Robust and Scalable Flux Crosstalk Calibration Approach in Superconducting Qubit-Coupler System

Myrron Albert Aguila^1*, Nien-Yu Li^2,3, Chen-Hsuan Ma^2,3, Li-Chieh Hsiao^1,4, Chii-Dong Chen^2,1, Chung-Ting Ke^2,1

¹Research Center for Critical Issues, Academia Sinica, Tainan, Taiwan
²Institute of Physics, Academia Sinica, Taipei, Taiwan
³Department of Physics, National Taiwan University, Taipei, Taiwan
⁴Department of Physics, National Cheng Kung Univeristy, Tainan, Taiwan

* Presenter:Myrron Albert Aguila, email:maguila@as.edu.tw

Magnetic flux crosstalk is a classical issue in superconducting quantum processors (SQPs) with flux-tunable elements. It occurs when magnetic flux from one flux line (Z-line) causes an unwanted frequency detuning on neighboring flux-tunable qubits and couplers, disrupting their target frequencies for quantum operations. If uncorrected, this crosstalk scales with the number of flux lines, complicating two-qubit gate tuning, reducing calibration speed, and limiting the overall quantum gate performance needed for error correction.

This report is an extension of our preliminary work on mitigating flux crosstalk in tunable qubit-coupler pairs (presented at TPS2025, Poster ID: P1-QI-025). Our original method, the Multi Z-Line Control (MZLC) protocol, reduced flux crosstalk from 27 ± 19 ‰ to 0.2 ± 0.1 ‰. In this presentation, we present new findings that validate and enhance MZLC [1]. We show that the flux crosstalk matrix generated by MZLC is highly consistent with results from the Ramsey sequence, maintaining comparable precision. MZLC proves robust even with coarse optimization of single-qubit and readout fidelities, and can precisely differentiate frequency shifts caused by flux crosstalk from other sources, benefiting from its 2D scan approach for increased precision.

Furthermore, MZLC incorporates an empirical Rabi model as its digital twin to analyze imperfections in the conditional-phase (CZ) SWAP gates. Since MZLC simplifies the tune-up of the CZ gate through tunable coupling, which is critical for optimizing quantum gates in SQP, we also discuss key protocol enhancements necessary for its scalability in large SQP systems.

[1] C. H, Ma, et. al. (2025) https://arxiv.org/abs/2508.03434 (submitted)

Keywords: superconducting qubits, tunable couplers, flux crosstalk matrix and cancellation, CZ-SWAP modelling, pulsed two-tone spectroscopy