How cryogenic illumination resets gate defined semiconductor quantum devices

Illumination is widely used to "reset" quantum-dot qubit devices at cryogenic temperatures. While highly effective, this technique has not been well studied and is often discussed informally as poorly understood lore. We present the results of systematic measurements of threshold voltage shifts in gate defined Si/SiGe Hall bars using a near infrared (780 nm) laser diode. We find that illumination under an applied gate voltage can be used to dial in a specific, stable, and reproducible threshold voltage in the range of ±0.5 V. Outside this range the threshold voltage can still be tuned, however the tunability diminishes due to the finite density of interface traps at the oxide interface, eventually saturating near ±1 V. We present a simple and intuitive model that explains both these results and why cryogenic illumination is successful at resetting quantum dot qubit devices after charging events.