Decoherence puzzle in measurements of photons originating from electron-positron annihilation

TL;DR

This paper investigates the inconsistent experimental results regarding photon polarization entanglement from electron-positron annihilation, focusing on decoherence effects and their implications for fundamental physics and PET technology.

Contribution

It compares different experimental setups and discusses the parameters affecting polarization correlation measurements, proposing methods to resolve the decoherence puzzle.

Findings

Experiments show conflicting results on polarization correlation after decoherence.

The importance of experimental parameters in measuring polarization correlation is highlighted.

Potential impact on PET imaging if polarization correlation remains unchanged after scattering.

Abstract

Entanglement of photons originating from the electron-positron annihilation has not been proven experimentally. Though the independent experiments performed so far unanimously confirm that correlation between the linear polarizations of back-to-back photons from the electron-positron annihilation is consistent with the assumption that these photons are entangled in polarization. Yet, unexpectedly, recent experiments differ as regards the correlation of polarization direction of back-to-back photons after the decoherence induced by scattering of one of these photons on the electron in the scattering material. In one of the experiments, the correlation before and after the decoherence of the photon state is the same and in the other experiment the scattering of one photon leads to a significant decrease in this correlation. Here we discuss this puzzle. The decoherent states were ensured by forcing one of the annihilation photons through prior scattering before the Compton kinematics based measurement of the polarization correlation. A comparison between the experimental setups used for the different measurements and the results obtained are briefly discussed, highlighting the parameters that are important for performing such measurements. Finally, the main features of the J-PET detector are presented with the schemes for performing similar measurements so that the results can be used as conclusive remarks for solving this puzzle. Solving the decoherence puzzle will have crucial consequences for basic studies of entanglement, as well as for the proposed application of polarisation of photons in positron emission tomography. In case if the correlation of polarisation of back-to-back photons from the electron-positron annihilation is the same before and after the scattering of these photons, then it will not be useful for the reduction of the scatter fraction in PET diagnostics.