  We report a joint experimental and theoretical study of the interference
properties of a single photon source based on a In(Ga)As quantum dot embedded
in a quasi-planar GaAs-microcavity. Using resonant laser excitation with a
pulse separation of 2 ns, we find near-perfect interference of the emitted
photons, and a corresponding indistinguishability of $\mathcal{I} =
(99.6\,^{+\,0.4}_{-\,1.4})\%$. For larger pulse separations, quasi-resonant
excitation conditions, increasing pump power or with increasing temperature,
the interference contrast is progressively and notably reduced. We present a
systematic study of the relevant dephasing mechanisms, and explain our results
in the framework of a microscopic model of our system. For strictly resonant
excitation, we show that photon indistinguishability is independent of pump
power, but strongly influenced by virtual phonon assisted processes which are
not evident in excitonic Rabi oscillations.

Quantum Physics;Mesoscale and Nanoscale Physics