Shinsei Ryu (Princeton University) Room: Physics/208

Fractional quantum anomalous Hall effects in rhombohedral pentalayer graphene

• Xiao Li

Room: Physics/208 The fractional quantum anomalous Hall (FQAH) effect in rhombohedral pentalayer graphene (PLG) has attracted significant attention due to its potential for observing exotic quantum states [1-3]. This talk will discuss two projects exploring the FQAH effect in PLG. First, we present a self-consistent Hartree-Fock theory focusing on the convergence of the calculation with various reference fields and the stability of the FQAH states [4-5]. We demonstrate that the charge neutrality scheme ensures convergence with respect to the momentum cutoff and provides an unambiguous result. Based on the Hartree-Fock band structure, we perform exact diagonalization calculations to investigate the stability of the FQAH states in PLG. The second project examines the intriguing experimental observation of FQAH states at various fractional fillings giving way to integer quantum anomalous Hall (IQAH) states as the temperature is lowered [3]. We propose a mechanism for the appearance of FQAH states within a finite temperature range using a toy model consisting of a flat Chern band and impurities [6]. The effects of impurities on the system's behavior at finite temperatures are analyzed, and we posit that the crossover may arise from the competition between the energy penalty for thermal excitations and the increase in entropy. Numerical calculations using exact diagonalization support our theoretical argument, suggesting that impurities may play a crucial role in the crossover from FQAH to IQAH states in rhombohedral PLG. Together, these projects provide an improved and unified theoretical framework for understanding the FQAH effect in rhombohedral PLG and pave the way for future studies on this captivating quantum phenomenon. References: [1] Z. Lu, T. Han, Y. Yao, A. P. Reddy, J. Yang, J. Seo, K. Watanabe, T. Taniguchi, L. Fu, and L. Ju, Fractional quantum anomalous Hall effect in multilayer graphene, Nature 626, 759 (2024). [2] D. Waters, A. Okounkova, R. Su, B. Zhou, J. Yao,K. Watanabe, T. Taniguchi, X. Xu, Y.-H. Zhang, J. Folk, and M. Yankowitz, Interplay of electronic crystals with integer and fractional Chern insulators in moiré pentalayer graphene, arXiv:2408.10133. [3] Z. Lu, T. Han, Y. Yao, J. Yang, J. Seo, L. Shi, S. Ye, K. Watanabe, T. Taniguchi, and L. Ju, Extended Quantum Anomalous Hall States in Graphene/hBN Moiré Superlattices, arXiv:2408.10203. [4] K. Huang, X. Li, S. Das Sarma, and F. Zhang, Self-consistent theory of fractional quantum anomalous Hall states in rhombohedral graphene, Phys. Rev. B 110, 115146 (2024).

Non-Hermitian quantum systems and non-unitary criticality

• Po-Yao Chang

Room: Physics/208 The characterization of non-unitary conformal field theories (CFTs) via entanglement measures is often hindered by the appearance of negative central charges, which lack a clear interpretation in standard entanglement theory. We address this by formulating a generalized entanglement entropy that remains well-defined in the non-unitary regime. Through numerical and analytical validation—including applications to the PT-symmetric Su-Schrieffer-Heeger model and quantum-group-invariant XXZ chains—we show that this measure accurately captures the scaling of non-unitary CFTs. We further apply this framework to investigate non-Hermitian critical phenomena. Our results reveal that local non-Hermitian perturbations can induce exceptional points that significantly alter the system's criticality, specifically shifting the central charge from c=1 to c=−2. Additionally, we present a novel PT-symmetric gapless SPT phase characterized by the presence of boundary modes that exhibit an unconventional degree of robustness, suggesting new directions for the study of topology in non-Hermitian critical systems.

Dualities and Exact Many-Body Scars

• Masahito Yamazaki (University of Tokyo)

Room: Physics/208 I will first discuss new examples of quantum many-body scars in the two-dimensional XY model. I will then discuss a dual $Z_2$ gauge theory obtained by gauging a global symmetry of the XY model, and discuss corresponding scars therein. This discussion suggests the possibility of systematic exploration of quantum many-body scars in the web of theories related by dualities. The talk is based on the paper arXiv:2505.21921, in collaboration with Yuan Miao, Linhao Li, and Hosho Katsura.

Interference, topology, and new Hilbert-space routes to quantum non-ergodicity

• Yi-Ping Huang (National Tsing Hua University)

Room: Physics/208 A central challenge in nonequilibrium quantum physics is to understand why certain many-body systems fail to thermalize even in the absence of disorder or integrability. In this talk, I will outline a different perspective in which non-ergodicity is governed by hidden geometric structures in Hilbert space rather than by conventional real-space mechanisms. This viewpoint leads to the concept of interference-caged quantum many-body scars (ICQMBS), where exact many-body destructive interference confines eigenstates to small regions of the Fock-space graph. Remarkably, interference zeros and graph automorphisms emerge as universal organizing principles, revealing a class of topological ICQMBS whose robustness originates from local Fock-space topology rather than symmetries or constraints. This framework not only explains diverse non-ergodic phenomena from one-dimensional systems to two-dimensional gauge models but also provides new tools for systematically identifying them. In addition to recent advances of using Fock-space graph to explore quantum ergodicity breaking, I will also summarize the recent applications of caged states in different contexts.

Quantum speed limits and selected applications in quantum many-body physics and quantum information science

• Jyong-Hao Chen (National Central University)

Room: Physics/208 Quantum speed limits furnish fundamental bounds on the rate of quantum evolution and thus provide a natural framework for analyzing quantum state preparation. In this talk, I will review the geometric formulation of quantum speed limits and discuss how these bounds can be used to constrain fidelities in driven many-body systems. I will focus in particular on applications to adiabatic dynamics, including bounds on the adiabaticity of pure states [1,2], lower bounds on the runtime of adiabatic quantum computation [3], and recent extensions to mixed states [4]. These examples illustrate how geometric constraints on quantum dynamics lead to useful bounds on fidelity and timescales in many-body physics and quantum information science. References: [1] J.-H. Chen and V. Cheianov, Phys. Rev. Research 4, 043055 (2022). [2] J.-H. Chen and V. Cheianov, SciPost Phys. Core 8, 084 (2025). [3] J.-H. Chen, Phys. Rev. Research 5, 033175 (2023). [4] L.-Y. Chou and J.-H. Chen, arXiv:2602.01943 [quant-ph].