Counting, timing and tracking with a single-photon germanium detector

We report the performance of a germanium quad-cell, composed of four avalanche photodiodes operated in the photon-counting regime, biased above the breakdown voltage. Each pixel detects a single photon in the wavelength range 1–1.5 µm with ~10% quantum efficiency and measures the photon arrival time with a time resolution of 100 ps. By adopting a multiplexed gating of the avalanche photodiodes we avoided the optical cross talk among pixels, thus achieving a noise-equivalent power of 3 × 10-15 W/Hz½ for each pixel and demonstrating the tracking capabilities of such a detector.