Title: [Joint CQSE & NCTS Seminar] Characterizing and Mitigation of the Parity Switching Properties of Superconducting Transmon Qubits
Speaker: Dr. Chung-Ting KE (Assistant Research Fellow, Institute of Physics, Academia Sinica)
Time: 2026/5/8 (Fri.) 14:30-15:30
Place: NCTS Physics Lecture Hall, 4F, Chee-Chun Leung Cosmology Hall, NTU
Online: https://nationaltaiwanuniversity-zbh.my.webex.com/nationaltaiwanuniversity-zbh.my/j.php?MTID=madb2c3b9d23a56faace3593823ecfdee 

Abstract
High-quality superconducting qubits are extremely sensitive to their surroundings; in particular, radiation from the cryogenic environment can profoundly affect their performance. Conversely, this high sensitivity can be utilized to characterize the radiation environment within the cryogenic system. Ensuring a "hygienic" cryogenic environment is key to superconducting qubit performance, therefore paving the way to a fault-tolerant quantum computer.

In this work, we employ high-quality superconducting qubits to investigate the impact of the cryogenic environment. By reducing the EJ/EC ratio of a superconducting transmon qubit, we measure qubit parity switching events induced by radiation and material strain relaxation [1,2]. These highly sensitive qubits enable detailed examination of the cryogenic environment and material behavior. We compare several setups that demonstrate an improvement in qubit temperature from 61.55 mK down to 17 mK, corresponding to a residual excited state population of 0.01% for a 3 GHz qubit, by reducing the radiation-induced transition. Furthermore, we analyze parity switching mechanisms to quantify the quasiparticle events. Building upon recent work [3], our optimized sample packaging achieves an ultra-low switching rate of 0.069 Hz. By monitoring switching events, we observe a continuous decay in switching rate, which was reported recently in Ref. 2.

Our results present a comprehensive approach to establishing a radiatively hygienic, low-IR environment for characterizing high-quality superconducting qubits [4]. Addressing these environmental factors is crucial for advancing the development of robust quantum computing systems.

References
[1] Ristè, D. et al. Millisecond charge-parity fluctuations and induced decoherence in a superconducting transmon qubit. Nat Commun 4, (2013).
[2] E. Yelton et al. Correlated Quasiparticle Poisoning from Phonon-Only Events in Superconducting Qubits. Physical Review Letters 135, 123601 (2025)
[3] Nho, H. et al. Recovery dynamics of a gap-engineered transmon after a quasiparticle burst. http://arxiv.org/abs/2505.08104 (2025).
[4] Wei-En Lin et al. Suppression of Quasiparticle Poisoning to Sub-10^-10 Levels in Superconducting Qubits via Infrared Shielding, in preparation

Biography
Assistant Research fellow, Institute of Physics, Academia Sinica, Taiwan
Postdoc, QuTech, Delft University of Technology, the Netherlands
Ph. D., Duke University, United States of America