Practical Methods for Distance-Adaptive Continuous-Variable Quantum Key Distribution

TL;DR

This paper investigates methods to extend the operational distance of continuous-variable quantum key distribution by evaluating modulation tuning, trusted loss addition, and rate-adaptive error correction, with experimental validation and analysis of trade-offs.

Contribution

It introduces and experimentally validates three strategies—modulation tuning, trusted loss, and rate-adaptive FEC—to overcome distance limitations in CV-QKD systems.

Findings

Rate-adaptive FEC achieves near-optimal secret key rates over various distances.

Modulation tuning and trusted loss extend operational distance but reduce key rate.

All methods are experimentally validated and compared for practical implementation.

Abstract

Continuous-variable quantum key distribution (CV-QKD) is a promising quantum-safe alternative to classical asymmetric cryptography that enables two authenticated parties to establish a shared secret over a potentially eavesdropped quantum channel. A key step in CV-QKD post-processing is information reconciliation, which leverages forward error correction (FEC) techniques to extract identical bit strings from noisy correlated data. In this work, we analyze the strict limitations on operating distance that are imposed by constant-rate FEC, severely limiting the practicability of CV-QKD systems in deployed optical networks. To overcome the distance limitations, we evaluate three strategies: (i) tuning modulation variance, (ii) adding controlled amounts of trusted detector loss, and (iii) the use of rate-adaptive FEC. All approaches are validated experimentally, compared in terms of performance, and we discuss implementation aspects. Our results show that while methods (i) and (ii) extend the operational distance of constant-rate FEC without the need for additional hardware components, they incur a significant penalty in secret key rate (SKR). In contrast, rate-adaptive FEC enables CV-QKD operation with performance close to the asymptotic SKR over a wide range of distances, provided that the reconciliation efficiency is chosen appropriately.