# Towards Searching for Entangled Photons in the CMB Sky

## TL;DR

This paper investigates the potential for detecting entangled photon pairs originating from the cosmic microwave background or other cosmological sources by analyzing polarization correlations in sky patches, aiming to identify signatures of quantum entanglement.

## Contribution

It proposes a novel method for searching for entangled photons in the CMB sky using polarization correlation measurements and null tests, extending quantum entanglement detection to cosmological scales.

## Key findings

- A null test can distinguish unentangled from entangled photon sources.
- Photon polarization correlations are derived to compare with Bell inequality expectations.
- Coincidence measurements are unlikely to violate Bell inequalities in this context.

## Abstract

We explore the possibility of detecting entangled photon pairs from cosmic microwave background or other cosmological sources coming from two patches of the sky. The measurements use two detectors with different photon polarizer directions. When two photon sources are separated by a large angle relative to the earth, such that each detector has only one photon source in its field of view, a null test of unentangled photons can be performed. The deviation from this unentangled background is, in principle, the signature of photon entanglement. To confirm whether the deviation is consistent with entangled photons, we derive a photon polarization correlation to compare with, similar to that in a Bell inequality measurement. However, since photon coincidence measurement cannot be used to discriminate unentangled cosmic photons, it is unlikely that the correlation expectation value alone can violate Bell inequality to provide the signature for entanglement.