# Rate-Adaptive Information Reconciliation for CV-QKD Systems at Low Signal-to-Noise Ratios

## TL;DR

This paper introduces new protocols for improving key distribution in quantum systems by adapting to low signal quality.

## Contribution

The paper proposes two novel rate-adaptive information reconciliation protocols (TIR and SIR) for CV-QKD systems.

## Key findings

- The proposed protocols achieve over 98.5% reconciliation efficiency at SNRs below −20 dB.
- The protocols maintain a secret key rate in long-distance quantum key distribution.
- TIR and SIR outperform existing rate-adaptive IR protocols in reconciliation efficiency.

## Abstract

In continuous-variable quantum key distribution (CV-QKD) systems, information reconciliation (IR) is a crucial step that significantly affects the secret key rate (SKR). The fixed-rate error-correcting codes used in IR are highly sensitive to changes in the signal-to-noise ratio (SNR) and cannot maintain a high reconciliation efficiency in practical CV-QKD systems. To address this issue, we first propose a rate-adaptive IR protocol, namely Threshold-based IR (TIR), which changes the code rate of low-density parity-check (LDPC) codes by selectively revealing bits with lower reliability and adjusting their log-likelihood ratios (LLRs). Then, we propose a rate-adaptive IR protocol, namely Sorting-based IR (SIR), which not only adjusts the code rate according to variations in SNR, but also enables the CV-QKD systems to achieve high reconciliation efficiency over a wide range of SNRs. Furthermore, we perform an analysis of the protocols in terms of code rate, reconciliation efficiency, and complexity. The simulation results demonstrate that the proposed protocols outperform other rate-adaptive IR protocols, achieving a reconciliation efficiency higher than 98.5% in the SNR range below −20 dB and maintaining a certain SKR in long-distance transmission.