We demonstrate polarization-entangled biphotons in a cold-atom double-$\mathrm{\Lambda }$ system by coherently mapping a selected two-dimensional orbital-angular-momentum (OAM) subspace onto the polarization basis. In a standard double-$\mathrm{\Lambda }$ configuration, atomic selection rules often suppress direct polarization correlations and favor OAM entanglement. Our approach bypasses this constraint through external mode conversion and postselection without modifying the atomic level structure or the underlying four-wave-mixing dynamics. 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. An average Clauser-Horne-Shimony-Holt parameter of $S=2.44$ verifies the survival of nonlocal correlations. This work presents the demonstration of OAM-to-polarization entanglement transfer in a cold-atom spontaneous four-wave-mixing platform and establishes a coherent interface between atomic OAM resources and polarization-based quantum communication networks.

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• Angular momentum of light
• Entanglement production
• Quantum communication, protocols & technology
• Quantum control
• Quantum optics
• Quantum state transfer
• Quantum tomography