Quasiparticle Poisoning in Superconducting Qubits from Focused Optical Illumination

Impacts of ionizing radiation on superconducting qubit chips result 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 on the device layer. Without explicit mitigation of this phonon-mediated poisoning process, such an impact event leads to correlated errors across a qubit array, which poses a significant challenge for quantum error correction schemes. A technique for local deposition of ionizing radiation in the chip would allow for the characterization of the dynamics of the quasiparticle poisoning and evaluation of different mitigation strategies. We have developed a cryogenic optical system that delivers a beam of light focused to a spot with a diameter below 0.1 mm to the backside of a chip containing an array of superconducting qubits on the millikelvin stage of a dilution refrigerator. The system, which incorporates a micro-electro-mechanical system (MEMS) mirror, can steer the position of the beam on the chip using control signals provided by room-temperature electronics interfaced with a PC, which allows us to coordinate the timing of optical pulses with qubit control and readout signals. With the ability to adjust the spatial location, intensity, and duration of the optical pulse, we can characterize the response of the qubits through measurements of quasiparticle charge-parity switching, local offset-charge shifts, and transient degradation of qubit relaxation time.