High Count-Rate 8 × 8 SPAD Arrays for Quantum Key Distribution

We developed a SPAD detector implementing an array structure to achieve short deadtime and high detection efficiency, to be employed in ground-based optical receivers for enhancing key rates in Quantum Key Distribution (QKD) channels at 800 nm. The device detects over 2 billion photons per second, thus enabling transmission rates beyond 2 GHz. The detector features two 8 × 8 SPAD arrays, named Trial A and Trial B, sharing 4 outputs. Trial A combines its 64 SPADs’ outputs in an OR-tree connected to a frequency divider, distributing the pulses to the 4 outputs. Trial B has 4 independent quadrants, each with its own OR-tree feeding a toggle flip-flop and connected to a single output. A pulse shrinker reduces the pile-up effect. The detector exhibits a Photon Detection Efficiency (PDE) of 39.6% at 800 nm, a median Dark Count Rate (DCR) of 300 cps/pixel, afterpulsing probability below 0.1% and a 230 ps Full-Width at Half-Maximum (FWHM) jitter per SPAD.