Security loophole in free-space quantum key distribution due to spatial-mode detector-efficiency mismatch

TL;DR

This paper reveals a security vulnerability in free-space quantum key distribution caused by spatial-mode detector-efficiency mismatch, demonstrating an attack method and proposing a practical countermeasure to enhance security.

Contribution

The study identifies a specific spatial-mode efficiency mismatch in standard polarization QKD receivers and experimentally demonstrates an effective countermeasure using a spatial filter.

Findings

Efficiency mismatch enables security breach via intercept-and-resend attack

Adding a spatial filter mitigates the detector-efficiency mismatch

Experimental validation confirms the countermeasure's effectiveness

Abstract

In free-space quantum key distribution (QKD), the sensitivity of the receiver's detector channels may depend differently on the spatial mode of incoming photons. Consequently, an attacker can control the spatial mode to break security. We experimentally investigate a standard polarization QKD receiver, and identify sources of efficiency mismatch in its optical scheme. We model a practical intercept-and-resend attack and show that it would break security in most situations. We show experimentally that adding an appropriately chosen spatial filter at the receiver's entrance is an effective countermeasure.