量子計算の効率と使い易さを高める回路最適化手法
— 制御量子ゲートを“まとめて”最適化する理論的提案 —Circuit optimization techniques to improve the efficiency and ease of use of quantum computing
— A theoretical proposal for collectively optimizing control quantum gates —
近年注目されている変分量子アルゴリズム（VQA）
しかしその一方で、回路が複雑になるほど最適化が難しくなり、
本研究では、
• 必要な測定回数を大幅に抑えつつ効率的な最適化
• より浅い（短い）
• 量子計算になじみのない人を量子回路設計の煩わしさから解放
することが可能になります。
「幅広い応用で有効性を確認」
提案手法は、以下のような代表的な量子計算タスクに適用され、
• イジング模型などの物理モデルの変分計算
• 分子の電子状態計算（分子の基底エネルギー計算）
• 量子回路の忠実度最大化
• 量子状態の時間発展を表す量子回路のコンパイル（回路圧縮）
特に、量子化学で重要な粒子数保存ゲートにも自然に拡張でき、
「現行の量子コンピュータの性能を引き出す重要な一歩」
現在の量子コンピュータはノイズや回路深さに制限がある「
• 計算を安定に進める
• 回路を短く保つ
• 実用的な量子アルゴリズムを実装する
ための基盤理論を提供するものです。
量子計算の実用化に向けて、「どのように量子回路を設計・
Quantum computers perform desired calculations by designing circuits composed of “quantum gates” that act on qubits.
Variational quantum algorithms (VQA), which have attracted attention in recent years, optimize quantum circuit parameters using classical computers, and are expected to be applied to quantum chemistry calculations and physical property simulations.
However, as circuits become more complex, optimization becomes increasingly difficult, leading to computational stalls, a major challenge known as the “Barren-Plateau problem.”
To address this issue, this research proposes a new theoretical method that allows controlled quantum gates, which were previously not considered a degree of freedom for optimization, to be given parameters and optimized accordingly.
This method enables:
• Efficient optimization while significantly reducing the number of measurements required
• Achieving versatility by using shallower (shorter) quantum circuits, allowing application to a variety of problem settings
• Relieving people unfamiliar with quantum computing from the hassle of quantum circuit design.
“Validity Confirmed in a Wide Range of Applications”
The proposed method was applied to the following representative quantum computing tasks, and its effectiveness was numerically demonstrated.
• Variational calculations of physical models such as the Ising model
• Calculation of molecular electronic states (calculation of molecular ground state energies)
• Maximizing the fidelity of quantum circuits
• Compiling quantum circuits that represent the time evolution of quantum states (circuit compression)
In particular, it has been shown that this method can be naturally extended to number-conserving gates, which are important in quantum chemistry, enabling high-precision calculations with fewer gates than conventional methods.
“An important step toward unlocking the performance of current quantum computers”
Current quantum computers are “NISQ (Non-Fault-Tolerant)” devices, which are limited in terms of noise and circuit depth. The results of this research provide a fundamental theory for:
• Stable computation
• Maintaining short circuit lengths
• Implementing practical quantum algorithms
This research result offers a new solution to the fundamental question of “how to design and optimize quantum circuits” toward the practical application of quantum computing.
Article Information
Publication: Quantum Science and Technology 11, 015046 (2026)
Title: Optimizing a parameterized controlled gate using free quaternion selection
Authors: Hiroyoshi Kurogi, Katsuhiro Endo, Yuki Sato, Michihiko Sugawara, Kaito Wada, Kenji Sugisaki, Shu Kanno, Hiroshi C Watanabe, Haruyuki Nakano
DOI: https://iopscience.iop.org/article/10.1088/2058-9565/ae379d
