# Polarization Entanglement from Parametric Down-Conversion with a LED Pump

**Authors:** Wuhong Zhang, Diefei Xu, Lixiang Chen  
**arXiv:** [2211.00841](https://arxiv.org/abs/2211.00841)  
**Date:** 2022-11-02  
**Summary page:** https://www.summarizepaper.com/en/paper/2211.00841v1/

## Key points

- Spontaneous parametric down-conversion (SPDC) is a method for generating entangled photon pairs
- Researchers have successfully generated polarization entangled photon pairs using a commercial LED as the pump source
- LED sources have low spatial coherence, which is usually considered unsuitable for SPDC
- Specific frequencies and wavelengths of LED light are allowed to participate in the down-conversion process due to type-II phase-matching condition
- The researchers achieved a Bell value of $S=2.33\pm 0.097$, surpassing the classical bound of $S=2$
- This opens up possibilities for generating polarization entanglement using natural light sources like sunlight or bio-light
- Promising applications for electricity-free quantum communications in outer space where laser-based systems may not be feasible
- Funding was provided by organizations including the National Natural Science Foundation of China and Xiamen University's Fundamental Research Funds

## Summary

Spontaneous parametric down-conversion (SPDC) is a well-established method for generating entangled photon pairs. Typically, a coherent laser beam is used as the pump beam to initiate the down-conversion process in a nonlinear crystal. However, in a recent breakthrough, researchers have successfully generated polarization entangled photon pairs using a commercial light-emitting diode (LED) as the pump source. This result is counterintuitive because LED sources have extremely low spatial coherence, which would normally be considered unsuitable for SPDC. Spatial coherence refers to the degree of correlation between different points in space and it plays an essential role in determining the quality of entanglement generation. Nevertheless, due to the type-II phase-matching condition inherent in SPDC only specific frequencies and wavelengths of LED light are allowed to participate in the down-conversion process. This selective filtering ensures that localized polarization Bell states can still be generated even when there is global incoherence over the entire transverse plane. In their experiment, the researchers characterized the degree of polarization entanglement induced by LED light using the standard framework of Bell inequality violation and achieved a Bell value of $S=2.33\pm 0.097$, clearly surpassing the classical bound of $S=2$ and providing strong evidence for quantum entanglement. The implications of this work are significant as it opens up possibilities for generating polarization entanglement using other natural light sources such as sunlight or bio-light with promising applications for electricity-free quantum communications in outer space where traditional laser-based systems may not be feasible. The researchers acknowledge previous studies on SPDC and related topics [15][16] and express gratitude to various individuals for their contributions and support throughout this project. Funding for this research was provided by several organizations including the National Natural Science Foundation of China and Xiamen University's Fundamental Research Funds. Overall, this study demonstrates the potential of using LED sources for generating polarization entanglement and paves the way for further advancements in quantum communication technologies.