Event
Ph.D. Dissertation Defense: Zhiyin Tu
Friday, November 6, 2026
2:00 p.m.
PSC 3150
ANNOUNCEMENT: Ph.D. Dissertation Defense
Name: Zhiyin Tu
Committee:
Professor Alicia J. Kollár, Chair
Professor Ronald Walsworth
Professor Thomas Antonsen
Professor Nathan Schine
Professor You Zhou, Dean's Representative
Date/time: Friday, November 6, 2026 at 2:00 PM
Location: PSC 3150
Title: Engineering Nonreciprocal Coupling in Superconducting Circuits with Spatiotemporal Modulation
Abstract: Superconducting circuits provide a versatile platform for quantum computing and analog quantum simulation, offering precise control over the design of and the coupling between microwave resonators and quantum bits. In particular, networks of coupled superconducting circuit elements enable the realization of synthetic quantum materials with engineered Hamiltonians. Nonreciprocal coupling, which distinguishes the two directions of photon propagation, is an important ingredient for both controlling quantum information flow and exploring unconventional physical phenomena in synthetic quantum systems.
In this dissertation, we first investigate nonreciprocal coupling in a pair of superconducting microwave resonators subject to phased frequency modulation. By controlling the differential phase and strength of the modulation, we experimentally realize strong microwave isolation at exceptional points, and pure gyration, where forward and reverse transmission have equal magnitudes but differ in phase by π. These linear responses arise from interference between multiple pathways in the synthetic frequency dimension and can be described by an effective non-Hermitian Hamiltonian.
We then extend the study of spatiotemporal modulation from harmonic resonators to a pair of coupled superconducting qubits, where nonlinearity introduces new features into the system's response. Through microwave spectroscopy, we experimentally investigate the Floquet spectrum of periodically modulated, degenerate transmons. Using a Wannier-Stark description in a synthetic frequency lattice, we identify modulation-induced avoided crossings and symmetry-protected crossings that govern the observed spectral features. We further demonstrate that the manifestation of nonreciprocity depends on the observable being measured, highlighting important differences between linear transmission and total power/qubit excitation observables.
Together, these results demonstrate how spatiotemporal modulation can engineer nonreciprocal coupling in superconducting circuits across both harmonic and anharmonic systems. This dissertation establishes a framework for understanding and controlling modulated superconducting systems, providing opportunities for directional quantum devices and the exploration of synthetic quantum materials with engineered coupling.
