# Experimental generation of polarization entanglement from spontaneous parametric down-conversion pumped by spatiotemporally highly incoherent light

**Authors:** Cheng Li, Boris Braverman, Girish Kulkarni, Robert W. Boyd  
**arXiv:** [2210.16229](https://arxiv.org/abs/2210.16229)  
**Date:** 2022-10-28  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2210.16229v1/

## Key points

- Spontaneous parametric down-conversion (SPDC) is a fundamental process in quantum optics
- SPDC has practical applications in controlled generation of entangled states
- Pump coherence in a specific degree of freedom (DOF) sets an upper limit on generated entanglement in the same DOF without postselection
- Cross-influence of pump coherence on generated entanglement in a different DOF is poorly understood
- Researchers experimentally investigate the effect of using a spatiotemporally highly-incoherent (STHI) light-emitting diode (LED) pump on polarization entanglement generated through SPDC
- STHI sources are more accessible than lasers and offer a wider range of available wavelengths
- Quantum state tomography measurements using multimode collection fibers are performed to assess the impact of STHI pumps on polarization entanglement
- Experimental results show reduced entanglement and purity with STHI pumps compared to theoretically predicted values
- Despite reduced entanglement, SPDC with STHI pumps remains valuable due to the availability of STHI sources at various wavelengths and robustness to scattering in long-distance quantum communication schemes.

## Summary

The generation of entangled states through spontaneous parametric down-conversion (SPDC) is a fundamental process in quantum optics and has practical applications in controlled generation of entangled states. Understanding the influence of pump coherence on the entanglement produced in SPDC is crucial for both theoretical understanding and practical implementation. It is known that the pump coherence in a specific degree of freedom (DOF) sets an upper limit on the generated entanglement in the same DOF when postselection is not applied. However, the cross-influence of pump coherence on the generated entanglement in a different DOF remains poorly understood. In this study, conducted by Cheng Li, Boris Braverman, Girish Kulkarni, and Robert W. Boyd, the researchers experimentally investigate the effect of using a spatiotemporally highly-incoherent (STHI) light-emitting diode (LED) pump on polarization entanglement generated in SPDC. The choice to use an STHI source as a pump is motivated by its ubiquity and wider range of available wavelengths compared to lasers. To assess the impact of STHI pumps on polarization entanglement, quantum state tomography measurements are performed using multimode collection fibers to minimize postselection effects. The experimental results reveal that the output polarization two-qubit state exhibits reduced entanglement and purity when an STHI pump is used. Specifically, a concurrence value of 0.531+/-0.006 and a purity value of 0.647+/-0.005 are obtained, which closely align with the theoretically predicted values of 0.536 for concurrence and 0.643 for purity. Despite observing reduced entanglement and purity with STHI pumps, this study highlights two important reasons why SPDC with STHI pumps remains valuable: First, STHI sources are readily available at various wavelengths, making them more accessible than lasers. Second, the polarization-entangled two-photon states generated using STHI pumps exhibit robustness to scattering, suggesting potential utility in long-distance quantum communication schemes. In summary, this research provides experimental insights into the influence of pump coherence on polarization entanglement generated through SPDC.