Entanglement does not always require one particle per party. It was predicted some thirty years ago that a single photon traversing a beam splitter could violate a Bell inequality. Although initially debated, single-photon nonlocality was eventually demonstrated via homodyne measurements. Here, we present an alternate realisation that avoids the complexity of homodyne measurements and potential loopholes in their implementation. We violate a Bell inequality by performing joint measurements on two copies of the same single-photon entangled state, where one photon acts as a phase reference for the other, making it self-referential. We observe CHSH parameters of 2.71±0.09 and 2.23±0.07, depending on the joint measurements implemented. This offers a new perspective on single-photon nonlocality and a more accessible experimental route, potentially applicable to general mode-entangled states in diverse platforms.
@article{arxiv.2511.21819,
title = {Testing Single Photon Entanglement using Self-Referential Measurements},
author = {Daniel Kun and Teodor Strömberg and Borivoje Dakić and Philip Walther and Lee A. Rozema},
journal= {arXiv preprint arXiv:2511.21819},
year = {2026}
}