Exploring Fundamentals of Circuit Quantum Electrodynamics Using Fluxonium

Abstract

In recent years, superconducting quantum computing has been established as a potential candidate for building a scalable quantum computer. This is exemplified by recent claims of quantum supremacy and by significant improvements in qubit coherence times in the past several years. Superconducting quantum computing is understood through the field of circuit quantum electrodynamics (cQED). Improving understanding of the field’s fundamentals is critical to push further into the future of superconducting quantum computing. Given the rich energy level structure of the fluxonium qubit, it can probe various properties of cQED devices and thus enhance our understanding of the basics of cQED. In this thesis, we explore two such examples. It was long believed in cQED that the allocation of external flux to different circuit elements was equivalent until theory work in 2019 suggested that under the influence of time-dependent flux, the choice affects the resulting Hamiltonian. Performing a fast flux ramping experiment, we verify the theory for properly handling time-dependent flux in cQED. Additionally, there is an open question in the field as to why superconducting qubits exhibit anomalously high excited state populations when in thermal equilibrium. We explore the relationship between effective qubit temperature and qubit frequency using fluxonium’s tunable energy level structure. Our initial results suggest that the effective qubit temperature depends on the transition frequency, with higher frequencies exhibiting higher effective temperatures than lower frequencies.