# Bounding light source side channels in QKD via Hong-Ou-Mandel interference

## TL;DR

This paper introduces a Hong-Ou-Mandel interference-based method to estimate and bound the total passive side-channel information leakage in practical quantum key distribution systems, enhancing security certification.

## Contribution

It presents a novel approach to quantify side-channel leakage using interference visibility, applicable to real-world QKD systems, and integrates this into security proofs.

## Key findings

- Experimental interference visibility values used in security bounds
- Lower bounds on secure key rate for realistic sources
- Tool for certification and loophole-free QKD design

## Abstract

Side-channel attacks on practical quantum key distribution systems compromise its security. Although some of these attacks can be taken into account, the general recipe of how to eliminate all side-channel flaws is still missing. In this work, we propose a method for estimation of the total passive side-channel information leakage from the Alice's light source. The method relies on Hong-Ou-Mandel interference between different signals emitted by Alice, which reveals their overall mode mismatch without the necessity to measure all individual degrees of freedom independently. We include experimental values of interference visibility in the security proof for the decoy-state BB84 protocol, and lower-bound the secure key rate for realistic light sources. The obtained results provide a tool that can be used for certification of the current QKD systems and pave the way towards the loophole-free design of the future ones.