  We demonstrate the in-plane emission of highly-polarized single photons from
an InAs quantum dot embedded into a photonic crystal waveguide. The spontaneous
emission rates are Purcell-enhanced by the coupling of the quantum dot to a
slow-light mode of the waveguide. Photon-correlation measurements confirm the
sub-Poissonian statistics of the in-plane emission. Under optical pulse
excitation, single photon emission rates of up to 19 MHz into the guided mode
are demonstrated, which corresponds to a device efficiency of 24%. These
results herald the monolithic integration of sources in photonic quantum
circuits.
