Today's quantum computer prototypes based on superconductors and/or semiconductors consist of a handful of quantum bits. High-fidelity operation of these prototypes rely on our understanding of the physical mechanisms governing initialisation, coherent control, readout, as well as decoherence of the quantum bits. The PhD research of the applicant will aim to further develop such quantum computers using the toolkit of theoretical physics. Activities will focus on qubit-level and holistic benchmarks of quantum computers, microscopic and phenomenological modelling of initialisation, control and readout, optimal control techniques, quantum error correction, quantum algorithms, quantum simulation, as well as geometrical concepts and methods in quantum information and quantum matter. Research will be carried out in an international network of leading European research organisations in the framework of multiple Horizon Europe projects (IGNITE, ONCHIPS, OpenSuperQPlus, QLSI2).