Metasurface-Based Dual-Basis Polarization Beam Splitter for efficient entanglement witnessing

Entanglement witnessing is essential for quantum technologies such as computing, key distribution, and networking. Conventional bulk-optics methods require sequential reconfiguration across multiple polarization bases, limiting efficiency and scalability. We propose a metasurface-based analyzer that performs dual-basis σ_z and σ_y projections simultaneously by mapping them to orthogonal spatial modes. This allows direct access to the commuting two-photon correlators \langle σ_z \otimes σ_z \rangle and \langle σ_y \otimes σ_y \rangle required for entanglement witnessing. The metasurface design employs meta-atoms engineered to impart independent linear and circular phase delays through anisotropy and geometric control, resulting in polarization-dependent beam deflection that separates H/V and R/L components. This approach halves the measurement overhead compared to sequential analysis while offering a compact, integrable platform for chip-scale quantum photonics. The proposed scheme provides a path toward efficient entanglement verification with applications in quantum key distribution, quantum repeaters, and scalable quantum networks.