近日，美国加州大学洛杉矶分校的Shane P. Kelly与德国美因茨大学的Jamir Marino合作并取得一项新进展。经过不懈努力，他们将测量引起的相变推广到信息交换对称性破缺。相关研究成果已于2025年1月16日在国际知名学术期刊《物理评论A》上发表。

在这项工作中，研究人员研究了导致被监测系统的信息动力学发生相变的扰动条件。研究人员提出了一个框架，该框架涵盖了广泛的实验，包括由投影测量构成的探测以及更一般地将量子信息从系统传输到量子计算机的探测。后一种情况与最近引入的量子增强实验相关，这类实验在量子计算机上进行量子后处理，相较于投影测量和经典后处理，可以提供指数级的采样优势。该框架明确考虑了环境的影响，使用了将系统、装置和环境耦合在一起的幺正演化模型。

研究人员通过研究演化状态的Rényi熵和von Neumann熵来探讨信息动力学，并构建了一个副本理论来研究这些量。研究人员确定了实验可能具有的副本对称性，并讨论了它们自发对称性破缺的含义。特别是，研究人员确定了一个最小子群，其自发对称性破缺会导致纠缠相变。这种对称性仅在装置中的信息关于系统动力学的信息量，与传输到环境中的信息量一样多时才可能存在。研究人员称这一要求为信息交换对称性，并通过熵之间的关系对其进行量化。

然后，他们引入了一个广义的自发对称性破缺概念，使得纠缠相变可以被理解为信息交换对称性的自发破缺，而无需引用副本理论。接着，研究人员证明了信息交换对称性破缺概括了测量诱导相变（MIPT）的现象学。他们将这一理论应用于探测Haar砖块量子电路幺正动力学的量子增强实验，并确定临界点的普适类与MIPT的普适类不同。信息交换对称性破缺的这一概念概括了MIPT，并为理解量子增强实验中量子信息的动力学提供了一个框架。

附：英文原文

Title: Generalizing measurement-induced phase transitions to information exchange symmetry breaking

Author:Shane P. Kelly^1,2,* and Jamir Marino²

Issue&Volume: 2025-01-16

Abstract: Probing a quantum system disrupts its state in a phenomenon known as backaction. In this work we investigate the conditions for this disruption to result in a phase transition in the information dynamics of a monitored system. We introduce a framework that captures a wide range of experiments encompassing probes comprised of projective measurements and probes which more generally transfer quantum information from the system to a quantum computer. The latter case is relevant to the recently introduced quantum-enhanced experiments in which quantum postprocessing is performed on a quantum computer and which can offer an exponential sampling advantage over projective measurements and classical postprocessing. Our framework explicitly considers the effects of an environment using a model of unitary evolution which couples the system, apparatus, and environment. Information dynamics is investigated using the Rényi and von Neumann entropies of the evolving state, and we construct a replica theory for studying these quantities. We identify the possible replica symmetries an experiment can possess and discuss the meaning of their spontaneous symmetry breaking. In particular, we identify a minimum subgroup whose spontaneous symmetry breaking results in an entanglement transition. This symmetry is only possible when the information in the apparatus is as informative about the dynamics of the system as the information transferred to the environment. We call this requirement the information exchange symmetry and quantify it by a relationship between the entropies. We then introduce a generalized notion of spontaneous symmetry breaking such that the entanglement transition can be understood as the spontaneous breaking of the information exchange symmetry and without referring to the replica theory. Information exchange symmetry breaking is then shown to generalize the phenomenology of the measurement-induced phase transition (MIPT). We apply this theory to a quantum-enhanced experiment probing the unitary dynamics of a Haar brickwork quantum circuit, and we determine that the universality class of the critical point is distinct from that for the MIPT. This notion of information exchange symmetry breaking generalizes the MIPT and provides a framework for understanding the dynamics of quantum information in quantum-enhanced experiments.

DOI: 10.1103/PhysRevA.111.012425

Source: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.111.012425