Optimization of Experimental Setups and Readout for Superconducting Qubits

Abstract

Superconducting qubit based architectures have gained substantial attention throughout recent years as a potential platform for quantum computation, where the circuit properties can be engineered by tailoring the components and their corresponding energies. However, to facilitate the use of large-scale multiqubit architectures, it is imperative to understand and optimize control on the single qubit level. Two variants that have been at the forefront of this research are the transmon, and more recently, the fluxonium qubit. While the fluxonium is attractive in part because of its noise protection, affording it with long coherence times, this protection simultaneously renders it difficult to readout with high speed and fidelity. In an effort to address this challenge, the principal focus of this work is the proposal and implementation of a flux-pulse-assisted readout scheme for fluxonium qubits. Enabled by the many features within the dispersive shift landscape arising from the lack of strict selection rules, we suggest employing a flux pulse while the readout tone is on in order to shift the qubit to a point where it possesses a large dispersive shift. We find that we can achieve 99.9% (94%) SNR-limited (assignment) fidelity without a quantum-limited parametric amplifier in our readout chain in under 400 ns through implementation of our readout scheme. This is done without sacrificing protection at the standard operating point, and we observe that our experimental findings are well supported by our theoretical model. Further to this, the specifics of the cryogenic wiring setup greatly impact overall qubit performance, including readout. Therefore, we also report our preliminary findings with regard to optimal attenuation and filtering in the context of transmon qubit performance, which can be extrapolated to use with fluxoniums, as well.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jorge Barreto (Supervisor) & Christian Kraglund Andersen (Supervisor)