  NV centers in diamond are generally recognized as highly promising as
indefinitely stable highly efficient single-photon sources. We report an
experimental quantification of the brightness, radiative decay rate,
nonradiative decay rate and quantum efficiency of single NV centers in diamond
nanocrystals. Our experiments show that the commonly observed large spread in
fluorescence decay rates of NV centers in nanodiamond is inconsistent with the
common explanation of large nanophotonic mode-density variations in the
ultra-small high-index crystals at near-unity quantum efficiency. We report
that NV centers in 25 nm nanocrystals are essentially insensitive to local
density of states (LDOS) variations that we induce at a dielectric interface by
using liquids to vary the refractive index, and propose that quantum
efficiencies in such nanocrystals are widely distributed between 0% and 20%.
For single NV centers in larger 100 nm nanocrystals, we show that decay rate
changes can be reversibly induced by nanomechanically approaching a mirror to
change the LDOS. Using this scanning mirror method, for the first time we
report calibrated quantum efficiencies of NV centers, and show that different
but nominally identical nanocrystals have widely distributed quantum
efficiencies between 10% and 90%. Our measurements imply that nanocrystals that
are to be assembled into hybrid photonic structures for cavity QED should first
be individually screened to assess fluorescence properties in detail.
