Strong Charge-Photon Coupling and High-Efficiency Microwave Photodetection in Hybrid Quantum Dot-Resonator Systems

This thesis investigates hybrid circuit quantum electrodynamics (cQED) as a versatile platform that unites superconducting resonators with semiconductor quantum dots. The central motivation is to harness strong light-matter coupling at the single-photon level, which enables coherent photon-qubit interactions and paves the way for quantum technologies in computation, communication, and sensing. The first part of this work combines hybrid cQED with holes in planar Ge/SiGe, a material system that has only recently become accessible for quantum devices. Planar Ge offers unique advantages for this purpose and has attracted considerable attention in the context of quantum computing. In contrast, its potential for hybrid cQED has remained largely unexplored, making the present work one of the first demonstrations of strong light-matter interaction in this platform. Within this framework, we demonstrate strong charge-photon coupling in planar Ge by observing vacuum Rabi splitting between a hole charge qubit based on a double quantum dot (DQD) and a microwave resonator. We employ a frequency-tunable high-impedance resonator to perform qubit spectroscopy, enabling systematic exploration of coupling regimes and confirming device controllability. In addition, we use this setup to carry out excited-state spectroscopy of strongly correlated states in the DQD, revealing the formation of Wigner molecules and illustrating how the hybrid architecture can be harnessed to access many-body physics. While these experiments in Ge demonstrate how strong coupling can be harnessed to explore qubit physics and correlated states, they also exemplify the wider potential of hybrid cQED. A particularly pressing application is in quantum optics, where efficient single-photon detectors are still lacking in the microwave regime. Such detectors are essential for enabling experiments in quantum communication and microwave quantum optics. In the second part of this thesis, we address this challenge by realizing a photodetector based on a GaAs DQD coupled to a superconducting resonator. The detector operates via photon-to-charge conversion and achieves efficiencies approaching unity. Strong coupling plays a central role by ensuring that incoming photons are absorbed before being lost to the environment, which is key to the high performance of the detector. We further exploit the frequency tunability of the resonator to investigate device performance and identify the parameters that optimize detection efficiency. The broader impact of this work lies in advancing quantum technologies that rely on coherent microwave photons. Applications include spin qubit readout, long-distance photon-mediated coupling, quantum communication, and microwave quantum optics. Looking forward, planar Ge is identified as a highly competitive platform for hybrid cQED. Future directions include the development of single-photon sources, quantum- limited detectors, and analog quantum simulators based on engine