iTHEMS Theoretical Physics Seminar
102 events
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Seminar
Mesoscopic transport via one-dimensional chain with Localized two-body loss
July 23 (Wed) at 10:00 - 11:30, 2025
Kensuke Kakimoto (Ph.D. Student, Faculty of Science and Engineering, School of Fundamental Science and Engineering, Waseda University)
Abstract: Mesoscopic transport has long served as a powerful probe into the quantum behavior of matter; however, the role of dissipation in such systems remains unresolved. In recent years, quantum simulations of mesoscopic systems with ultracold atomic gases have made significant progress, particularly through the use of optical tweezers to induce local dissipation via atom loss. In this talk, we discuss a two-terminal mesoscopic system in which two-body loss occurs locally at the center of a one-dimensional chain, modeling a dissipative quantum point contact. To analyze this setup, we employ the Keldysh Green’s function formalism in combination with a noise-field representation of Lindblad dynamics. Our analysis reveals that the dissipation strength depends on the occupation number of the central dissipative site, leading to a weaker suppression of particle current in the weakly dissipative regime compared to the case of one-body loss.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Boundary Scattering and Non-invertible Symmetries in 1+1 Dimensions
July 4 (Fri) at 14:00 - 15:00, 2025
Soichiro Shimamori (Ph.D. Student, Osaka University)
Recent studies by Copetti, Córdova and Komatsu have revealed that when non-invertible symmetries are spontaneously broken, the conventional crossing relation of the S-matrix is modified by the effects of the corresponding topological quantum field theory (TQFT). We extend these considerations to (1+1)-dimensional quantum field theories (QFTs) with boundaries. In the presence of a boundary, one can define not only the bulk S-matrix but also the boundary S-matrix, which is subject to a consistency condition known as the boundary crossing relation. We show that when the boundary is weakly-symmetric under the non-invertible symmetry, the conventional boundary crossing relation also receives a modification due to the TQFT effects. As a concrete example of the boundary scattering, we analyze kink scattering in the gapped theory obtained from the Φ(1,3)-deformation of a minimal model. We explicitly construct the boundary S-matrix that satisfies the Ward-Takahashi identities associated with non-invertible symmetries. This talk is based on the collaboration with Satoshi Yamaguchi.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Black hole states at finite N
June 18 (Wed) at 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
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Seminar
Charting the landscape of gauge-fermion dynamics
June 16 (Mon) at 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
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Seminar
A New Holographic Entanglement Entropy in the de Sitter space
June 13 (Fri) at 16:00 - 17:00, 2025
Yuki Suzuki (Ph.D. Student, Yukawa Institute for Theoretical Physics, Kyoto University)
We propose a new holographic entanglement entropy in the three-dimensional de Sitter space. It is known that the holographic entanglement entropy via Ryu-Takayanagi prescription violates the entropic inequalities that they should satisfy. We propose a kind of extensions of the Ryu-Takayanagi formula so that they satisfy the strong subadditivity. We fix consistent parameter regions of the entropy and finally comment on the implications to the static patch holography.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Gravitational Lensing in the Schwarzschild Spacetime: Photon Rings in Vacuum and in the Presence of a Plasma
June 12 (Thu) at 15:00 - 16:30, 2025
Torben Christian Frost (Postdoctoral Researcher, Kavli Institute for Astronomy and Astrophysics, Peking University, China)
Astrophysical black hole candidates are often surrounded by an accretion disk. In particular the interior region of this accretion disk can consist of a plasma and the trajectories of light rays travelling through this plasma can deviate significantly from the trajectories of light rays travelling through vacuum. While usually these environments are very complex we can already learn a lot about the observable features using simple plasma models. In the context of general relativity for some of these plasma models the equations of motion are fully separable and even analytically solvable. In my talk I will illustrate what we can learn from such models using the Schwarzschild spacetime as background. I will assume that the black hole is surrounded by an inhomogeneous, pressureless, and non-magnetised plasma and solve the equations of motion analytically exact. Then I will assume that we have a luminous disk in the equatorial plane and discuss the impact of the plasma on the so-called photon rings. I will discuss the changes of their geometrical structure, the redshift, and the travel time of the photons compared to photon rings in vacuum and what we can learn about the properties of the plasma. I will also discuss how the presence of the plasma may be able to help us to constrain gravity in the strong field regime. Finally, I will discuss how the obtained results will contribute to designing a multimessenger approach for probing gravity in the strong field regime in the context of the Maxwell-Einstein-Pauli Observatory.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
A New Measure of Genuine Multipartite Entanglement
June 6 (Fri) at 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
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Seminar
Exotic Pairing Nature and Charge Ordering in Kagome Superconductor
June 5 (Thu) at 15:00 - 16:15, 2025
Wu Xianxin (Associate Professor, CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, China)
Abstract：Recent experimental investigations have identified fascinating electronic orders in kagome metals, such as intriguing superconductivity, charge density wave (CDW) and nematicity. Notably, there is various evidence of spontaneous time-reversal symmetry (TRS) breaking within the CDW phase, pointing to a long-pursued loop current order, though its underlying mechanism remains elusive. In this talk, I will first review the exotic properties of these kagome superconductors. Then, I will discuss the effective model and the unique sublattice texture associated with van Hove singularities in the kagome lattice [1], which turns out to have significant effect on correlated states. Finally, I will present our scenario for realizing a TRS breaking CDW within the kagome lattice and discuss loop-current fluctuation induced unconventional pairing [2-4]. Potential experimental implications will be also discussed.
Venue: via Zoom / #359, Main Research Building
Event Official Language: English
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Seminar
Generalized symmetry from Type IIB superstring theory
May 27 (Tue) at 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
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Seminar
2d Cardy-Rabinovici model with the modified Villain lattice formulation
May 9 (Fri) at 14:00 - 15:00, 2025
Nagare Katayama (Ph.D. Student, Yukawa Institute for Theoretical Physics, Kyoto University)
One of the most famous scenarios of the quark confinement problem is the dual superconductor picture. In this picture, the quark confinement is induced by monopole condensation, but in the theory with a θ term, we expect that not only monopole but also dyon condensation is induced, as suggested by Cardy and Rabinovici through their intuitive arguments. In this study, the Witten effect of the theory of two-dimensional compact bosons with the θ term is examined using a modified Villain-type lattice theory that can treat the θ term and dion in a rigorous manner. In addition, we construct the 2d Cardy-Rabinovici model and analyze the phase diagram through the scaling dimension argument and the anomaly matching constraint.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English
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Seminar
Supernova axion emissivity with Δ(1232) resonance in heavy baryon chiral perturbation theory
May 1 (Thu) at 16:00 - 17:30, 2025
Shu-Yu Ho (Postdoctoral Researcher, Institute of Physics, Academia Sinica, Taiwan)
Abstract: In this talk, we evaluate the energy loss rate of supernovae induced by the axion emission process π− + p → n + a with the Δ(1232) resonance in the heavy baryon chiral perturbation theory for the first time. Given the axion-nucleon-∆ interactions, we include the previously ignored Δ-mediated graphs to the π− + p → n + a process. In particular, the Δ_0-mediated diagram can give a resonance contribution to the supernova axion emission rate when the center-of-mass energy of the pion and proton approaches the Δ(1232) mass. With these new contributions, we find that for the typical supernova temperatures, compared with the earlier work with the axion-nucleon (and axion-pion-nucleon contact) interactions, the supernova axion emissivity can be enhanced by a factor of ∼ 4(2) in the Kim-Shifman-Vainshtein-Zakharov model and up to a factor of ∼ 5(2) in the Dine-Fischler-Srednicki-Zhitnitsky model with small tanβ values. Remarkably, we notice that the Δ(1232) resonance gives a destructive contribution to the supernova axion emission rate at high supernova temperatures, which is a nontrivial result in this study.
Venue: #345-347, 3F, Main Research Building (Main Venue) / via Zoom
Event Official Language: English
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Seminar
On IR/UV divergence of inflationary decoherence
April 22 (Tue) at 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
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Seminar
A Strategy for Proving the Strong Eigenstate Thermalization Hypothesis: Chaotic Systems and Holography
April 3 (Thu) at 16:00 - 17:00, 2025
Taishi Kawamoto (Ph.D. Student / JSPS Research Fellow DC, Yukawa Institute for Theoretical Physics, Kyoto University)
The strong eigenstate thermalization hypothesis (ETH) provides a sufficient condition for thermalization and equilibration. Although it is expected to hold in a wide class of highly chaotic theories, there are only a few analytic examples demonstrating the strong ETH in special cases, often through methods related to integrability. In this talk, I will explore sufficient conditions for the strong ETH that may apply to a broad range of chaotic theories. These conditions are expressed as inequalities involving the long-time averages of real-time thermal correlators. Specifically, I will discuss bottom-up holographic models that satisfy these conditions under certain assumptions, which are expected to hold in such models. This talk is based on the preprint 2411.09746 [hep-th].
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
How to define a Majorana fermion?
February 20 (Thu) at 13:00 - 14:30, 2025
Kazuo Fujikawa (Professor Emeritus, The University of Tokyo)
It is fundamental in particle physics if the neutrino is a Dirac fermion or a Majorana fermion, and the seesaw model gives naturally a Majorana neutrino in an extension of the Standard Model. However, the commonly used chirality changing \(pseudo-C symmetry \) \(\nu^{\tilde C}_L=C\overline{\nu_L}^T\) of a chiral fermion is not defined in Lagrangian field theory. Precisely speaking, the neutrinoless double beta decay is not described by the pseudo-C symmetry. The Majorana neutrino obtained after a Bogoliubov-type canonical transformation, which is the one originally defined by Majorana using a Dirac-type fermion, describes consistently all the properties expected for the Majorana neutrino. Physical implication of this fact is briefly discussed.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
New topological quantum order in 2D lattices from non-invertible symmetries
February 18 (Tue) at 15:00 - 16:00, 2025
Ayan Mukhopadhyay (Associate Professor, Valparaiso University, Chile)
I will introduce an exactly solvable 2D lattice model which reveals a large number of distinct topological phases with non-invertible (generalized) symmetries. In all these topological phases, which have topological ground state degeneracy, a commutative stabilizer monoid of Hermitian operators leave the ground state invariant and can also distinguish *all* local excitations, (These symmetries are indeed symmetry operations.) There exists novel confined fractonic excitations which change the nature of deconfined excitatons profoundly. The fusion rules form an associative but noncommutative. non-Abelian and non-unital category, and are distinct for each of these phases. A class of these phases are adiabatically connected to a limit which can be described in terms of generalized free field theories. I will describe systematic ways to construct such phases. I will also discuss phases which do not have generalized free field limits. These phases have novel forms of non-local entanglement as many of them share the same topological entanglement entropy. They also violate the entanglement bootstrap axioms. When the phases do not have a generalized free field limit, the violation of the entanglement bootstrap axioms can happen for arbitrary large subregions signifying new forms of long-range entanglement.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Linking quantum error correction and gauge theories with quantum reference frames
February 14 (Fri) at 10:30 - 11:30, 2025
Philipp Hoehn (Assistant Professor, Okinawa Institute of Science and Technology Graduate University (OIST))
Redundancy is the hallmark of both quantum error correction and gauge theories. In this talk, I will show that this analogy is not merely a coincidence but that there is a deeper underlying structural relationship. The key ingredient to this observation is quantum reference frames (QRFs), which constitute a universal tool for dealing with symmetries in quantum systems. They define a split between redundant and physical information in gauge systems, thereby establishing a notion of encoding in that context. This leads to an exact dictionary between (group-based) quantum error correcting codes and QRF setups. In stabilizer codes, this uncovers a correspondence between errors and QRFs: every maximal set of correctable errors generates a unique QRF, and each QRF is associated with a unique class of correctable errors. This allows for a reinterpretation of the Knill-Laflamme condition and novel insights into the relation between correctability and redundancy. The dictionary also reveals a novel error duality, based on Pontryagin duality, and somewhat akin to electromagnetic duality. Time permitting, I will illustrate these findings in surface codes, which can be understood as both codes and lattice gauge theories. These findings may find use in code design and quantum simulations of gauge theories.
Venue: #345-347, 3F, Main Research Building
Event Official Language: English
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Seminar
Reheating after a cosmological constant relaxation and gravitational waves lensed by a supermassive black hole
January 30 (Thu) at 14:00 - 15:30, 2025
Paul Martens (Postdoctoral Fellow, Department of Physics, The Chinese University of Hong Kong, China)
This presentation will be articulated in two parts. In a first part, I will present the a reheating mechanism that follows a dynamically relaxed cosmological constant. The latter is achieved by the dynamics of a scalar field whose kinetic term is modulated by an inverse power of spacetime curvature. While it is at work against radiative corrections to the dark energy, this process alone would wipe out not only the vacuum energy, but also all other matter contents. A reheating phase is thus introduced, which exploits a null-energy-condition violating sector. In a second part, I shall present a more recent and still ongoing project to describe and characterize the lensing of gravitational waves by an active galactic nuclei (or any supermassive black hole), in the geometric limit. Such systems are simple enough for constraints to be derived with only few assumptions. Yet, they present interesting features that could provide information on e.g. on binary black hole formation mechanisms and quasinormal modes.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Krylov subspace method for quantum dynamics
December 23 (Mon) at 14:00 - 15:00, 2024
Kazutaka Takahashi (Postdoctoral Researcher, Department of Physics and Materials Science, University of Luxembourg, Luxembourg)
For a given system, the structure of the minimal subspace where the state unfolds determines the static and dynamical properties of the state. The Krylov subspace method is a mathematical framework for constructing the space systematically and has been applied to a wide variety of problems. The method was applicable only for systems with time-indepedent generators. As applications to quantum dynamics with time-dependent Hamiltonians, we discuss the constrution of the adiabatic gauge potential and the generalization of the Krylov algorithm to time-dependent generators.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Perturbative unitarity of Higgs inflation in the Riemannian and generalized geometry
December 13 (Fri) at 16:00 - 17:30, 2024
Yusuke Mikura (Ph.D. Student, C-Lab, Department of Physics, Institute for Advanced Research, Nagoya University)
In a simple Higgs inflation model in metric-affine gravity, it is known that its UV cutoff is much smaller than the Planck scale. While it calls for UV completion, a concrete example has not yet been found, even with the large-N limit known as a successful technique to complete an original Higgs inflation defined on the Riemannian geometry. In this talk, after a review of the unitarity issue and previous attempts to complete Higgs inflation models, we investigate how small deformation of the simple Higgs inflation affects the emergence and properties of dynamical fields particularly in the large-N limit.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Dark matter from inflationary quantum fluctuations
December 9 (Mon) at 14:00 - 15:30, 2024
Mohammad Ali Gorji (Junior Faculty, Center for Theoretical Physics of the Universe, Institute for Basic Science, Republic of Korea)
We explore a scenario in which dark matter is a massive bosonic field, arising solely from quantum fluctuations generated during inflation. In this framework, dark matter exhibits primordial isocurvature perturbations with an amplitude of O(1) at small scales that are beyond the reach of current observations, such as those from the CMB and large-scale structure. Assuming a monochromatic initial power spectrum, we identify the viable parameter space defined by dark matter mass and the length scale of perturbations. A key prediction of this scenario is copious formation of subsolar dark matter halos at high redshifts.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
102 events