Focused Optical Illumination of Superconducting Qubit Arrays: Quasiparticle Poisoning

The impact of ionizing radiation, such as gamma rays and cosmic ray muons, on superconducting qubit chips results in the generation of electron-hole pairs in the substrate, as well as a cascade of pair-breaking phonons. These energetic phonons spread efficiently throughout the chip and create excess quasiparticles in the junction electrodes of any qubits present on the device layer. This phonon-mediated poisoning from these impacts can cause correlated errors across the qubit array, an impediment to quantum error correction. In order to characterize this poisoning process, we have implemented a cryogenic micro-electro-mechanical system (MEMS) mirror capable of adjusting the spatial location of a focused optical pulse on the back side of a chip containing an array of charge-sensitive transmons. We study quasiparticle dynamics by observing the transient degradation of the qubit relaxation time as we vary the position of the optical beam for different optical pulse widths, delay times, optical attenuation, and wavelength. We analyze our experimental results alongside numerical simulations of phonon and quasiparticle dynamics in our system.