Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Schoelkopf, Robert

Abstract

Quantum computing has garnered a lot of attention due to the belief that it would beable to solve certain kinds of problems which are intractable to classical computers. One of the leading candidate platforms for quantum computing is superconducting circuits. Within this platform, there are two main methods of storing quantum infor- mation: Josephson-junction based artificial atoms and harmonic oscillators. The large Hilbert space of the bosonic excitations of a harmonic oscillator allows for redundant storage of quantum information. Quantum error correction in a single mode using bosonic codes has been demonstrated beyond break-even. However, devices with more than a handful of oscillators have not yet been demonstrated. One of the main challenges in scaling up devices is oscillator design. Currently, coaxial stub cavities machined from high-purity aluminium are most commonly used. Fully-lithographic micromachined cavities have previously been demonstrated, but had short lifetimes due to loss in the seams. In this thesis, I describe a way to fabricate and measure ultra-high-quality microwave seams using indium bump-bonding. I then discuss the application of this to micromachined cavities, improving their lifetime hundredfold and exceeding that of stub cavities. I also demonstrate suspended coaxial resonators, which have a demountable centre conductor. They are easier to make than micro- machined cavities and can slightly exceed stub cavities’ lifetime without the use of high-purity aluminium. Finally, I discuss recent work on ways of measuring the losses of different materials or interfaces using multimode resonators, and the comparison of this to traditional materials studies methods.

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