Quantum Pulse Gate Attack on IM/DD Optical Key Distribution Exploiting Symbol Shape Distortion

Intensity modulation/direct detection (IM/DD) optical key distribution (OKD) is a method to generate secret whose security against passive eavesdropping guaranteed by the shot noise inherent photodetection process. Here effects of intensity-dependent symbol shape distortion on IM/DD OKD are investigated assuming that eavesdropper can implement temporal mode demultiplexing using e.g. quantum pulse gating technique. The quantitative analysis includes generation based either hard- or soft-decoding detected signal as well impact excess noise. A simple, rule-of-thumb relation between severity and strength required ensure presented.