[Special Joint CQSE-NCTS seminar] Waveguide Quantum Electrodynamics with Superconducting Artificial Atoms and Spin Ensembles

Title: [Special Joint CQSE-NCTS seminar] Waveguide Quantum Electrodynamics with Superconducting Artificial Atoms and Spin Ensembles
Speaker: Prof. Io-Chun HOI (City University of Hong Kong)
Time: 2026/7/10 (Fri.) 15:30-16:30
Place: Rm. 312, CCMS & New Physics Building, NTU
Online: https://nationaltaiwanuniversity-zbh.my.webex.com/nationaltaiwanuniversity-zbh.my/j.php?MTID=mdfd6cb4bd8ff5152ba6c9cd7d83ed873
 

Abstract
In this talk, I will present recent advances in waveguide quantum electrodynamics using superconducting artificial atoms and spin ensembles.
In the first set of experiments [1], we show that a single artificial atom can dynamically control the velocity of microwave light via two distinct mechanisms, both leveraging the interplay between radiative and non-radiative decay rates near a mirror. The first method exploits mirror-induced interference to tune the radiative decay, while the second uses the Autler–Townes effect to manipulate non-radiative decay through atom pumping. When half the radiative decay rate exceeds the non-radiative rate, a positive group delay is observed; the reverse condition yields a negative group delay.
In the second set of experiments [2], we exploit resonant dipole–dipole interactions between distant spin ensembles coupled to a mirror-terminated coplanar waveguide (CPW) to realize an on-demand, all-to-all coherent network. This platform enables selective coupling and decoupling among four spin ensembles. Moreover, by adjusting their positions along the CPW, we achieve collective coupling and demonstrate coherent energy exchange among multiple ensembles in the time domain. These results highlight the potential of this device as a medium-scale all-to-all network architecture, well suited for advancing the study of many-body physics.

References:
[1] Y.-T. Cheng et al., Physical Review Letters, 135(7), 073601 (2025)
[2] C.-X. Run et al., Physical Review Letters, 136 (20), 206905 (2026)
 
Biography
Io-Chun Hoi was born in Macau, China. He received B.S. and Ph.D. from the Electro-Physics Department at National Chiao Tung University, Taiwan, and from the Department of Microtechnology and Nanoscience at Chalmers University of Technology, Sweden, in 2007 and 2013, respectively. He conducted his postdoctoral research with Prof. John Martinis, Nobel Laureate in Physics 2025, at the University of California, Santa Barbara, U.S.A., from 2013 to 2015. He was an assistant professor and later an associate professor at National Tsing Hua University, Taiwan, from 2015 to 2021. He joined the City University of Hong Kong as a Tenured Associate Professor in 2021. He is the author or co-author of over 30 publications, including 1 first-authored Nature Physics, 3 first-authored Physical Review Letters, and 4 corresponding-authored Physical Review Letters. His research has gained more than 4,800 citations, according to Google Scholar.