- Open Access
Topological Mixed States: Phases of Matter from Axiomatic Approaches
Phys. Rev. X 16, 041002 – Published 2 October, 2026
DOI: https://doi.org/10.1103/3bpt-f9pd
Abstract
For closed quantum systems, topological orders are understood through the equivalence classes of ground states of gapped local Hamiltonians. The generalization of this conceptual paradigm to open quantum systems, however, remains elusive, often relying on operational definitions without fundamental principles. Here, we fill this gap by proposing an approach based on three axioms: (i) local recoverability, (ii) absence of long-range correlations, and (iii) spatial uniformity. States that satisfy these axioms are fixed points; requiring the axioms only after coarse-graining promotes each fixed point to an equivalence class, i.e., a phase, presenting the first step toward the axiomatic classification of mixed-state phases of matter: the mixed-state bootstrap program. From these axioms, a rich set of topological data naturally emerges; importantly, these data are robust under relaxation of axioms. For example, each topological mixed state supports locally indistinguishable classical and/or quantum logical memories with distinct responses to topological operations. These data label distinct mixed-state phases, allowing one to distinguish them. We further uncover a hierarchy of secret-sharing constraints: In non-Abelian phases, reliable recovery—even of information that looks purely classical—demands a specific coordination among spatial subregions, a requirement different across non-Abelian classes. This originates from non-Abelian fusion rules that can stay robust under decoherence. Finally, we performed large-scale numerical simulations to corroborate stability: Weakly decohered fixed points respect the axioms once coarse-grained. Surprisingly, the axiom violations at different length scales exhibit an exact crossing at the critical point, enabling its identification without any rescaling or data-collapse ambiguities. These results lay the foundation for a systematic classification of topological states in open quantum systems.
Physics Subject Headings (PhySH)
Popular Summary
Realistic quantum systems interact with their surroundings and are therefore described by mixed states, for which the usual theory of topological phases is incomplete. We address this conceptual gap by developing a quantum-information-based framework that defines when two mixed states belong to the same topological phase, enabling a systematic classification of mixed-state phases of matter. The framework identifies phase invariants—properties that remain unchanged within a phase—including the amount of quantum information that can be robustly stored (memory capacity) and the ways in which it can be distributed among spatial regions (secret sharing). It also provides a direct numerical diagnostic of phase transitions that avoids ambiguities associated with finite-size scaling. These results establish a general foundation for classifying topological order in open quantum systems.
Article Text
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