Bell-inequality violation by entangled single-photon states generated from a laser, an LED, or a halogen lamp

In single-particle or intraparticle entanglement, two degrees of freedom of a single particle, e.g., momentum and polarization of a single photon, are entangled. Single-particle entanglement is a resource that can be exploited both in quantum communication protocols and in experimental tests of noncontextuality based on the Kochen-Specker theorem. Here we show that single-particle entangled states of single photons can be produced from attenuated classical sources of light. To experimentally certify the single-particle entanglement, we perform a Bell test, observing a violation of the Clauser, Horne, Shimony, and Holt inequality. We show that single-particle entanglement can be achieved even in a classical light beam, provided that first-order coherence is maintained between the degrees of freedom involved in the entanglement. This demonstrates that cheap, compact, and low-power photon sources can be used to generate single-particle entangled photons which could be a resource for quantum technology applications.