Correlated Errors in Qubit Arrays due to Phonon-mediated Quasiparticle Poisoning

Phonons in the substrate of a superconducting qubit array with energy above the superconducting gap can lead to correlated errors between qubits. Mitigating such errors is challenging for quantum error correcting codes when the correlations extend across a significant fraction of the qubits in the array. Errors arise when energetic phonons spread throughout the substrate and impact the superconducting electrodes on the device layer, where the phonons can break Cooper pairs and generate dissipative quasiparticle excitations above the superconducting ground state. This process leads to transient degradation of the qubit relaxation time. High-energy particles, such as gamma rays from background radioactivity or muons generated by cosmic rays, impacting the qubit substrate produce a large number of electron-hole pairs as well as a burst of phonons. Besides particle impacts, bursts of pair-breaking phonons without an accompanying charge signal can also be generated through relaxation of stresses built up in the device thin films, substrate, and attachment points to the sample packaging. We characterize the phonon-mediated quasiparticle poisoning process in arrays of charge-sensitive qubits through monitoring of the qubit relaxation time, the switching of quasiparticle charge-parity on each qubit island, and the shift in the offset-charge environment of the qubit when electrons and holes are liberated in the substrate. Through a series of experiments, we controllably induce phonon-mediated poisoning using direct injection of phonons with on-chip tunnel junctions, active irradiation with a gamma-ray source located outside the cryostat, and depositions of energy with a focused spot of pulsed light on the back of the substrate controlled with a cryogenic micro-electromechanical system (MEMS) mirror. We use these techniques to characterize various mitigation strategies for making superconducting qubit processors resilient in the face of phonon-mediated poisoning.