Correlated quasiparticle poisoning from gamma irradiation in superconducting qubit arrays

When a gamma ray impacts a superconducting qubit chip, it generates many electron-hole pairs and pair-breaking phonons. Some electron-hole pairs diffuse from the impact site, which alters the local charge environment for nearby qubits. The phonons can travel throughout the chip and have energy above the superconducting gap of the device layer, leading to quasiparticle generation. Elevated quasiparticle density degrades qubit coherence and can cause correlated errors that challenge standard error-correction schemes that are vital for future quantum processors. By using a gamma-ray source outside the dilution refrigerator, we can observe the response of two qubit-array devices: one without and one with phonon mitigation via an array of 1-μm-thick Cu islands on the back side of the chip. We use charge-sensitive transmons to detect offset-charge jumps and quasiparticle poisoning from gamma radiation. We characterize correlations in offset-charge jumps and quasiparticle poisoning between qubits in the array. We also study correlations between offset-charge jumps following a gamma-ray impact with quasiparticle poisoning throughout the chip. Lastly, we compare our experimental results to detailed numerical modeling of the charge and phonon dynamics in our system. Our use of active gamma irradiation allows us to characterize various phonon-mitigation strategies for developing superconducting qubit designs that are robust against external radiation.