Backflashes in fast-gated avalanche photodiodes in quantum key distribution

TL;DR

This study investigates backflashes in GHz-gated InGaAs avalanche photodiodes used in quantum key distribution, showing that while present, their information leakage is minimal and has negligible impact on security.

Contribution

First experimental characterization of backflashes in fast-gated APDs, demonstrating reduced leakage and analyzing their origin in the device.

Findings

Backflash-induced information leakage is lower in GHz-gated APDs.

The effect of backflashes on secure key rate is negligible.

Backflash rate correlates with APD dark current, indicating origin in the InP multiplication region.

Abstract

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.