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<p>Demonstrating subthreshold scaling of a surface-code quantum memory on hardware whose native connectivity does not match the code remains a central challenge. We address this on IBM heavy-hex superconducting processors by co-designing the code embedding and control: a depth-minimizing SWAP-based “fold-unfold” embedding that uses bridge ancillas, together with robust, gap-aware dynamical decoupling (DD). We show that anisotropic scaling from distance 3 to (<i>d</i><sub><i>x</i></sub>&#xa0;=&#xa0;3,&#xa0;<i>d</i><sub><i>z</i></sub>&#xa0;=&#xa0;5) and (<i>d</i><sub><i>x</i></sub>&#xa0;=&#xa0;5,&#xa0;<i>d</i><sub><i>z</i></sub>&#xa0;=&#xa0;3) improves protection of <i>Z</i>- and <i>X</i>-basis logical states, respectively, and a calibrated &#xa0;~&#xa0;30% noise reduction would enable isotropic (5,&#xa0;5)-versus-(3,&#xa0;3) scaling on this heavy-hex layout. We show that DD suppresses coherent <i>Z</i><i>Z</i> crosstalk and non-Markovian dephasing during idle gaps, eliminating spurious subthreshold claims. To quantify performance, we derive an entanglement fidelity metric which reveals that widely used single-parameter suppression-factor fits can mischaracterize code performance. Our results provide a path to robust subthreshold surface-code scaling through optimized DD on non-native architectures.</p>

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Surface code scaling on heavy-hex superconducting quantum processors

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月29日

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  • Arian Vezvaee,
  • Cesar Benito,
  • Mario Morford-Oberst,
  • Alejandro Bermudez,
  • Daniel A. Lidar

摘要

Demonstrating subthreshold scaling of a surface-code quantum memory on hardware whose native connectivity does not match the code remains a central challenge. We address this on IBM heavy-hex superconducting processors by co-designing the code embedding and control: a depth-minimizing SWAP-based “fold-unfold” embedding that uses bridge ancillas, together with robust, gap-aware dynamical decoupling (DD). We show that anisotropic scaling from distance 3 to (dx = 3, dz = 5) and (dx = 5, dz = 3) improves protection of Z- and X-basis logical states, respectively, and a calibrated  ~ 30% noise reduction would enable isotropic (5, 5)-versus-(3, 3) scaling on this heavy-hex layout. We show that DD suppresses coherent ZZ crosstalk and non-Markovian dephasing during idle gaps, eliminating spurious subthreshold claims. To quantify performance, we derive an entanglement fidelity metric which reveals that widely used single-parameter suppression-factor fits can mischaracterize code performance. Our results provide a path to robust subthreshold surface-code scaling through optimized DD on non-native architectures.