Quantum soft filtering for the improved security analysis of the coherent one-way quantum-key-distribution protocol

TL;DR

This paper introduces quantum soft filtering operations to enhance security analysis of the coherent one-way QKD protocol, revealing that such attacks can yield more information for eavesdroppers than traditional methods, especially without monitoring control states.

Contribution

The study develops a generalized quantum soft filtering approach for security analysis, bridging beam-splitting and unambiguous state discrimination attacks, and provides new security bounds for the COW QKD protocol.

Findings

Quantum soft filtering can increase Eve's information gain.

Attack efficiency depends on control state monitoring.

Best-case attack scenarios occur without control state statistics maintenance.

Abstract

A precise security analysis of practical quantum key distribution (QKD) systems is an important step for improving their performance. Here we consider a class of quantum soft filtering operations, which generalizes the unambiguous state discrimination (USD) technique. These operations can be applied as a basis for a security analysis of the original coherent one-way (COW) QKD protocol since their application interpolates between beam-splitting (BS) and USD attacks. We demonstrate that a zero-error attack based on quantum soft filtering operations gives a larger amount of the information for Eve at a given level of losses. We calculate the Eve information as a function of the channel length. The efficiency of the proposed attack highly depends on the level of the monitoring under the maintenance of the statistics of control (decoy) states, and best-case results are achieved in the case of the absence of maintenance of control state statistics. Our results form additional requirements for the analysis of practical QKD systems based on the COW QKD protocol and its variants by providing an upper bound on the security.