  The fundamental process limiting the coherence of quantum-dot based
single-photon sources is the interaction with phonons. We study the effect of
phonon decoherence on the indistinguishability of single photons emitted from a
quantum dot embedded in a suspended nanobeam waveguide. At low temperatures,
the indistinguishability is limited by the coupling between the quantum dot and
the fundamental vibrational modes of the waveguide and is sensitive to the
quantum-dot position within the nanobeam cross-section. We show that this
decoherence channel can be efficiently suppressed by clamping the waveguide
with a low refractive index cladding material deposited on the waveguide. With
only a few microns of cladding material, the coherence of the emitted single
photons is drastically improved. We show that the degree of
indistinguishability can reach near unity and become independent of the
quantum-dot position. We finally show that the cladding material may serve dual
purposes since it can also be applied as a means to efficiently outcouple
single photons from the nanophotonic waveguide into an optical fiber. Our
proposal paves the way for a highly efficient fiber-coupled source of
indistinguishable single photons based on a planar nanophotonic platform.
