# Laser damage attack against optical attenuators in quantum key distribution

## TL;DR

This paper demonstrates that high-power laser attacks can permanently damage optical attenuators in quantum key distribution systems, potentially compromising security by increasing photon numbers in the prepared states.

## Contribution

The study experimentally shows laser damage can alter optical attenuators in QKD systems, revealing a new security vulnerability not previously addressed.

## Key findings

- Two of four tested attenuators suffer permanent damage
- Damaged attenuators lead to higher photon numbers
- Laser attack can compromise QKD security

## Abstract

Many quantum key distribution systems employ a laser followed by an optical attenuator to prepare weak coherent states in the source. Their mean photon number must be pre-calibrated to guarantee the security of key distribution. Here we experimentally show that this calibration can be broken with a high-power laser attack. We have tested four fiber-optic attenuator types used in quantum key distribution systems, and found that two of them exhibit a permanent decrease in attenuation after laser damage. This results in higher mean photon numbers in the prepared states and may allow an eavesdropper to compromise the key.