Correlated quasiparticle poisoning of superconducting qubits from stress-relaxation events

Differential thermal contraction between the superconducting qubit device layer and substrate and between the substrate and sample packaging at millikelvin temperatures causes stresses in the system that relax in time. Relaxation of thermally-induced stress occurs through stick-slip events that create bursts of pair-breaking phonons that travel throughout the substrate. When these phonons impact the device layer, they break Cooper pairs, generating quasiparticles (QPs) in the qubit electrodes. QPs near the Josephson junction can tunnel across the barrier, absorbing energy from the qubit, which leads to relaxation errors. Because the phonons can travel throughout the entire chip, these events cause correlated errors, a roadblock for implementing quantum error correction. Ionizing radiation, a known source of correlated errors in superconducting qubits, incident on the substrate generates bursts of both charge and pair-breaking phonons. We monitor the generation of both of these particle populations with charge-sensitive transmons. Following the start of the cryostat cooldown, we observe a power-law reduction of multi-qubit correlated charge-parity switching event rates until this reaches a constant level set by background ionizing impact events; over the same period, the rate of shifts in qubit offset charge remains constant. This behavior, which we observe for multiple cooldowns and device configurations, is consistent with the expected dynamics of stress-relaxation events.