  Fast gated avalanche photodiodes (APDs) are the most commonly used single
photon detectors for high bit rate quantum key distribution (QKD). Their
robustness against external attacks is crucial to the overall security of a QKD
system or even an entire QKD network. Here, we investigate the behavior of a
gigahertz-gated, self-differencing InGaAs APD under strong illumination, a
tactic Eve often uses to bring detectors under her control. Our experiment and
modelling reveal that the negative feedback by the photocurrent safeguards the
detector from being blinded through reducing its avalanche probability and/or
strengthening the capacitive response. Based on this finding, we propose a set
of best-practice criteria for designing and operating fast-gated APD detectors
to ensure their practical security in QKD.
