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arXiv:2607.23463

Purcell-Engineered Hybrid Coupler for Leakage-Suppressed Robust CZ Gates

Hui Wang, Feng Bao, Yan-Jun Zhao, Xun-Wei Xu

quant-phSubmitted Jul 26, 2026• Announced Jul 28, 2026• 0 citations• Revised Oct 9, 2026 (v3)
v1 · Jul 26, 2026v2 · Jul 28, 2026v3 · Oct 9, 2026
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Abstract

We propose a Purcell-engineered notch-filter hybrid coupler for superconducting controlled-$Z$ (CZ) gates that combines coherent interaction engineering with leakage-selective dissipation. The architecture integrates a nonlinear transmon coupler with a coupled Purcell-filter and notch-resonator subsystem, providing additional control over both the coherent interaction pathways and the engineered dissipative environment. The filter branch reshapes the effective interaction pathways, while the notch resonator further tailors the frequency response of the coupled filter network and preserves strong leakage-selective dissipation. Using dressed-eigenstate analysis together with Lindblad master-equation simulations, we show that the proposed architecture substantially reduces leakage and retains high-fidelity, low-leakage operation compared with an optimized single-transmon coupler while remaining robust over a broad range of coherence assumptions and device parameters. The gate analysis reconstructs the projected computational quantum channel and therefore retains the phase information required to distinguish a CZ operation from identity. The nominal operating point occurs at $t_{\rm CZ}=327.46~\mathrm{ns}$ with $F_{\rm avg}^{\rm CZ}=99.714\%$, $L_{\rm avg}=8.78\times10^{-4}$, and $L_{\rm max}=2.04\times10^{-3}$. A minimax timing-robust operating point at $327.20~\mathrm{ns}$ maintains $F_{\rm avg}^{\rm CZ}\geq99.590\%$ over a $\pm0.50~\mathrm{ns}$ timing window. These results demonstrate that engineered dissipation complements conventional coherent interaction engineering and provides an additional design degree of freedom for realizing robust, high-fidelity superconducting CZ gates.

Comments
13 page, 13 figures

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We propose a Purcell-engineered notch-filter hybrid coupler for superconducting controlled-ZZZ (CZ) gates that combines coherent interaction engineering with leakage-selective dissipation. The architecture integrates a nonlinear transmon coupler with a coupled Purcell-filter and notch-resonator subsystem, providing additional control over both the coherent interaction pathways and the engineered dissipative environment. The filter branch reshapes the effective interaction pathways, while the notch resonator further tailors the frequency response of the coupled filter network and preserves strong leakage-selective dissipation.
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Unlocking a fast adiabatic CZ gate and exact residual ZZZZZZ cancellation between fixed-frequency transmons using a floating tunable coupler
Tunable couplers in superconducting qubit architectures enable strong qubit-qubit interactions for two-qubit gates while suppressing unwanted coupling during single-qubit operations. However, achieving low error rates for fast two-qubit gates remains challenging, as suppressing leakage and non-adiabatic errors typically requires specialized qubit, coupler, or pulse designs, often at the expense of an idling ZZ=0ZZ=0ZZ=0 condition. In this work, we demonstrate that a symmetric floating tunable coupler provides a natural platform for fast, high-fidelity adiabatic controlled-Z (CZ) gates. Its favorable energy-level structure eliminates the conventional trade-off between rapid conditional-phase accumulation and adiabatic evolution while preserving exact cancellation of residual ZZZZZZ interaction at idling. This architecture exhibits intrinsic robustness to non-adiabatic transitions, even under simple flux modulation waveforms. To push performance at short gate durations, where maintaining adiabaticity becomes more challenging despite the favorable level structure, we introduce pulse-shaping techniques based on the instantaneous adiabatic factor that further suppress non-adiabatic errors. We experimentally realize a 24 ns adiabatic CZ gate with fidelity exceeding 99.9% and stable operation over several hours.
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