Cooperating on networks: inequality and social structure

May 27 (Wed) 14:00 - 15:00, 2026

Manuel Staab (Lecturer, University of Queensland, Australia)

We analyse how inequality in endowments and social structure jointly affect individuals' ability to cooperate. Individuals repeatedly invest in a local public good ("cooperation'') in an environment that is described by a distribution of endowments and a network of beneficiaries. We measure the cooperativeness of an environment by the minimum discount factor needed to sustain (any) cooperation in equilibrium. We characterise the endowment distribution that maximises cooperativeness for any given network and the corresponding minimum discount factor. The latter is shown to be inversely proportional to the maximal index of the graph describing the network. The corresponding dominant eigenvalue of the adjacency matrix characterises the most cooperative income distribution. Moreover, we show that if an environment maximises cooperativeness (over all income distributions and networks of a certain size), then the network is described by a nested split graph. We further show that this is the same class of graphs that maximise welfare for any given discount factor, and yet, the most cooperative graph need not be equal to the most efficient.

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

Event Official Language: English

Harnessing inequality for cooperation

May 26 (Tue) 14:00 - 15:00, 2026

Maria Kleshnina (Senior Lecturer, School of Mathematical Sciences, Queensland University of Technology, Australia)

Inequality in resources is widely thought to undermine cooperation in social dilemmas. Yet cooperation among unequals is ubiquitous: between senior and junior colleagues, firms of different sizes, nations with asymmetric stakes. Here, we offer a resolution to this puzzle and derive a novel prediction: if the returns from cooperation are shared in accordance with the individuals' strategic incentives, inequality enables and strengthens cooperation. We develop a strategic framework to systematically explore cooperation when the returns of a joint project can be shared unevenly. We characterise the optimal sharing rule, which we call resilient sharing, that can sustain cooperation in repeated interactions when no other rule can. Resilient sharing equalises incentives to defect across players, but is neither egalitarian nor proportional. Surprisingly, it typically rewards weaker partners beyond their relative contributions. We show that cooperation can be sustained through direct reciprocity in any environment whenever individual contributions are sufficiently unequal. Evolutionary simulations and a behavioural experiment confirm the central prediction: under resilient sharing, cooperation succeeds among unequal partners where it fails among equals. This suggests that cooperation is more likely to evolve and thrive when individuals can vary contributions and divide returns flexibly, pointing to the role of institutions and norms in harnessing inequality to stabilize cooperation.

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

Event Official Language: English

The First RIKEN Quantum International Workshop on Frontiers of Quantum Computing Applications and Quantum-HPC Integration

May 25 (Mon) - 26 (Tue) 2026

This two-day workshop will bring together leading experts from academia, industry, and national laboratories to explore the rapidly evolving frontiers of quantum computing applications and their integration with high-performance computing (HPC) platforms. Hosted by RIKEN Quantum, the event will provide a forum for discussing recent advances, practical challenges, and future directions toward achieving utility-scale quantum computations and robust quantum–HPC hybrid workflows. The workshop is primarily an in-person event, but a special session on quantum computing in chemistry and life sciences will also be accessible via Zoom.

Venue: 2F Large Conference Room, Administrative Headquarters, RIKEN Wako Campus

Event Official Language: English

Singularities of differentiable maps and Thom polynomials

May 22 (Fri) 15:00 - 17:30, 2026

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

Singularities are locations where something is exceptional. In particular, singularities of differentiable maps are mathematical concepts corresponding to stationary points of functions and apparent contours of surfaces under projection onto the retina. These are unavoidable in general, but important to study the shape of spaces and behavior of maps. The theory for them was initiated by R. Thom in 1950's, and have been deeply studied by many researchers.

Venue: Room 359, RIKEN Wako Campus (Main Venue) / via Zoom

Event Official Language: English

Mathematical Application Research Team Meeting #15

May 22 (Fri) 10:30 - 12:00, 2026

Pietro De Lellis (Associate Professor, Department of Electrical Engineering and Information Technology, University of Naples Federico II, Italy)

Mathematical Application Research Team is delighted to welcome Dr. Pietro De Lellis from University of Naples Federico II, for an upcoming team meeting. We warmly invite everyone to join us for this opportunity to hear about his latest research. Title: Synchronization and Control in Higher-Order Network Dynamics Abstract Understanding how network structure shapes coordinated behavior in coupled dynamical systems is a central question in network science. Although pairwise interactions among identical nodes are well understood, much less is known about the effects of directed higher-order interactions, especially in systems with heterogeneous nodes. In this seminar, I will discuss recent results on the emergence and control of collective behavior in such systems. I will first present an extension of the standard pairwise framework to higher-order directed interactions and derive conditions for synchronization in the identical-node setting. I will then introduce a mathematically grounded heuristic for steering the system toward a prescribed trajectory. Moving to heterogeneous dynamics, I will present a new notion of network synchronizability that quantifies how hypergraph topology can promote trajectory alignment even when node parameters differ. The resulting metric is analytically derived, agrees closely with numerical observations, and scales efficiently to large networks. This makes it possible to compare higher-order and pairwise interactions systematically, and to identify when multi-body interactions favor or obstruct synchronization. I will conclude with an application to opinion dynamics, where the framework highlights which hyperedges are most conducive to consensus formation.

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

Event Official Language: English

Entanglement entropy and conformal bounds for five-dimensional CFTs

May 21 (Thu) 15:00 - 16:00, 2026

Javier Moreno (Project Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)

Abstract: The entanglement entropy of spatial regions in odd-dimensional conformal field theories contains a universal constant contribution that encodes important information about the theory. This quantity can be defined in a robust way using mutual information between slightly deformed versions of a given region. In three-dimensional conformal field theories, it is known that this quantity is always positive and bounded from below by the value corresponding to a spherical region. There is also strong evidence that, for any region, its normalized value is maximized by the free scalar theory. In this work, we show that the situation changes significantly in five dimensions. Although the spherical region remains a local minimum under small shape deformations, more general regions can lead to values that become arbitrarily large in magnitude, with either sign. This implies that, in five-dimensional conformal field theories, the quantity is not bounded from above or below. We also demonstrate that the analogous maximization property observed in three dimensions does not hold in five dimensions when considering general regions. Despite this, we find that existing evidence is consistent with a weaker statement: for small deformations of a spherical region, the normalized quantity remains bounded above by the free scalar result across all five-dimensional conformal field theories. This leads to a new conjectured universal bound relating two key physical quantities—the coefficient governing stress-tensor correlations and the sphere free energy—which appears to hold for all currently known examples.

Venue: via Zoom / Seminar Room #359, Seminar Room #359

Event Official Language: English

Stochastic Thermodynamics: noise and energetics of nanoscale systems

May 21 (Thu) 13:00 - 14:00, 2026

Jean-Charles Delvenne (Professor, Applied Mathematics, Université Catholique de Louvain, Belgium)

Stochastic thermodynamics, initiated three decades ago, aims at quantifying the fluctuations of physical observables in relation with thermodynamic quantities (such as heat or entropy production). A typical result is the Thermodynamic Uncertainty Relation, which states that a high entropy production is required to obtain a favorable noise-to-signal ratio for some observables (such as displacement of a molecular motor) in stationary out-of-equilibrium systems. It is especially relevant for nanoscale systems, where the fluctuations cannot be neglected. This includes biological systems (e.g. biological motors such as kinesin along a microtubule), electronic systems (transistor-based memories), chemical reactions, etc. The talk will be both a tutorial on some basic results or applications, and a presentation of some recent results and perspectives.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

On Mean-Field Games

May 21 (Thu) 10:30 - 11:30, 2026

Antoine Diez (Research Scientist, Mathematical Application Research Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Stochastic differential games with a large number of players are notoriously challenging, both theoretically and numerically, typically when it comes to computing Nash equilibria. Yet, when many players interact somehow symmetrically by responding only to the average behavior of the others, the game can surprisingly become more tractable by taking the limit of an infinite number of players. This is in direct analogy with the so-called « mean-field theory » which simplifies the analysis of large systems of interacting particles in statistical physics. Introduced independently about two decades ago by Lasry and Lions (mathematics) and Caines, Huang and Malahamé (engineering), the theory of Mean-Field Games has since been greatly developed with various applications in engineering, economical, social and biological sciences. The goal of this short lecture is to introduce the key concepts, particularly the deep connections between game theory, Partial Differential Equations and stochastic analysis, and to showcase a few striking recent applications.

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

Event Official Language: English

Positivity constraints for the gravitational path integral

May 21 (Thu) 10:00 - 11:50, 2026

Gabriele Di Ubaldo (Postdoctoral Researcher, RIKEN-Berkeley Center, Division of Global Collaborations and Research Talent Development, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

For a quantum theory of gravity to have a well-defined Hilbert space, the inner product between different states of open and closed universes must be positive semi-definite. Positivity however is not manifest in the low-energy effective theory and in fact imposes nontrivial constraints on the theory. Working in the Gravitational Path Integral (GPI) approach, we derive the general set of positivity constraints on the closed and open universe Hilbert spaces. In the case of AdS gravity, open universe positivity in principle follows from CFT unitarity, however the holographic description of closed universes remains unclear. Strikingly, we exhibit positivity of closed universes across many theories and prove that open positivity implies closed positivity, showing that the CFT 'knows' about the closed universe hilbert space. We then analyze positivity constraints on gravitational theories coupled to axions. We present a method to compute off-shell axion wormholes in AdS and flat space which we use to show that positivity is violated if the axion shift symmetry is exact. In low-energy EFTs where these wormholes are perturbatively stable, to restore positivity the wormhole must have a non-perturbative instability due to instantons that breaks the shift symmetry. Positivity then leads to a proof of a sharp version of the Axion Weak Gravity Conjecture A-WGC, including precise numerical constants. For the QCD axion this provides a bound on the axion decay constant which has phenomenological and experimental consequences for axion searches. In string theory, positivity gives a bound on the coupling between the axion and the dilaton in the low energy effective action.

Venue: via Zoom

Event Official Language: English

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

May 19 (Tue) 15:00 - 16:30, 2026

Kazuya Shinjo (Research Scientist, Computational Quantum Matter Research Team, RIKEN Center for Emergent Matter Science (CEMS))

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

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

Event Official Language: English

Synthetic Data from Domain Knowledge: Pretraining Medical Deep Networks under Data Scarcity

May 18 (Mon) 14:00 - 15:00, 2026

Naoki Nonaka (Senior Research Scientist, Medical Science Deep Learning Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Training deep learning models typically requires large-scale data, yet in the medical domain such data are often difficult to obtain due to privacy constraints, the rarity of certain diseases, and the high cost of acquisition. In this talk, I present one approach to this challenge: pretraining with synthetic data generated from domain knowledge. As concrete examples, I introduce the synthesis of electrocardiograms (ECG) and phonocardiograms (PCG). For ECG, each waveform component (P, Q, R, S, and T) is modeled with Gaussian functions; for PCG, synthetic signals are generated by combining S1 and S2 heart sounds with modulated noise. I show that pretraining a model on such synthetic data and then fine-tuning on a small amount of real data substantially improves classification performance compared to training on real data alone, and that this improvement becomes more pronounced as the size of the real dataset decreases. I will also touch on extensions such as self-supervised learning with synthetic data and a comparison between knowledge-driven simulators and learned generative models, and discuss the broader potential of domain knowledge as a data source for medical applications where real data are limited.

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

Event Official Language: English

Basics of chiral lattice fermion

May 18 (Mon) 13:00 - 14:00, 2026

Arata Yamamoto (Senior Research Scientist, Quantum Mathematical Science Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

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

Event Official Language: English

Introduction to quantum resource theories (3)

May 15 (Fri) 9:00 - 17:00, 2026

Ryuji Takagi (Associate Professor, Graduate School of Arts and Sciences, The University of Tokyo)

[Registration Closed] Due to high demand and venue capacity limits, registration for this course is now closed as of April 25. If you wish to be placed on a waiting list in case of cancellations, please contact us via the inquiry form at the bottom of this page. One of the central goals of quantum information theory is to quantitatively clarify the relationship between the performance of quantum information processing and the valuable quantum features that underlie it. In this lecture, we will discuss quantum resource theories, a framework that provides a useful approach to this question. By presenting concrete examples—starting with entanglement theory, the most representative resource theory—as well as recent research results, we will see how perspectives and tools from information theory enable the quantification of quantum resources and the characterization of their convertibility. Beyond entanglement theory, we plan to discuss other key settings such as quantum thermodynamics, resource theory of asymmetry, and quantum magic—relevant resource in fault-tolerant quantum compuation. The overall aim of this lecture is to provide new analytical viewpoints that can be applied to a wide range of systems and quantum information processing tasks. While we do not plan to change the overall start and end times for each day, the detailed lecture schedule is subject to change. The intensive course will be held over three days. Please register for the course using the form. The registration deadline is May 7 (Thu). Please note that the registration form is the same for all three days, so you only need to register once. The 3rd day: May 15 (Fri) 9:00–10:30 Lecture 7 10:30–11:00 Coffee break 11:00–12:30 Lecture 8 12:30-13:30 Lunch time 13:30-15:00 Lecture 9 15:00-15:30 Coffee break 15:30-17:00 Seminar (or Lecture 10) This event is in-person only.

Venue: #435-437, 4F, Main Research Building

Event Official Language: English

Sexual conflict over floral receptivity? — theory-guided experiments of evolutionary ecology

May 14 (Thu) 16:00 - 17:00, 2026

Daisuke Kyogoku (Associate Professor, Faculty, Division of Natural Sciences, Nara Women's University)

The theory of evolutionary biology predicts that the interests (in terms of adaptation) of different individuals can conflict with one another. Specifically, mating partners can have different optima in traits such as mating rates, number of mates, number of offspring, and resource allocation to the offspring. Botanists have long recognized that pollination induces floral closure or wilting, which is typically seen as the adaptation of the pollen recipients (i.e., getting rid of costly flowers after achieving their function). However, it is also possible that floral closure or wilting is, at least in part, the outcome of the manipulation by selfish pollen. For example, pollen may secure their paternity by preventing additional pollination. Fewer seed production by the recipients can result in so much allocation of maternal resources to each fertilized egg cell that is maternally maladaptive (but paternally adaptive). Being guided by these theoretical predictions, I have been testing the hypothesis using both Taraxacum dandelions and Arabidopsis. In this talk, I will show our recent (mostly unpublished) results. Although the projects are halfway, results so far generally support the hypothesis. The ideas of related future projects and the philosophy behind the projects may also be discussed.

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

Event Official Language: English

Stochastic Schrödinger Diffusion Models for Pure-State Ensemble Generation

May 14 (Thu) 14:30 - 15:30, 2026

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

In quantum machine learning (QML), classical data are often encoded as quantum pure states and processed directly as quantum representations, motivating \emph{representation-level generative modeling} that samples new quantum states from an underlying pure-state ensemble rather than re-preparing them from perturbed classical inputs. However, extending \emph{score-based} diffusion models with well-defined reverse-time samplers to quantum pure-state ensembles remains challenging, due to the non-Euclidean geometry of the complex projective space $\mathbb{CP}^{d-1}$ and the intractability of transition densities. We propose \emph{Stochastic Schr\"odinger Diffusion Models} (SSDMs), an intrinsic score-based generative framework on $\mathbb{CP}^{d-1}$ endowed with the Fubini--Study (FS) metric. SSDMs formulate a forward Riemannian diffusion with a stochastic Schr\"odinger equation (SSE) realization, and derive reverse-time dynamics driven by the Riemannian score $\nabla_{\mathrm{FS}} \log p_t$. To enable training without analytic transition densities, we introduce a local-time objective based on a local Euclidean Ornstein--Uhlenbeck approximation in FS normal coordinates, yielding an analytic teacher score mapped back to the manifold. Experiments show that SSDMs faithfully capture target pure-state ensemble statistics, including observable moments, overlap-kernel MMD, and entanglement measures, and that SSDM-generated quantum representations improve downstream QML generalization via representation-level data augmentation.

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

Event Official Language: English

Tropical geometry as a tool in algebraic geometry and beyond

May 13 (Wed) 15:30 - 17:00, 2026

Paul Alexander Helminck (Assistant Professor, Mathematical Institute of the Polish Academy of Sciences, Poland)

Tropical geometry is a field in mathematics that lies at the interface of algebraic geometry and combinatorics. One of the main goals in tropical geometry is to uncover the combinatorial patterns hidden in algebraic geometry. This basic principle can already be found in Bézout’s theorem, which counts the roots of sufficiently generic polynomial equations, and its generalization in the form of the BKK theorem, both of which can be proven tropically. The abstract combinatorial ideas that come out of this study have since also seen applications in economics, machine learning, chemical reaction networks and mathematical physics, among others. In this talk I will give an introduction to tropical geometry and I will discuss some of the main results. I will also discuss some of my latest work on finding the topology of an algebraic variety using tropical methods. In particular, I will discuss how this gives rise to a CW complex structure on a K3 surface. This structure for instance gives us a quick way to see various phenomena from mirror symmetry such as the monodromy of integral affine structures.

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

Event Official Language: English

From Birkhoff's Polytope to Petz Recovery: Unistochastic Matrices, Quantum Channels, and Approximate Markov Chains

May 13 (Wed) 13:30 - 15:00, 2026

Claude Gravel (Assistant Professor, Department of Computer Science, Toronto Metropolitan University, Canada)

A doubly stochastic matrix is unistochastic if its entries correspond to the squared moduli of a unitary matrix. Determining which n × n doubly stochastic matrices admit such a representation remains an open problem at the intersection of convex geometry, combinatorics, and quantum information. For 3 × 3 matrices, elegant triangle inequalities provide a complete characterization: the unistochastic set occupies approximately 75% of the Birkhoff polytope and exhibits deltoid cross-sections. For n ≥ 4, the characterization problem remains unresolved and is influenced in unexpected ways by the prime factorization of n via the defect of the Fourier matrix. This presentation surveys these results and then establishes a connection to a second, seemingly unrelated question: given a tripartite quantum state with small conditional mutual information, to what extent can one subsystem be recovered from the others? The Petz recovery map and its rotated variants offer a universal solution. These two topics are linked through coherification, which concerns when a classical stochastic process can be elevated to coherent quantum dynamics, and through the conditional mutual information as a continuous measure of non-unistochasticity. The talk concludes with open problems at this interface, including the star-shapedness conjecture for n = 4 and the pursuit of tighter recovery bounds.

Venue: #359, Seminar Room #359

Event Official Language: English

Universal Laws of Nonequilibrium Quantum Many-Body Systems: From Understanding to Control

May 12 (Tue) 15:00 - 16:00, 2026

Ryusuke Hamazaki (RIKEN Hakubi Team Leader, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team, RIKEN Cluster for Pioneering Research (CPR))

(Note: This lecture will be given in Japanese. This seminar is also designated as part of the Pioneering Research Institute (PRI) Lecture Series.) Recent advances in quantum simulators and quantum computers have made it possible to realize and manipulate quantum many-body systems with high precision and to directly observe their dynamics. This progress has renewed interest in a fundamental question dating back to John von Neumann: how macroscopic statistical mechanics emerges from microscopic quantum mechanics. At the same time, there is growing momentum toward harnessing the quantum nature of such systems through control, with the aim of realizing devices and functionalities that surpass those of classical systems. In this talk, I will discuss our research and future perspectives from the viewpoint of understanding universal laws governing nonequilibrium quantum many-body systems from microscopic quantum dynamics, and theoretically elucidating their controllability. In particular, I will focus on topics such as the characterization of phases unique to open quantum many-body systems, the emergence of thermal statistical mechanics in isolated quantum systems, and the establishment of rigorous universal laws in nonequilibrium dynamics. Building on these insights, I will also discuss how we may open up the unexplored frontier of the statistical and many-body physics of control. Finally, I would like to touch upon the possibility that this universal framework of nonequilibrium statistical mechanics may find applications beyond quantum condensed matter physics and extend to other fields as well.

Venue: 2F Large Conference Room, Administrative Headquarters, RIKEN Wako Campus

Event Official Language: Japanese

Introduction to quantum resource theories (2)

May 12 (Tue) 9:00 - 17:00, 2026

Ryuji Takagi (Associate Professor, Graduate School of Arts and Sciences, The University of Tokyo)

[Registration Closed] Due to high demand and venue capacity limits, registration for this course is now closed as of April 25. If you wish to be placed on a waiting list in case of cancellations, please contact us via the inquiry form at the bottom of this page. One of the central goals of quantum information theory is to quantitatively clarify the relationship between the performance of quantum information processing and the valuable quantum features that underlie it. In this lecture, we will discuss quantum resource theories, a framework that provides a useful approach to this question. By presenting concrete examples—starting with entanglement theory, the most representative resource theory—as well as recent research results, we will see how perspectives and tools from information theory enable the quantification of quantum resources and the characterization of their convertibility. Beyond entanglement theory, we plan to discuss other key settings such as quantum thermodynamics, resource theory of asymmetry, and quantum magic—relevant resource in fault-tolerant quantum compuation. The overall aim of this lecture is to provide new analytical viewpoints that can be applied to a wide range of systems and quantum information processing tasks. While we do not plan to change the overall start and end times for each day, the detailed lecture schedule is subject to change. The intensive course will be held over three days. Please register for the course using the form. The registration deadline is May 7 (Thu). Please note that the registration form is the same for all three days, so you only need to register once. The 2nd day: May 12 (Tue) 9:00–10:30 Lecture 3 10:30–11:00 Coffee break 11:00–12:30 Lecture 4 12:30-13:30 Lunch time 13:30-15:00 Lecture 5 15:00-15:30 Coffee break 15:30-17:00 Lecture 6 This event is in-person only.

Venue: #435-437, 4F, Main Research Building

Event Official Language: English