Modeling phonon-mediated quasiparticle poisoning in superconducting qubit devices

When ionizing radiation, such as cosmic ray muons and gamma-rays, impacts superconducting qubit device substrates, a burst of charge (electron-hole pairs) is created, which subsequently generates phonons above the pair-breaking energy of typical superconducting films. Pair-breaking phonons generate quasiparticles (QPs), broken Cooper pairs, when they scatter inside superconducting films. When QPs tunnel across the Josephson junction, they can absorb energy from the qubit, causing a relaxation error. As a result, bursts of pair-breaking phonons can cause correlated errors in superconducting qubit arrays which are detrimental in realizing quantum error correct schemes. Therefore, understanding the spatial and temporal characteristics of such events is crucial for engineering a fault-tolerant quantum computer. We model these events using a Monte-Carlo based simulation toolkit called Geant4 Condensed Matter Physics (G4CMP). We calibrate simulation parameters to experiments where we controllably inject phonons into the substrate by biasing an on-chip tunnel junction. We measure and simulate various device configurations of ground plane material and back-side metalization material to quantify device performance in response to a burst of pair-breaking phonons in the device substrate. The calibrated simulation parameters are used to model a device response to an ionizing radiation impact event for a dense array of qubits, allowing us to resolve the spatial and temporal device response to an ionizing impact. These results inform future fault-tolerant superconducting qubit device design and proposed quantum error correction algorithms that are robust against correlated errors.