  The nitrogen-vacancy (NV) center in diamond has been established as a prime
building block for quantum networks. However, scaling beyond a few network
nodes is currently limited by low spin-photon entanglement rates, resulting
from the NV center's low probability of coherent photon emission and
collection. Integration into a cavity can boost both values via the Purcell
effect, but poor optical coherence of near-surface NV centers has so far
prevented their resonant optical control, as would be required for entanglement
generation. Here, we overcome this challenge, and demonstrate resonant
addressing of individual, fiber-cavity-coupled NV centers, and collection of
their Purcell-enhanced coherent photon emission. Utilizing off-resonant and
resonant addressing protocols, we extract Purcell factors of up to 4,
consistent with a detailed theoretical model. This model predicts that the
probability of coherent photon detection per optical excitation can be
increased to 10% for realistic parameters - an improvement over state-of-the
art solid immersion lens collection systems by two orders of magnitude. The
resonant operation of an improved optical interface for single coherent quantum
emitters in a closed-cycle cryogenic system at T $\sim$ 4 K is an important
result towards extensive quantum networks with long coherence.
