Channel Capacity Gain in Entanglement-Assisted Communication Protocols Based Exclusevly on Linear Optics and Single Photon Inputs

TL;DR

This paper investigates entanglement-assisted communication protocols using only linear optics and single photon inputs, demonstrating capacity gains beyond the traditional 0.585 bits limit of Bell state-based dense coding.

Contribution

The study introduces a new class of bipartite four-mode two-photon entangled states that enhance channel capacity using linear optics and single photon inputs.

Findings

Capacity gain exceeds 0.585 bits with new entangled states

Proposes generalization to N-mode two-photon entangled states for N=6,8

Shows feasibility of capacity enhancement without Bell state discrimination

Abstract

Entanglement can effectively increase communication channel capacity as evidenced by dense coding that predicts a capacity gain of 1 bit when compared to entanglement-free protocols. However, dense coding relies on Bell states and when implemented using photons the capacity gain is bounded by $0.585$ bits due to one's inability to discriminate between the four optically encoded Bell states. In this paper we study the following question: Are there alternative entanglement-assisted protocols that rely only on linear optics, coincidence photon counting and separable single photon input states and at the same time provide a greater capacity gain than $0.585$ bits. We show that besides the Bell states there is a class of bipartite four-mode two-photon entangled states that facilitate an increase in channel capacity. We also discuss how the proposed scheme can be generalized to the case of two-photon $N$-mode entangled states for $N=6,8$.