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arXiv:2501.04679 · Communications Physics

Exploring nontrivial topology at quantum criticality on a superconducting processor

Ziqi Tan, Ke Wang, Sheng Yang, Fanhao Shen, Feitong Jin, Xuhao Zhu, Yujie Ji, Shibo Xu, Jiachen Chen, Yaozu Wu, Chuanyu Zhang, Yu Gao, Ning Wang, Yiren Zou, Aosai Zhang, Tingting Li, Zehang Bao, Zitian Zhu, Jiarun Zhong, Zhengyi Cui, Yihang Han, Yiyang He, Han Wang, Yanzhe Wang, Yanzhe Wang, Jiayuan Shen, Gongyu Liu, Zixuan Song, Hang Dong, Hang Dong, Pengfei Zhang, Shao-Kai Jian, Hekang Li, Zhen Wang, Junxiang Zhang, Qiujiang Guo, Hai‐Qing Lin, Chao Song, Xue-Jia Yu, H. Wang, Fei Qing Wu

Published Mar 7, 2026• 4 citations
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Abstract

The discovery of nontrivial topology in quantum critical states has revised the classification of quantum phase transitions and opened a new direction for exploiting topological phases. However, the experimental investigation of gapless many-body systems is challenging due to the inherent complex quantum entanglement. Here, we demonstrate an experimental study of the topological properties in the critical cluster Ising model on a superconducting processor with up to 100 qubits. By applying entanglement Hamiltonian tomography to the experimentally prepared low-lying states, we successfully reconstruct the reduced density matrices of the underlying ground states for subsystems with up to 12 qubits, based on which we extract the scaling dimension and central charge of the critical cluster Ising model. Furthermore, we recognize a robust two-fold topological degeneracy as well as the conformal-tower structure in the entanglement spectrum, experimentally revealing the bulk-boundary correspondence in topological critical systems. Our results demonstrate the low-lying states at critical points as useful quantum resources for investigating the interplay between topology and quantum criticality. Nontrivial topologies are long believed to survive only in gapped systems, yet recent progress suggests their existence even at quantum critical points. By preparing low-lying states on a superconducting processor up to 100 qubits and applying entanglement Hamiltonian tomography, the authors experimentally observe these nontrivial topological properties and suggest low-lying states as useful quantum resources.

DOI
10.1038/s42005-026-02569-9

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The discovery of nontrivial topology in quantum critical states has revised the classification of quantum phase transitions and opened a new direction for exploiting topological phases. However, the experimental investigation of gapless many-body systems is challenging due to the inherent complex quantum entanglement. Here, we demonstrate an experimental study of the topological properties in the critical cluster Ising model on a superconducting processor with up to 100 qubits. By applying entanglement Hamiltonian tomography to the experimentally prepared low-lying states, we successfully reconstruct the reduced density matrices of the underlying ground states for subsystems with up to 12 qubits, based on which we extract the scaling dimension and central charge of the critical cluster Ising model. Furthermore, we recognize a robust two-fold topological degeneracy as well as the conformal-tower structure in the entanglement spectrum, experimentally revealing the bulk-boundary correspondence in topological critical systems. Our results demonstrate the low-lying states at critical points as useful quantum resources for investigating the interplay between topology and quantum criticality. Nontrivial topologies are long believed to survive only in gapped systems, yet recent progress suggests their existence even at quantum critical points. By preparing low-lying states on a superconducting processor up to 100 qubits and applying entanglement Hamiltonian tomography, the authors experimentally observe these nontrivial topological properties and suggest low-lying states as useful quantum resources.
Abstract The discovery of nontrivial topology in quantum critical states has revised the classification of quantum phase transitions and opened a new direction for exploiting topological phases. However, the experimental investigation of nontrivial topology in gapless many-body systems is challenging due to the inherent complex quantum entanglement. Here, we experimentally explore the topological properties in the critical cluster Ising model by preparing its low-lying critical states on a superconducting processor with up to 100 qubits. We develop an efficient method to probe the boundary g-function, which allows us to uniquely identify the nontrivial topology of the critical systems under study. Furthermore, we probe and recognize two-fold topological degeneracy in the entanglement spectrum under periodic boundary conditions and experimentally reveal the novel bulk-boundary correspondence in topological critical systems. Our results demonstrate the low-lying critical states as useful quantum resources for investigating the interplay between topology and quantum criticality.
Quantum entanglement can be an effective diagnostic tool for probing topological phases protected by global symmetries. Recently, the notion of nontrivial topology in critical systems has been proposed and is attracting growing attention. In this work, as a concrete example, we explore the quantum entanglement properties of fermionic symmetry-enriched quantum critical points by constructing exactly solvable models based on stacked multiple Kitaev chains. We first analytically establish the global phase diagram using entanglement entropy and reveal three topologically distinct gapped phases with different winding numbers, along with three topologically distinct transition lines separating them. Importantly, we unambiguously demonstrate that two transition lines exhibit fundamentally different topological properties despite sharing the same central charge. Specifically, they display nontrivial topological degeneracy in the entanglement spectrum under periodic boundary conditions, thereby generalizing the Li-Haldane bulk-boundary correspondence to a broader class of fermionic symmetry-enriched criticality. Additionally, we identify a novel Lifshitz multicritical point at the intersection of the three transition lines, which also exhibits nontrivial topological degeneracy. This work provides a valuable reference for investigating gapless topological phases of matter from the perspective of quantum entanglement.
Topology is a fascinating phenomena in condensed-matter physics typically associated with a bulk gap. However, recent research shifts focus to quantum critical points or phases that exhibit nontrivial topological properties. Here we explore a cluster-Ising chain with long-range antiferromagnetic interactions that decay as a power law with distance. Using large-scale density matrix renormalization group simulations, we demonstrate that the nontrivial topology at the critical point remains stable against long-range interactions, resulting in a topologically nontrivial critical line. Moreover, even within the gapped region, the interplay between topology and long-range interaction can give rise to a topological phase featuring algebraically decaying correlations and edge modes, similar to gapless topological phases. We refer to this phase as the algebraic topological phase, which exhibits nontrivial gapless topological behaviors and arises solely from long-range interactions without short-range counterparts. The findings pave the way for more studies on topological states in long-range systems. Long-range interactions play a crucial role in condensed-matter systems and can lead to range of emerging and exotic physical phenomenon. Here using simulations, the authors investigate the non-trivial topological properties that can arise in a cluster Ising chain with long-range antiferromagnetic interactions, finding evidence for what they classify as an algebraic symmetry-protected topological phase.
Topology and symmetry play critical roles in characterizing quantum phases of matter. Recent advancements have unveiled symmetry-protected topological (SPT) phases in many-body systems as a unique class of short-range entangled states, notable for their nontrivial edge modes and characteristic ground-state entanglement gap. In this study, we demonstrate the robust simulation of many-body ground states of an Ising-cluster model on a quantum computer. By employing the method of quantum imaginary-time evolution (QITE) combined with enhanced zero-noise extrapolation techniques, we achieve accurate measurements of the transition between trivial and cluster SPT phases. Furthermore, we measured the characteristic edge modes and their associated topological entanglement properties, such as the second Rényi entropy, reduced density matrix, and entanglement spectral gap. Our work demonstrates the potential of using QITE in investigating sophisticated quantum phase transitions and critical phenomena on quantum computers.
A quantum phase transition is usually achieved by tuning physical parameters in a Hamiltonian at zero temperature. Here, we show that the ground state of a topological phase itself encodes critical properties of its transition to a trivial phase. To extract this information, we introduce an extensive partition of the system into two subsystems both of which extend throughout the bulk in all directions. The resulting bulk entanglement spectrum has a low-lying part that resembles the excitation spectrum of a bulk Hamiltonian, which allows us to probe a topological phase transition from a single wave function by tuning either the geometry of the partition or the entanglement temperature. As an example, this remarkable correspondence between the topological phase transition and the entanglement criticality is rigorously established for integer quantum Hall states.
We introduce topological invariants for gapless systems and study the associated boundary phenomena.More generally, the symmetry properties of the low-energy conformal field theory (CFT) provide discrete invariants establishing the notion of symmetry-enriched quantum criticality.The charges of nonlocal scaling operators, or more generally, of symmetry defects, are topological and imply the presence of localized edge modes.We primarily focus on the 1 þ 1d case where the edge has a topological degeneracy, whose finite-size splitting can be exponential or algebraic in system size depending on the involvement of additional gapped sectors.An example of the exponential case is given by tuning the spin-1 Heisenberg chain to a symmetry-breaking Ising phase.An example of the algebraic case arises between the gapped Ising and cluster phases: This symmetry-enriched Ising CFT has an edge mode with finite-size splitting scaling as 1=L 14 .In addition to such new cases, our formalism unifies various examples previously studied in the literature.Similar to gapped symmetry-protected topological phases, a given CFT can split into several distinct symmetry-enriched CFTs.This raises the question of classification, to which we give a partial answer-including a complete characterization of symmetry-enriched 1 þ 1d Ising CFTs.Nontrivial topological invariants can also be constructed in higher dimensions, which we illustrate for a symmetry-enriched 2 þ 1d CFT without gapped sectors.