Modeling and Suppressing Unwanted Parasitic Interactions in Superconducting Circuits

TL;DR

This paper investigates parasitic interactions in superconducting qubits, develops methods to eliminate or utilize ZZ interactions, and proposes a new parasitic-free gate to improve quantum gate fidelity.

Contribution

It introduces a dynamical ZZ freedom strategy, demonstrates a parasitic-free (PF) gate, and analyzes multi-qubit interactions to enhance superconducting quantum circuits.

Findings

Theory accurately matches experimental results.

Dynamical ZZ freedom can zero the total ZZ interaction.

Proposed PF gate achieves high fidelity and suppresses parasitic effects.

Abstract

Superconducting qubits are among the most promising candidates for building quantum computers. Despite significant improvements in qubit coherence, achieving a fault-tolerant quantum computer remains a major challenge, largely due to imperfect gate fidelity. A key source of this infidelity is the parasitic interaction between coupled qubits, which this thesis addresses in two- and three-qubit circuits. This parasitic interaction causes a bending between computational and non-computational levels, leading to a parasitic ZZ interaction. The thesis first investigates the possibility of zeroing the ZZ interaction in two qubit combinations: a pair of interacting transmons, and a hybrid pair of a transmon coupled to a capacitively shunted flux qubit (CSFQ). The theory developed is used to accurately simulate experimental results from our collaborators, who measured a CSFQ-transmon pair with and without a cross-resonance (CR) gate. The strong agreement between theory and experiment motivated further study of a CR gate that achieves 99.9% fidelity in the absence of static ZZ interaction. Since the CR pulse adds an additional ZZ component to the static part, a new strategy called dynamical ZZ freedom is proposed to zero the total ZZ interaction. This strategy can be applied in all-transmon circuits to enable perfect entanglement. Based on these findings, a new two-qubit gate, the parasitic-free (PF) gate, is proposed. Additionally, the thesis explores how to utilize the ZZ interaction to enhance the performance of a controlled-Z gate. Lastly, the impact of a third qubit on two-qubit gate performance is examined, with several examples illustrating the properties of two-body ZZ and three-body ZZZ interactions in circuits with more than two qubits.