A Purity Benchmarking Study of Superconducting Single-qubit Fluctuations

Abstract

Quantum processes are susceptible to errors. Over the years, numerous noise models, such as two-level system noise and flux noise, have been proposed by physicists to describe the mechanisms behind the error sources affecting quantum processes. However, a compre- hensive understanding of the quantum noise landscape, particularly on longer timescales, is still under active exploration. This thesis contributes to this ongoing effort, exploring long-term quantum noise through the lens of a superconducting Xmon transmon qubit. In our study, we explore the long-term qubit noises by conducting continuous purity benchmarking experiments, utilizing a set of established metrics to gauge the quantum errors. These metrics, namely the average gate fidelity and unitarity, provides a more detailed characterization of quantum error compared to the commonly studied variables such as T1 and frequency detuning, including characterization of the coherence property. These metrics are also the subject of intense discussions, particularly in the fields of quan- tum algorithms and quantum information processing hardware development. We measured the coherent and incoherent quantum error for very long time periods, up to 440 hours. Through these experiments. we gain valuable insights into the nature of the quantum noise and its impact on qubit coherence. Following the experiments, we further attempt to reconcile our observations with well- established models, namely the two-level system and flux noise, through simultaneous measurements and comprehensive simulations. While we succeed in explaining certain aspects of the experimental results, our findings also highlight intriguing discrepancies between experimental observations and simulations, thus prompting further research.