Researchers at Japan's University of Electro-Communications (UEC) have made a groundbreaking discovery in quantum many-body physics, revealing how the shape of a lattice can inhibit the relaxation of quantum systems toward equilibrium. Published in Physical Review B on April 20, 2026, the study titled "Dynamics of entanglement asymmetry for space-inversion symmetry of free fermions on honeycomb lattices" demonstrates that in honeycomb lattices—structures akin to graphene's atomic arrangement—equilibration does not always occur, depending crucially on the lattice's dimensions.
This finding challenges conventional assumptions in quantum dynamics, where isolated systems are expected to thermalize over time. Instead, UEC's team showed that for certain lattice configurations, symmetry breaking persists indefinitely, preventing the system from reaching thermal equilibrium. The research, led by a collaborative effort from UEC's Department of Fundamental Science and Engineering, opens new avenues for controlling quantum states in materials and simulations.
🧬 Understanding Quantum Many-Body Systems and Relaxation
Quantum many-body systems consist of numerous interacting particles governed by quantum mechanics, exhibiting collective behaviors like superconductivity or magnetism. In these systems, quantum many-body relaxation refers to the process where an initial non-equilibrium state evolves toward thermal equilibrium, distributing energy evenly according to the system's Hamiltonian.
Traditionally, the eigenstate thermalization hypothesis (ETH) posits that most quantum systems thermalize. However, exceptions exist, such as many-body localization (MBL), where disorder traps the system in non-ergodic states. The UEC study explores free fermions—non-interacting particles with fermionic statistics—on a honeycomb lattice, a model relevant to topological insulators and graphene-like materials.
Honeycomb lattices feature Dirac cones in their band structure, leading to linear dispersion relations. The team's innovation lies in using entanglement asymmetry, a quantum information measure quantifying symmetry breaking in subsystems, to track relaxation dynamics.
🔬 The Novel Mechanism: Lattice Shape's Role
The core discovery is that equilibration depends on the lattice's vertical length parity (odd or even number of unit cells). For odd lengths (Ly odd), space-inversion symmetry in subsystems recovers, allowing thermalization. For even lengths (Ly even), symmetry breaking endures, inhibiting relaxation.
This arises from flat bands in the single-particle energy spectrum, influenced by boundary conditions and parity. Flat bands, with zero dispersion, localize quasiparticles, preventing energy redistribution. The researchers employed quasiparticle representations and numerical simulations to map entanglement asymmetry's time evolution, confirming persistent oscillations for even Ly.
This shape-dependent inhibition provides a tunable knob for engineering non-equilibrium quantum states, contrasting with interaction-driven MBL.
📊 Methodology: Entanglement Asymmetry and Simulations
Entanglement asymmetry (A) measures deviation from maximally mixed states respecting symmetry, computed via charged moments of the reduced density matrix. For U(1) symmetries, it's linked to particle number fluctuations; here, adapted for space-inversion (Z2 symmetry).
The team simulated finite honeycomb lattices with periodic boundaries in one direction and open in the other, quenching from symmetry-broken initial states. Exact diagonalization and quasiparticle propagation revealed parity-selective dynamics. Flat bands at zero energy for even Ly trap modes, sustaining asymmetry.
For more details, see the original paper in Physical Review B.
🏛️ University of Electro-Communications: A Quantum Powerhouse
Established in 1919 as a training institute for electrical communications, UEC in Chofu, Tokyo, evolved into a national university specializing in information science, engineering, and frontier physics. With ~4,000 students and strong industry ties (e.g., NTT), UEC ranks high in Japan's quantum research ecosystem.
Japan's Moonshot R&D Program and Q-LEAP allocate billions to quantum tech, with UEC contributing via its Quantum Science Center. Recent initiatives include collaborations with RIKEN and global partners, fostering quantum simulation and computing. This discovery exemplifies UEC's role in advancing non-equilibrium quantum physics amid ¥1 trillion+ national quantum investments by 2026.
👥 Spotlight on the Researchers
Ryogo Hara, a first-year master's student, performed key simulations, marking his entry into publishable research.
Shimpei Endo, Associate Professor, specializes in quantum information and dynamics, with prior work on quantum Mpemba effects and error correction. His lab bridges theory and experiment.
Shion Yamashika, Assistant Professor since 2023, focuses on quantum many-body scars and Mpemba effects. With a PhD from University of Tokyo and postdoc at RIKEN, Yamashika's expertise in entanglement drove this study. Active on X (@Mathemashika), he bridges academia and outreach.
Their collaboration highlights UEC's mentorship model, nurturing students in cutting-edge quantum physics.
🌐 Implications for Quantum Technologies
This lattice shape control offers strategies for stabilizing quantum states against decoherence, vital for quantum computing. Honeycomb platforms simulate topological phases; inhibiting relaxation could realize robust qubits or sensors.
In Japan, where quantum simulators using cold atoms or Rydberg arrays advance rapidly (e.g., RIKEN's 100-qubit systems), UEC's findings guide experimental designs. Globally, parallels to moiré lattices in twisted bilayer graphene suggest applications in exotic matter engineering.
Read UEC's announcement: News Release.
🔮 Future Directions and Experimental Realization
Extending to interacting fermions or bosons could reveal richer dynamics. Experiments with ultracold fermions in optical honeycomb lattices (achieved by groups at Hokkaido University) may verify predictions. Yamashika envisions hybrid systems blending theory with UEC's quantum optics expertise.
Japan's 2026 quantum roadmap emphasizes simulation; UEC aims for NISQ-era validations.
📈 Japan's Higher Education in Quantum Frontier
Universities like UEC, Tokyo, Kyoto, and Tohoku lead Japan's quantum surge, backed by MEXT's Q-LEAP (¥30B+). UEC's interdisciplinary focus positions it uniquely for quantum info-communications.
Challenges include talent retention amid global competition, but initiatives like JSPS fellowships sustain momentum. This PRB publication underscores Japan's rising quantum research output, rivaling Europe/US.
Photo by Kieran Wood on Unsplash
In summary, UEC's lattice shape discovery redefines quantum relaxation control, promising advances in simulation and tech. As Japan invests heavily in quantum higher ed, watch UEC for more breakthroughs.
- Explore quantum research roles: links below.