Seminars / Visitors

12th PIS Lab Seminar [2025/4/3 (Thu.) 16:00 - 17:00 @ 3F902 Seminar Room]

皆川 慎太郎 [筑波大学 システム情報系 博士研究員]

量子測定過程における両立不可能性および熱力学

測定によって系の状態が変化するという一連のプロセスを測定過程という。特に量子測定においては測定が系の状態を乱すケースが多いため、測定過程の議論がしばしば重要になる。本セミナーの第一部では量子測定過程の両立不可能性を、第二部では量子測定過程と熱力学の関係を扱う。

(第一部)複数のプロセスが両立可能であるとは、それらをまとめた一つのプロセスが存在することを意味する。量子論には両立可能ではない測定が存在し、これは古典論と著しく違う性質である。ところで量子測定過程においては、これまで複数の異なる両立可能性の概念が提案されている[1]。本発表では、動的リソース理論の観点から、異なる両立可能性の概念を統合し両立不可能性を定量化する方法を紹介する[2]。

(第二部)測定によって取得した情報を用いたフィードバック制御という操作が、熱力学第二法則を破るように見えるという問題は、「マクスウェルの悪魔のパラドックス」として熱・統計力学の基礎において長年議論されてきた。このパラドックスの解決として広く受け入れられているのは、フィードバック制御で第二法則を破った分は、測定と測定で得たデータのリセットによって埋め合わされるというものである[3]。本発表では、フィードバック制御のための測定過程を、量子論で実現可能な一般の測定過程の数学的表現であるCP-インストルメント[4]として解析を行い、測定 +フィードバック制御+リセットの一連のプロセスと第二法則が整合する条件を議論する[5]。

<参考文献>

[1] A. Mitra and M. Farkas, Phys. Rev. A 105, 052202 (2022).

[2] F. Buscemi, K. Kobayashi, S. Minagawa, P. Perinotti, and A. Tosini, Quantum 7, 1035 (2023).

[3] T. Sagawa and M. Ueda, Phys. Rev. Lett. 102, 250602 (2009).

[4] M. Ozawa, J. Math. Phys. 25, 79 (1984).
[5] S. Minagawa, M. H. Mohammady, K. Sakai, K. Kato, and F. Buscemi, npj Quantum Information 11, 18 (2025). 

13th PIS Lab Seminar [2025/6/4 (Wed.) 15:30 - 16:30  @ 3F902 Seminar Room] CS Colloquium

Sandro Wimberger  [Dipartimento di Scienze Matematiche, Fisiche Informatiche, Università di Parma & INFN, Sezione Milano-Bicocca, Parma group]

Quantum Walks with Spinor Bose-Einstein Condensate

Quantum randomness is intrinsically different from classical stochasticity since it is affected by interference and entanglement. Both make quantum walks promising candidates for the implementation of quantum computational algorithms and as a sensitive detector of interference. We present a discrete-time quantum walk that uses the momentum of ultra-cold rubidium atoms as the walk space and two internal atomic states as the coin degree of freedom [1]. We demonstrate the distinctive features of a quantum walk, contrasting them to a classical walk. By manipulating either the walk or coin operator we show how the walk dynamics can be biased and even reversed. Our quantum walk provides a platform for a wide range of applications such as quantum search, the investigation of decoherence [2], and the observation of topological phases [3].

References:

[1] S. Dadras, A. Gresch, C. Groiseau, S. Wimberger, G.S. Summy, Quantum Walk in Momentum Space with a Bose-Einstein Condensate, Phys. Rev. Lett. 121, 070402 (2018)

[2] J.H. Clark, C. Groiseau, Z.N. Shaw, S. Dadras, C. Binegar, S. Wimberger, G.S. Summy, Y. Liu, Quantum to Classical Walk Transitions Tuned by Spontaneous Emissions, Phys. Rev. Research 3, 043062 (2021)

[3] N. Bolik, C. Groiseau, J.H. Clark, G.S. Summy, Y. Liu, S. Wimberger, Detecting Topological Phase Transitions in a Double Kicked Quantum Rotor, Phys. Rev. A 106, 043318 (2022)

14th PIS Lab Seminar [2025/6/19 (Thu.) 18:15 - 19:30 @ SB 0112 Lecture Room] CS Colloquium

谷 茉莉  [京都大学　大学院理学研究科  物理学・宇宙物理学専攻　助教]

身近な現象に潜む物理〜弾性ひも／シートが作るかたち〜

 私たちは、子どもの頃から弾性が関わった様々な自然現象を目にしてきている。たとえば、葉の上の朝露や、朝顔の蔓の巻き付き、濡れた髪の毛の接着などは、表面張力や弾性変形、その両者が効いている現象である。いずれも、小さな力で大きくしなやかに変形して、多彩なかたちを生み出す。このようなかたちと力の関係は、古典的な力学の問題でありつつ、その物理的な奥の深さと応用性から、近年改めて注目が集まっている。特に、表面張力と弾性変形がカップリングした現象はelasto-capillarity （弾性毛管現象）として2000年頃から研究が盛んに行われてきた[1]。本セミナーでは、弾性ひもやシートが見せるかたちに着目して[2-5]、そこに潜む物理法則とメカニズムについて議論する。また、動く「ひも」状の分子（微小管）に関する最近の結果についても、簡単に紹介する予定である。

[1] J. Bico, E. Reyssat and B. Roman, Annual Review of Fluid Mechanics 50, 629 (2018).

[2] M. Tani and H. Wada, Phys. Rev. Lett. 132(5), 058204 (2024).

[3] 谷茉莉, 和田浩史, 日本物理学会誌80, 62 (2025).

[4] H. Bense, M. Tani, M. Saint-Jean, E. Reyssat, B. Roman and J. Bico, Soft matter 16, 1961 (2020).

[5] M. Tani, J.-W. Hong, T. Tomizawa, E. Lepoivre, J. Bico and B. Roman, Extreme Mech. Lett. 71, 102 (2024).

15th PIS Lab Seminar [2025/7/18 (Fri.) 18:30 - 21:00 @ 3B棟プレゼンテーションルーム (3B213)]

18:30 - 19:00 田中 翔大 [Stanford University, freshman undergraduate student]

非線形現象としてのバイオリン

バイオリンは身近な楽器でありながら、複雑で多様な振る舞いをする。その背景にあるのは、摩擦の非線形性を伴う擦弦振動である。バイオリンの物理学は、古くからH. V. Helmholtz[1]、Lord Rayleigh[2]、C. V. Raman[3]を含む多くの物理学者の関心を集めてきた。寺田寅彦[4]もまた、自らがバイオリンの演奏者であると共にその物理学に惹かれた一人である。本セミナーでは、私が興味を持って研究している、バイオリンにおけるハーモニクス奏法(フラジオレット奏法)という演奏法の解析[5]を中心に、バイオリンの背後に潜む物理学について紹介する。

[1] H. V. Helmholtz. “Die Lehre von den Tonempfindungen als physiologische Grundlage für die Theorie der Musik (On the Sensations of Tone as a Physiological Basis for the Theory of Music)”, 2. Aufl. Braunschweig: CK und Verlag von Friedrich Vieweg und Sohn, 1865.

[2] L. Rayleigh. “The Theory of Sound.” UK, Macmillan, 1894.

[3] C. V. Raman. “On the mechanical theory of the vibrations of bowed strings and of musical instruments of the violin family, with experimental verification of the results.” Indian Association for the Cultivation of Science, 1918.

[4] 寺田寅彦. “手首の問題.” 中央公論. 1932.

[5] S. Tanaka, H. Kori, A. Ozawa; A mathematical study about the sustaining phenomenon of overtone in flageolet harmonics on bowed string instruments. Proc. Mtgs. Acoust. 4 December 2023; 52 (1): 035006 

19:00 - 19:30 清水 紘輔  [筑波大学 情報学群 情報メディア創成学類 2年]

Human Computer Interactionは何が新しいのか

Human Computer Interaction (HCI)が創出する「新しさ」は、なぜ理学系の研究者に伝わりにくいのか。本発表は、HCIが単に新しいモノを作るのではなく、人間と技術の「新しい関係性」を具体的に構成し、その価値を問う研究分野であることを考察する。その独自の研究プロセス──試作と評価の反復、数値化できない「体験」の重視、新しい「当たり前」の社会実装──が持つ内在的な関係性を、様々な事例を用いて議論する。これにより、HCIという知的な営みが、いかにして未来の可能性を具体的に構成していくかを示し、分野間の深い対話を目指す。

19:30 - 21:00 意見交換会 (軽食・飲み物あり)

共催：株式会社Litable

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6th PIS Lab Seminar [2024/5/27 (Mon.) 16:00 - 17:00 @ 3F 902 セミナー室] CS Colloquium

早川 龍 [京都大学白眉センター, 特定助教]

ガイド付きローカルハミルトニアンの計算複雑性

量子計算の有望な応用として、量子化学が挙げられる。本セミナーでは、量子化学に関係するタスクとして、物理に関係するようなハミルトニアンの基底状態のエネルギーを推定する問題の計算複雑性を議論する。通常の「ローカルハミルトニアン問題」では、問題の入力はローカルハミルトニアンだけであり、問題の難しさはQMA困難であるが、これに加えて、「ガイド状態」と呼ばれる、基底状態を近似することが保証されているような状態の古典記述が与えられている場合を考える。ガイド付きローカルハミルトニアン問題に関して、BQP完全性を示すことを通じて、量子計算量理論がどのように量子計算の古典計算の高速性について示唆を与えることができるかを紹介する。

7th PIS Lab Seminar [2024/8/23 (Fri.) 16:00 - 17:00 @ 3F 902 セミナー室] CS Colloquium

Takano Taira [the University of Tokyo, Institute of Industrial Science, JSPS Research fellow]

Non-Hermitian Physics in Open and Closed Quantum Systems

In this talk, I will provide an overview of my research contributions to two seemingly opposite disciplines that have gained attention through the common framework of non-Hermitian Hamiltonians: closed non-Hermitian quantum mechanics (or PT-symmetric quantum mechanics) and non-Hermitian physics as an effective theory of open quantum systems.

PT-symmetric quantum mechanics (PTQM) reconsiders one of the fundamental assumptions of quantum mechanics: that physical quantities must be Hermitian. Instead, PTQM introduces the concept of quasi-Hermitian operators. This concept, first introduced by Geyer et al. in 1990 [1] and popularized by Carl Bender and Stefan Boettcher in 1998 [2], has found successful applications in classical optical waveguide experiments. I will demonstrate that a time-dependent pseudo-Hermitian Hamiltonian is equivalent to a time-independent Hermitian Hamiltonian with moving boundary conditions [3].

Non-Hermitian Hamiltonians also emerge as effective theories in open quantum systems under several assumptions, such as the Born-Markov approximation and post-selection. I will introduce a lesser-known method to derive non-Hermitian Hamiltonians via the Feshbach formalism [4], which does not require approximations on the system. This approach leads to a non-linear non-Hermitian eigenvalue problem that can reveal non-Markovian effects, unobservable under the Born-Markov approximation. As a concrete example, I will briefly discuss our work on the spontaneous decay of a massless Dirac particle in an open system [5], illustrating how non-Markovianity manifests in the survival probability of the decaying particle.

[1] Scholtz, F. G., Geyer, H. B., & Hahne, F. J. W. (1992). Annals of Physics, 213(1), 74-101.
[2]   Bender, C. M., & Boettcher, S. (1998). Physical Review Letters, 80(24), 5243.
[3]   Fring, A., & Taira, T. (2023). Physical Review A, 108(1), 012222.
[4]   Feshbach, H. (1962). Annals of Physics, 19(2), 287-313.
[5]   Taira, T., Hatano, N., & Nishino, A. (2024). arXiv preprint arXiv:2406.17436.

8th PIS Lab Seminar [2024/11/18 (Mon.) 11:30 - 12:30 @ 3F 902 Seminar Room]

Sandu Popescu [University of Bristol, Professor of Physics]

A new look at quantum conservation laws

Conservation laws are some of the most important laws of physics. Having their origin in the symmetries of nature, conservation laws are present in all our physical theories. There are, however, significant differences between what conservation laws mean in these various theories. In quantum mechanics, which is a theory that is nondeterministic at a fundamental level, the conservation laws, as they are standardly formulated, do not refer to individual experiments but only to the statistics over a large ensemble of repeated identical experiments. Here, we take a step toward extending quantum conservation laws to individual cases.
[1] Y. Aharonov, S. Popescu, and D. Rohrlich, Proc. Natl. Acad. Sci. U.S.A.118 e1921529118 (2021)
[2] Y. Aharonov, S. Popescu, and D. Rohrlich, Proc. Natl. Acad. Sci. U.S.A.120 e2220810120 (2023)

9th PIS Lab Seminar [2025/1/9 (Thu.) 11:00 - 12:00 @ 3F 902 Seminar Room] CS Colloquium

上西 慧理子[慶応義塾大学量子コンピューティングセンター 特任講師]

サンプリングノイズを活用した変分量子アルゴリズムの鞍点脱出解析

現状の量子コンピュータでは、NISQ（Noisy Intermediate-Scale Quantum）デバイスが実用化に向けた有力なアプローチの一つとされており、その中で量子と古典を組み合わせたアルゴリズムが注目されています。特に、変分量子アルゴリズム（VQE）[1,2,3]は、 量子化学計算や分子シミュレーションといった中短期的な実用において有望な手法として位置づけられています。 しかし、VQEの課題として、局所解や鞍点に閉じ込められる問題が指摘されています。NISQデバイスでは測定回数が有限であるため、不可避なサンプリングノイズが存在します。我々は、このサンプリングノイズを「積極的に活用する」視点から、確率的勾配法（SGD）を用いたVQE最適化における鞍点脱出を解析しました。解析の結果、ノイズ強度が鞍点脱出時間に与える影響を明らかにし、ノイズ強度が増加するにつれて脱出時間がべき乗的に減少することを示しました。 また、VQEの離散的なダイナミクスを連続時間近似、いわゆる確率微分方程式 (SDE) を用いて理論的に説明し、この理論モデルがVQEの最適化におけるパラメータ調整に重要な指針を与えることを明らかにしました[4]。
　本講演では、量子コンピュータの現状を概観し、変分量子アルゴリズムの概略と問題点の解説、そして我々の最新の研究成果について紹介します。
[1] M. Cerezo, A. Arrasmith, R. Babbush et al. "Variational quantum algorithms", Nat Rev Phys 3, 625–644 (2021).
[2] H. C. Watanabe, R. Raymond, Y. Ohnishi, E. Kaminishi and M. Sugawara, "Optimizing Parameterized Quantum Circuits With Free-Axis Single-Qubit Gates", IEEE Transactions on Quantum Engineering 4,  3101016 (2023)
[3] K. Wada, R. Raymond, Y. Ohnishi, E. Kaminishi, M. Sugawara, N. Yamamoto and H. C. Watanabe, "Simulating time evolution with fully optimized single-qubit gates on parametrized quantum circuits", Physical Review A 105, 062421 (2022)
[4] E. Kaminishi, T. Mori, M. Sugawara and N. Yamamoto, "Impact of Measurement Noise on Escaping Saddles in Variational Quantum Algorithms", arXiv:2406.09780.

10th PIS Lab Seminar [2025/1/23 (木) 17:00 - 18:00 @ 3F902 Seminar Room] 

Satoya Imai [QSTAR, INO-CNR, and LENS in Florence, Postdoctoral Fellow]

Multipartite entanglement and chiral symmetry

A central problem in the theory of quantum information is the characterization and quantification of entanglement. One approach to advance this direction is the use of symmetries in quantum systems. For bipartite states, the situation becomes simpler, since there are only symmetric (bosonic) and antisymmetric (fermionic) states. In my talk, I will cover three topics: First, I will explain how to quantify multipartite entanglement from the geometric perspective (see, e.g., [1]). Second, I will present our results on subspaces in which all states are highly entangled, using chiral symmetries [2]. Third, I will show the usefulness of our results for applications in quantum information processing [3].

[1] T-C. Wei and P. M. Goldbart, PRA 68, 042307 (2003).
[2,3] In preparation, coming soon: permutation (Sophia Denker, Otfried Gühne, and Satoya Imai)

Bio about Satoya Imai:
Sep. 2023, PhD at Siegen in Germany with Otfried Gühne
Nov. 2023, posdoc at Florence in Italy with Augusto Smerzi and Luca Pezzé

11th PIS Lab Seminar [2025/2/5 (水) 13:45 - 14:45 @ 3B棟プレゼンテーションルーム (3B213) ] CS Colloquium

岸田 昌子 [国立情報学研究所 情報学プリンシプル研究系 准教授]

情報系研究者のための制御理論入門とリスクアウェア制御

制御理論は、動的システムを意図通りに操作するための数学的手法であり、その応用範囲は情報通信、医療、交通、経済など多岐にわたります。従来の物理システム制御に加え、情報系の研究分野でもその手法が注目され、さまざまな応用可能性が広がりつつあります。本講演では、情報系の研究者に向けて、制御理論の基礎を概説するとともに、情報系の研究でも有用とされる状態空間表現やモデル予測制御（MPC）を取り上げます。また、不確実性が存在する環境下で、システムの性能と安全性を両立することを目指すリスクアウェア制御の考え方について、私の研究成果を交えて紹介します。
[1] M. Kishida, "Risk-aware control of discrete-time stochastic systems: Integrating Kalman filter and worst-case CVaR in control barrier functions," Proc. of IEEE Conference on Decision and Control, 2024 to appear
[2] M. Kishida, “Risk-aware stability, ultimate boundedness, and positive invariance,” IEEE Transactions on Automatic Control, vol. 69, no. 1, pp. 681-688, 2024
[3] M. Kishida, “Risk-aware self-triggered linear quadratic control,” IET Control Theory & Applications, vol. 17, no. 9, pp. 1167-1183, 2023
[4] M. Kishida and M. Nagahara, “Risk-aware maximum hands-off control using worst-case conditional value-at-risk,” IEEE Transactions on Automatic Control, vol. 68, no. 10, pp. 6353-6360, 2023
[5] M. Kishida and A. Cetinkaya, “Risk-aware linear quadratic control using conditional value-at-risk,” IEEE Transactions on Automatic Control, vol. 68, no.1, pp.416-423, 2023

1st PIS Lab Seminar [2023/11/9 16:00 - 17:00 @ 3B Presentation Room (2nd floor)] CS Colloquium

Vikash Mittal [National Tsing Hua University, Taiwan, Postdoctoral Research Fellow]

Deterministic generation of hybrid entangled states using quantum walks

Recently, hybrid entanglement (HE), which involves entangling a qubit with a coherent state, has demonstrated superior performance in various quantum information processing tasks, particularly in quantum key distribution [arXiv:2305.18906(2023)].
Despite its theoretical advantages, the practical generation of these states in the laboratory has been a challenge. In this context, we introduce a deterministic and efficient approach for generating HE states using quantum walks. Our method achieves a remarkable fidelity of 99.90 % with just 20 time steps in a one-dimensional split-step quantum walk. This represents a significant improvement over prior approaches that yielded HE states only probabilistically, often with fidelities as low as 80%. Our scheme not only provides a robust solution to the generation of HE states but also highlights a unique advantage of quantum walks, thereby contributing to the advancement of this burgeoning field. Moreover, our scheme is experimentally feasible with the current technology.

2nd PIS Lab Seminar [2023/11/13 (Mon.) 10:00 - 11:00 @ 3B Presentation Room (2nd floor)] CS Colloquium

Mostafa Sabri [New York University, Abu Dhabi campus, Reseaerch Associate]

Flat bands of periodic graphs

The discrete Laplacian of an infinite, Z^d-periodic graph (e.g. Z^d  itself, the honeycomb lattice, infinite strips, face-centered cubic lattices...etc) has a band spectrum which is  typically absolutely continuous, but for certain geometries, infinitely degenerate eigenvalues appear, which are known as ``flat bands''. They have corresponding eigenvectors of compact support. Flat bands have been extensively studied by physicists in recent years, with hundreds  of papers on the subject. Mathematically they have been much less explored, and many fundamental questions remain completely open. I will introduce this exciting topic, give some recent results with Pierre Youssef, and discuss some open problems.

3rd PIS Lab Seminar [2023/11/13 (Mon.) 11:00 - 12:00 @ 3B Presentation Room (2nd floor)] CS Colloquium

Taira Giordani [Sapienza University of Rome, Italy, Lecturer]

Certification of coherence, dimension and contextuality in integrated optical quantum circuits

Path encoding for photonic qubits and qudits is the most suitable degree of freedom to be employed in integrated circuits. Integrated photonic devices are essential to process in a precise and scalable way such quantum states. This seminar focused on the correct manipulation and characterization of high-dimensional path-encoded single photon states in integrated optical circuits. In particular, it will be discussed the certification of such kinds of experiments since the exponential complexity of tomographic processes makes this problem a challenging task. One of the last results in this field showed the verification of coherence and dimension witnesses tailored for quantum systems of increasing dimension, using pairwise overlap measurements. Such a procedure has been enabled by the precise control of a reconfigurable six-mode universal photonic chip fabricated with the femtosecond laser writing technology. In particular, the device enabled the verification of coherence and dimension witnesses for qudits of dimensions up to 5. 

4th PIS Lab Seminar [2023/11/13 (Mon.) 14:00 - 15:00 @ 3B Presentation Room (2nd floor)]

Davide Poderini [Sapienza University of Rome, Italy, Postdoctoral Research Fellow]

Interventional causal inequalities

Quantum correlations, those exhibited between two or more quantum systems and that cannot be explained in classical terms, are at the core of quantum theory and its applications for information processing. Previous research has shown that the causal modeling framework offers a powerful set of techniques to study and analyze nonclassicality, and to extend this notion to general networks with independent sources of correlations and communication between parties. Interventions are an essential tool in causal inference, enabling us to determine the causal relationships between variables in a given process.  Unlike passive observations, interventions involve locally changing the underlying causal structure of an experiment, by erasing all external influences that a given variable might have and putting it under the exclusive control of an observer. Interestingly, through interventions, we can demonstrate the quantum behavior of a system that may appear classical at the observational level. One paradigmatic example is the instrumental causal structure, where violations of causal bounds that rely on a specific measure of causal influence, called the average causal effect (ACE), can be violated even when no violation is possible with observational data.This suggests a new approach that takes into account all available interventional and observational data in a given experiment, and can be applied to arbitrary scenarios.
This approach extends the notion of classical correlations to include interventional data, defining a set that is now bounded by hybrid (observational and interventional) inequalities, which subsumes all Bell-like and causal bounds, and provides a general method to better detect and characterize nonclassical behavior.

5th PIS Lab Seminar [2023/11/13 (Mon.) 15:00 - 16:00 @ 3B Presentation Room (2nd floor)]

Pramod Padmanabhan [Indian Institute of Technology Bhubaneswar, India, Assitant Professor]

Disorder-free localisation in continuous-time quantum walks

We systematically construct permutation symmetric Hamiltonians for N fermions. In this setup we find that the time-evolved basis states in each number operator sector have perfect overlap with the initial state for large N showing that the states localise in Hilbert space without the introduction of any disorder coefficients. We then show that this is not an isolated effect and is true for a large class of perturbations and also for bosonic and spin systems. We also probe a connection to graph theory that could possibly provide a mechanism for this type of localisation. The models discussed here can be experimentally realised in ion traps, ultra-cold atom systems and in superconducting circuits.