Prof. Britton Plourde

High-energy particles, such as gamma rays from background radioactivity or muons generated by cosmic rays, impacting the qubit substrate produce a large number of electron-hole pairs as well as a burst of phonons. The charge generation and subsequent motion lead to offset-charge shifts for qubits near the impact site; the phonons that are produced have energy well above the superconducting gap and can lead to correlated errors between qubits through generation of dissipative quasiparticle excitations. Mitigating such errors is challenging for quantum error correcting codes when the correlations extend across a significant fraction of the qubits in the array. Metallic structures on the back side of the chip can downconvert the phonons to energies below the superconducting gap, thus limiting the spread of poisoning following an ionizing impact. We induce phonon-mediated poisoning in arrays of qubits with active irradiation from a gamma-ray source. We characterize the poisoning and mitigation strategies through measurements of quasiparticle charge-parity switching and offset-charge shifts. Combined with numerical modeling of the phonon, charge, and quasiparticle dynamics, we discuss approaches for making large-scale superconducting quantum processors resilient to ionizing impacts.