Dissertation: Drilon Zenelaj, Lund University

Abstract

Nanoscale systems coupled to superconducting circuits offer a platform to study a variety of research questions ranging from more fundamental investigations on the quantum nature of light interacting with matter to more application-oriented ones such as quantum computing and communication. These hybrid systems have thus gained a great deal of attention in the community in the recent years and have led to a whole new research field called circuit quantum electrodynamics (cQED).

In this thesis, the cQED system of interest is a semiconductor double quantum dot (DQD) coupled to a superconducting microwave resonator (MR). The main focus lies on the study of photo-assisted electron transport and the potential applicability of the DQD-MR hybrid system as a detector for single microwave photons.

In Paper I, we study the zero-frequency full counting statistics (FCS) and the finite-frequency noise (FFN) of the photo-assisted electric current through the DQD-MR system in the case when the system is driven by a monochromatic, coherent source of microwave radiation. We find analytical solutions to the FCS and the FFN in the limits of low and large drive strengths.

In Paper II, we study the photo-assisted electron transport in the case when the system is driven by a single, coherent microwave pulse. In the limit of low drive, we develop a Wigner-function formalism for the drive and the detector, giving a visually compelling tool to study the interaction between the drive and the system. We use this formalism to compute the photo-assisted current and analyze the performance of the detector in different parameter regimes.

In Paper III, we demonstrate the conversion of microwave to electrical power in the DQD-MR system by running the photo-assisted current against an applied bias between the source and drain of the DQD. The device reaches a maximum power-harvesting efficiency of 2% for an incoming microwave power of 2 fW.

In Paper IV, we investigate the role of charge transport on the coherence of light-matter coupling in a DQD strongly coupled to a high-impedance microwave resonator by applying a finite voltage bias across the DQD. An inherent asymmetry in the tunnel couplings to the electronic leads shows how different charge occupations effect the decoherence in this system. We also find an electron-hole symmetry between different bias polarities.