# Laser seeding attack in quantum key distribution

## TL;DR

This paper demonstrates a laser seeding attack on practical quantum key distribution systems, revealing a vulnerability in trusted sources that could compromise the security of quantum cryptography protocols.

## Contribution

It experimentally shows a new attack on semiconductor laser sources used in QKD, with a general security analysis applicable to various protocols.

## Key findings

- Laser seeding increases pulse intensity unnoticed
- The attack can compromise MDI-QKD security
- Security analysis applies broadly to intensity-increasing vulnerabilities

## Abstract

Quantum key distribution (QKD) based on the laws of quantum physics allows the secure distribution of secret keys over an insecure channel. Unfortunately, imperfect implementations of QKD compromise its information-theoretical security. Measurement-device-independent quantum key distribution (MDI-QKD) is a promising approach to remove all side channels from the measurement unit, which is regarded as the "Achilles' heel" of QKD. An essential assumption in MDI-QKD is however that the sources are trusted. Here we experimentally demonstrate that a practical source based on a semiconductor laser diode is vulnerable to a laser seeding attack, in which light injected from the communication line into the laser results in an increase of the intensities of the prepared states. The unnoticed increase of intensity may compromise the security of QKD, as we show theoretically for the prepare-and-measure decoy-state BB84 and MDI-QKD protocols. Our theoretical security analysis is general and can be applied to any vulnerability that increases the intensity of the emitted pulses. Moreover, a laser seeding attack might be launched as well against decoy-state based quantum cryptographic protocols beyond QKD.