We present a rigorous mathematical framework for verifying effective entanglement in a Coherent One-Way (COW) quantum key distribution setup. In particular, we introduce a two-parameter family of entanglement witnesses, identify the parameter ranges where they constitute valid witnesses, and demonstrate their ability to reveal effective entanglement in the COW protocol. Additionally, we analyze previously obtained experimental data from a COW implementation and report clear signatures of effective entanglement.
@article{arxiv.2512.01359,
title = {Witnessing the Effective Entanglement in the COW Protocol},
author = {F. Rezazadeh and A. Mani and M. Khodabandeh and M. Jaberi and S. A. Madani},
journal= {arXiv preprint arXiv:2512.01359},
year = {2025}
}