High-performance quantum entanglement generation via cascaded second-order nonlinear processes

TL;DR

This paper demonstrates a high-performance, multi-degree-of-freedom entangled photon-pair source using cascaded nonlinear processes in a PPLN waveguide, achieving high fidelity and Bell inequality violations.

Contribution

The work introduces a novel integrated PPLN waveguide system with noise filtering to generate high-quality entangled photon pairs in multiple degrees of freedom.

Findings

Achieved a CAR higher than 52,600 with a photon-pair rate of 52.3 kHz.

Generated energy-time, frequency-bin, and time-bin entanglement with high fidelity and Bell inequality violations.

Demonstrated potential for quantum photonics applications with a compact, efficient source.

Abstract

In this paper, we demonstrate the generation of high-performance entangled photon-pairs in different degrees of freedom from a single piece of fiber pigtailed periodically poled LiNbO$_3$ (PPLN) waveguide. We utilize cascaded second-order nonlinear optical processes, i.e. second-harmonic generation (SHG) and spontaneous parametric down conversion (SPDC), to generate photon-pairs. Previously, the performance of the photon pairs is contaminated by Raman noise photons from the fiber pigtails. Here by integrating the PPLN waveguide with noise rejecting filters, we obtain a coincidence-to-accidental ratio (CAR) higher than 52,600 with photon-pair generation and detection rate of 52.3 kHz and 3.5 kHz, respectively. Energy-time, frequency-bin and time-bin entanglement is prepared by coherently superposing correlated two-photon states in these degrees of freedom, respectively. The energy-time entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S=2.708$\pm$0.024 with a two-photon interference visibility of 95.74$\pm$0.86%. The frequency-bin entangled two-photon states achieve fidelity of 97.56$\pm$1.79% with a spatial quantum beating visibility of 96.85$\pm$2.46%. The time-bin entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S=2.595$\pm$0.037 and quantum tomographic fidelity of 89.07$\pm$4.35%. Our results provide a potential candidate for quantum light source in quantum photonics.