Strong-to-Weak Spontaneous Symmetry Breaking in Mixed Quantum States

• URL: http://arxiv.org/abs/2405.03639v2
• Date: Wed, 3 Jul 2024 20:04:25 GMT
• Title: Strong-to-Weak Spontaneous Symmetry Breaking in Mixed Quantum States
• Authors: Leonardo A. Lessa, Ruochen Ma, Jian-Hao Zhang, Zhen Bi, Meng Cheng, Chong Wang,
• Abstract summary: We show that a thermal state at a nonzero temperature in the canonical ensemble (with fixed symmetry charge) should have spontaneously broken strong symmetry. We argue that a thermal state at a nonzero temperature in the canonical ensemble (with fixed symmetry charge) should have spontaneously broken strong symmetry.
• Score: 10.383582684153945
• License: http://creativecommons.org/licenses/by-nc-nd/4.0/
• Abstract: Symmetry in mixed quantum states can manifest in two distinct forms: \textit{strong symmetry}, where each individual pure state in the quantum ensemble is symmetric with the same charge, and \textit{weak symmetry}, which applies only to the entire ensemble. This paper explores a novel type of spontaneous symmetry breaking (SSB) where a strong symmetry is broken to a weak one. While the SSB of a weak symmetry is measured by the long-ranged two-point correlation function $\mathrm{Tr}(O_xO^{\dagger}_y\rho)$, the strong-to-weak SSB (SW-SSB) is measured by the fidelity $F(\rho, O_xO^{\dagger}_y\rho O_yO^{\dagger}_x)$, dubbed the \textit{fidelity correlator}. We prove that SW-SSB is a universal property of mixed-state quantum phases, in the sense that the phenomenon of SW-SSB is robust against symmetric low-depth local quantum channels. { We also show that the symmetry breaking is "spontaneous " in the sense that the effect of a local symmetry-breaking measurement cannot be recovered locally.} We argue that a thermal state at a nonzero temperature in the canonical ensemble (with fixed symmetry charge) should have spontaneously broken strong symmetry. Additionally, we study non-thermal scenarios where decoherence induces SW-SSB, leading to phase transitions described by classical statistical models with bond randomness. In particular, the SW-SSB transition of a decohered Ising model can be viewed as the "ungauged" version of the celebrated toric code decodability transition. We confirm that, in the decohered Ising model, the SW-SSB transition defined by the fidelity correlator is the only physical transition in terms of channel recoverability. We also comment on other (inequivalent) definitions of SW-SSB, through correlation functions with higher R\'enyi indices.

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