Efficient Source of Indistinguishable Single-Photons based on Phonon-Assisted Excitation

Semiconductor quantum dots in cavities are high-performance single-photon sources. Thus far the most efficient sources utilise resonant excitation of an unpolarized quantum emitter coupled to a highly birefringent cavity. However, this demands very high polarization extinction, and challenging experimental operation. Here, we remove these requirements by using off-resonant phonon-assisted excitation of a linear exciton dipole, exploiting the quantum dot's vibrational environment and natural asymmetry. This allows the collection of single photons that are spectrally separated from the excitation laser, and intrinsically present a very high degree of linear polarization up to 0.994 $\pm$ 0.007. This phonon-assisted excitation scheme enables very high single-photon purity and indistinguishability, and only reduces the emitter population by (15 $\pm$ 1) %, as compared to resonant excitation. Overall, we simultaneously demonstrate a polarized first lens brightness of 0.51 $\pm $ 0.01, with a single-photon purity of 0.939 $\pm$ 0.001 and corrected single-photon indistinguishability of 0.915 $\pm$ 0.003.