Site-controlled quantum dots as sources of quantum light

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Moroni, Stefano T.
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University College Cork
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Quantum information is at its infancy. Though several different approaches are being pursued, the ability of manipulating a quantum state and make two quantum systems interact in a controlled way is still a great challenge, especially in terms of reproducibility and fidelity to the expected theoretical state. Among the possible implementations of quantum information, photonics seems to offer many technological advantages, the biggest challenge being the availability of a pure, scalable and integrable source of photons with all the required properties. Epitaxial semiconductor quantum dots (QDs) have been exploited to deliver such quantum light, e.g. indistinguishable single-photons and polarization-entangled photon pairs, by both optical and electrical injection, generated on demand. However, conventional self-assembled QDs are necessarily characterized by random positioning and randomly distributed optical properties, which limit the feasibility of a technology based on this kind of system. In this context, pyramidal quantum dots (PQDs) are one very valid alternative to conventional semiconductor QD-based quantum light sources. In fact, the precise control over the position of the PQDs, together with the uniformity of properties granted by the combination of lithographic methods and metalorganic vapor-phase epitaxy (MOVPE), make this source one of the few scalable systems which have been proven in recent years to emit photons with very interesting properties, polarization-entangled photons, for instance, upon optical excitation. In this work, all the main relevant aspects regarding PQDs are addressed through the most recent results obtained studying the system, starting from fundamental aspects regarding the epitaxy step. A growth model is presented which was also employed in the past as a practical tool to predict the result of the MOVPE in terms of geometry and composition of AlGaAs and GaAs structures inside a pyramidal recess. Here the model is extended in its simulation capabilities in order to include also the epitaxy of InGaAs, the actual material of which the optically active QD layer is made. This is then employed to simulate and understand the physical reason for a previously observed experimental behavior, henceforth confirming the applicability of the extended model to the simulation of InGaAs. Segregation, one fundamental epitaxy-related phenomenon which is predicted as well by the abovementioned growth model, is the key element allowing selective injection of carriers into a PQD, when its structure is embedded into a PIN junction. The whole fabrication process is described, including a selective-contacting technique that allows the realization of the electrical excitation of PQDs, one of the requirements for an efficient integration of PQDs on a photonic platform. On-demand generation of both single photons and entangled photon pairs is demonstrated from this device, the latter also importantly reaching a record fidelity to the ideal entangled state of 0.82 (upon the application of an appropriate time-filtering technique) and violating Bell’s inequalities. Among the unique possibilities offered by the PQD system is the capability of precisely stacking one quantum dot over another, allowing the formation of interacting multiple-QD systems. A first study of the statistics resulting from the fabrication of multiple PQDs with different distance and number of QD is here presented, showing how the QDs affect each other and offer further “tuning-knobs” for controlling their optical properties. For example, stacking two PQDs at the right distance can result in the generation of two subsequent photons with the same energy, which was previously reported only in a specific case of self-assembled QDs. However, limitations in the quality of the optical properties of PQDs are still in place for actual technological implementations. Among these is the distribution of emission energies and the residual fine structure splitting (FSS) affecting the quality of the entangled photon pairs. A piezoelectric stress-based tuning technique is used to tune both the emission energy of PQDs and their FSS, demonstrating also restoral of entanglement upon the application of the proper stress. Finally, a new method for releasing PQDs is shown, which allows their manipulation on several substrates and opens up new possibilities for the integration of the QDs onto functional platforms. As an example, integration over a multimode fiber is demonstrated as well as the emission of single-photons directly through the fiber.
Entanglement , Site-control , Quantum dots
Moroni, S. 2018. Site-controlled quantum dots as sources of quantum light. PhD Thesis, University College Cork.
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