# Eavesdropper's ability to attack a free-space quantum-key-distribution receiver in atmospheric turbulence

## TL;DR

This paper experimentally and theoretically examines how atmospheric turbulence affects an eavesdropper's ability to perform intercept-resend attacks on free-space quantum key distribution systems, highlighting the importance of turbulence mitigation.

## Contribution

It provides experimental data on turbulence impact and theoretical limits on attack feasibility, incorporating adaptive optics and entanglement-breaking analysis.

## Key findings

- Turbulence significantly impairs Eve's attack effectiveness.
- Adaptive optics can help Eve compensate for turbulence effects.
- A theoretical turbulence threshold limits attack success.

## Abstract

The ability of an eavesdropper (Eve) to perform an intercept-resend attack on a free-space quantum key distribution (QKD) receiver by precisely controlling the incidence angle of an attack laser has been previously demonstrated. However, such an attack could be ineffective in the presence of atmospheric turbulence due to beam wander and spatial mode aberrations induced by the air's varying index of refraction. We experimentally investigate the impact turbulence has on Eve's attack on a free-space polarization-encoding QKD receiver by emulating atmospheric turbulence with a spatial light modulator. Our results identify how well Eve would need to compensate for turbulence to perform a successful attack by either reducing her distance to the receiver, or using beam wavefront correction via adaptive optics. Furthermore, we use an entanglement-breaking scheme to find a theoretical limit on the turbulence strength that hinders Eve's attack.