  The nitrogen-vacancy (NV) center in diamond has an optically addressable,
highly coherent spin. However, an NV center even in high quality
single-crystalline material is a very poor source of single photons: extraction
out of the high-index diamond is inefficient, the emission of coherent photons
represents just a few per cent of the total emission, and the decay time is
large. In principle, all three problems can be addressed with a resonant
microcavity. In practice, it has proved difficult to implement this concept:
photonic engineering hinges on nano-fabrication yet it is notoriously difficult
to process diamond without degrading the NV centers. We present here a
microcavity scheme which uses minimally processed diamond, thereby preserving
the high quality of the starting material, and a tunable microcavity platform.
We demonstrate a clear change in the lifetime for multiple individual NV
centers on tuning both the cavity frequency and anti-node position, a Purcell
effect. The overall Purcell factor $F_{\rm P}=2.0$ translates to a Purcell
factor for the zero phonon line (ZPL) of $F_{\rm P}^{\rm ZPL}\sim30$ and an
increase in the ZPL emission probability from $\sim 3 \%$ to $\sim 46 \%$. By
making a step-change in the NV's optical properties in a deterministic way,
these results pave the way for much enhanced spin-photon and spin-spin
entanglement rates.
