Abstract: Yang-Lee zeros provide a powerful way to understand the phase transitions in terms of the analytic structure of the partition functions. However, traditional Yang-Lee theory has mainly focused on thermal phase transitions. In this talk, I will discuss recent progress in extending this perspective to quantum phase transitions and nonunitary dynamics. In the first part, I will introduce how the distribution of Yang-Lee zeros reveals the essential singularity of Bardeen-Cooper-Schrieffer (BCS) superconductivity by extending the interaction strength to the complex plane. Furthermore, we present the semicircle theorem, which unveils a universal relation between the Fermi-surface instability and the geometric structure of the distribution of Yang-Lee zeros. In the second part, I will discuss the Yang-Lee theory for general quantum phase transitions from the perspective of quantum entanglement. We show that the edges of Yang-Lee zeros always correspond to the entanglement transitions in universal quantum many-body systems. In the last part, I will introduce the Yang-Lee theory for nonunitary quantum dynamics. We show that chaotic renormalization-group flows emerge in postselected nonunitary dynamics and that the measurement-induced parity-time transition belongs to the universality class of Yang-Lee edge singularity. In addition, we find that the chaotic RG flows can also arise in quantum channels with decoherence-free subspaces.

About the speaker: Hongchao Li is a Ph.D. student from the University of Tokyo under the supervision of Prof. Masahito Ueda. His main research interests are in quantum many-body theory in open quantum systems, quantum optics, quantum simulation, and quantum algorithm.

Inviter: 孙孝奇 xqsun@iphy.ac.cn