Polarization engineering of entangled photons from a lithium niobate nonlinear metasurface

TL;DR

This paper introduces a novel ultrathin nonlinear metasurface on lithium niobate that can generate and shape polarization-entangled photon states, offering a compact and tunable alternative to bulky traditional crystals for quantum technologies.

Contribution

It demonstrates the fabrication of a multiplexed nonlinear metasurface capable of producing arbitrary polarization-entangled states, advancing miniaturized quantum photonic devices.

Findings

Successful fabrication of orthogonal metagratings with identical resonance

Experimental control of two-photon polarization states via metagrating orientation

Theoretical framework for generating arbitrary entangled qutrit states

Abstract

Complex polarization states of photon pairs are indispensable in various quantum technologies. Conventional methods for preparing desired two-photon polarization states are realized through bulky nonlinear crystals, which can restrict the versatility and tunability of the generated quantum states due to the fixed crystal nonlinear susceptibility. Here we present a solution using a nonlinear metasurface incorporating multiplexed silica metagratings on a lithium niobate film of 300 nanometer thickness. We fabricate two orthogonal metagratings on a single substrate with an identical resonant wavelength, thereby enabling the spectral indistinguishability of the emitted photons, and demonstrate in experiments that the two-photon polarization states can be shaped by the metagrating orientation. Leveraging this essential property, we formulate a theoretical approach for generating arbitrary polarization-entangled qutrit states by combining three metagratings on a single metasurface, allowing the encoding of desired quantum states or information. Our findings enable miniaturized optically controlled quantum devices using ultrathin metasurfaces as polarization-entangled photon sources.