Ivo Cools, Quantum Device Physics

Main supervisor: Professor Saroj Prasad Dash, QDP

Examiner: Professor Dag Winkler, QDP

Assistant supervisor; Associate Professor Thilo Bauch

Abstract: 

Quantum computing based on superconducting qubits is maturing at a rapid rate and will lead to important innovations in finance, pharmaceuticals, and the chemical industry. In this field, the co-planar waveguide (CPW) resonator is an ideal workhorse to route microwave signals on-chip through a conducting lead separated from two ground planes. 

Despite their low-loss characteristics, CPW resonators are highly susceptible to external magnetic field, which limits their broader functionality. Additionally, electrical voltage gating in coupled devices requires modifications to the ground planes, changing the resonator characteristics. 
To address these limitations, we introduce quarter wavelength differentially driven coplanar stripline resonators made of NbTiN. We characterise devices with varying geometries in these novel resonators and analyse changes in their internal quality factor and two-level system losses. Furthermore, we demonstrate the resilience of the resonators to an applied magnetic field by tracking the resonance frequency, without optimisation of the resonator geometry.

Due to their flexible design and high magnetic field resilience, coplanar stripline resonators are particularly well-suited to study superconductor-semiconductor heterophysics where voltage gating or a magnetic field are important. Finally, we show such an implementation where we strongly couple an InAs nanowire with Al coating to the resonator and measure the Andreev bound state spectrum, paving the way for studies of hybrid quantum systems.