Curator's Take
AI Commentary
This article shows that many dynamic‑circuit primitives—such as fan‑out gates and multi‑qubit Pauli rotations—can be equipped with a lightweight error‑detection layer that trades a modest post‑selection cost for dramatically higher fidelity without adding extra ancilla qubits. By demonstrating an error‑detected Bell‑pair preparation across a 100‑qubit superconducting chain with F≈0.59, the work proves that long‑range entanglement can be generated in constant depth even on noisy hardware, a key step toward practical low‑depth algorithms and quantum‑enhanced sensing. The approach dovetails with recent pushes to integrate mid‑circuit measurement and feedforward into near‑term devices, though its reliance on postselection means scaling will still require careful management of success probabilities.
— Mark Eatherly
Summary
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.