2025-11-26 東京大学
2種類の量子エラー訂正符号を用いて規模の抑制と高速な計算を実現する誤り耐性型量子コンピュータ
＜関連情報＞
- https://www.t.u-tokyo.ac.jp/press/pr2025-11-26-002
- https://www.t.u-tokyo.ac.jp/hubfs/press-release/2025/1126/002/text.pdf
- https://www.nature.com/articles/s41567-025-03102-5
多重対数時間と定数の空間オーバーヘッドを持つフォールトトレラントな量子計算 Fault-tolerant quantum computation with polylogarithmic time and constant space overheads
Shiro Tamiya,Masato Koashi & Hayata Yamasaki
Nature Physics Published:26 November 2025
DOI:https://doi.org/10.1038/s41567-025-03102-5
Abstract
A major challenge in fault-tolerant quantum computation is to reduce both the space overhead, that is, the large number of physical qubits per logical qubit, and the time overhead, that is, the long physical gate sequences needed to implement a logical gate. Here we prove that a protocol using non-vanishing-rate quantum low-density parity-check (QLDPC) codes, combined with concatenated Steane codes, achieves constant space overhead and polylogarithmic time overhead, even when accounting for the required classical processing. This protocol offers an improvement over existing constant-space-overhead protocols. To prove our result, we develop a technique that we call partial circuit reduction, which enables error analysis for the entire fault-tolerant circuit by examining smaller parts composed of a few gadgets. With this approach, we resolve a logical gap in the existing arguments for the threshold theorem for the constant-space-overhead protocol with QLDPC codes and complete its proof. Our work establishes that the QLDPC-code-based approach can realize fault-tolerant quantum computation with a negligibly small slowdown and a bounded overhead of physical qubits.
