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Demonstrating Coherent Quantum Routers for Bucket-Brigade Quantum Random Access Memory on a Superconducting Processor

Sheng Zhang1,2,3, Yun-Jie Wang4, Peng Wang1,2,3, Ren-Ze Zhao1,2, Xiao-Yan Yang1,2, Ze-An Zhao1,2, Tian-Le Wang1,2, Hai-Feng Zhang1,2, Zhi-Fei Li1,2 et al.

Yuan Wu1,2, Hao-Ran Tao5, Liang-Liang Guo5, Lei Du5, Chi Zhang5, Zhi-Long Jia5, Wei-Cheng Kong5, Zhuo-Zhi Zhang1,2,3, Xiang-Xiang Song1,2,3, Yu-Chun Wu1,2,6, Zhao-Yun Chen6,*, Peng Duan1,2,†, and Guo-Ping Guo1,2,5,‡

  • *Contact author: chenzhaoyun@iai.ustc.edu.cn
  • †Contact author: pengduan@ustc.edu.cn
  • ‡Contact author: gpguo@ustc.edu.cn
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Phys. Rev. X 16, 031051 – Published 25 August, 2026

DOI: https://doi.org/10.1103/h5m3-qrn9

Abstract

Quantum routers (QRouters) are essential components of bucket-brigade quantum random access memory (QRAM), enabling quantum applications such as Grover’s search and quantum machine learning. Despite significant theoretical advances, achieving scalable and coherent QRouters experimentally remains challenging. Here, we demonstrate coherent quantum routers using a superconducting quantum processor, laying a practical foundation for scalable QRAM systems. The quantum router at the core of our implementation utilizes the transition composite gate scheme, wherein auxiliary energy levels temporarily mediate conditional interactions. By leveraging two-qubit primitives in its construction, the quantum router preserves straightforward initialization and calibration while significantly reducing circuit depth and duration relative to traditional gate-based decompositions. Moreover, by encoding routing addresses in the nonadjacent qutrit states |0⟩ and |2⟩, our design inherently enables erasure-detection capability, providing efficient postselection to mitigate routing errors. Experimentally, we achieve an average fidelity of 94.8% across three individual QRouters, and we validate scalability through a two-layer quantum routing network achieving a fidelity of 82.4%. Our results represent a significant advancement in quantum routing technology, providing enhanced fidelity, built-in error resilience, and practical scalability crucial for the development of future QRAM and large-scale quantum computing architectures.

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Physics Subject Headings (PhySH)

Popular Summary

Quantum random access memory requires coherent quantum routers to direct information along superposed address paths, but physical implementations suffer from deep circuit overheads and hardware noise. We demonstrate coherent quantum routers on a superconducting processor using a transition composite gate scheme that leverages auxiliary energy levels to streamline gate operations and enables inherent error erasure via postselection. Our approach reduces circuit depth and operational complexity, allowing us to achieve high routing fidelity across individual devices and a cascaded two-layer network. Furthermore, we show that quantum information transfers accurately when routing addresses are prepared in arbitrary quantum superpositions. Our work establishes a hardware-efficient framework that advances the development of scalable quantum memory and large-scale quantum networks.

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Phys. Rev. X 16, 031051– Published 25 August, 2026
  • Received 27 May 2025
  • Revised 16 April 2026
  • Accepted 24 July 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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