Polarization entanglement enabled by orthogonally stacked van der Waals NbOCl2 crystals

TL;DR

This paper demonstrates a method to generate tunable polarization-entangled photon pairs using twisted stacking of NbOCl2 van der Waals crystals, enabling new control over quantum light states at the nanoscale.

Contribution

It introduces a novel approach to produce and control polarization entanglement in vdW crystals through twist-stacking, overcoming previous limitations.

Findings

Tunable polarization entanglement achieved in NbOCl2 vdW crystals.

Twist-stacking enables control over entanglement states.

New nanoscale method for engineering quantum light states.

Abstract

Polarization entanglement holds significant importance for photonic quantum technologies. Recently emerging subwavelength nonlinear quantum light sources, e.g., GaP and LiNbO3 thin films, benefiting from the relaxed phase-matching constraints and volume confinement, has shown intriguing properties, such as high-dimensional hyperentanglement and robust entanglement anti-degradation. Van der Waals (vdW) NbOCl2 crystal, renowned for its superior optical nonlinearities, has emerged as one of ideal candidates for ultrathin quantum light sources [Nature 613, 53 (2023)]. However, polarization-entanglement is inaccessible in NbOCl2 crystal due to its unfavorable nonlinear susceptibility tensor. Here, by leveraging the twist-stacking degree of freedom inherently in vdW systems, we showcase the preparation of tunable polarization entanglement and quantum Bell states. Our work not only provides a new and tunable polarization-entangled vdW photon-pair source, but also introduces a new knob in engineering the entanglement state of quantum light at the nanoscale.