Design of a superconducting Bias-Tee

Abstract

The tight integration of qubits is a significant challenge for the development of modern quantum computers. Closely placed qubits will be subjected to frequency crowding. Flux tuning of qubits can reduce the frequency crowding by changing the resonance frequency of a qubit by changing the magnetic flux, which requires the coupling of DC and RF signals. A bias-tee implements the diplexing of DC power and RF signals. In this thesis, We report findings on a microwave bias-tee designed to be fabricated as an integrated passive device in a superconducting process on silicon or quartz. A bias-tee requires an inductive branch and a capacitive branch to diplex DC signals and RF signals. Therefore, passive components such as inductors and capacitors have been modelled and simulated. Moreover, the effect of the wafer, dielectrics, and packaging techniques are studied to describe the non-ideal performance of passive components. One of the challenges of simulating thin film devices in the Finite Element Method (FEM) solvers is the issue of dielectric thickness compared to the rest of the structure. The large difference in dimensions leads to erroneous mesh elements. This thesis also introduces a novel detailed explanation of the meshing of thin films in FEM solvers that has not been thoroughly covered in the existing literature. The simulation results show capacitors with a quality factor of 7000 realised in a silicon process and a quality factor of 4000 realized in a quartz process. We also report the design of planar coils, which have a quality factor of 2400 for the silicon process and 2000 for the quartz process.