Experimental violation of a Bell inequality with two different degrees of freedom of entangled particle pairs

TL;DR

This paper demonstrates hybrid entanglement of photon pairs across different degrees of freedom, violating Bell inequalities and supporting the fundamental nature of entanglement regardless of physical realization.

Contribution

It presents the first experimental violation of a Bell inequality using two different degrees of freedom of entangled photon pairs, combining path and polarization.

Findings

Achieved S=2.653±0.027 in CHSH inequality

Violates local realism by more than 24 standard deviations

Supports entanglement's independence from physical degrees of freedom

Abstract

We demonstrate hybrid entanglement of photon pairs via the experimental violation of a Bell inequality with two different degrees of freedom (DOF), namely the path (linear momentum) of one photon and the polarization of the other photon. Hybrid entangled photon pairs are created by Spontaneous Parametric Down Conversion and coherent polarization to path conversion for one photon. For that photon, path superposition is analyzed, and polarization superposition for its twin photon. The correlations between these two measurements give an S-parameter of S=2.653+/-0.027 in a CHSH inequality and thus violate local realism for two different DOF by more than 24 standard deviations. This experimentally supports the idea that entanglement is a fundamental concept which is indifferent to the specific physical realization of Hilbert space.