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Superconducting transmon qubits play a pivotal role in contemporary superconducting quantum computing systems. These nonlinear devices are typically composed of a Josephson junction shunted by a large capacitor and the bottom two energy eigenstates serve as qubits. When a qubit is placed in its excited state, it decays to its ground state with a relaxation timescale $T_1$. However, recent studies have suggested that cosmic rays or ambient gamma radiation could significantly degrade the relaxation times of transmon qubits, leading to detrimental correlated errors that impede quantum error correction processes [1,2]. In this study, we explore the potential of utilizing transmon qubits as radiation detectors by investigating the impact of radioactivity on transmons fabricated at the Superconducting Quantum Materials and Systems (SQMS) center, Fermilab. We develop a fast detection protocol based on rapid projective measurements and active reset to perform detection with milli-second time resolution. We utilize the underground facility at INFN-Gran Sasso and controlled radioactive sources (such as Thorium) to validate our scheme. Additionally, we investigate the possibility of enhancing detection efficiency by evaluating transmons fabricated with various superconducting materials and improved signal analysis schemes.
[1] Matt McEwen et al., Nature Physics18, 107–111 (2022)
[2] C.D. Wilen et al., Nature 594, 369–373 (2021)
*The work was supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems (SQMS) Center under the contract No. DE-AC02-07CH11359, by the Italian Ministry of Foreign Affairs and International Cooperation, grant number US23GR09, and by the Italian Ministry of Research under the PRIN Contract No. 2020h5l338.
