# An Effective Parameter Analysis for Sending-or-Not-Sending Quantum Key Distribution with Untrusted Light Sources

## TL;DR

This paper proposes a method to improve the security of quantum key distribution by monitoring untrusted light sources, enabling longer transmission distances.

## Contribution

A novel light-source monitoring solution using untagged bits is introduced to enhance the security and performance of quantum key distribution protocols.

## Key findings

- The untagged bits-based solution achieves ideal monitoring performance in untrusted light source environments.
- The method derives a tight bound for secure bit rate and approximates the performance of ideal light sources.
- The solution outperforms existing methods in transmission distance and mitigates light-source fluctuations.

## Abstract

The twin-field (TF) protocol is a key protocol in quantum key distribution (QKD) that enables remote key distribution, achieving a maximum secure transmission distance of over 500 km. However, the TF protocol still faces several security issues in real-world environments. To address the issue of untrusted sources, one effective solution is to introduce a light-source monitoring module into the system. Analysis shows that a solution based on untagged bits (UBs) can achieve ideal monitoring performance. This solution can capture UB signals to accurately estimate key parameters in the protocol’s security analysis, ultimately deriving a tight bound for the secure bit rate. Simulations show that this solution approximates the performance of ideal light sources in the presence of untrusted sources and effectively mitigates the impact of light-source fluctuations. It outperforms other solutions in key performance metrics, such as transmission distance.