Polarization Entanglement with highly non-degenerate photon pairs enhanced by effective walk-off compensation method

TL;DR

This paper reports on generating highly non-degenerate polarization-entangled photon pairs with high visibility and rate, using a novel birefringent crystal wedge compensation method to address walk-off issues, for quantum communication applications.

Contribution

The introduction of an effective birefringent crystal wedge compensation method to eliminate spatial and temporal walk-offs in non-degenerate entangled photon pairs.

Findings

Achieved a coincidence rate of 33.33 kHz with high entanglement visibility.

Generated pairs at a rate of approximately 2.92 MHz with 96.6% visibility.

Demonstrated suitability for ground-to-satellite quantum key distribution.

Abstract

We demonstrate polarization entanglement in highly non-degenerate photon pairs, generated through Type-0 spontaneous parametric down conversion (SPDC) using bulk periodically poled Lithium Niobate (PPLN) crystals. Through the utilization of both a beam displacer interferometer scheme and a Sagnac interferometer, we ensure high polarisation contrast and stable interference of the highly non-degenerate photon pairs, which however causes substantial spatial and temporal walk-offs of the photon paths which poses a formidable challenge. We introduce an effective compensation method using birefringent crystal wedges to eliminate spatial and temporal walkoffs simultaneously. This method is implemented in our entangled photon source (EPS) designed specifically for testing entanglement-based quantum key distribution (EBQKD) between ground and satellite, as part of the Quantum Encryption and Science Satellite (QEYSSat) mission funded by the Canadian Space Agency (CSA). We observed a coincidence rate of N = (33.33+-0.05)kHz, a significant improvement compared to the absence of the spatial compensation. We also observed an estimated pair generation rate of (2.92+-0.12)MHz and an entanglement visibility of (96.6+-0.3)% from only 1.0mW of pump power, making it a promising source for long-distance quantum communication for ground-to-satellite and fiber optic links.