Coherent State Quantum Key Distribution with Entanglement Witnessing

TL;DR

This paper proposes a quantum key distribution method using entanglement witnesses to detect eavesdropping, analyzing its robustness over distance and its practical implementation advantages.

Contribution

It introduces a practical entanglement witness-based approach for quantum key distribution that can detect eavesdropping through sign changes of witness functions.

Findings

Eavesdropping can be detected by sign changes in witness functions.

The system's performance degrades with distance due to loss and eavesdropping.

The eavesdropping witness W is easier to implement than the true entanglement witness S.

Abstract

An entanglement witness approach to quantum coherent state key distribution and a system for its practical implementation are described. In this approach, eavesdropping can be detected by a change in sign of either of two witness functions, an entanglement witness S or an eavesdropping witness W. The effects of loss and eavesdropping on system operation are evaluated as a function of distance. Although the eavesdropping witness W does not directly witness entanglement for the system, its behavior remains related to that of the true entanglement witness S. Furthermore, W is easier to implement experimentally than S. W crosses the axis at a finite distance, in a manner reminiscent of entanglement sudden death. The distance at which this occurs changes measurably when an eavesdropper is present. The distance dependance of the two witnesses due to amplitude reduction and due to increased variance resulting from both ordinary propagation losses and possible eavesdropping activity is provided. Finally, the information content and secure key rate of a continuous variable protocol using this witness approach are given.