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Spectral flow and applications

June 23 (Mon) 14:00 - 16:00, 2025

Christopher Bourne (Associate Professor, Institute of Liberal Arts and Sciences, Nagoya University)

Given a family of symmetric matrices indexed by a parameter (e.g. time, external field), changing this parameter will cause the eigenvalues to move along the real axis. The spectral flow tracks these eigenvalues and counts how many cross the point 0. This idea turns out to be very useful for both pure mathematics as well as applications to physics and elsewhere. In this talk, I will introduce the spectral flow and how it can be generalised to a variety of settings that are also relevant for applications in quantum physics.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building (Main Venue) / via Zoom

Event Official Language: English

Black hole states at finite N

June 18 (Wed) 16:30 - 17:30, 2025

Sunjin Choi (Postdoctoral Fellow, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

We study new cohomologies for the local BPS operators of the maximal super-Yang-Mills theory to better understand the black hole microstates. We first analyze the index of these black hole operators and explicitly construct their cohomologies to study how they imitate the quantum black holes. We find many towers of states and partial no-hair behaviors where certain gravtions are forbidden to dress these black hole operators. This qualitatively agrees with the behavior of the perturbative hairy BPS black holes or the so-called grey galaxies. Throughout this talk, we mainly focus on a subsector of the field theory corresponding to the BMN matrix theory, which exhibits a black hole-like entropy growth at large N.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Charting the landscape of gauge-fermion dynamics

June 16 (Mon) 13:30 - 15:00, 2025

Álvaro Pastor Gutiérrez (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Gauge–fermion quantum field theories are central to our understanding of nature, from QCD to the electroweak sector. Beyond the Standard Model, strongly coupled gauge dynamics offer compelling avenues to address open puzzles. In this talk, I present a cartographic study of gauge–fermion theories across varying numbers of colours and flavours, focusing on the interplay between colour confinement and chiral symmetry breaking. We determine the flavour and colour dependence of the corresponding dynamical scales, providing a unified picture that interpolates between QCD-like regimes—where we recover quantitative agreement with lattice results—and the perturbative conformal limit. The analysis is based on the functional renormalisation group and employs a novel approximation scheme that allows for controlled and flexible access to the non-perturbative dynamics. We further explore the near-conformal regime with walking behaviour and estimate the lower boundary of the Caswell–Banks–Zaks conformal window. This framework enables a self-consistent mapping of the theory space of strongly coupled gauge–fermion systems and yields first-principles results with direct relevance for physics beyond the Standard Model.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

A New Measure of Genuine Multipartite Entanglement

June 6 (Fri) 14:00 - 15:00, 2025

Jaydeep Kumar Basak (Post-doctoral Fellow, Gwangju Institute of Science and Technology (GIST), Republic of Korea)

In this talk, I will introduce ``Latent entropy" (L-entropy) as a novel measure to characterize genuine multipartite entanglement in pure states, applicable to quantum systems with both finite and infinite degrees of freedom. This measure, derived from an upper bound on reflected entropy, attains its maximum for three-party GHZ states and $n=4,5$-party $2$-uniform states. I will also show the generalization of this measure for higher party states. Furthermore, I will discuss an analogue of the Page curve in multiboundary wormholes. If time permits, I will show the behaviour of multipartite entanglement in random states.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Extracting particle mass on quantum computers: state preparation and measurement

June 3 (Tue) 11:00 - 12:30, 2025

Xiaoyang Wang (Postdoctoral Researcher, Quantum Mathematical Science Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

In this seminar, I will introduce the procedure of extracting particle mass from the ab initio calculation using quantum computers, including two essential steps: state preparation and measurement. For the measurement process, in our recent work "Computing n-time correlation functions without ancilla qubits" [arXiv:2504.12975], we developed a measurement method for correlation functions without ancilla qubits, circumventing longstanding hardware constraints of limited qubit connectivity and short-range control operations. We demonstrate our method using IBM quantum hardware and successfully reproduce the noiseless results of the Schwinger model hadron mass within a relative error of 0.18%, even in the presence of realistic hardware limitations and noise. For the state preparation process, another work "Performance guarantees of light-cone variational quantum algorithms for the maximum cut problem" [arXiv:2504.12896] focused on the accuracy of the state preparation using variational quantum algorithms (VQAs). We propose a light-cone VQA with provable performance guarantees, whose single round has higher accuracy than the 3-round standard VQA for the maximum cut problem. We experimentally validated the single-round light-cone VQA using IBM quantum hardware with solution accuracy that exceeds the known classical hardness threshold in both a 72-qubit demonstration and a 148-qubit demonstration.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Non-perturbative formulation of resonant quantum mechanics within unified exact WKB framework

May 28 (Wed) 16:00 - 17:30, 2025

Okuto Morikawa (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

We study quasi-stationary states in quantum mechanics using the exact WKB analysis as a non-perturbative framework. We explore unstable states such as resonances by analyzing the inverted Rosen-Morse potential, which exhibits barrier resonance. This model allows exact solutions, enabling a direct comparison with exact WKB predictions. We provide a simple analytic picture of resonance and demonstrate consistency between exact and WKB-based results. A unified exact WKB framework is developed for exploring the equivalence and complementarity of different well-established regularizations \`a la Zel'dovich, complex scaling, and rigged Hilbert space within this framework.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Generalized symmetry from Type IIB superstring theory

May 27 (Tue) 16:00 - 17:00, 2025

Masashi Kawahira (Postdoctoral Fellow, Department of Physics, Graduate School of Science, Kobe University)

Recently, generalized symmetries have enabled the systematic analysis of various quantum systems. In this talk, we focus on global generalized symmetries that appear in the low-energy effective theory of type IIB superstring theory (i.e., type IIB supergravity). Specifically, we highlight the SL(2,ℤ) gauge symmetry (self-duality) in type IIB supergravity. We see that a global ℤ₁₂ eight-form symmetry arises as the quantum symmetry of the SL(2,ℤ) gauge symmetry. And we discuss its topological operator and its relation to 7-branes. This talk is based on joint work with Hiroki Wada (Tohoku University) and Naoto Kan (Osaka University).

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building (Main Venue) / via Zoom

Event Official Language: English

A review of incompatibility in quantum information science (Topical Review Meeting #1)

May 26 (Mon) 16:00 - 17:30, 2025

Haruki Emori (Ph.D. Student, Graduate School of Information Science and Technology, Hokkaido University)

As the first activity of the Quantum Foundation Study Group, we will hold a Topical Review Meeting. In quantum mechanics and quantum information science, the exploration of “quantumness” is of fundamental importance. One key concept that captures quantumness is “incompatibility”. In this meeting, I will give a review on incompatibility. If time permits, I would also like to introduce some of my recent research on this topic and engage in discussions with the participants.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Beyond-classical simulations using a quantum annealer

May 23 (Fri) 10:00 - 11:15, 2025

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].

Venue: via Zoom

Event Official Language: English

Stable homotopy theory of invertible quantum spin systems

May 16 (Fri) 16:00 - 18:00, 2025

Yosuke Kubota (Associate Professor, Graduate School of Science, Kyoto University)

In the past decade, A. Kitaev proposed that the set of invertible gapped quantum spin systems would form an \Omega-spectrum. This conjecture is considered to have potentially significant application to the study of SPT phases. Recently, we give a mathematically rigorous realization of this proposal with the language of functional analysis and operator algebra. This gives a unified proof of a series of existing researches. The proof also suggests to understand Kitaev's proposal from the viewpoint of coarse geometry of metric spaces. This association leads us to the concept of localization flow.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Mini-Workshop on Neuro Science

May 16 (Fri) 9:00 - 12:00, 2025

Satoshi Iso (Director, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Takuya Isomura (Unit Leader, Brain Intelligence Theory Unit, RIKEN Center for Brain Science (CBS))
Taro Toyoizumi (Team Director, Laboratory for Neural Computation and Adaptation, RIKEN Center for Brain Science (CBS))
Kazuo Okanoya (Professor, Advanced Comprehensive Research Organization (ACRO), Teikyo University)
Atsushi Iriki (Specially Appointed Professor, Advanced Comprehensive Research Organization, Teikyo University)
Gary Shiu (Professor, Department of Physics, University of Wisconsin-Madison, USA)

[Program] 9:00am – 9:05am Satoshi Iso (iTHEMS RIKEN) “Opening Remarks” 9:05am – 9:35am Takuya Isomura (CBS RIKEN) “Triple equivalence integrates neural network dynamics, statistical inference, and computation” 9:45m – 10:15am Taro Toyoizumi (CBS RIKEN) “Chaotic neural dynamics facilitate probabilistic computations through sampling” 10:25am – 10:55am Kazuo Okanoya (Teikyo University) “Brain topography and auditory processing in birds and rats” 11:05am – 11:30am Atsushi Iriki (Teikyo University, iTHEMS RIKEN) “From Classical to Quantum: Rethinking Science, Consciousness, and Civilization” 11:30am – Gary Shiu (University of Wisconsin) “Summary: Mathematical Science and Brain” Followed by a seminar 2:00pm – 3:30pm Prof. Gary Shiu (University of Wisconsin) Topology and Brain Science Room 359, 3rd floor, Main Research Building, RIKEN

Venue: #345-347, Main Research Building, RIKEN Wako Campus (Main Venue) / via Zoom

Event Official Language: English

On IR/UV divergence of inflationary decoherence

April 22 (Tue) 14:00 - 15:30, 2025

Fumiya Sano (Ph.D. Student, Institute of Science Tokyo)

Supported by observational evidence indicating that cosmological scalar perturbations were nearly Gaussian at the beginning of the universe, it is anticipated that the origin of these perturbations is quantum fluctuations. Consequently, cosmic inflation provides a valuable laboratory for testing the quantum nature with/of gravity. Evaluation of the quantumness of the primordial perturbations is an inevitable step for the purpose. However, quantum states of the perturbations are suffered from IR/UV divergence, resulting in fully classical states. In this presentation, I will first review the evaluation of the quantum coherence in de Sitter spacetime as a measure of quantumness, and then show how to regularize the divergence.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Gauge subtleties and the finiteness of loop corrections beyond slow roll

April 3 (Thu) 14:00 - 15:30, 2025

Danilo Artigas (JSPS Postdoctoral Research Fellow, Department of Physics Ⅱ, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)

The early universe undergoes a phase of exponential expansion called inflation, under which quantum fluctuations are amplified and later seed cosmological structures. A long-standing question is whether interactions of these quantum fields may significantly affect the n-point statistics of cosmological observables. These corrections are known as loop corrections. Recently, Kristiano and Yokoyama claimed that, in scenarios beyond slow-roll inflation, the one-loop correction of super-Hubble fluctuations could become non-negligible and violate cosmological-perturbation theory. This result is highly debated, and in this talk we will use a non-linear approach known as delta N formalism to evaluate these loop corrections. We find the existence of loop corrections for modes close to the Hubble scale, however, these corrections are quickly suppressed for long-wavelength modes. We also show how the result of Kristiano and Yokoyama may arise when truncating the perturbative expansion, and how this result depends on the chosen gauge.

Venue: Seminar Room #359

Event Official Language: English

Omega Meson from Lattice QCD

April 2 (Wed) 15:00 - 16:00, 2025

Haobo Yan (Ph.D. Student, School of Physics, Peking University, China)

The three-body problem, renowned for its unsolvable nature in celestial mechanics and homonymous science fiction, is not only solvable in the quantum realm regarding spectra but also offers profound insights into QCD. In this talk, I will present the first-ever lattice calculation of the resonance parameters for the lightest hadron decaying into three particles, the -meson. By mapping finite-volume energy levels to infinite-volume scattering amplitude, a pole position trajectory is obtained that, when extrapolated to the physical point, shows good agreement with the experiment.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Stability of nonsingular black holes

March 27 (Thu) 15:00 - 16:30, 2025

Shinji Tsujikawa (Professor, Graduate School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University)

We show that nonsingular black holes (BHs) realized in nonlinear electrodynamics are always prone to Laplacian instability around the center because of a negative squared sound speed in the angular direction. This is the case for both electric and magnetic BHs, where the instability of one of the vector-field perturbations leads to enhancing a dynamical gravitational perturbation in the even-parity sector. Thus, the background regular metric is no longer maintained in a steady state. We also generalize our analysis to the case in which a scalar field is present besides the U(1) gauge field and find no explicit examples of linearly stable nonsingular BHs. Our results suggest that the construction of regular BHs without instabilities is generally challenging within the scheme of classical field theories.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Third Workshop on Density Functional Theory: Fundamentals, Developments, and Applications (DFT2025)

March 25 (Tue) - 27 (Thu) 2025

The density functional theory (DFT) is one of the powerful methods to solve quantum many-body problems, which, in principle, gives the exact energy and density of the ground state. The accuracy of DFT is, in practice, determined by the accuracy of an energy density functional (EDF) since the exact EDF is still unknown. Currently, DFT has been used in many communities, including nuclear physics, quantum chemistry, and condensed matter physics, while the fundamental study of DFT, such as the first principle derivations of an accurate EDF and methods to calculate many observables from obtained densities and excited states, is still ongoing. However, there has been little opportunity to have interdisciplinary communication. On December 2022, we had the first workshop on this series (DFT2022) at Yukawa Institute for Theoretical Physics, Kyoto University, and several interdisciplinary discussions and collaborations were started. On February 2024, we had the second workshop on this series (DFT2024) at RIKEN Kobe Campus, and more stimulated discussion occured. To keep and extend collaborations, we organize the third workshop. Since the third workshop, we extend the scope of the workshop to the development and application of DFT as well. In this workshop, the current status and issues of each discipline will be shared towards solving these problems by meeting together among researchers in mathematics, nuclear physics, quantum chemistry, and condensed matter physics. This workshop mainly comprises lectures/seminars on cutting-edge topics and discussion, while sessions composed of contributed talks are also planned.

Venue: 8F, Integrated Innovation Building (IIB) / via Zoom

Event Official Language: English

A Century of Quantum Mechanics

March 24 (Mon) 14:00 - 15:30, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a RIKEN iTHEMS - The Univ. of Tokyo, Phys. Dept. Joint Seminar. This year, 2025, the "International Year of Quantum Science and Technology (IYQ)," is the 100th anniversary of the "formal" start of quantum mechanics, the description of the microscopic world. 1925 is the year in which Werner Heisenberg and others formulated "matrix mechanics," and physicists began to understand how to accurately predict microscopic phenomena. In this talk I will describe how quantum mechanics came about, starting with physicists in the late nineteenth century trying to understand the colors of hot metals and other hot objects, noting crucial advances leading to the fully developed wave and matrix quantum mechanics in the mid 1920's, to steps towards understanding real materials, culminating with spectacular applications such as smartphones, scarcely a century later.

Venue: The Univ. of Tokyo, Faculty of Science Building #4, room 1220 (Main Venue) / via Zoom

Event Official Language: English

The puzzle of angular momentum conservation in beta decay and related processes.

March 21 (Fri) 14:00 - 15:30, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a iTHEMS-FQSP joint seminar. We ask the question of how angular momentum is conserved in a number of related processes, from elastic scattering of a circularly polarized photon by an atom, where the scattered photon has a different spin direction than the original photon; to scattering of a fully relativistic spin-1/2 particle by a central potential; to inverse beta decay in which an electron is emitted following the capture of a neutrino on a nucleus, where the final spin is in a different direction than that of the neutrino – an apparent change of angular momentum. The apparent non-conservation of angular momentum arises in the quantum measurement process in which the measuring apparatus does not have an initially well-defined angular momentum, but is localized in direction in the outside world. We generalize the discussion to massive neutrinos and electrons, and examine nuclear beta decay and electron-positron annihilation processes through the same lens, enabling physically transparent derivations of angular and helicity distributions in these reactions.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Asymptotically flat black hole spacetimes with multiple injections

March 14 (Fri) 15:30 - 17:00, 2025

Yuta Saito (Ph.D. Student, Graduate School of Science and Technology, Nihon University)

In quantum gravity, Hawking radiation presents several fundamental problems. One of the problems is the black hole (BH) information paradox, in which the entanglement entropy (EE), which quantifies quantum entanglement, exceeds its upper bound. In the absence of the paradox, EE follows the Page curve. Recent progress has been made in resolving this paradox using the island formula, a method for computing EE that successfully reproduces the expected Page curve. In this approach, a portion of the black hole interior is treated as part of the radiation region. Meanwhile, an alternative scenario has been proposed where multiple collapsing shells prevent the formation of a well-defined event horizon [1]. In this case, radiation is emitted throughout the collapse process, shifting dynamically the Schwarzschild radius inward, and a surface structure is formed just outside. This leads to a distinction between the conventional event horizon and the surface, introducing an intermediate region between the Schwarzschild radius and the surface. Interestingly, this model also suggests that part of the black hole interior effectively belongs to the radiation region, drawing a possible parallel to the island formula. In this talk, we explore spacetimes with multiple energy injections in asymptotically flat two-dimensional black hole backgrounds and analyze the entanglement entropy in such scenarios. Since considering backreaction in gravitational collapse in two dimensions is difficult, we instead construct a spacetime solution with multiple energy injections and analyze EE within this background. The main focus of this talk is to derive the spacetime and examine its properties. Additionally, we perform EE calculations in parallel with previous studies [2], which consider the case of α single injection, and confirm that the behavior of EE depends on the interval between energy injections.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English