Measuring the effect of gap engineering on radiation-induced decoherence in transmon qubits

When ionizing radiation strikes the substrate of a superconducting qubit circuit, it generates numerous quasiparticles that decohere the qubit. Although these quasiparticle bursts are infrequent, they can degrade the performance of quantum error correction. Engineering the superconducting gap of the qubit films is a promising approach to mitigate such catastrophic impacts, as the gap difference at the junction impedes tunneling of quasiparticles when their energy lies close to the gap edge. In this work, we measure the influence of the gap asymmetry on the burst events, their duration, as well as the qubit relaxation rate during the events. By individually measuring three transmon qubits with different gap asymmetries, we find that gap asymmetry can suppress the number of events and the qubit relaxation rate by a factor of a few when it sufficiently exceeds the qubit energy. These results confirm that gap engineering can be used to mitigate the radiation-induced decoherence in superconducting qubits.