Near-Noiseless Single-Photon Detection in the Infrared with a Megapixel Semiconductor Array for Low-Background Astronomy

Low-flux astronomical imaging and spectroscopy is limited by sensor noise, since ideal detectors would cleanly register individual photon arrivals. Optical semiconductor arrays can already reach single-photon sensitivity, but comparable infrared performance has mostly required superconducting detectors that operate near absolute zero and consume substantially more power, creating major system-level challenges. HgCdTe avalanche photodiodes (APDs) offer a semiconductor alternative by amplifying charge before readout, yet dark current and tunneling effects have so far restricted their use in the faintest regimes.

The authors demonstrate single-photon-resolving operation in a megapixel-format HgCdTe linear-mode avalanche photodiode (LmAPD) array operated at conventional cryogenic temperature. Avalanche gain lifts photon-induced steps in non-destructive up-the-ramp data above the readout-noise floor, while the dark signal remains largely unamplified. This yields a false-positive rate of approximately 2 false counts per thousand reads per pixel.

The results establish LmAPDs as a promising route toward infrared photon-counting focal planes for future low-background astronomical observatories.