand sources of noise. We find that surface morphology is
crucial for realizing reproducible chemical termination, and use these insights
to achieve a highly ordered, oxygen-terminated surface with suppressed noise.
We observe NV centers within 10 nm of the surface with coherence times extended
by an order of magnitude.
