发表机构
IBM Quantum, IBM Research Cambridge; IBM Quantum, Thomas J. Watson Research Center; University of Wisconsin-Madison（IBM量子，IBM剑桥研究院; IBM量子，托马斯·J·沃森研究中心; 威斯康星大学麦迪逊分校）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文提出统一错误检测框架，通过后选择开销换取保真度，无需额外辅助比特，使动态电路原语（如扇出门、W态制备）在超导处理器上实现超越光锥的长程纠缠，实验验证了100比特Bell态保真度超越认证阈值。
AI 中文摘要
动态电路通过中途测量和经典前馈增强酉操作，能够在恒定深度内产生长程纠缠，从而实现从非平凡态制备到多量子比特纠缠门等低深度原语。然而，摆脱酉电路的光锥约束是有代价的：这些原语通常需要随系统规模扩展的中途测量次数，这些测量与前馈延迟一起可能引入误差，从而削弱它们所依赖的长程纠缠。在此，我们通过展示许多此类原语在统一框架下可支持一种错误检测方案来缓解这一矛盾，该方案以保真度换取后选择开销，且无需额外辅助量子比特。我们的框架因此统一并升级了包括扇出门、多量子比特泡利旋转、W态和更高权重Dicke态的制备以及某些非正规矩阵积态在内的一大类原语。我们还引入了Hadamard测试的降深度、错误检测实现，将动态电路的应用场景扩展到关键算法原语。最后，我们通过在超导量子处理器上的实验确立了该方案的实际效用。我们演示了在100量子比特链上跨越长程纠缠Bell态的错误检测制备，保真度为$F=0.59\pm0.02$，超过了基线动态电路实现无法达到的纠缠认证阈值$F>0.5$（基线为$0.39\pm0.01$）。此外，我们通过消耗最多40量子比特的GHZ态，演示了最多20量子比特W态的恒定深度制备，在研究的最大规模上发现绝对保真度提升$\Delta F\approx 0.2$。总之，这些结果使低深度动态电路原语在当前硬件上触手可及。
英文摘要
Dynamic circuits, which augment unitary operations with mid-circuit measurements and classical feedforward, can generate long-range entanglement in constant depth, enabling low-depth primitives ranging from nontrivial state preparation to many-qubit entangling gates. Escaping the light-cone constraints of unitary circuits, however, comes at a cost: these primitives typically require a number of mid-circuit measurements that scales with system size and that, together with feedforward latency, can introduce errors that degrade the long-range entanglement they rely on. Here, we alleviate this tension by showing that many such primitives, when cast into a common framework, admit an error-detection scheme that trades infidelity for postselection overhead with no additional ancillas. Our framework thus unifies and upgrades a broad class of primitives including fan-out gates, multi-qubit Pauli rotations, the preparation of W and higher-weight Dicke states, and certain non-normal matrix product states. We also introduce a reduced-depth, error-detected implementation of the Hadamard test, extending the use cases of dynamic circuits to a key algorithmic primitive. Finally, we establish the practical utility of our scheme through experiments on a superconducting quantum processor. We demonstrate the error-detected preparation of a long-range entangled Bell pair spanning a 100-qubit chain with fidelity $F=0.59\pm0.02$, surpassing the entanglement-certification threshold $F>0.5$ that the baseline dynamic-circuit implementation fails to reach ($0.39\pm0.01$). Separately, we demonstrate constant-depth preparation of W states of up to 20 qubits by consuming GHZ states of up to 40 qubits, finding absolute fidelity improvements of $ΔF\approx 0.2$ across the largest sizes studied. Altogether, these results bring low-depth dynamic-circuit primitives within practical reach on present-day hardware.
