PhD defence by Abdulmalik A. Madigawa

Abstract

The development of high-quality single-photon sources (SPSs) with near-unity single-photon purity, efficiency, and indistinguishability is crucial for advancing photonic quantum information processing applications, including optical quantum computing, secure quantum communication, and quantum networks.

This thesis focuses on the experimental realization of self-assembled quantum dot (QD)-based micropillar SPSs, investigating some of the key experimental challenges to optimize device performance. One of the primary challenges associated with self-assembled QDs is the random spatial and spectral distribution inherent to their growth process. This randomness complicates the fabrication of photonic structures with spatially and spectrally resonant QDs, often leading to discrepancies between theoretically predicted and experimentally measured Purcell factors. These discrepancies hinder the realization of optimal single-photon collection efficiency.

In the first part of this thesis, we developed a precise QD localization technique based on photoluminescence (PL) imaging to enable the deterministic integration of QDs into photonic structures. The final alignment accuracy of this marker-based method was evaluated and compared to other state-of-the-art deterministic fabrication techniques, including marker-based cathodoluminescence (CL) imaging and marker-free in-situ electron beam lithography.

Our results revealed that marker-based techniques achieve a final positioning offset of < 100 nm from the center of the structures, while the marker-free in-situ EBL approach achieves a positioning offset of < 60 nm.

In the second part of the thesis, we studied structural imperfections in micropillar devices, which introduce optical losses and degrade SPS performance. We optimized the micropillar fabrication process, including the etching profile, to achieve straight and smooth sidewalls and maximize the quality factor of the devices.

Finally, we applied our deterministic fabrication approach to integrate GaAs QDs into micropillar structures and experimentally investigated their performance in comparison with theoretical predictions. The study included an analysis of decay dynamics and photon indistinguishability, alongside a discussion of the challenges that remain in achieving optimal device performance. We identified and discussed the discrepancy between the theoretically predicted and experimentally measured source efficiency. Additionally, we introduced two cylindrical rings around the micropillar structure to enhance the collection efficiency.

While this design was theoretically predicted to significantly improve collection efficiency, experimental measurements showed only a slight enhancement, highlighting the sensitivity of the design to fabrication dimension variations. Overall, this thesis represents a foundational step toward realizing optimal SPS performance and provides valuable insights into the various challenges that must be addressed to achieve this goal.

Supervisors

• Main Supervisor: Professor Niels Gregersen, DTU Electro, Denmark.
• Co-supervisor: Associate Professor, Battulga Munkhbat, DTU Electro, Denmark.

Assessment committee

• Senior Researcher, Elizaveta Semenova, DTU Electro, Denmark (chair).
• Associate Professor Leonardo Midolo, Niels Bohr Institute, Denmark.
• Senior Researcher Julien Claudon, CEA Grenoble, France.

Master of the Ceremony

• Assistant Professor, Luca Vannucci, DTU Electro, Denmark.