Site-controlled quantum dots integrated with photonic crystal waveguides and cavities

Quantum mechanics did not only deeply transform our world view down to a philosophical level, it is also expected to be key ingredient of future so-called quantum technologies. Indeed, quantum properties of matter such as isolated single particles or entanglement, can provide a technological resource for faster computers or perfectly secure communication protocols. In this new paradigm, arbitrary quantum states of light and matter would be deterministically controlled to perform operations that are not feasible in the realm of classical physics. A promising platform proposed for quantum technologies consists of quantum dots (QD) embedded in photonic crystal (PhC) circuits. While QDs exhibit high oscillator strengths, and are a promising solution as single photon sources, PhCs represent an ideal platform for light processing due to their capacity to enhance light-matter interactions. However, most previous experiments relied on self-assembled QDs nucleating at random position, which prevents their scaling to larger systems with multiple QDs. The subject of this thesis is the study of site-controlled quantum dots and their interaction with photonic crystal systems made of cavities and waveguides. A first challenge lies in the free propagation of light in simple PhC waveguides. Elongated PhC cavities were first harnessed to measure the mode reflectivity at the edge of the linear cavities, and the propagation losses in PhC waveguides. The impact of disorder on the density of states, mode localization and dispersion in long linear PhC cavities was then investigated. The direct imaging of the modes permitted to identify localized modes. A statistical analysis of the measured group index clarified the boundary between the diffusive and dispersive regime. The site-control of the QDs permitted the in-situ probing of the the local density of states, from which the weak field distortions in the dispersive regime were analyzed. In a second part, the integration of five site controlled QDs in a PhC waveguide is demonstrated together with the corresponding on-chip single photon transfer over macroscopic distances. The efficiency of coupling QDs to waveguides in these structures is measured while taking into account the statistical variations of the QDs intrinsic properties. Broad fluctuations of the coupling efficiencies were observed and attributed to the formation of Fabry-Pérot modes in the waveguides. An optimal design to reach reproducibly a broadband high efficiency of coupling is then proposed. The final part focuses on a system in which a QD is embedded in a cavity, itself coupled to a PhC waveguide. First, a design for coupling light out of plane from a PhC waveguide with reduced back-reflection in the waveguide is presented. This coupler is then used for collecting light from the QD-cavity-waveguide system. The existence of a cavity to waveguide coupling which optimizes the coupling efficiency of single photons to the waveguide is then shown theoretically. The parameters controlling the coupling between the QD cavity and waveguide are measured, from which we infer a single photon collection efficiency close to its optimal value. This work represents the first experimental implementation of site-controlled QDs in PhC circuits beyond simple cavities. It focuses on the limitations and possibilities opened by such QD systems as on-chip efficient single photon sources, useful for on-chip quantum circuits.

Kapon, Elyahou
Lausanne, EPFL

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 Record created 2018-02-28, last modified 2020-10-25

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