2GHz single photon avalanche diode detector for quantum key distribution applications

We present a Single-Photon Avalanche Diode (SPAD) detector capable of reaching 2 GHz of maximum count rate, thanks to an array architecture. This detector will be employed in optical receivers from satellite to ground for quantum communication links, based on Quantum Key Distribution (QKD), at 800 nm wavelength. The equivalent low deadtime of the detector allows to push transmission speed at a very high data rate (2 GHz), despite its large active area (163 µm × 163 µm). The detector comprises 8-by-8 SPADs, whose outputs are merged through an OR tree and sent to four output pads. Two 8-by-8 arrays with different OR trees and output pad configurations have been implemented for testing purposes. The first merges into a single signal the pulses coming from the respective 64 SPADs, and feeds this to a frequency divider, which evenly distributes the pulses to the four outputs. The second array is divided into four quadrants, and each quadrant is connected through an OR tree to one of the four outputs. Thus, in this array, the four outputs behave independently. The deadtime of each SPAD is lower than 3 ns, controlled by externally provided voltages to the Variable Load Quenching Circuits (VLQCs). Furthermore, pulse-shrinker circuits have been employed in the readout chain to mitigate the pile-up effects. Experimental results of maximum count rate, afterpulsing, Dark Count Rate (DCR), timing jitter and Photon Detection Efficiency (PDE) are reported.