Resisting light-induced quasiparticle poisoning in superconducting qubit arrays

Quantum error correction assumes that physical qubit errors are sufficiently uncorrelated in time and space. In superconducting qubit arrays, that assumption can be broken from high-energy impact events. These events produce phonons in the device substrate which in turn generate quasiparticles (QPs) in the superconducting metal. Subsequently, QP tunneling across the qubit's Josephson junction results in suppressed coherence times and long-lived correlated errors. By engineering the superconducting gap energy difference at the junction, these tunneling processes can be reduced, allowing the device to resist the effects of high-energy impact events. Here, we evaluate the efficacy of gap engineering by generating QPs in a controlled and on-demand manner using optical light with energies well above the superconducting gap. We demonstrate that strong gap engineering enables qubits to maintain coherence even when considerable densities of QPs are introduced. Furthermore, we show that small superconducting gap differences can be measured using qubit coherence spectroscopy and injected optical light. These results confirm that gap engineering is a highly effective technique to mitigate correlated errors from QP poisoning events in superconducting qubit arrays.