This is a past event.

Qi Zhong from St. Louis University will present at this Thursday Physics Colloquium. Qi Zhong's presentation is titled "Non-Hermiticity and Exceptional Points in Photonics: From Classical to Quantum". 

The seminar will be presented at 11:00 a.m. on Thursday (Mar. 13) in Fisher 325. 

Abstract  In quantum mechanics, Hermiticity is a fundamental principle, ensuring energy conservation in closed systems. However, the discovery of parity-time (PT) symmetry sparked interest in systems governed by non-Hermitian Hamiltonians. Originally introduced in quantum theory, PT symmetry quickly found experimental realizations in classical optics, later extending to other fields. One particularly intriguing phenomenon in non-Hermitian systems is the emergence of non-Hermitian singularities, known as exceptional points (EPs), where two or more eigenvalues and their corresponding eigenvectors coalesce.

In this talk, I will discuss how PT symmetry and unidirectional coupling enable the realization of EPs in optical systems. I will introduce a novel design strategy for an exceptional surface that balances robustness for practical implementation with their hallmark sensitivity required for optical sensing. Beyond classical optics, I will explore how non-Hermiticity provides new tools for engineering quantum states of light, including entanglement and polarization control. I will present a scheme where an optical site coupled to a one-dimensional photonic array serves as an artificial environment, enabling an anti-PT-symmetric dimer that functions as an entanglement filter for two-photon excitations. Finally, I will demonstrate how single-photon polarization states can be manipulated using a non-Hermitian Mach-Zehnder interferometer, where unidirectional coupling between horizontal and vertical polarization states paves the way for promising applications in quantum sensing.

Bio  Qi Zhong is a postdoctoral researcher in the Department of Electrical and Computer Engineering at St. Louis University. His research focuses on quantum-inspired photonics, non-Hermitian photonics and electronics, quantum optics, nonlinear optics, and integrated nanophotonics. He earned his Ph.D. in Physics from Michigan Technological University in 2019. Before joining St. Louis University in 2025, he was a postdoctoral fellow at Pennsylvania State University and CREOL at University of Central Florida.