Impact of Ionizing Radiation on Energy Relaxation in Superconducting Fluxonium Qubits*

Measuring and quantifying the influence of environmental factors on superconducting qubit performance is a powerful tool to shed light on their decoherence mechanisms. We explore the effect of ionizing radiation on a superconducting fluxonium qubit, focusing on the statistics of quantum jumps. We operate the qubit in a deep underground facility, where it is protected from cosmic radiation. Moreover, we partially shield the cryostat from local ionizing sources by using a led brick shield. This allows us to start from a low background and to controllably increase the level of radioactivity experienced by the qubit. We show that the statistics of quantum jumps changes with radiation exposure. Furthermore, the long-lived two level system bath, which in our case forms the dominant loss mechanism of the qubit, is unaffected by exposure to background radiation.