  A coupled quantum dot--nanocavity system in the weak coupling regime of
cavity quantumelectrodynamics is dynamically tuned in and out of resonance by
the coherent elastic field of a $f_{\rm SAW}\simeq800\,\mathrm{MHz}$ surface
acoustic wave. When the system is brought to resonance by the sound wave,
light-matter interaction is strongly increased by the Purcell effect. This
leads to a precisely timed single photon emission as confirmed by the second
order photon correlation function $g^{(2)}$. All relevant frequencies of our
experiment are faithfully identified in the Fourier transform of $g^{(2)}$,
demonstrating high fidelity regulation of the stream of single photons emitted
by the system.
