Fast CZ gate in hybrid fluxonium-transmon systems with tunable couplers
Abstract
Hybrid superconducting architectures combining different types of qubits offer a promising platform for exploiting their complementary advantages, yet high-fidelity entangling gates remain challenging because of strong nonlinearities and residual qubit-qubit interactions. Here, we propose a high-fidelity controlled-Z (CZ) gate for a hybrid circuit comprising a fluxonium qubit, a fixed-frequency transmon qubit, and a flux-tunable transmon coupler. By modulating only the external magnetic flux applied to the coupler, the qubit-qubit interaction is dynamically engineered for conditional-phase accumulation while the residual interaction is suppressed at idle, mitigating spectator-induced errors. We employ a low-dimensional Fourier-cosine pulse parameterization and a physically motivated cost function to independently suppress conditional-phase errors and leakage from the computational subspace. Numerical simulations demonstrate that a microwave-free CZ gate can be realized within 25 ns, with an average gate fidelity exceeding 99.99% and leakage below 10−5. Using experimentally relevant superconducting-qubit parameters and accounting for decoherence, the proposed scheme maintains a CZ-gate fidelity of approximately 99.9%. We further extend the analysis to larger coupled architectures and find that the CZ-gate infidelity remains below 10−4 in the presence of spectator qubits. These results establish single-parameter flux control as a simple and robust approach for realizing high-fidelity entangling gates in heterogeneous superconducting quantum architectures.
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BibTeX
@misc{xu2026fast,
title = {{Fast CZ gate in hybrid fluxonium-transmon systems with tunable couplers}},
author = {Peng Xu and Yunlong Wang and Yuechen Mu and Ling Jiang and Peng Zhao and Shengjun Wu and Xiaohong Yan},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.14197},
eprint = {2609.14197},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.14197},
}
RIS
TY - UNPB
TI - Fast CZ gate in hybrid fluxonium-transmon systems with tunable couplers
AU - Peng Xu
AU - Yunlong Wang
AU - Yuechen Mu
AU - Ling Jiang
AU - Peng Zhao
AU - Shengjun Wu
AU - Xiaohong Yan
PY - 2026
DA - 2026/09/13
PB - arXiv
DO - 10.48550/arxiv.2609.14197
UR - https://arxiv.org/abs/2609.14197
AB - Hybrid superconducting architectures combining different types of qubits offer a promising platform for exploiting their complementary advantages, yet high-fidelity entangling gates remain challenging because of strong nonlinearities and residual qubit-qubit interactions. Here, we propose a high-fidelity controlled-Z (CZ) gate for a hybrid circuit comprising a fluxonium qubit, a fixed-frequency transmon qubit, and a flux-tunable transmon coupler. By modulating only the external magnetic flux applied to the coupler, the qubit-qubit interaction is dynamically engineered for conditional-phase accumulation while the residual interaction is suppressed at idle, mitigating spectator-induced errors. We employ a low-dimensional Fourier-cosine pulse parameterization and a physically motivated cost function to independently suppress conditional-phase errors and leakage from the computational subspace. Numerical simulations demonstrate that a microwave-free CZ gate can be realized within 25 ns, with an average gate fidelity exceeding 99.99% and leakage below 10^−5. Using experimentally relevant superconducting-qubit parameters and accounting for decoherence, the proposed scheme maintains a CZ-gate fidelity of approximately 99.9%. We further extend the analysis to larger coupled architectures and find that the CZ-gate infidelity remains below 10^−4 in the presence of spectator qubits. These results establish single-parameter flux control as a simple and robust approach for realizing high-fidelity entangling gates in heterogeneous superconducting quantum architectures.
ER -