Gigahertz-gated InGaAs SPAD system with avalanche charge sensitivity approaching the fundamental limit

We present an InGaAs/InP single-photon detection system operating at 1.25 GHz with detection efficiency above 50 % and per-gate afterpulse probability, measured 24.8 ns after an avalanche, below 0.2 %. The high efficiency and low afterpulse probabilities we observe are achieved with an avalanche discrimination system whose threshold for detection approaches the fundamental limit imposed by Johnson noise on a 50 Ohm load; we measure the threshold to be less than 8 fC. We discuss the design and performance of our approach, and tradeoffs between detection efficiency, afterpulse probability, and maximum count rate.