Spin and topological-qubit based quantum computers require an easily scalable and highly sensitive method for readout and manipulation, for which Dispersive Gate Sensing (DGS) is a promising candidate [1, 2]. DGS enables sensing of single electron tunneling events in a mesoscopic system by measuring a reflected Radio Frequency (RF) signal from a resonator capacitively coupled to one of the system’s gate electrodes. From the resonator’s perspective, the system is modeled as an effective ‘parametric capacitance’ [3]. Motivated by its viability for qubit readout and control, we measure transport characteristics of a semiconducting-superconducting hybrid charge island system using this method. Doing so, we observe a spin degeneracy modulation of interdot tunnel couplings resulting from hybridization of a dot orbital with a subgap state of the superconductor, measured entirely within Coulomb blockade. Without DC measurement, we also track a subgap state’s energy via voltage intervals between island charge states as a function of field. Subsequently, we attempt to correlate changes in the resonator’s internal dissipation with coherence of tunneling into the superconducting island. In particular, we collapse a superconducting island’s lowest energy state to the Fermi level by applying a magnetic field. Contrary to expectations, internal dissipation in the resonator did not change dramatically with field, in fact decreasing at charge degeneracy points at all field strengths. We corroborate measurements with existing theory, augmented by simple analytical and numerical models demonstrating that coherence factors of superconducting quasiparticle states and the degeneracy of dot orbitals modulate parametric capacitance. A master equation model of finite temperature interdot tunneling is solved, agreeing with experimental evidence that Sisyphus dissipation is negligible when a dot orbital is hybridized with multiple independent quasiparticle states. To be employed in quantum computers, a full understanding of all phenomena contributing to a DGS signal in these hybrid systems is critical. Hence, these results mark a step towards unambiguous readout of Majorana parity in topological qubits [4].

This thesis presents the design, development and first spectroscopy measurements of nanowire-based fluxonium devices. We demonstrate the strong external flux and gate voltage tunability of their spectrum, which allows to accurately tune their first transition frequency over a range of more than 10 GHz. We also show the nanowire fluxonium resilience to magnetic fields up to 800 mT, demonstrating its compatibility with the creation of Majorana bound states (MBSs) at the junction ends, what would open the door to the exploration of new physics and new technological applications. First, the emergence of MBSs in a nanowire fluxonium would result in new Majorana signatures, obtained by radio-frequency spectroscopy techniques. This would complement the current experimental evidence for the creation of MBSs in semiconducting nanowires and would allow to characterize their coupling energy scales, that are, up to date, unknown. And second, the nanowire fluxonium devices presented here can be used for addressing a qubit whose state is topologically protected from local perturbations. Integrating topological qubits into a cQED platform would solve the currently existing problems of the lack of a universal set of quantum gates and reliable methods for qubit operation and readout, establishing a path for the development of topological quantum computing.