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CREST HEP-QC seminar: PM14:00-, 05th Feb. (Thu) 2026 (Zoom online)

Title: Neural network wavefunctions for SU(2) lattice gauge theory in the Hamiltonian formulation

Speaker: Thomas Spriggs (Kavli Institute of Nanoscience and QuTech)

Abstract: In this talk I will cover our preprint arXiv:2509.12323 where we propose a neural network approach to finding the ground state wavefunction of SU(2) lattice gauge theory. Specifically, we demonstrate that the use of bespoke SU(2)-gauge-equivariant neural network layers increases the extent to which our variational ansatz can represent the ground state of this system. During this talk I will contrast the Hamiltonian and Euclidean formalisms of lattice gauge theories, highlighting the promises that the former offers but also the difficulties: noting briefly the issues of parameterising the continuous Hilbert space that plague tensor network and quantum simulation approaches and how our approach alleviates this. I will try and present our method pedagogically as we are very interested in learning its uses but also the limits of its validity, before closing with some remarks on scaling to larger systems and different gauge groups.

CREST HEP-QC seminar: PM14:00-, 06th Jan. (Tue) 2026 (K202)

Title: Clarifying the Quantum Simulation of Non-Abelian Gauge Theory: Two Common Misconceptions

Speaker: Masanori Hanada (Queen Mary, U. of London)

Abstract:For a long time, there was almost no progress toward quantum simulations of non-Abelian gauge theory because of two fatal misconceptions --- (1) the gauge field on a lattice must be described by unitary link variables, and (2) physical states are gauge-invariant. The truth is that we don't have to use unitary link variables, and gauge-invariant states are just one of many possible representations of physical states. Once we free ourselves from those misconceptions, it becomes straightforward to design efficient quantum simulation protocols.

CREST HEP-QC seminar: PM16:00-, 23rd Dec. (Tue) 2025 (K202)

Title: Logical Gates by Gauge Field Formalism of Quantum Error Correction

Speaker: Junichi Haruna (The University of Osaka)

Abstract:The gauge field formalism, or operator-valued cochain formalism, has recently emerged as a powerful framework for describing quantum Calderbank-Shor-Steane (CSS) codes. In this work, we extend this framework to construct a broad class of logical gates for general CSS codes, including the S, Hadamard, T, and (multi-)controlled-Z gates, under the condition where fault-tolerance or circuit-depth optimality is not necessarily imposed. We show that these logical gates can be expressed as exponential of polynomial functions of the electric and magnetic gauge fields, which allows us to derive explicit decompositions into physical gates. We further prove that their logical action depends only on the (co)homology classes of the corresponding logical qubits, establishing consistency as logical operations. Our results provide a systematic method for formulating logical gates for general CSS codes, offering new insights into the interplay between quantum error correction, algebraic topology, and quantum field theory. This talk is based on arXiv:2511.15224.

CREST HEP-QC seminar: PM16:30-, 04th Nov. (Tue) 2025 (K202)

Title: Infinite matrix product states for 1+1-dimensional gauge theories

Speaker: Ross Dempsey (MIT department of physics)

Abstract:Gauge theories in 1+1 dimensions serve as models for many interesting phenomena, including (de)confinement, supersymmetry, and non-invertible symmetry. This has motivated substantial effort towards numerical simulation of these theories. In this talk, I will review existing approaches for applying matrix product states to gauge theories, and present a new construction called a link-enhanced matrix product operator (LEMPO). I will show how LEMPOs naturally encode the Hamiltonians of lattice gauge theories with abelian or non-abelian gauge groups, and allow us to work on infinite lattices. I will show examples of the results we can obtain with this method, focusing on the Schwinger model and on adjoint QCD_2.

CREST HEP-QC seminar: PM15:00-, 09th Oct. (Thu) 2025 (K202)

Title: Correlation-spreading dynamics in cold-atom systems

Speaker: Ippei Danshita (Kindai University)

Abstract: Thanks to their controllability and long coherence time, quantum simulators built with cold atoms serve as ideal platforms for the studies of quantum many-body dynamics far from equilibrium. In this work, we investigate dynamics of spatial correlations after quantum quenches in a few many-body models which can be simulated with cold atom systems. We first analyze the two-dimensional (2D) Bose-Hubbard model [1], which quantitatively describes Bose gases in optical lattices. Using the projected entangled pair states (PEPS) algorithm, we compute time evolution of the single-particle and density-density correlation functions after a quantum quench starting from a Mott insulating state with unit filling. Comparing our numerical results of the single-particle correlation function with the outputs from the experiments of Ref. [2], we show that PEPS can quantitatively capture the correlation-spreading dynamics of this system. We calculate the propagation velocities of the correlations for a wide range of the interaction parameter. We next focus on the spin-1/2 Ising model with a transverse field [3], which can be simulated with Rydberg atoms in an optical tweezer array. We use several methods, including the exact analytical treatment for 1D, PEPS for 2D, and linear-spin-wave theory (LSWT) for 1D and 2D, in order to compute the spin-spin correlation functions after a quantum quench of the transverse field from infinity to a finite value. In contrast to the case of the Bose-Hubbard model, it is difficult to extract the propagation velocities from the PEPS results because of irregular oscillatory behavior of the correlation functions. Instead, from a thorough comparison between the LSWT and exact analytical treatment for 1D, we show that the LSWT can quantitatively capture the propagation velocities of the correlations as long as the final transverse field is sufficiently large. Our findings will be useful as benchmarks for quantum-simulation experiments in the future and theoretical refinement of the Lieb-Robinson bound of these models. [1] R. Kaneko and I. Danshita, Commun. Phys. 5. 65 (2022). [2] Y. Takasu et al., Sci. Adv. 6, eaba9255 (2020). [3] R. Kaneko and I. Danshita, Phys. Rev. A 108, 023301 (2023).

CREST HEP-QC seminar: AM10:00-, 26th Sep. (Fri) 2025 (K202)

Title: Modulated symmetries in anomalous spin systems

Speaker: Hiromi Ebisu (RIKEN iTHEMS)

Abstract: Recently, there has been considerable amount of interests in updating concept of symmetry, which is the most fundamental guiding principal in physics. One such an example is modulated symmetry, described by spatially inhomogeneous symmetry operations. The key insight of the modulated symmetry is that it imposes a mobility constraint on excitations, leading to intriguing physical consequences, such as the breaking of ergodicity. In this talk, we address the following question; ``how is the modulated symmetry emerged?" To do so, we explore the interplay between quantum spin systems with anomaly in the sense that two global symmetries (including higher form ones) exhibit nontrivial commutation relation, depending on the system size in a manner akin to the Lieb-Schultz-Mattis type anomaly, and modulated symmetry. We demonstrate that by gauging one of the global symmetries in an anomalous spin model, there exit modulated symmetries, especially dipole symmetries associated with conservation of dipole. Moreover, these modulated symmetries form unusual dipole algebra – p-form and q-form symmetry operators are related with one another via translational operators. Our consideration provides a new insight of emergence of modulated symmetries in a concrete quantum system on a lattice with anomaly, making better understanding of these exotic symmetries.

CREST HEP-QC seminar: AM10:00-, 15th Jul. (Tue) 2025 (K202)

Title: Computational complexity of unitary and state design properties

Speaker: Yoshifumi Nakata (YITP)

Abstract: Randomness is extremely useful resource in information processing. This is extended even to the quantum regime, where randomness is represented by a random unitary (or pure state) that is chosen uniformly at random. However, generating uniform randomness by quantum circuits is highly inefficient. This necessitates the concept of its approximation, commonly formulated as unitary and state designs. In this study, we introduce and study computational problems to decide whether a given distribution forms a unitary (or state) design. We formulate this problem in various ways and show that they belong to the computational classes that are computationally hard to solve. That is, efficient algorithms for checking unitary (or state) designs are highly unlikely to exist, whether classical or quantum.

CREST HEP-QC seminar: PM14:00-, 24th Jun. (Tue) 2025 (K202)

Title: Tensor renormalization group study of pure Yang-Mills theories in 2+1 dimensions

Speaker: Atis Yosprakob (YITP)

Abstract: We present a tensor renormalization group (TRG) simulation of pure SU(N) gauge theories in 2+1 dimensions for N=2 and N=3. The simulations are carried out on a dual lattice using the armillary sphere formalism, which analytically eliminates unphysical degrees of freedom in the lattice. The reduced model enables high-precision calculations compared to the original model at both zero and finite temperatures. In particular, we identify the deconfinement transition, consistent with those obtained from Monte Carlo simulations.

CREST HEP-QC seminar: AM10:00-, 12th May (Mon) 2025

Title: Critical behavior of the Schwinger model via gauge-invariant variational uniform matrix product states

Speaker: Kohei Fujikura (YITP)

Abstract: We study the lattice Schwinger model by combining the variational uniform matrix product state (VUMPS) algorithm with a gauge-invariant matrix product ansatz that locally enforces the Gauss law constraint. Both the continuum and lattice versions of the Schwinger model with θ=π are known to exhibit first-order phase transitions for the values of the fermion mass above a critical value, where a second-order phase transition occurs. Our algorithm enables a precise determination of the critical point in the continuum theory. We further analyze the scaling in the simultaneous critical and continuum limits and confirm that the data collapse aligns with the Ising universality class to remarkable precision.

CREST HEP-QC seminar: AM10:00-, 9th Apr. (Wed) 2025

Title: Z_N lattice gauge theory: toy model for quantum simulation

Speaker: Arata Yamamoto (University of Tokyo)

Abstract: In high energy physics, realistic gauge theory has continuous gauge symmetry such as U(1) and SU(N). Quantum simulation of continuous gauge theory is however difficult due to its infinite-dimensional Hilbert space. Discrete gauge theory is used for benchmarking near-term devices and algorithms. In this seminar, I will present the introduction of Z_N lattice gauge theory and its application to quantum simulation.

CREST HEP-QC seminar: AM10:00-, 19th Feb. (Wed) 2025

Title: Improving threshold and decoding for fault-tolerant color code quantum computing

Speaker: Yugo Takada (Osaka University)

Abstract: Color codes are promising quantum error-correcting codes because they have an advantage over surface codes in that all Clifford gates can be implemented transversally. However, they also have drawbacks: decoding is difficult, and the error threshold is low. In this seminar, I will talk about two works that address these drawbacks. In Ref. [1], an Ising model formulation for decoding is proposed, and numerical results demonstrate that a decoder using this formulation achieves high accuracy. Ref. [2] presents a method to improve the error threshold by optimizing decoder weights using flag qubits. In numerical simulations, we show that the threshold has been improved compared to previous studies.
[1] Y Takada, Y Takeuchi, and K Fujii, Phys. Rev. Research 6, 013092 (2024).
[2] Y Takada and K Fujii, PRX Quantum 5, 030352 (2024).
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