Eavesdropping and countermeasures for backflash side channel in quantum cryptography

TL;DR

This paper investigates the backflash side channel in quantum key distribution systems, demonstrating how emitted photons can leak information and proposing countermeasures to enhance security.

Contribution

It identifies backflash emissions in commercial detectors, characterizes their properties, and demonstrates how they can compromise QKD security, proposing countermeasures.

Findings

Silicon avalanche photodiodes emit backflashes during detection

Backflashes can be distinguished to identify detector clicks

Countermeasures can mitigate backflash information leakage

Abstract

Quantum key distribution (QKD) promises information theoretic secure key as long as the device performs as assumed in the theoretical model. One of the assumptions is an absence of information leakage about individual photon detection outcomes of the receiver unit. Here we investigate the information leakage from a QKD receiver due to photon emission caused by detection events in single-photon detectors (backflash). We test commercial silicon avalanche photodiodes and a photomultiplier tube, and find that the former emit backflashes. We study the spectral, timing and polarization characteristics of these backflash photons. We experimentally demonstrate on a free-space QKD receiver that an eavesdropper can distinguish which detector has clicked inside it, and thus acquire secret information. A set of countermeasures both in theory and on the physical devices are discussed.