Intrinsic mitigation of the after-gate attack in quantum key distribution through fast-gated delayed detection

TL;DR

This paper investigates how fast-gated semiconductor avalanche photodiodes can inherently mitigate the after-gate attack in quantum key distribution by leveraging carrier trapping mechanisms that increase error rates under attack.

Contribution

It provides experimental evidence that trapped carriers in specific detector regions can enhance security against the after-gate attack in QKD systems.

Findings

Trapped carriers cause delayed detection in avalanche photodiodes.

Release of trapped carriers increases quantum bit error rate under attack.

This increase surpasses the security threshold, supporting intrinsic mitigation.

Abstract

The information theoretic security promised by quantum key distribution (QKD) holds as long as the assumptions in the theoretical model match the parameters in the physical implementation. The superlinear behaviour of sensitive single-photon detectors represents one such mismatch and can pave the way to powerful attacks hindering the security of QKD systems, a prominent example being the after-gate attack. A longstanding tenet is that trapped carriers causing delayed detection can help mitigate this attack, but despite intensive scrutiny, it remains largely unproven. Here we approach this problem from a physical perspective and find new evidence to support a detector's secure response. We experimentally investigate two different carrier trapping mechanisms causing delayed detection in fast-gated semiconductor avalanche photodiodes, one arising from the multiplication layer, the other from the heterojunction interface between absorption and charge layers. The release of trapped carriers increases the quantum bit error rate measured under the after-gate attack above the typical QKD security threshold, thus favouring the detector's inherent security. This represents a significant step to avert quantum hacking of QKD systems.