  Single photon avalanche diodes (SPADs) are the most commercially diffused
solution for single-photon counting in quantum key distribution (QKD)
applications. However, the secondary photon emission, arising from the
avalanche of charge carriers during a photon detection, may be exploited by an
eavesdropper to gain information without forcing errors in the transmission
key. In this paper, we characterise such backflash light in gated InGaAs/InP
SPADs, and its spectral and temporal characterization for different detector
models and different operating parameters. We qualitatively bound the maximum
information leakage due to backflash light, and propose a solution.
