  InGaAs single-photon avalanche photodiodes (APDs) are key enablers for
high-bit rate quantum key distribution. However, the deviation of such
detectors from ideal models can open side-channels for an eavesdropper, Eve, to
exploit. The phenomenon of backflashes, whereby APDs reemit photons after
detecting a photon, gives Eve the opportunity to passively learn the
information carried by the detected photon without the need to actively
interact with the legitimate receiver, Bob. Whilst this has been observed in
slow-gated detectors, it has not been investigated in fast-gated APDs where it
has been posited that this effect would be lessened. Here, we perform the first
experiment to characterise the security threat that backflashes provide in a
GHz-gated self-differencing APD using the metric of information leakage. We
find that, indeed, the information leakage is lower than that reported for
slower-gated detectors and we show that its effect on the secure key rate is
negligible. We also relate the rate of backflash events to the APD dark
current, thereby suggesting their origin is the InP multiplication region in
the APD.
