Center for Theory of Quantum Matter, University of Colorado Boulder

Abstract：

Recently the exploration of phases of matter in open quantum systems has garnered substantial interest across various disciplines including condensed matter physics and quantum information science, especially those phenomena that are intrinsically mixed without analogues in closed systems. In particular, the energy gap is a crucial concept in physics, particularly in the study of solid-state physics and quantum mechanics.

In this talk, based on the correlation behaviors of density matrix, we first generalize the concept of “gap” to mixed quantum states. For gapped mixed states, we introduce the concept of “average symmetry-protected topological (ASPT) phases”, together with a systematic construction and classification paradigm. In particular, a plethora of ASPT states that cannot be realized in pure states are provided, dubbed “intrinsic ASPT” phases.

For “non-gapped” mixed states, we systematically introduce a new concept of “strong-to-weak spontaneous symmetry breaking” (SWSSB) in mixed quantum states. As a universal mixed-state quantum phase, we demonstrate the universality of SWSSB through the stability against symmetric finite-depth local channel perturbation, and the spontaneity implies that a local perturbation may lead to a global change of the whole density matrix.

Refs:

1. 1. R. Ma, J.-H. Zhang, Z. Bi, M. Cheng, C. Wang, Topological Phases with Average Symmetries: the Decohered, the Disordered, and the Intrinsic; Physical Review X in press.

2. 2. L. A. Lessa, R. Ma, J.-H. Zhang, Z. Bi, M. Cheng, C. Wang, Strong-to-Weak Spontaneous Symmetry Breaking in Mixed Quantum States; PRX Quantum 6, 010344 (2025).

3. 3. Y. Guo, J.-H. Zhang, S. Yang, Z. Bi, Locally Purified Density Operators for Symmetry-Protected Topological Phases in Mixed States; Physical Review X in press.

4. 4. D. Stephen, R. Nandkishore, J.-H. Zhang, Many-body quantum catalysts for transforming between phases of matter; PRX Quantum in press.

5. 5. S. Sun, J.-H. Zhang, Z. Bi, Y. You, Holographic View of Mixed-State Symmetry-Protected Topological Phases in Open Quantum Systems; PRX Quantum in press.

Brief CV of Dr. Jianhao Zhang:

Jianhao Zhang is a postdoctoral fellow at Center for Theory of Quantum Matter, University of Colorado Boulder, working with Andy Lucas and Rahul Nandkishore. He pursued his Ph.D. studies with Prof. Zheng-Yu Weng at Tsinghua University, receiving his Ph.D. degree in Aug. 2020. After postdoctoral fellowships at CUHK (2020.11-2022.04, with Zheng-Cheng Gu) and Penn State University (2022.05-2024.05, with Zhen Bi), he joined CU Boulder. Jian-Hao’s research mainly focuses on the interdisciplinary direction on the crossroad of quantum information and quantum matter, especially the exotic symmetry and topological properties of mixed quantum states.

报告地点：M楼830会议室

腾讯会议ID: 391-644-672

会议密码：0515

主持人：方辰 研究员

联系人：傅琦 （82649469）