Special AEP Seminar: Qi Zhong (St. Louis Univ.)
Location
Clark Hall 701
Description
Non-Hermiticity and Exceptional Points in Photonics: From Classical to Quantum
In quantum mechanics, Hermiticity is a core concept, and a Hermitian Hamiltonian describes a closed system in which energy is conserved. However, the discovery of parity-time (PT) symmetry sparked interest in systems governed by non-Hermitian Hamiltonians. Initially introduced in quantum theory, PT symmetry quickly found experimental realizations in classical optics and photonics and has since expanded into other fields. In recent years, non-Hermitian photonics—exemplified by PT-symmetric photonics—has seen rapid development, leading to breakthrough advancements in both theory and experiment. Among these, one particularly notable phenomenon 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. While EPs exhibit extreme sensitivity to external perturbations—making them promise for next-generation optical sensors—this same sensitivity poses significant fabrication challenges. To address this issue, we introduce a novel design strategy for a photonic exceptional surface that balances robustness for practical implementation with their hallmark sensitivity. Specifically, we realize an exceptional surface using an optical microcavity coupled to a waveguide, where one end exhibits back reflection. We further demonstrate how this approach enables applications such as chiral perfect absorption and control over spontaneous emission.
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 acts as an effective "environment," enabling a passive PT-symmetric dimer within a Hermitian system. By tuning the coupling to this environment, we achieve an entanglement filter for two-photon excitations. Finally, I will show 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 postdoc 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 micro/nanophotonics. Before starting at St. Louis University in 2025, he was a postdoc fellow at The Pennsylvania State University and CREOL at University of Central Florida.