RQC Seminar

246th RQC Seminar

  • Speaker

    Dr. Chung-Ting Ke
    ( Academia Sinica, Taiwan )

  • Date

    13:00-14:00, (1:00p.m.-2:00p.m.) Thursday, December 4, 2025

  • Venue

    Hybrid( Zoom,
    Large Meeting Room, 2F, Welfare and Conference Building, Wako Campus / 統合支援施設2階 大会議室 (C61))

  • Title

    Characterizing and Mitigating Environmental Radiation in Superconducting Transmon Qubits

  • Inquiries

    yasunobu.nakamura[at]riken.jp

Abstract
High-quality superconducting qubits are highly sensitive to their surroundings; specifically, radiation from the cryogenic environment can profoundly impact their performance. Conversely, this high sensitivity can be leveraged to characterize the radiation within the cryogenic environment itself. Ensuring a "hygienic" cryogenic environment is key to optimizing superconducting qubit performance.
In this talk, we employ high-quality superconducting qubits to investigate the impact of environmental radiation. By reducing the EJ/EC ratio of a transmon, we measure qubit parity switching events caused by radiation1. These highly sensitive qubits allow for a detailed examination of the cryogenic environment, complementing standard qubit temperature measurements. We compare several setups that demonstrate an improvement in qubit temperature from 61.55 mK down to 18.32 mK, corresponding to a residual excited state population of 0.04%. Furthermore, we analyze parity switching mechanisms to quantify the switching rate. Building upon recent work2, our optimized sample packaging achieves an ultra-low switching rate of 0.88 Hz, improved from 3kHz. This performance relies not only on careful package design but also on the application of additional IR absorption coatings within the cryogenic system.
Our results present a comprehensive approach to establishing a radiatively hygienic, low-IR environment for characterizing high-quality superconducting qubits. 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. Nho, H. et al. Recovery dynamics of a gap-engineered transmon after a quasiparticle burst. http://arxiv.org/abs/2505.08104 (2025).


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