Measurement of superconducting qubits in an ultra-low radioactivity environment.

Over the past decades, superconducting qubits have emerged as one of the most promising technologies for the realization of a fully-functioning quantum computer. Over these years their performance has improved dramatically, paving the way both for applications in fundamental research and for the first commercial solutions featuring this technology. However, recent years have seen growing concerns in the scientific community about the sensitivity of superconducting qubits to ionizing radiation and the errors in quantum computation that particle impacts

can induce. Previous studies have already proved that radioactivity can affect the energy-relaxation time of qubits and induce correlated errors in multi-qubit chips that could make present-day error correction algorithms fail. In this talk I will illustrate the research activity carried out at the Gran Sasso National Laboratories (LNGS), Italy, to assess quantitatively the impact of ionizing radiation on present-day qubits. Measurements were done in a deep-underground facility below the Gran Sasso mountain, which acts as a natural shield for cosmic rays and allows to operate qubits in an unprecedented low-radioactivity environment. To disentangle ionizing radiation from other phenomena affecting the behavior of superconducting qubits, the same measurements were repeated while exposing the qubit to a gamma radiation source, so to compare results. Measurements were done on two different types of qubits: granular-aluminum-based fluxonium qubits developed at the Karlsruhe Institute of Technology (KIT, Germany), and transmon qubits developed by the Superconducting Quantum Materials and Systems (SQMS) Center at Fermilab, US.