发表机构
Nanjing University of Posts and Telecommunications; Nanjing University; Shanxi University; Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（南京邮电大学; 南京大学; 山西大学; 粤港澳大湾区量子科学中心）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
针对磁通量子比特与Transmon的混合电路，通过仅调制耦合器磁通实现25纳秒内保真度超99.99%的无微波CZ门，并有效抑制旁观者误差。
AI 中文摘要
结合不同类型量子比特的混合超导架构为利用其互补优势提供了一个有前景的平台，然而由于强非线性和残余的量子比特间相互作用，高保真纠缠门仍然具有挑战性。在此，我们提出了一种针对混合电路的高保真受控-Z（CZ）门，该电路包含一个磁通量子比特、一个固定频率的Transmon量子比特和一个磁通可调的Transmon耦合器。通过仅调制施加在耦合器上的外部磁通，量子比特间的相互作用被动态设计用于条件相位累积，同时在空闲时抑制残余相互作用，从而减轻旁观者引起的错误。我们采用低维傅里叶-余弦脉冲参数化和物理动机驱动的代价函数，以独立抑制条件相位误差和从计算子空间的泄漏。数值模拟表明，可以在25纳秒内实现无微波的CZ门，平均门保真度超过99.99%，泄漏低于10^-5。使用实验相关的超导量子比特参数并考虑退相干，所提出的方案保持了约99.9%的CZ门保真度。我们进一步将分析扩展到更大的耦合架构，发现在存在旁观者量子比特的情况下，CZ门的不保真度保持在10^-4以下。这些结果确立了单参数磁通控制作为在异质超导量子架构中实现高保真纠缠门的简单而稳健的方法。
英文摘要
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 $25 \mathrm{ns}$, 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.
