Developing and validating models of open quantum systems in solid-state nanostructures

Realistic quantum devices are open systems, and their behaviour depends not only on the subsystem of interest but also on how the environment is modelled and how the relevant observables are extracted. This thesis studies that problem in three settings. First, we study cavity-assisted qubit readout beyond the dispersive regime. We show that once coherent excitation exchange and dissipation become relevant, a microscopic treatment is needed to describe the readout quantitatively, and the optimal readout is set by a trade-off between signal accumulation and the distinguishability of the readout states. Second, we compare fermionic and spin baths in non-interacting transport models and show that spin baths reproduce fermionic reduced dynamics only in the large-bath weak-coupling limit, while clear deviations appear for smaller baths and near half-filling. Third, we study charge transport in molecular junctions using Automated Compression of Environments (ACE). There, the main difficulty is not the time evolution itself, but the extraction of the transport current, since this observable depends on system-environment correlations and is therefore not contained in the reduced system density matrix alone. We show that a probe-mode construction provides a viable route to reformulate the current as a system observable. This thesis advances our understanding of what is required for accurate modelling of open quantum dynamics across a range of solid-state settings, from cavity-assisted qubit readout to charge transport in molecular junctions.