Using Cascade in Quantum Key Distribution

TL;DR

This paper identifies a critical flaw in the analysis of Quantum Key Distribution protocols using the Cascade error correction method and proposes a corrected approach that ensures accurate key rate calculations.

Contribution

The authors introduce a new method that properly accounts for all two-way communication in Cascade, correcting previous incomplete analyses.

Findings

The new approach produces valid key rates in QKD protocols.

In many cases, the corrected and old methods agree on key rates.

In some scenarios, the new method yields lower, more accurate key rates.

Abstract

We point out a critical flaw in the analysis of Quantum Key Distribution (QKD) protocols that employ the two-way error correction protocol Cascade. Specifically, this flaw stems from an incom-plete consideration of all two-way communication that occurs during the Cascade protocol. We present a straightforward and elegant alternative approach that addresses this flaw and produces valid key rates. We exemplify our new approach by comparing its key rates with those generated using older, incorrect approaches, for Qubit BB84 and Decoy-State BB84 protocols. We show that in many practically relevant situations, our rectified approach produces the same key rate as older, incorrect approaches. However, in other scenarios, our approach produces valid key rates that are lower, highlighting the importance of properly accounting for all two-way communication during Cascade.