Bachelor's Defense

Christian Behrendorff Madsen

Title: Characterization and Readout of superconducting Quantum Dots in Two-Dimensional Semiconductor-Superconductor Heterostructures

Heterostructures are a powerful platform to investigate electron transport through confined geometries, including hybrid quantum dots. At low temperatures, the different band structures associated with different semiconducting layers allow the formation of a two-dimensional electron gas, with can further be confined by the local electric field produced by gate electrodes of various shapes. An active field or research investigates the interplay of semiconductor physics and superconductivity, in terms of the resulting energy spectrum (such as the emergence of subgap states), coherence properties, and parity effects, which may all depend on the exact shape of the hybrid device. In this project, I characterize the properties of epitaxial semi-super quantum dots at low temperatures using transport measurements and charge sensing at subkelvin temperatures. Unlike previous studies that explored elongated dots, our dots are defined by sub-micron circular top gates. Depending on the choice of gate voltages, electron flow in the presence of Coulomb blockade is studied, at low and high magnetic fields, thereby allowing the identification of 1e or 2e periodic Coulomb oscillations. We also explore the use of proximal quantum dots to serve as charge sensors of super-semi quantum dots, finding a strong capacitive coupling mediated by a dog-bone-like floating gate electrode. Our findings are useful for understanding and applying the properties of quasiparticle poisoning and parity protection for new types of quantum devices.