Fast-gated detectors afford more secure communication

Quantum key distribution (QKD) is a method of secure communication between two parties. Because QKD relies on quantum mechanics, a third party attempting to eavesdrop creates detectable disturbances in the system. While QKD is perfectly secure in theory, security loopholes arise in practice because system components stray from their ideal behavior.

The high detection efficiency and potential for room-temperature operation of avalanche photodiodes (APDs) make them promising single-photon detectors for QKD systems. But APDs can also create a side-channel when reemitting photons after a photon detection. This light, known as a backflash, can allow a third party to eavesdrop without interacting with the legitimate receiving party.

Previous work has examined backflashes in slow-gated detectors, but not fast-gated detectors. Koehler-Sidki et al. measured the blackflash rate on fast-gated detectors, specifically GHz-gated InGaAs/InP APDs, for the first time. They found that backflashes occur less often in fast-gated APDs than their slower counterparts, because the gates are shorter so the avalanches have less charge.

A lower backflash rate means that the information leakage is lower in fast-gated APDs than in slow-gated ones. Quantifying the information leakage for GHz-gated APDs, the authors found it to be an order of magnitude lower than MHz-gated APDs. They determined that the low backflash rate – about 0.5% – does not practically affect the security of QKD and there is no need for a countermeasure to this side-channel.

“We showed the first explicit effect of backflashes on the secure key rate in QKD,” said author Alexander Koehler-Sidki. “By demonstrating fast-gated APDs emit fewer backflashes than slower-gated ones, our work takes a significant step in closing a QKD loophole.”

Source: “Backflashes from fast-gated avalanche photodiodes for quantum key distribution,” by A. Koehler-Sidki, J. F. Dynes, T. K. Paraïso, M. Lucamarini, A. W. Sharpe, Z. L. Yuan, and A. J. Shields, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/1.5140548 .