  The nitrogen vacancy center in diamond in its negative charge state is a
promising candidate for quantum optic experiments that require single photon
emitters. Important benefits of the NV center are its high brightness and
photo-stability, even at room temperature. Engineering the emission properties
of NV centers with optical resonators is a widely followed approach to meet the
requirements for quantum technological applications, but the effect on
non-radiative decay paths is yet to be understood. Here we report on modifying
the internal quantum efficiency of a single NV center in a nanodiamond coupled
to a 1D photonic crystal cavity. We assess the Purcell enhancement via three
independent measurement techniques and perform autocorrelation measurements at
elevated excitation powers in confocal microscopy. Employing a three-level
model allows us to extract the setup efficiency, individual transition rates
and thus the internal quantum efficiency of our system. Combining our results,
we find that the enhancement of the radiative decay rate via the Purcell effect
results in an internal quantum efficiency of 90 % for cavity-coupled NV
centers. Our findings will facilitate the realization of nano-scale single
photon sources with near-unity internal quantum efficiencies operating at high
repetition rates.
