The main goal of the project has been to realize an efficient source of coherent single photons by coupling a self-assembled quantum dot to a photonic crystal waveguide. Such a source would have a wide range of applications in the field of quantum information processing.

By studying the coupling of single quantum dots to the waveguide, we demonstrate that the emitters are coupled with near-unity efficiency to the waveguide mode. We measure a coupling efficiency (β-factor) as high as 98.4% close to the band-edge of the waveguide mode, and β-factors above 90% over a bandwidth of 20 nm. Based on this result, we improved the design of the photonic crystal waveguide, and we characterized in detail the efficiency of the device and the coherence of the emitted single photons.

We investigate the decoherence mechanisms affecting the quantum dots by performing resonance fluorescence experiments on emitters not embedded in nanostructures. From the study of a sample without electrical gates, we observe that the linewidth of the emitters is significantly broadened by spectral diffusion. Nevertheless, we demonstrate that the quantum dot can efficiently probe changes in the electrostatic environment at the level of a single charge. A very high degree of coherence can be achieved by embedding quantum dots in electrically gated samples. We show that a single quantum dot behaves like a nearly-ideal two-level system in a sample with electrical gates, and single photons emitted up to 1 μs apart show indistinguishability in a Hong-Ou-Mandel experiment.

Finally, we demonstrate that a coherent quantum dot coupled to a photonic crystal waveguide is not only a promising single-photon source, but also a highly nonlinear system sensitive at the single-photon level. By performing resonant transmission measurements through a photonic crystal waveguide, we observe that a single quantum dot can extinguish a weak resonant laser with 30% contrast. Measurements of the statistics of the field transmitted through the waveguide reveal that the suppression of the laser is due to a single-photon nonlinearity, which can potentially be applied to realize deterministic photon-photon switches and transistors.