  Semiconductor avalanche photodiodes (APDs) are commonly used for single
photon detection in quantum key distribution. Recently, many attacks using
bright illumination have been proposed to manipulate gated InGaAs APDs. In
order to devise effective counter-measures, careful analysis of these attacks
must be carried out to distinguish between incorrect operation and genuine
loopholes. Here, we show that correctly-operated, gated APDs are immune to
continuous-wave illumination attacks, while monitoring the photocurrent for
anomalously high values is a straightforward counter-measure against attacks
using temporally tailored light.
