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Physical review. B./Physical review. B

Intrinsic symmetry-protected topological mixed state from modulated symmetries and hierarchical structure of boundary anomaly

Yizhi You, Masaki Oshikawa

Published Oct 25, 2024• 13 citations
Publisher

Abstract

We introduce a class of intrinsic symmetry-protected topological mixed state (mSPT) in open quantum systems that feature modulated symmetries, such as dipole and subsystem symmetries. Intriguingly, these mSPT phases cannot be realized as the ground states of a gapped Hamiltonian under thermal equilibrium. The microscopic form of the density matrix characterizing these intrinsic mixed-state SPT ensembles is constructed using solvable coupled-wire models that incorporate quenched disorder or quantum channels. A detailed comparison of the hierarchical structure of boundary anomalies in both pure and mixed states is presented, utilizing flux insertion and Laughlin's charge pumping arguments. Finally, we explore the salient features of boundary anomalies in the mixed-state ensemble, which can be detected through the R\'enyi-$N$ correlation function of charged observables.

DOI
10.1103/physrevb.110.165160

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We introduce a class of intrinsic symmetry-protected topological mixed state (mSPT) in open quantum systems that feature modulated symmetries, such as dipole and subsystem symmetries. Intriguingly, these mSPT phases cannot be realized as the ground states of a gapped Hamiltonian under thermal equilibrium. The microscopic form of the density matrix characterizing these intrinsic mixed-state SPT ensembles is constructed using solvable coupled-wire models that incorporate quenched disorder or quantum channels. A detailed comparison of the hierarchical structure of boundary anomalies in both pure and mixed states is presented, utilizing flux insertion and Laughlin's charge pumping arguments. Finally, we explore the salient features of boundary anomalies in the mixed-state ensemble, which can be detected through the R'enyi-NNN correlation function of charged observables.
We introduce a class of intrinsic symmetry-protected topological mixed-state(mSPT) in open quantum systems that feature modulated symmetries, such as dipole and subsystem symmetries. Intriguingly, these mSPT phases cannot be realized as the ground states of a gapped Hamiltonian under thermal equilibrium. The microscopic form of the density matrix characterizing these intrinsic mixed-state SPT ensembles is constructed using solvable coupled-wire models that incorporate quenched disorder or quantum channels. A detailed comparison of the hierarchical structure of boundary anomalies in both pure and mixed states is presented, utilizing flux insertion and Laughlin's charge pumping arguments. Finally, we explore the salient features of boundary anomalies in the mixed-state ensemble, which can be detected through the Renyi-N correlation function of charged observables.
The interplay of symmetry and topology in quantum many-body mixed states has recently garnered significant interest. In a phenomenon not seen in pure states, mixed states can exhibit average symmetries—symmetries that act on component states while leaving the ensemble invariant. In this work, we systematically characterize symmetry-protected topological (SPT) phases of short-range entangled (SRE) mixed states of spin systems—protected by both average and exact symmetries—by studying their pure Choi states in a doubled Hilbert space, where the familiar notions and tools for SRE and SPT pure states apply. This advantage of the doubled space comes with a price: extra symmetries as well as subtleties around how hermiticity and positivity of the original density matrix constrain the possible SPT invariants. Nevertheless, the doubled-space perspective allows us to obtain a systematic classification of mixed-state SPT (MSPT) phases. We also investigate the robustness of MSPT invariants under symmetric finite-depth quantum channels, the bulk-boundary correspondence for MSPT phases, and the consequences of the MSPT invariants for the separability of mixed states and the symmetry-protected sign problem. In addition to MSPT phases, we study the patterns of spontaneous symmetry breaking (SSB) of mixed states, including the phenomenon of exact-to-average SSB, and the order parameters that detect them. Mixed-state SSB is related to an ingappability constraint on symmetric Lindbladian dynamics.
We establish a holographic duality between d -dimensional mixed-state symmetry-protected topological (mSPT) phases and ( d + 1 ) -dimensional subsystem symmetry-protected topological (SSPT) states. Specifically, we show that the reduced density matrix of the boundary layer of a ( d + 1 ) -dimensional SSPT state with subsystem symmetry S and global symmetry G corresponds to a d -dimensional mSPT phase with strong S and weak G symmetries. Conversely, we demonstrate that the wave function of an SSPT state can be constructed by replicating the density matrix of the corresponding lower-dimensional mSPT phase. This mapping links the density matrix in lower dimensions to the entanglement properties of higher-dimensional wave functions, providing an approach for analyzing nonlinear quantities and quantum information metrics in mixed-state systems. Our duality offers a new perspective for studying intrinsic mSPT phases that are unique to open quantum systems, without pure-state analogues. We show that strange correlators and twisted Rényi- N correlators can diagnose these nontrivial phases and we explore their connection to strange correlators in pure-state SSPT phases. Furthermore, we discuss several implications of this holographic duality, including a method for preparing intrinsic mSPT phases through the duality.
We generalize the topological response theory to detect the boundary anomalies of linear subsystem symmetries. This approach allows us to distinguish different subsystem symmetry-protected topological (SSPT) phases and uncover new ones. We focus on the cases where the mixed anomaly exists within the adjacent subsystems. Using numerical simulations, we demonstrate the power of this method by identifying strong and weak ${Z}_{2}^{\ensuremath{\tau}}\ifmmode\times\else\texttimes\fi{}{Z}_{2}^{\ensuremath{\sigma}}$ SSPT phases in a tunable tensor network state. Our analysis reveals an intrinsic Z2{Z}_{2}Z2​ SSPT phase characterized by its degenerate entanglement spectrum. Furthermore, we extend the anomaly indicator to mixed-state density matrices and show that quantum anomalies of subsystem symmetry can persist under both uniform and alternating disorders. This finding establishes a connection between boundary quantum anomalies in pure and mixed states. Our work provides a numerical method to detect quantum anomalies of subsystem symmetries, offering new insights into the study of topological quantum phases.
We investigate mixed states exhibiting nontrivial topological features, focusing on symmetry-protected topological (SPT) phases under various types of decoherence. Our findings demonstrate that these systems can retain topological information from the SPT ground state despite decoherence. In the ''doubled Hilbert space,'' we define symmetry-protected topological ensembles (SPT ensembles) and examine boundary anomalies in this space. We generalize the concept of the strange correlator, initially used to diagnose SPT ground states, to identify anomalies in mixed-state density matrices. Through exact calculations of stabilizer Hamiltonians and field theory evaluations, we show that nontrivial features of SPT states persist in two types of strange correlators: type-I and type-II. The type-I strange correlator reveals SPT information that can be efficiently detected and used experimentally, such as in preparing long-range entangled states. The type-II strange correlator encodes the full topological response of the decohered mixed state, reflecting the SPT state's pre-decoherence presence. Our work offers a unified framework for understanding decohered SPT phases from an information-theoretic perspective.
We investigate the physics of one-dimensional symmetry protected topological (SPT) phases protected by symmetries whose symmetry generators exhibit spatial modulation. We focus in particular on phases protected by symmetries with linear (i.e., dipolar), quadratic and exponential modulations. We present a simple recipe for constructing modulated SPT models by generalizing the concept of decorated domain walls to spatially modulated symmetry defects, and develop several tools for characterizing and classifying modulated SPT phases. A salient feature of modulated symmetries is that they are generically only present for open chains, and are broken upon the imposition of periodic boundary conditions. Nevertheless, we show that SPT order is present even with periodic boundary conditions, a phenomenon we understand within the context of an object we dub a ``bundle symmetry''. In addition, we show that modulated SPT phases can avoid a certain no-go theorem, leading to an unusual algebraic structure in their matrix product state descriptions.