Linear-mode single-photon-sensitive avalanche photodiodes for GHz-rate near-infrared quantum communications

We present design and performance data for a high-speed telecom-band (1.3 mum) single-photon-sensitive receiver based on a new class of multi-stage InGaAs avalanche photodiode (APD) operated in the proportional mode (linear mode). Unlike Geiger-mode single-photon-sensitive APDs (SPADs), a multi-stage linear SPAD is operated below its avalanche breakdown voltage and need not be gated to suppress afterpulsing. Further, the linear-mode APD preserves signal amplitude information, whereas the response of a Geiger APD is binary. Unlike contemporary near-infrared (NIR) APDs, the engineered APDs have excess avalanche noise comparable to silicon, sufficient for quantum communication protocols.