Characterization of Radiation-Induced Errors in Superconducting Qubits Protected with Various Gap-Engineering Strategies

Impacts from high-energy particles cause correlated errors in superconducting qubits by increasing the quasiparticle density in the vicinity of the Josephson junctions (JJs). Such errors are particularly harmful as they cannot be easily remedied via conventional error correcting codes. Recent experiments reduced correlated errors by making the difference in superconducting gap energy across the JJ larger than the qubit transition energy. In this work, we assess gap engineering near the JJ (δΔJJ) and the capacitor/ground-plane (δΔM1) by exposing arrays of transmon qubits to two sources of radiation. For $α$-particles from an ²⁴¹Am source, we observe T₁ errors correlated in space and time, supporting a hypothesis that hadronic cosmic rays are a major contributor to the 10⁻¹⁰ error floor observed in Ref. 1. For electrons from a pulsed linear accelerator, we observe temporally correlated T₁ and T₂ errors, this measurement is insensitive to spatial correlations. We observe that the severity of correlated T₁ errors is reduced for qubit arrays with a greater degree of gap engineering at the JJ. For both T₁ and T₂ errors, the recovery time is hastened by an increased δΔM1, which we attribute to the trapping of quasiparticles into the capacitor/ground-plane. We construct a model of quasiparticle dynamics that qualitatively agrees with our observations. This work reinforces the multifaceted influence of radiation on superconducting qubits and provides strategies for improving radiation resilience.