  Semiconductor quantum dots in cavities are promising single-photon sources.
Here, we present a path to deterministic operation, by harnessing the intrinsic
linear dipole in a neutral quantum dot via phonon-assisted excitation. This
enables emission of fully polarized single photons, with a measured degree of
linear polarization up to 0.994 $\pm$ 0.007, and high population inversion --
85\% as high as resonant excitation. We demonstrate a single-photon source with
a polarized first lens brightness of 0.50 $\pm $ 0.01, a single-photon purity
of 0.954 $\pm$ 0.001 and single-photon indistinguishability of 0.909 $\pm$
0.004.
