Correlated Error Bursts in a Gap-Engineered Superconducting Qubit Array

One of the roadblocks towards the implementation of a fault-tolerant superconducting quantum processor is impacts of ionizing radiation with the qubit substrate. Such impacts temporarily elevate the density of quasiparticles across the device, leading to correlated qubit error bursts. The most damaging errors, T1 errors, stem from quasiparticle tunneling across the qubit Josephson junctions. Recently, we demonstrated that this type of error can be strongly suppressed by engineering the profile of superconducting gap at the Josephson junctions in a way that prevents quasiparticle tunneling. In this work, we identify a new type of impact-induced correlated error that persists in the presence of gap engineering. We observe that impacts shift the frequencies of the affected qubits, and thus lead to correlated phase errors. The frequency shifts are systematically negative, reach values up to 3MHz, and last for ∼1ms. We provide evidence that the shifts originate from quasiparticle-qubit interactions in the Josephson junction region. Further, we demonstrate that the shift-induced phase errors can be detrimental to the performance of quantum error correction protocols.