2026年6月16日（火）13:30- (13:30-, June 16, 2026)
理学部4号館1220教室 (Room No. 1220, Science 4th Bldg.)
Speaker: Yuxuan GUO (Department of Physics, UTokyo)
Title: Strong-to-Weak Spontaneous Symmetry Breaking in Decohered Quantum Systems: From Foundations to Field Theory Approach
Abstract:
Quantum phases and phase transitions in open systems have attracted widespread attention in recent years. In this talk, we explore how quantum phases can be rigorously defined for mixed states. Starting from a pedagogical introduction to the foundational concepts, we introduce the classification of mixed-state phases based on the equivalence relation of two-way connectivity via local quantum channels, alongside relevant quantum informational diagnosis. In the landscape of mixed state phases, spontaneous symmetry breaking (SSB)—the cornerstone of conventional phase classification in many-body physics—manifests in uniquely rich ways. Depending on whether a symmetry is satisfied on average by the ensemble or strictly by each individual pure state component, the concept of SSB is generalized into strong to weak spontaneous symmetry breaking (SWSSB). We will discuss the appropriate order parameters designed to diagnose these distinct symmetry-breaking patterns. Finally, we will illustrate the physics of SWSSB with concrete examples, focusing on a 1+1D Luttinger liquid and a 2+1D transverse-field Ising model subjected to decoherence.
References:
[1] S. Sang, Y. Zou, and T. H. Hsieh, Mixed-State Quantum Phases: Renormalization and Quantum Error Correction, Phys. Rev. X 14, 031044 (2024).
[2] L. A. Lessa, R. Ma, J.-H. Zhang, Z. Bi, M. Cheng, and C. Wang, Strong-to-Weak Spontaneous Symmetry Breaking in Mixed Quantum States, PRX Quantum 6, 010344 (2025).
[3] Y. Guo, S. Yang, and X.-J. Yu, Quantum strong-to-weak spontaneous symmetry breaking in decohered one-dimensional critical states, PRX Quantum 6, 040311 (2025).
[4] Y.-M. Ding, Y. Guo, Z. Bi, and Z. Yan, Strong-to-Weak Spontaneous Symmetry Breaking in a 2+1 D Transverse-Field Ising Model under Decoherence, arXiv:2603.24342 (2026).