Irradiating superconducting qubits with gamma rays to characterize quasiparticle poisoning mitigation strategies

High-energy gamma-rays, which are ubiquitous in typical lab environments, impacting superconducting qubit devices generate many electron–hole pairs and energetic phonons that propagate throughout the substrate. The resulting charge dynamics produces offset-charge shifts for nearby charge-sensitive qubits, while the pair-breaking phonons create quasiparticles in the superconducting device layer, degrading coherence leading to correlated errors across the qubit array. A general requirement for quantum error correction (QEC) is that logical error rate is reduced as code distance is increased. However, when high-weight correlated errors are present in an array, they inhibit this reduction, posing obstacles for QEC and challenging the scaling of superconducting quantum processors. In this work, we use a controlled gamma-ray source outside the dilution refrigerator to characterize the response of two devices, a control device with no phonon mitigation structures and another device with a 1-μm-Cu layer on the chip back side for phonon downconversion. We employ charge-sensitive transmons that allow us to simultaneously monitor offset-charge shifts and quasiparticle poisoning across the array over a range of irradiation doses. By correlating offset-charge jumps near the impact site with the level of quasiparticle poisoning for the qubit array, we can assess the characteristic quasiparticle poisoning footprint for each device. We compare these results with numerical modeling of electron-hole motion and phonon dynamics in the substrate following a gamma-ray impact. Thus, we establish a framework for characterizing strategies for mitigating errors due to correlated quasiparticle poisoning in superconducting qubit arrays, providing a pathway to the development of designs that are resilient to radiation.