Comparative study of cosmic ray-induced decay in Transmon qubits in surface and deep underground experiments using fast decay detection*

A fundamental requirement for practical quantum computing is long-lived qubits with stable parameters to ensure non-degrading performance and facilitate error correction. Recently, cosmic rays or ambient gamma radiation have been proposed to deposit large amounts of energy, generating phonons that can create quasiparticles in superconducting circuits. Contemporary studies either claim a drastic reduction in relaxation times of transmon qubits resulting in correlated errors that are harmful for quantum error correction [1,2] or conclude fluctuations in qubit lifetimes [3] causing operational instability due to such events. To clearly understand the effect of radioactivity on the transmon qubits fabricated at the Superconducting Quantum Materials and Systems (SQMS) center, Fermilab, we utilize the underground facility at INFN-Gran Sasso and controlled radioactive sources (Thorium).  We first characterize the action of gamma-ray events on the qubits utilizing the change in decay probability with sub-millisecond time resolution and then compare the occurrence of such events above and below ground. We will present our preliminary data including simulations and discuss its implications on quantum computing.

[1] Matt McEwen et al., Nature Physics18, 107–111 (2022) 

[2] C.D. Wilen et al., Nature 594, 369–373 (2021) 

[3] Ted Thorbeck et al., PRX Quantum 4, 020356 (2023)