and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and biolight, which holds promise for electricity-free quantum communications in outer space.

DOI: 10.1103/physrevapplied.19.054079

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S=2.33\ifmmode\pm\else\textpm\fi{}0.097

, obviously surpassing the classical bound

S=2

S=2.33\pm 0.097

, obviously surpassing the classical bound

S=2

and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and bio-light, which holds promise for electricity-free quantum communications in outer space.