Focused Optical Illumination of Superconducting Qubit Arrays: Substrate Charge Dynamics

High-energy events such as gamma-ray or muon impacts on superconducting qubit chips generate cascades of electron-hole pairs and pair-breaking phonons in the silicon substrate. The motion and trapping of these charges modify the local electrostatic environment, leading to measurable offset-charge shifts in nearby charge-sensitive transmons. To study these effects in a controlled manner, we developed a cryogenic optical system that delivers a focused beam of light, roughly 100 µm in diameter, to the back side of a qubit chip at millikelvin temperatures. A micro-electro-mechanical system (MEMS) mirror enables dynamic beam steering of the light source, and we can synchronize the optical pulses with qubit control and readout. By tuning the pulse position, intensity, and duration, we can characterize the temporal and spatial evolution of offset-charge dynamics across multiple qubits, observing correlated jumps and charge relaxation following localized excitation. Complementary measurements of qubit relaxation times and quasiparticle charge-parity switching provide additional insight into phonon-mediated quasiparticle generation. We compare our experimental results with numerical simulations of charge and phonon transport in the substrate, advancing understanding of radiation-induced charge fluctuations and their coupling to superconducting qubits.