Polarization Entanglement in Atomic Biphotons via OAM-to-Spin Mapping

We demonstrate polarization-entangled biphotons in a cold-atom double-Λ system, overcoming atomic selection rules that suppress polarization correlations and favor orbital angular momentum (OAM) entanglement. Using spatial light modulators, we coherently map a selected two-dimensional OAM subspace onto the polarization basis and thereby open an otherwise inaccessible polarization channel. Quantum-state tomography confirms that the mapping preserves the biphoton coherence. The four polarization Bell states are generated with fidelities of 92-94\% with few-percent statistical uncertainties, and an average Clauser-Horne-Shimony-Holt parameter of S=2.44 verifies the survival of nonlocal correlations. To the best of our knowledge, this work presents the first demonstration of OAM-to-polarization entanglement transfer in a cold-atom spontaneous four-wave mixing platform and establishes a practical interface for integrating atomic OAM resources with polarization-based quantum communication networks.