C5: Constrained quantum many-body dynamics
Mari-Carmen Bañuls, Michael Knap, Frank Pollmann
When quantum many-body systems are brought out of equilibrium, global conserved quantities commonly lead to diffusive transport at late times according to the laws of classical hydrodynamics. However, recently various counter-examples have been identified in which the simple assumption of diffusive hydrodynamic transport fails. In these systems, constraints appear to play an essential role for understanding their unconventional dynamics. Our main goals are to investigate the role of different constraints on the quantum dynamics in condensed matter relevant settings using novel methods and to provide experimentally relevant predictions.
Publications
2025 |
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Isbrandt, G.; Pollmann, F.; Knap, M. Anisotropic spin ice on a breathing pyrochlore lattice Journal Article Phys. Rev. B 112, 165110 (2025). @article{isbrandt_anisotropic_2025,Spin ice systems represent a prime example of constrained spin systems and exhibit rich low-energy physics. In this study, we explore how introducing a tunable anisotropic spin coupling to the conventional Ising spin ice Hamiltonian on the breathing pyrochlore lattice affects the ground state properties of the system. Significant changes are observed in the ground state structure, reflected in the spin structure factor and in a reduction of residual entropy at low temperatures. We theoretically uncover a rich phase diagram by varying the anisotropy and demonstrate how this modification reduces the ground state degeneracy across different phases. Numerical simulations reveal that, at sufficiently low temperatures, the system either undergoes a crossover into a constrained spin ice manifold, characterized by an entropy density that drops below the Pauling entropy of conventional spin ice, or a phase transition into a symmetry-broken state, depending on the perturbations. Additionally, we compute the spin structure factors for the anisotropic model and compare these results to analytical predictions from a self-consistent Gaussian approximation, finding good agreement. This work develops the understanding of spin ice in anisotropic limits, which may be experimentally realized by strain, providing, among others, key signatures in entropy and specific heat. |  |
Will, M.; Cochran, T. A.; Rosenberg, E.; Jobst, B.; Eassa, N. M.; Roushan, P.; Knap, M.; Gammon-Smith, A.; Pollmann, F. Probing non-equilibrium topological order on a quantum processor Journal Article Nature 645, 348 (2025). @article{will_probing_2025,Out-of-equilibrium phases in many-body systems constitute a new paradigm in quantum matter—they exhibit dynamical properties that may otherwise be forbidden by equilibrium thermodynamics. Among these non-equilibrium phases are periodically driven (Floquet) systems1–5, which are generically difficult to simulate classically because of their high entanglement. Here we realize a Floquet topologically ordered state theoretically proposed in ref. 6, on an array of superconducting qubits. We image the characteristic dynamics of its chiral edge modes and characterize its emergent anyonic excitations. Devising an interferometric algorithm allows us to introduce and measure a bulk topological invariant to probe the dynamical transmutation of anyons for system sizes up to 58 qubits. Our work demonstrates that quantum processors can provide key insights into the thus-far largely unexplored landscape of highly entangled non-equilibrium phases of matter. | |
Papaefstathiou, I.; Malz, D.; Cirac, J. I.; Bañuls, M. C. Efficient tensor network simulation of multiemitter non-Markovian systems Journal Article Phys. Rev. A 112, 013721 (2025). @article{papaefstathiou_efficient_2025, | |
Cochran, T. A.; Jobst, B.; Rosenberg, E.; Lensky, Y. D.; Gyawali, G.; Eassa, N.; Will, M.; Szasz, A.; Abanin, D.; Acharya, R.; Beni, L. Aghababaie; Andersen, T. I.; Ansmann, M.; Arute, F.; Arya, K.; Asfaw, A.; Atalaya, J.; Babbush, R.; Ballard, B.; Bardin, J. C.; Bengtsson, A.; Bilmes, A.; Bourassa, A.; Bovaird, J.; Broughton, M.; Browne, D. A.; Buchea, B.; Buckley, B. B.; Burger, T.; Burkett, B.; Bushnell, N.; Cabrera, A.; Campero, J.; Chang, H. -S.; Chen, Z.; Chiaro, B.; Claes, J.; Cleland, A. Y.; Cogan, J.; Collins, R.; Conner, P.; Courtney, W.; Crook, A. L.; Curtin, B.; Das, S.; Demura, S.; Lorenzo, L. De; Paolo, A. Di; Donohoe, P.; Drozdov, I.; Dunsworth, A.; Eickbusch, A.; Elbag, A. Moshe; Elzouka, M.; Erickson, C.; Ferreira, V. S.; Burgos, L. Flores; Forati, E.; Fowler, A. G.; Foxen, B.; Ganjam, S.; Gasca, R.; Genois, É.; Giang, W.; Gilboa, D.; Gosula, R.; Dau, A. Grajales; Graumann, D.; Greene, A.; Gross, J. A.; Habegger, S.; Hansen, M.; Harrigan, M. P.; Harrington, S. D.; Heu, P.; Higgott, O.; Hilton, J.; Huang, H. -Y.; Huff, A.; Huggins, W.; Jeffrey, E.; Jiang, Z.; Jones, C.; Joshi, C.; Juhas, P.; Kafri, D.; Kang, H.; Karamlou, A. H.; Kechedzhi, K.; Khaire, T.; Khattar, T.; Khezri, M.; Kim, S.; Klimov, P.; Kobrin, B.; Korotkov, A.; Kostritsa, F.; Kreikebaum, J.; Kurilovich, V.; Landhuis, D.; Lange-Dei, T.; Langley, B.; Lau, K. -M.; Ledford, J.; Lee, K.; Lester, B.; Guevel, L. Le; Li, W.; Lill, A. T.; Livingston, W.; Locharla, A.; Lundahl, D.; Lunt, A.; Madhuk, S.; Maloney, A.; Mandrà, S.; Martin, L.; Martin, O.; Maxfield, C.; McClean, J.; McEwen, M.; Meeks, S.; Megrant, A.; Miao, K.; Molavi, R.; Molina, S.; Montazeri, S.; Movassagh, R.; Neill, C.; Newman, M.; Nguyen, A.; Nguyen, M.; Ni, C. -H.; Ottosson, K.; Pizzuto, A.; Potter, R.; Pritchard, O.; Quintana, C.; Ramachandran, G.; Reagor, M.; Rhodes, D.; Roberts, G.; Sankaragomathi, K.; Satzinger, K.; Schurkus, H.; Shearn, M.; Shorter, A.; Shutty, N.; Shvarts, V.; Sivak, V.; Small, S.; Smith, W. C.; Springer, S.; Sterling, G.; Suchard, J.; Sztein, A.; Thor, D.; Torunbalci, M.; Vaishnav, A.; Vargas, J.; Vdovichev, S.; Vidal, G.; Heidweiller, C. Vollgraff; Waltman, S.; Wang, S. X.; Ware, B.; White, T.; Wong, K.; Woo, B. W. K.; Xing, C.; Yao, Z. Jamie; Yeh, P.; Ying, B.; Yoo, J.; Yosri, N.; Young, G.; Zalcman, A.; Zhang, Y.; Zhu, N.; Zobrist, N.; Boixo, S.; Kelly, J.; Lucero, E.; Chen, Y.; Smelyanskiy, V.; Neven, H.; Gammon-Smith, A.; Pollmann, F.; Knap, M.; Roushan, P. Visualizing dynamics of charges and strings in (2 + 1)D lattice gauge theories Journal Article Nature 642, 315 (2025). @article{cochran_visualizing_2025,Lattice gauge theories (LGTs)1–4 can be used to understand a wide range of phenomena, from elementary particle scattering in high-energy physics to effective descriptions of many-body interactions in materials5–7. Studying dynamical properties of emergent phases can be challenging, as it requires solving many-body problems that are generally beyond perturbative limits8–10. Here we investigate the dynamics of local excitations in a $$textbackslashmathbbZ_2$$LGT using a two-dimensional lattice of superconducting qubits. We first construct a simple variational circuit that prepares low-energy states that have a large overlap with the ground state; then we create charge excitations with local gates and simulate their quantum dynamics by means of a discretized time evolution. As the electric field coupling constant is increased, our measurements show signatures of transitioning from deconfined to confined dynamics. For confined excitations, the electric field induces a tension in the string connecting them. Our method allows us to experimentally image string dynamics in a (2+1)D LGT, from which we uncover two distinct regimes inside the confining phase: for weak confinement, the string fluctuates strongly in the transverse direction, whereas for strong confinement, transverse fluctuations are effectively frozen11,12. We also demonstrate a resonance condition at which dynamical string breaking is facilitated. Our LGT implementation on a quantum processor presents a new set of techniques for investigating emergent excitations and string dynamics. | |
Zerba, C.; Seidel, A.; Pollmann, F.; Knap, M. Emergent Fracton Hydrodynamics of Ultracold Atoms in Partially Filled Landau Levels Journal Article PRX Quantum 6, 020321 (2025). @article{zerba_emergent_2025,The realization of synthetic gauge fields for charge neutral ultracold atoms and the simulation of quantum Hall physics have witnessed remarkable experimental progress. Here, we establish key signatures of fractional quantum Hall systems in their nonequilibrium quantum dynamics. We show that in the lowest Landau level the system generically relaxes subdiffusively. The slow relaxation is understood from emergent conservation laws of the total charge and the associated dipole moment that arises from the effective Hamiltonian projected onto the lowest Landau level, leading to subdiffusive fracton hydrodynamics. We discuss the prospect of rotating quantum gases as well as ultracold atoms in optical lattices for observing this unconventional relaxation dynamics. | |
Li, Y.; Pollmann, F.; Read, N.; Sala, P. Highly Entangled Stationary States from Strong Symmetries Journal Article Phys. Rev. X 15, 011068 (2025). @article{li_highly_2025,We find that the presence of strong non-Abelian symmetries can lead to highly entangled stationary states even for unital quantum channels. We derive exact expressions for the bipartite logarithmic negativity, Rényi negativities, and operator space entanglement for stationary states restricted to one symmetric subspace, with focus on the trivial subspace. We prove that these apply to open quantum evolutions whose commutants, characterizing all strongly conserved quantities, correspond to either the universal enveloping algebra of a Lie algebra or the Read-Saleur commutants. The latter provides an example of quantum fragmentation, whose dimension is exponentially large in system size. We find a general upper bound for all these quantities given by the logarithm of the dimension of the commutant on the smaller bipartition of the chain. As Abelian examples, we show that strong U(1) symmetries and classical fragmentation lead to separable stationary states in any symmetric subspace. In contrast, for non-Abelian SU(𝑁) symmetries, both logarithmic and Rényi negativities scale logarithmically with system size. Finally, we prove that, while Rényi negativities with 𝑛 >2 scale logarithmically with system size, the logarithmic negativity (as well as generalized Rényi negativities with 𝑛 <2) exhibits a volume-law scaling for the Read-Saleur commutants. Our derivations rely on the commutant possessing a Hopf algebra structure in the limit of infinitely large systems and, hence, also apply to finite groups and quantum groups. | |
Papaefstathiou, I.; Knolle, J.; Bañuls, M. C. Real-time scattering in the lattice Schwinger model Journal Article Phys. Rev. D 111, 014504 (2025). @article{papaefstathiou_real-time_2025, | |
2024 |
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Adler, D.; Wei, D.; Will, M.; Srakaew, K.; Agrawal, S.; Weckesser, P.; Moessner, R.; Pollmann, F.; Bloch, I.; Zeiher, J. Observation of Hilbert space fragmentation and fractonic excitations in 2D Journal Article Nature 636, 80 (2024). @article{adler_observation_2024,The relaxation behaviour of isolated quantum systems taken out of equilibrium is among the most intriguing questions in many-body physics1. Quantum systems out of equilibrium typically relax to thermal equilibrium states by scrambling local information and building up entanglement entropy. However, kinetic constraints in the Hamiltonian can lead to a breakdown of this fundamental paradigm owing to a fragmentation of the underlying Hilbert space into dynamically decoupled subsectors in which thermalization can be strongly suppressed2–5. Here we experimentally observe Hilbert space fragmentation in a two-dimensional tilted Bose–Hubbard model. Using quantum gas microscopy, we engineer a wide variety of initial states and find a rich set of manifestations of Hilbert space fragmentation involving bulk states, interfaces and defects, that is, two-, one- and zero-dimensional objects. Specifically, uniform initial states with equal particle number and energy differ strikingly in their relaxation dynamics. Inserting controlled defects on top of a global, non-thermalizing chequerboard state, we observe highly anisotropic, subdimensional dynamics, an immediate signature of their fractonic nature6–9. An interface between localized and thermalizing states in turn shows dynamics depending on its orientation. Our results mark the observation of Hilbert space fragmentation beyond one dimension, as well as the concomitant direct observation of fractons, and pave the way for in-depth studies of microscopic transport phenomena in constrained systems. | |
Luo, M.; Trivedi, R.; Bañuls, M. C.; Cirac, J. I. Probing off-diagonal eigenstate thermalization with tensor networks Journal Article Phys. Rev. B 109, 134304 (2024). @article{luo_probing_2024, | |
Choi, W.; Knap, M.; Pollmann, F. Finite Temperature Entanglement Negativity of Fermionic Symmetry Protected Topological Phases and Quantum Critical Points in One Dimension Journal Article Phys. Rev. B 109, 115132 (2024). @article{choi_finite_2024, |  |
2023 |
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Kawano, M.; Pollmann, F.; Knap, M. Unconventional spin transport in strongly correlated kagome systems Journal Article Phys. Rev. B 109, L121111 (2023). @article{kawano_unconventional_2023, | |