Quantum sensing of charge states in hybrid quantum dot-microwave systems

Sammanfattning

Inspired by the experimental work performed in the research group led by Ville Maisi at Lund University, this thesis investigates a hybrid quantum setup in circuit quantum electrodynamics (cQED). The system consists of a microwave resonator coupled to a double quantum dot (DQD) acting as an internal sensor to indirectly detect the state of an external quantum system. Using input-output theory and quantum Langevin equations, we derive the reflection coefficient of the coupled system, utilizing a mean-field approximation to determine steady-state solutions. The performance and sensitivity of the measurement scheme are quantified using Quantum Fisher Information (QFI) to identify optimal probing regimes for an external system modeled as either a single quantum dot or a secondary two-level qubit. Our results show that the external system induces state-dependent frequency shifts in the sensor qubit, which produce distortions in the reflection spectrum. We find that sensitivity is maximized at probing frequencies corresponding to the steepest slopes of resonance features, in which the reflection is most sensitive to the external system. Ultimately, this work demonstrates that cQED-based hybrid systems pro- vide a versatile platform for indirect state discrimination and provides a foundation for future studies into the quantum limits of sensing.