We explore the possibility of detecting entangled photon pairs from two patches of the sky, such as cosmic microwave background or other cosmological sources. The measurements use two detectors with different photon polarizer directions. When two photon sources are separated by a large angle, a null test of unentangled photons can be performed. The deviation from this unentangled background is the signature of photon entanglement. To confirm whether the deviation is consistent with entangled photons, we derive the photon polarization correlation to compare with, similar to that in a Bell inequality measurement. Besides, with the use of holographic Schwinger–Keldysh correlators, we construct the CHSH (Clauser–Horne–Shimony–Holt) form of Bell inequality, and find that its violation can also be implemented in the holographic model of EPR (Einstein–Podolsky–Rosen) pair.　　

Contact Wang Shaojiang