Title: Entangled photon pair generation from an epsilon-near-zero metasurface

Abstract: Entangled photon pair sources are essential for diverse quantum technologies, such as quantum communication, computation, and imaging. Currently, most methods for generating and manipulating entangled photon pairs rely on bulk nonlinear crystals, with some requiring sophisticated engineering. In this work, we propose and experimentally demonstrate a 68-nm-thick entangled photon pair source using a plasmonic metasurface strongly coupled to an epsilon-near-zero (ENZ) material. By tailoring dual resonances at both pump and emission wavelengths and utilizing the field enhancement induced by the ENZ effect, the photon pair generation efficiency is boosted. The coincidences-to-accidentals ratio of the generated photon pairs reaches 40, well above the limit of classical light radiation. Moreover, the ENZ metasurface platform enables versatile manipulation of the system's anisotropic second-order nonlinear susceptibility tensor, allowing for direct control over the polarization states of the generated photon pairs. By conducting polarization-resolved second-harmonic generation measurements, we discuss the potential to achieve a polarization-entangled Bell state from the identical ENZ metasurface, based on the quantum-classical correspondence. Our approach opens a new avenue for simultaneously achieving the miniaturization of photon pair sources and quantum state engineering.