  We demonstrate a high-purity source of indistinguishable single photons using
a quantum dot embedded in a nanophotonic waveguide. The source features a
near-unity internal coupling efficiency and the collected photons are
efficiently coupled off-chip by implementing a taper that adiabatically couples
the photons to an optical fiber. By quasi-resonant excitation of the quantum
dot, we measure a single-photon purity larger than 99.4% and a photon
indistinguishability of up to 94+-1% by using p-shell excitation combined with
spectral filtering to reduce photon jitter. A temperature-dependent study
allows pinpointing the residual decoherence processes notably the effect of
phonon broadening. Strict resonant excitation is implemented as well as another
mean of suppressing photon jitter, and the additional complexity of suppressing
the excitation laser source is addressed. The study opens a clear pathway
towards the long-standing goal of a fully deterministic source of
indistinguishable photons, which is integrated on a planar photonic chip.
