Prof. Erhai Zhao
Department of Physics and Astronomy
George Mason University

Abstract

We were taught in quantum mechanics class that physical observables are described by Hermitian operators, and that the time evolution of a closed system is unitary. Then it must be nonsensical to write H = p²+x²+ix³. Or is it a really smart move? For open quantum systems, it turns out both legitimate and valuable to think about non-Hermitian operators and non-unitary dynamics. In this talk, I will introduce a few simple examples of non-Hermitian Hamiltonians which give rise to knots, exceptional rings and other exotic objects. Interestingly, familiar notions such as the band structure and the Brillouin zone require some nuanced "upgrade" to understand these systems. Several recent experiments to test the theory predictions will be discussed.

References:
Phys. Rev. Lett. 126, 010401 (2021)
Phys. Rev. B 107, 035101 (2023)

 
Brief bio: Erhai Zhao is a theorist working at the intersection of condensed matter physics and atomic, molecular, and optical physics. He got his PhD from Northwestern University, his thesis was on nonequilibrium superconductivity and quantum transport. As a postdoc at University of Toronto, he worked on strongly correlated quantum materials, before moving to University of Pittsburgh to focus on ultracold Fermi gases. He joined the faculty of George Mason University in 2009 and is now a professor of physics. His recent research interests include the many-body physics of ultracold quantum gases, topological phases of matter, and dynamics in open quantum systems.