  Correlated errors caused by ionizing radiation impacting superconducting
qubit chips are problematic for quantum error correction. Such impacts generate
quasiparticle (QP) excitations in the qubit electrodes, which temporarily
reduce qubit coherence significantly. The many energetic phonons produced by a
particle impact travel efficiently throughout the device substrate and generate
quasiparticles with high probability, thus causing errors on a large fraction
of the qubits in an array simultaneously. We describe a comprehensive strategy
for the numerical simulation of the phonon and quasiparticle dynamics in the
aftermath of an impact. We compare the simulations with experimental
measurements of phonon-mediated QP poisoning and demonstrate that our modeling
captures the spatial and temporal footprint of the QP poisoning for various
configurations of phonon downconversion structures. We thus present a path
forward for the operation of superconducting quantum processors in the presence
of ionizing radiation.
