Quantum Computation SGセミナー

Exploring Noisy Quantum Thermodynamical Processes via the Depolarizing-Channel Approximation

2026年10月28日(水) 16:00 - 17:00

Jian Li (Ph.D. Student, Center for Theoretical Physics, Polish Academy of Sciences, Poland)

Noise and errors are unavoidable in any realistic quantum process, including processes designed to reduce noise and errors in the first place. In particular, quantum thermodynamical protocols for cooling can be significantly affected, potentially altering both their performance and efficiency. Analytically characterizing the impact of such errors becomes increasingly challenging as the system size grows, particularly in deep quantum circuits where noise can accumulate in complex ways. To address this, we introduce a general framework for approximating the cumulative effect of gate-dependent noise using a global depolarizing channel. We specify the regime in which this approximation provides a reliable description of the noisy dynamics. Applying our framework to the thermodynamical two-sort algorithmic cooling (TSAC) protocol, we analytically derive its asymptotic cooling limit in the presence of noise. Using the cooling limit, the optimal cooling performance is achieved by a finite number of qubits --- distinguished from the conventional noiseless TSAC protocol by an infinite number of qubits --- and fundamental bounds on the achievable ground-state population are derived. This approach opens new avenues for exploring noisy quantum thermodynamical processes.

会場: via Zoom

イベント公式言語: 英語

Data assimilation with the full covariance on a fault-tolerant quantum computer

2026年10月21日(水) 15:00 - 16:30

三好 建正 (理化学研究所 計算科学研究センター (R-CCS) データ同化研究チーム チームプリンシパル)

Data assimilation (DA) is the core of numerical weather prediction, estimating an accurate state of the weather model by combining forecast and observations. DA provides a general approach and is being applied to various problems beyond weather. The state estimation is a linear solve where the background error covariance B is essential. Since B is an N × N matrix with N ~ 10^8 to 10^9 for the weather model, the full B has never been used. For decades the community has approximated B by parameterized covariance models or by low-rank ensembles with localization and thereby discards genuine structures that we know exist from large-ensemble experiments. Fault-tolerant quantum computing (FTQC) could remove this constraint, since quantum linear system solvers scale polylogarithmically in N. However, the DA equations cannot be applied naively: the observation-space form squares the block-encoding normalization factor for observations with wide footprints, and the state-space form contains B^{-1}, whose condition number ~10^8. This talk presents a combined state-observation formulation that avoids both difficulties: it contains no B^{-1}, and its condition number is governed by the observation error covariance R rather than by B. Numerical experiments with the Lorenz-96 model reproduce the exact solution with the full B and outperform localized ensemble Kalman filters. The formulation assumes that B and the observation operator are provided as block-encoded oracles, and how to construct such oracles without ever storing the matrices is the open problem. A first concrete target is an end-to-end demonstration with Lorenz-96 on a quantum simulator with only seven data qubits, which I would like to discuss with the iTHEMS community.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Magic state distillation and cultivation #2

2026年9月10日(木) 13:30 - 17:00

平野 裕 (Research Scientist, Nanofiber Quantum Technologies)

本セミナーでは、誤り訂正において重要な概念である"distillation"や"cultivation"について講義形式で説明していただきます。対面およびZoomのハイブリット形式ですが、機器の都合によりZoom配信が困難な可能性がありますので、対面参加を推奨しています。 講義にあたって、以下の事項の理解は前提とします。 A. 量子計算の基礎 (quantum state, quantum gate, measurement, quantum circuit, Pauli gate, Clifford gate, Non-Clifford gate) B. 量子エラー訂正の概念（quantum error correction, decoding） これらは、Nielsen and Chuang [1] が最もまとまっています。 以下の事項は講義で紹介しますが、理解しているとより良いかもしれません。 C. 論理ゲート及び FTQC [1] D. surface code [2], [3] E. lattice surgery [4] なお、B-E については、https://docs.qc500.org/ の "Quantum Error Correction" にもまとまっています。 [1]: M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information (Cambridge University Press, 2000). [2]: A. Yu. Kitaev, “Fault-tolerant quantum computation by anyons,” Ann. Phys. 303, 2–30 (2003). [3]: H. Bombin and M. A. Martin-Delgado, "Optimal Resources for Topological 2D Stabilizer Codes: Comparative Study," Phys. Rev. A 76, 012305 (2007). [4]: D. Horsman, A. G. Fowler, S. Devitt, and R. Van Meter, "Surface code quantum computing by lattice surgery," New J. Phys. 14, 123011 (2012).

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 日本語

Magic state distillation and cultivation #1

2026年9月10日(木) 10:30 - 12:00

平野 裕 (Research Scientist, Nanofiber Quantum Technologies)

本セミナーでは、誤り訂正において重要な概念である"distillation"や"cultivation"について講義形式で説明していただきます。対面およびZoomのハイブリット形式ですが、機器の都合によりZoom配信が困難な可能性がありますので、対面参加を推奨しています。 講義にあたって、以下の事項の理解は前提とします。 A. 量子計算の基礎 (quantum state, quantum gate, measurement, quantum circuit, Pauli gate, Clifford gate, Non-Clifford gate) B. 量子エラー訂正の概念（quantum error correction, decoding） これらは、Nielsen and Chuang [1] が最もまとまっています。 以下の事項は講義で紹介しますが、理解しているとより良いかもしれません。 C. 論理ゲート及び FTQC [1] D. surface code [2], [3] E. lattice surgery [4] なお、B-E については、https://docs.qc500.org/ の "Quantum Error Correction" にもまとまっています。 [1]: M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information (Cambridge University Press, 2000). [2]: A. Yu. Kitaev, “Fault-tolerant quantum computation by anyons,” Ann. Phys. 303, 2–30 (2003). [3]: H. Bombin and M. A. Martin-Delgado, "Optimal Resources for Topological 2D Stabilizer Codes: Comparative Study," Phys. Rev. A 76, 012305 (2007). [4]: D. Horsman, A. G. Fowler, S. Devitt, and R. Van Meter, "Surface code quantum computing by lattice surgery," New J. Phys. 14, 123011 (2012).

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 日本語

Overview of quantum error correcting codes

2026年7月7日(火) 15:00 - 16:30

松浦 孝弥 (理化学研究所 量子コンピュータ研究センター (RQC) 量子計算理論研究チーム 特別研究員)

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Gauge and Homological Structures in Quantum Error Correction

2026年7月2日(木) 16:00 - 17:00

春名 純一 (京都大学 大学院情報学研究科 特定研究員)

Gauge theory, quantum error correction, and homology theory share a common mathematical backbone that, when made explicit, becomes a practical toolkit for fault-tolerant quantum computation. A CSS code is naturally a length-2 chain complex in which the X-stabilizers act as Gauss-law generators and the code space is the gauge-invariant subspace, the toric code being the prototypical realization of a Z_2 lattice gauge theory. Building on this correspondence, I present two results. First, I introduce a gauge-field formalism in which logical gates are written as exponentials of polynomials of operator-valued cochains—the lattice gauge fields—on the underlying chain complex. Requiring no special structure on the code, the construction applies to general CSS codes and yields explicit physical-gate decompositions of logical S, H, CZ, and T gates whose action depends only on the cohomology class of the logical qubits. Second, I show that the transversal implementability of logical Pauli-Z rotations has a purely homological origin: their logical action is classified by a Z_{2^m}-module extending logical Pauli operators to higher levels of the Clifford hierarchy, and transversality is governed by compatibility and lifting obstructions on homology classes beyond the usual Z_2 coefficient. From a high-energy-physics viewpoint, a level-m transversal gate is a gauge-invariant "2^{m-1}-th root of a Wilson loop." Together these results offer a unifying language for designing logical gates and point toward fault-tolerance from lattice gauge theory and algebraic topology. This talk is based on arXiv:2511.15224 and arXiv:2602.14499.

会場: セミナー室 (359号室) 3階 359号室とZoomのハイブリッド開催

イベント公式言語: 英語

Realizing two-dimensional discrete time crystals on a digital quantum computer

2026年5月19日(火) 15:00 - 16:30

新城 一矢 (理化学研究所 創発物性科学研究センター (CEMS) 計算物性物理研究室 研究員)

This work was featured in a RIKEN press release. For details, please see the related link.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Uniform Matrix Product States for Hamiltonian Lattice Gauge Theories: Methods and Applications

2026年4月28日(火) 16:00 - 17:30

藤倉 浩平 (京都大学 基礎物理学研究所 基研特任助教)

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Quantum Computing of Molecular Properties for Fundamental Physics

2026年4月21日(火) 16:30 - 18:00

プラディヨット・プリタム・サフー (東京大学 大学院理学系研究科 大学院外国人研究生)

This is the self-introduction talk by Pradyot Pritam Sahoo. Pradyot is a Student Trainee in iTHEMS.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Quantum Computation SG seminar 2026

2026年4月7日(火) 15:00 - 17:00

Self-introductions (name + research interests) and discussion about study group activities in FY2026.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Quantum Electrodynamics of Strong Laser-Matter Interaction: The Ongoing Journey and Beyond

2026年2月10日(火) 10:00 - 12:00

Ciappina Marcelo (Professor, Guangdong Technion, China)

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Analog variational quantum eigensolver for neutral atomic quantum simulators

2026年1月20日(火) 10:00 - 12:00

長尾 一馬 (理化学研究所 計算科学研究センター (R-CCS) 量子系物質科学研究チーム 特別研究員)

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Phase transition in parametrized quantum circuits

2025年12月16日(火) 10:00 - 12:00

王 啸洋 (理化学研究所 数理創造研究センター (iTHEMS) 数理展開部門 量子数理科学チーム 特別研究員)

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Quantum Computing Journal Club #1

2025年12月2日(火) 10:00 - 12:00

Bring a recent paper on quantum computing for discussion. No need to prepare slides.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Chiral anomaly in Hamiltonian lattice gauge theory

2025年11月18日(火) 10:00 - 12:00

山本 新 (理化学研究所 数理創造研究センター (iTHEMS) 数理展開部門 量子数理科学チーム 上級研究員)

The 4th quantum computing gathering organized by Quantum Computing Study Group

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Simulating nonequilibirum quantum dynamics on Reimei

2025年10月21日(火) 10:00 - 12:00

早田 智也 (慶応義塾大学 医学部 准教授)

This is the third quantum computing gathering hold by quantum computing study group.

会場: via Zoom / セミナー室 (359号室)

イベント公式言語: 英語

Beyond-classical simulations using a quantum annealer

2025年5月23日(金) 10:00 - 11:15

Alberto Nocera (Senior Staff Scientist, Stewart Blusson Quantum Matter Institute, The University of British Columbia, Canada)

Join us for an electrifying online zoom talk featuring Dr. Alberto Nocera, the author of the groundbreaking Science article detailing D-Wave's quantum annealer outperforming classical supercomputers in simulating complex magnetic materials. D-Wave’s quantum annealer uses quantum annealing to efficiently solve optimization problems by finding low-energy states of complex systems. Unlike IBM and Quantinuum’s gate-based quantum computers, which are universal and execute algorithms via quantum circuits, D-Wave’s system is specialized but currently more scalable and practical for certain applications. While gate-based systems are still limited by noise and error rates, D-Wave’s annealer recently demonstrated a quantum advantage in simulating magnetic materials, outperforming classical supercomputers. This highlights a key difference: D-Wave excels in specific tasks today, while IBM and Quantinuum aim for broader, long-term quantum computational capabilities. Talk Abstract: Solving the equations governing the dynamics of interacting many-particle quantum systems is one of the biggest challenges in modern science. In 1982, Richard P. Feynman envisioned that the best way to emulate the behavior of many-particle quantum systems is to use another quantum system, starting the field of quantum simulation. Rather than seeking to solve the fundamental equations of quantum mechanics using conventional or classical computers, in quantum simulation one seeks to simulate a quantum system using an "analog device" mimicking its behavior, hoping to access solutions which are not easy to compute otherwise. In this talk, using the quantum annealing simulator device developed by D-Wave as a main tool, I will show that the use of tensor network methods has a key role in quantum simulation: besides benchmarking the quantum device, they can assess and help establishing its functionality beyond the classically simulatable regime [1].

会場: via Zoom

イベント公式言語: 英語

Exploiting hidden low-rank structures in quantum field theories

2025年2月24日(月) 13:00 - 14:30

品岡 寛 (埼玉大学)

Tensor networks are a powerful tool for compressing wave functions and density matrices of quantum systems in physics. Recent developments have shown that tensor network techniques can efficiently compress many functions beyond these traditional objects. Notable examples include the solutions to turbulence in Navier–Stokes equations [1] and the computation of Feynman diagrams [2,3]. These advancements have heralded a new era in the use of tensor networks for expediting the resolution of various complex equations in physics. This talk will provide an overview of our work utilizing tensor networks for computations based on quantum field theories. First, we will introduce the Quantics/quantized Tensor Train (QTT) representation [3,4] for compressing the space-time dependence of correlation functions in quantum systems [5], leveraging inherent length-scale separation for efficient representation. Second, we will present a robust tool named "Quantics Tensor Cross Interpolation" [6], which learns a quantics low-rank representation of a given function. Applications include the computation of Brillouin zone integrals [6] and integration of complex self-energy Feynman diagrams for multiorbital electron-phonon impurity models [7]. Finally, we will introduce new algorithms [8] and open-source libraries [9] for tensor cross interpolation.

会場: via Zoom / Hong Kong University Science and Technology

イベント公式言語: 英語