Master Thesis Defence by Cecilie Toftdahl Olesen

Efficient Generation of Pure and Indistinguishable Single Photons

High-quality single-photon sources are a vital part for realizing many quantum information protocols. In this thesis quantum dots embedded in photonic crystal waveguides have been studied for the purpose of generating high indistinguishable single photons. High indistinguishability is achieved by optically exciting the exciton in a quantum dot using a pulsed laser with frequency on resonance with the exciton transition. However, resonant excitation introduces the challenge of suppressing laser scattering in the collected emission. This directly limits the purity of the source. The waveguide creates a spatial separation between the excitation of the quantum dot and the collected emission. This allows for the purity to be high, as collection mainly consists of photons emitted from the quantum dot. This begs the question of how pure a single photon emission can be generated on this type of platform. This has been studied by estimating purity, indistinguishability, and efficiency of the single-photon source. The main focus of this thesis has been on investigating how the wavefront and shape of the excitation pulse influences the quality of the source. This includes studying how the pulse duration of the excitation laser influences the purity. The pulse duration should be short enough to not cause multi-photon emission, while not so short that it introduces laser scattering. Instead of changing the shape of the pulse one may also alter the wavefront. One approach is to adjust the polarization of the wavefront. The efficiency of which the quantum dot is being excited is directly affected by the overlap between the polarization of the excitation laser and the orientation of the quantum dot dipole. Therefore, this should be optimized for. However, for some spin-photon interface and multi-photon protocols it might be a requirement to selectively excite a given polarized dipole, where scattering is not naturally suppressed. It is in this work reported how modifying the phase of the incident wavefront can improve the purity at a given polarization.The best achieved single photon emission was measured to have a purity of g2(0) = 0.0086 ± 0.0013 and intrinsic indistinguishability of V = 98 ± 1 % with a total efficiency of 6 ± 1 %, which results in a detected photon rate of 2.6 million photons per second, using a pulsed laser of 76 MHz repetition rate. Concurrently achieving such a high-quality across all three parameters shows how quantum dots embedded in photonic crystal waveguides are promising candidates as state-of-the-art single-photon sources.