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Gabriel Kotliar (Rutgers University)

Title DMFT and g-RISB: some unexpected connections 
Abstract Quantum embedding methods have revolutionized our understanding of strongly correlated electron materials, and opened the way to compute their physical properties starting from first principles. Traditional auxilliary particle methods, such as the slave boson method preceded DMFT are computationally much faster, but at the same time less accurate than DMFT. Surprisingly, it's possible to formulate them in the language of quantum embedding methods and this new perspective gave rise to new extensions to make their accuracy closer to that of DMFT without increasing too much its computational cost.    We will review some of these developments and illustrate them with both first principles calculations of materials as well as model Hamiltonian studies and conclude with promising future directions. 

 

Chang-Jong Kang (Chungnam National University)

Title Electronic structure and physical properties of altermagnetic systems
Abstract  Altermagnetism is a recently identified fundamental form of magnetism characterized by a vanishing net magnetization and a broken electronic structure with time-reversal symmetry. In this talk, we employ a combination of symmetry analysis and first-principle calculations to reveal that the crystallographic symmetry groups of numerous magnetic materials, featuring negligibly small relativistic spin-orbit coupling (SOC), are significantly larger than conventional magnetic groups. Consequently, a symmetry description incorporating partially decoupled spin and spatial rotations, termed the spin group, becomes essential. We establish the classifications of spin point groups that describe collinear magnetic structures, encompassing altermagnetic phases. Using MnTe as an example, we provide direct evidence for altermagnetism in MnTe. 

 

Hongbin Zhang (Technical University of Darmstadt)

Title Spectroscopic and energetic features of electronic correlations in functional materials
Abstract Advanced functional materials are of pivotal importance for future sustainable developments of our society, where the rational design of such materials entails profound mechanistic understanding of their physical properties. In this work, taking permanent magnets and battery cathode materials as examples, I am going to showcase how to quantify the corresponding features of correlated electrons. On the one hand, for high-performance permanent magnets comprising 4f rear-earth and 3d transition-metal elements, it is critical to decipher the behavior of 4f electrons driven by the interplay of hierarchical interactions. Correspondingly, I will elaborate how an efficient workflow can be constructed to parameterize DFT results and how to solve the resulting local atomic Hamiltonian to obtain energetic (i.e., magnetic anisotropy) and spectroscopic (e.g., x-ray circular dichroism) features, validated with experimental results. On the other hand, for battery cathode materials based on transition-metal oxides, I am going to demonstrate how x-ray absorption and photoelectron spectroscopies can be evaluated considering both local and long-range charge fluctuations to elucidate on the local structural variations during the (dis-)charging processes. Lastly, combined with machine learning (in particular Bayesian optimization) techniques, it is demonstrated that the local electronic/spin Hamiltonians can be identified in an automatized manner based on the spectroscopic data, enabling not only more efficient sampling for experimental data acquisition but also autonomous characterization/understanding of the underlying correlated physics in the future.

 

Bongjae Kim (Kyungpook National University)

Title Role of nonlocal Coulomb interactions in perovskite transition metal oxides
Abstract Employing the density functional theory incorporating on-site and inter-site Coulomb interactions (DFT+U+V), we have investigated the role of the nonlocal interactions on the electronic structures of the transition metal oxide perovskites. Using constrained random phase approximation (cRPA) calculations, we derived screened Coulomb interaction parameters and revealed a competition between localization and screening effects, which results in nonmonotonic behavior with d-orbital occupation. We highlight the significant role and nonlocality of inter-site Coulomb interactions, V, comparable in size to the local interaction, U. Our DFT+U+V results exemplarily show the representative band renormalization, and deviations from ideal extended Hubbard models due to increased hybridization between transition metal d and oxygen p orbitals as occupation increases. We further demonstrate that the inclusion of the inter-site V is essential for accurately reproducing the experimental magnetic order in transition metal oxides. If time allows, we will also discuss methodological aspects of the cRPA approach.

 

Thomas Ayral (Eviden Quantum Lab)

Title Quantum computing with and for many-body physics
Abstract The many-body problem is ubiquitous in condensed-matter physics, quantum chemistry and even applied mathematics (where is it known as combinatorial optimization). It represents a very hard computational challenge that is met with sophisticated classical algorithms, which come with limitations in some regimes.

On the other hand, a new generation of processors called quantum processors has emerged in the last decade that can manipulate hundreds of quantum bits for thousands of operations. These processors are themselves a many-body problem.

In this presentation, I will show that this makes quantum processors a promising tool to accelerate the resolution of many-body problems, but also asks for a careful comparison with classical methods: physical effects like decoherence degrades quantum computations, and classical methods already bring powerful intuitions.

I will thus argue that classical methods to reach potential quantum advantage: for instance, tensor networks can be used as benchmarking tools [1] or warm-starters for quantum computations [2], while quantum embedding methods can be used to take

advantage of the natural decoherence of quantum computers [3].

 

[1] Thomas Ayral, Thibaud Louvet, Yiqing Zhou, Cyprien Lambert, E Miles Stoudenmire, Xavier Waintal (2022). A density-matrix renormalisation group algorithm for simulating quantum circuits with a finite fidelity. Physical Review X Quantum 4, 020304.

[2] Baptiste Anselme Martin, Thomas Ayral, François Jamet, Marko Rancic, Pascal Simon (2023). Combining Matrix Product States and Noisy Quantum Computers for Quantum Simulation, Physical Review A, 109, 062437.

[3] Corentin Bertrand, Pauline Besserve, Michel Ferrero, Thomas Ayral (2024). Turning qubit noise into a blessing: Automatic state preparation and long-time dynamics for impurity models on quantum computers, arXiv:2412.13711.

 

Sangkook Choi (Korea Institute for Advanced Study)

Title Quantum Zeno Monte Carlo for computing observables
Abstract The recent development of logical quantum processors marks a pivotal transition from the noisy intermediate-scale quantum (NISQ) era to the fault-tolerant quantum computing (FTQC) era. These devices have the potential to address classically challenging problems with polynomial computational time using quantum properties. However, they remain susceptible to noise, necessitating noise resilient algorithms. We introduce Quantum Zeno Monte Carlo (QZMC), a classical-quantum hybrid algorithm that demonstrates resilience to device noise and Trotter errors while showing polynomial computational cost for a gapped system. QZMC computes static and dynamic properties without requiring initial state overlap or variational parameters, offering reduced quantum circuit depth.

 

[1] M. Han, H. Park, and S. Choi, Quantum Zeno Monte Carlo for computing observables, Npj Quantum Inf 11, 1 (2025).

 

Synge Todo (The University of Tokyo)

Title Advanced Tensor Network Algorithms for Quantum Many-Body Problems and Quantum Computation
Abstract Tensor networks offer a powerful and versatile framework for addressing the complexity of quantum many-body systems and the challenges in quantum computing. This talk presents recent advances in applying tensor network algorithms within quantum-classical hybrid computing frameworks, particularly their integration with high-performance computing (HPC) environments. We explore how tensor networks serve as efficient representations for quantum states, enabling breakthroughs in quantum embedding schemes, error correction protocols, and circuit simulation techniques. We further highlight the emerging role of tensor-network-based Monte Carlo methods, which combine the strengths of stochastic sampling and structured representations to enhance simulation accuracy and scalability. Beyond quantum simulation, we discuss the synergy between tensor networks and quantum machine learning, emphasizing their shared mathematical structures and potential for mutual advancement.

 

Dong-Hee Kim (Gwangju Institute of Science and Technology)

Title Disordered flat-band superconductivity in the kagome Hubbard model
Abstract We investigate the fate of the flat-band superconductivity in the attractive Hubbard model on the kagome lattices under disorder. We consider two types of disorder: the uncorrelated random onsite potential and the correlated disorder [Phys. Rev. B 98, 235109 (2018)] designed to preserve the flat band in the parent noninteracting Hamiltonian. Using the Bogoliubov-de Gennes mean-field and exact diagonalization calculations, we find that superconductivity is significantly more resilient to the flat-band preserving disorder compared to the uncorrelated random potential. While both cases develop spatial inhomogeneity in the pairing as disorder increases, eventually leading to the superconductor-insulator transition, it turns out that the geometric contribution to the superfluid weight remains robust as long as the flat-band degeneracy is intact. We observe that the flat-band signature in the superfluid weight that is linearly proportional to the interaction strength persists with the flat-band-preserving disorder.

 

Chia-Min Chung (National Sun Yat-sen University)

Title Numerical studies of superconductivity with partially filled stripes in the Hubbard model
Abstract The Hubbard model is an foundational model in quantum many-body physics and has been intensely studied, especially since the discovery of high-temperature cuprate superconductors. The development of advanced numerical techniques, such as tensor networks and quantum Monte Carlo, now enables highly accurate simulations of correlated electron systems. In this talk, I will introduce our works in numerical studies on the two-dimensional Hubbard model. Recently, we found superconductivity in both the electron- and hole-doped regimes of the two-dimensional Hubbard model with next-nearest neighbor hopping. In the electron-doped regime, superconductivity was weaker and was accompanied by antiferromagnetic Néel correlations at low doping. The strong superconductivity on the hole-doped side coexisted with stripe order, which persisted into the overdoped region with weaker hole-density modulation. These stripe orders varied in fillings between 0.6 and 0.8. Our results suggest the applicability of the Hubbard model with next-nearest hopping for describing cuprate high–transition temperature (Tc) superconductivity.

 

Evgeny Kozik (King's College London)

Title Fast diagrammatic calculations on classical — and possibly quantum — computers
Abstract Feynman’s diagrammatic series is a common language for a formally exact theoretical description of systems of infinitely-many interacting quantum particles, as well as a foundation for precision computational techniques. I will present a versatile framework for efficient and numerically exact evaluation of diagrammatic expansions of arbitrary structure. It is based on an explicit combinatorial construction of the sum of the integrands of all diagrams by dynamic programming, at a computational cost that can be made only exponential in the diagram order on a classical computer and potentially polynomial on a quantum computer. I will illustrate the technique by its application to controlled solution of correlated systems that remain a challenge for unbiased methods, including the SU(N) Hubbard model in an experimentally relevant regime and electrons in the lowest Landau level exhibiting a fractional quantum Hall state.

 

Kun Woo Kim (Chung-Ang University)

Title Cavity quantum electrodynamics of photonic temporal crystals
Abstract Photonic temporal crystals host a variety of intriguing phenomena, from wave amplification and mixing to exotic band structures, all stemming from the time-periodic modulation of optical properties. While these features have been well described classically, their quantum manifestation has remained elusive. Here, we introduce a quantum electrodynamical model of PTCs that reveals a deeper connection between classical and quantum pictures: the classical momentum gap arises from a localization-delocalization quantum phase transition in a Floquet-photonic synthetic lattice. Leveraging an effective Hamiltonian perspective, we pinpoint the critical momenta and highlight how classical exponential field growth manifests itself as wave-packet acceleration in the quantum synthetic space. Remarkably, when a two-level atom is embedded in such a cavity, its Rabi oscillations undergo irreversible decay to a half-and-half mixed state-a previously unobserved phenomenon driven by photonic delocalization within the momentum gap, even with just a single frequency mode. Our findings establish photonic temporal crystals as versatile platforms for studying nonequilibrium quantum photonics and suggest new avenues for controlling light matter interactions through time domain engineering.

 

 

Hiroshi Shinaoka (Saitama University)

Title Dimensionality Reduction for Quantum Field Theories
Abstract Quantum field theories based on Green’s functions have been widely used to solve quantum many-body problems. However, many quantum systems exhibit widely varying energy, time, or length scales, which makes them challenging to treat numerically. In this talk, I present two complementary approaches: the intermediate representation (IR) for imaginary-time propagators [1,2] and the quantics tensor train (QTT) representation [3] for general space-time dependencies.

 

The IR provides a compact and general basis for representing the imaginary-time and imaginary-frequency dependence of propagators. It is constructed via a singular value decomposition of the analytic continuation kernel [1]. Moreover, sparse grids in the imaginary-time and Matsubara-frequency domains associated with the IR can be constructed [2], enabling efficient and stable transformations between the two domains. These techniques have led to significant acceleration of diagrammatic methods such as the GW approximation [2] and Migdal–Eliashberg theory [4,5]. The IR framework can be easily integrated into existing codes using our open-source libraries: sparse-ir (Python) and SparseIR.jl (Julia) [6]. A full implementation in C and Fortran is currently under development [7].

 

In contrast, the QTT representation enables compression of more general space-time dependencies, including imaginary-time, real-time, and real-space functions with widely varying scales. Importantly, QTTs support fundamental operations—such as integration, Fourier transforms, and convolution—directly in the compressed domain. The usefulness of QTTs for such operations can be further extended [8] by combining them with tensor-network-based machine learning frameworks, such as Tensor Cross Interpolation (TCI) [9,10]. I will highlight recent applications of these techniques, including solutions to the parquet equations at the two-particle level [11], multiorbital impurity models [12], and simulations of nonequilibrium quantum dynamics [13,14].

 

[1] H. Shinaoka, J. Otsuki, M. Ohzeki, K. Yoshimi, Phys. Rev. B 96, 035147 (2017).

[2] J. Li, M. Wallerberger, C.-N. Yeh, N. Chikano, E. Gull, H. Shinaoka, Phys. Rev. B 101, 035144 (2020).

[3] H. Shinaoka, M. Wallerberger, Y. Murakami, K. Nogaki, R. Sakurai, P. Werner, A. Kauch, Phys. Rev. X 13, 021015 (2023).

[4] T. Wang, T. Nomoto, Y. Nomura, H. Shinaoka, J. Otsuki, T. Koretsune, R. Arita, Phys. Rev. B 102, 134503 (2020).

[5] H. Mori, T. Nomoto, R. Arita, E. R. Margine, Phys. Rev. B 110, 064505 (2024).

[6] M. Wallerberger et al., SoftwareX 21, 101266 (2023).

[7] https://github.com/SpM-lab/libsparseir

[8] M. K. Ritter, Y. Núñez-Fernández, M. Wallerberger, J. von Delft, H. Shinaoka, X. Waintal, Phys. Rev. Lett. 132, 056501 (2024).

[9] Y. Núñez-Fernández, M. Jeannin, P. T. Dumitrescu, T. Kloss, J. Kaye, O. Parcollet, X. Waintal, Phys. Rev. X 12, 041018 (2022).

[10] Y. Núñez-Fernández, M. K. Ritter, M. Jeannin, J.-W. Li, T. Kloss, T. Louvet, S. Terasaki, O. Parcollet, J. von Delft, H. Shinaoka, X. Waintal, SciPost Phys. 18, 104 (2025).

[11] S. Rohshap, M. K. Ritter, H. Shinaoka, J. von Delft, M. Wallerberger, A. Kauch, Phys. Rev. Research 7, 023087 (2025).

[12] H. Ishida, N. Okada, S. Hoshino, H. Shinaoka, arXiv:2405.06440v2 (to appear in PRL).

[13] M. Murray, H. Shinaoka, P. Werner, Phys. Rev. B 109, 165135 (2024).

[14] M. Środa, K. Inayoshi, H. Shinaoka, P. Werner, arXiv:2412.14032.

 

Seung-Sup Lee (Seoul National University)

Title Hundness in twisted bilayer graphene: correlated gaps and quantum criticalities
Abstract We characterize gap-opening mechanisms and quantum criticalities in the topological heavy fermion (THF) model of magic-angle twisted bilayer graphene (MATBG), with and without electron-phonon coupling, using dynamical mean-field theory (DMFT) with the numerical renormalization group (NRG) impurity solver. In the presence of symmetry breaking associated with valley-orbital ordering (time-reversal-symmetric or Kramers intervalley coherent, or valley polarized), spin anti-Hund and orbital-angular-momentum Hund couplings, induced by the dynamical Jahn–Teller effect, result in a robust pseudogap at filling 2 ≲ | ν | ≲ 2.5. We identify quantum critical points near the boundaries of the pseudogap regions, which are of Berezinskii–Kosterlitz–Thouless (BKT) or heavy-fermion type, depending on the strengths of the effective Hund couplings determined by the valley-orbital ordering. The pairing susceptibilities are enhanced near these quantum critical points, which might be a precursor to the superconducting phases neighboring | ν | = 2.

 

Kyo-Hoon Ahn (Czech Academy of Sciences)

Title Exchange interactions and spin waves in double-layered antiferromagnets: semiclassical model and ab initio analysis
Abstract Traditional approaches to understanding the sign of exchange interactions rely on competition between antiferromagnetic (AFM) and ferromagnetic (FM) contributions by means of wavefunction overlap [1], electrostatic interactions [2], and orbital symmetries [3]. However, these analyses have primarily focused on pairs of actual ions, whereas a comparable study on elementary spins in chain models remains unexplored. This raises a key question: Does such a sign change in exchange parameters apply to pairs of elementary spins, or are there unknown constraints that might control it?

To address this issue, we introduce a novel semiclassical model of magnons in double-layered AFM chains with a collinear ↑↑↓↓ spin order. We underline that conventional FM and AFM chains contain only parallel or antiparallel spin pairs, respectively. In contrast, our model involves both types of pairs at each site, making it a minimal yet ideal framework for exploring the interplay between intra- and inter-sublattice couplings. We find that the surrounding spin configurations can control the sign of exchange parameters and even lead to an AFM sign for the intra-sublattice interactions in our model of railroad trestle (RT) geometry.

We substantiate our findings through direct calculations of the magnon dispersion relation and ab initio investigations on AFM CrN. We identify CrN as a unique example where a fully three-dimensional system can be effectively described by a one-dimensional model (RT). This successful application arises from the dominant target ↑↑↓↓ order, which exists exclusively along a single crystallographic direction in double-layered antiferromagnets.

[1] J. C. Slater, Phys. Rev. 36, 57 (1930).

[2] C. Zener, Phys. Rev. 81, 440 (1951); Phys. Rev. 83, 299 (1951).

[3] Y. O. Kvashnin, R. Cardias, A. Szilva, I. Di Marco, M. I. Katsnelson, A. I. Lichtenstein, L. Nordström, A. B. Klautau, and O. Eriksson, Phys. Rev. Lett. 116, 217202 (2016).

[Preprint] Seo-Jin Kim, Zdeněk Jirák, Jiří Hejtmánek, Karel Knížek, Helge Rosner, and Kyo-Hoon Ahn, arXiv:2412.04685.

 

Andreas Weichselbaum (Brookhaven National Laboratory)

Title Automated Fermionic signs in tensor networks via Z2 graded tensors.
Abstract It is well known that Fermionic signs in one-dimensional (1D) systems can be taken care of by a Jordan-Wigner transformation. In 2D lattice models, the fermionic projected entangled pair state (fPEPS) construction by Corboz et al. (PRB 2010) has shown that fermionic signs beyond 1D systems can also be treated efficiently locally, albeit with organizational overhead to properly include fermionic parity. More recently it was argued (Mortier et al, SciPost 2024; Gao et al., quant-ph/2024 ) that fermionic signs can be treated automatically in the tensor framework via Z2-graded tensors. I will briefly review this setting, together with the current status of its implementation in the QSpace tensor library.