We propose metasurfaces-based scheme to expand the polarization entanglement between two photons. Our approach increases the entanglement dimension, expanding the biphoton state into a hyperentangled state with polarization and OAM entanglement imprinted onto two photons.
Photon-based polarization entanglement in the form of Bell states and qutrits is an essential tool for understanding complex quantum phenomena and implementing various quantum technologies such as free-space quantum communication and quantum imaging. Polarization-entangled photons are conventionally generated in bulky nonlinear crystals through spontaneous parametric down-conversion (SPDC). In this approach, the photon polarization is intrinsically defined by the nonlinear tensor of the material, restricting the versatility of the generated quantum states. The recent developments of dielectric nanostructures with sub-wavelength thickness called metasurfaces [1] enable miniaturization of quantum light sources, while dramatically enhancing the photon-pair generation and allowing the preparation of complex quantum states with spectral [2] and spatial [3] entanglement. However, optically reconfigurable generation of biphoton polarization states with metasurfaces remained unexplored.
Conventional approaches to generating and distributing entangled quantum states usually rely on bulky traditional optical devices, posing significant challenges to system miniaturization and integration. Here we report an entanglement engineering scheme that utilizes quantum interference on a metasurface, combined with postselection, to generate and distribute multiple polarization-entangled photon pairs. Two unentangled identical photons, differing only in polarization, are incident on a multichannel metasurface where each channel supports a specific polarization transformation. The metasurface facilitates two-photon interference, generating postselected polarization-entangled states and forming a fully connected entanglement distribution among every two output channels. Experiments demonstrate the generation and distribution of four Bell states across 21 channel pairs, underscoring the potential of metasurfaces as compact, scalable entanglement sources for quantum networking.