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arXiv:2604.25034

Bell Test of Photons from Electron-Positron Annihilation via POVM-based Compton Polarimetry

Jack Clarke, Preslav Asenov, Jesse Smeets, Jia-Shian Wang, David B. Cassidy, Alessio Serafini

quant-phhep-exhep-phhep-thSubmitted Apr 27, 2026• Announced Apr 29, 2026• 0 citations
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Abstract

Quantum entanglement between gamma-ray photons emitted following electron-positron annihilation is expected to be maximal and may be characterized via non-classical polarization correlations. However, this is difficult to verify experimentally because there are no established schemes that approach ideal projective-polarization measurements for high-energy photons. Hence, polarization entanglement between MeV-scale annihilation photons has not yet been conclusively demonstrated. We develop here a framework that models polarization measurements of high-energy photons via Compton polarimetry, employing the formalism of positive operator-valued measures (POVMs). We extend the POVM description to sequences of repeated interactions and show that the measurement converges toward an ideal projective measurement of linear polarization as the number of interactions increases. We demonstrate that this progressive improvement in measurement sharpness can enable the experimental violation of CHSH inequalities.

Comments
Main: 8 pages, 2 figures. Supplemental: 11 pages

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Quantum entanglement between gamma-ray photons emitted following electron-positron annihilation is expected to be maximal and may be characterized via non-classical polarization correlations. However, this is difficult to verify experimentally because there are no established schemes that approach ideal projective-polarization measurements for high-energy photons. Hence, polarization entanglement between MeV-scale annihilation photons has not yet been conclusively demonstrated. We develop here a framework that models polarization measurements of high-energy photons via Compton polarimetry, employing the formalism of positive operator-valued measures (POVMs). We extend the POVM description to sequences of repeated interactions and show that the measurement converges toward an ideal projective measurement of linear polarization as the number of interactions increases. We demonstrate that this progressive improvement in measurement sharpness can enable the experimental violation of CHSH inequalities.
We present a new experimental study of the quantum entanglement of photon pairs produced in positron-electron annihilation at rest. Each annihilation photon has an energy that is five orders of magnitude higher than the energy of photons in optical experiments. It provides a unique opportunity for controlled Compton pre-scattering of initial photons before the polarization measurements. The experimental setup includes a system of Compton polarimeters to measure the angular correlations of annihilation photons in initial and thus prepared pre-scattered states. For the first time, a direct comparison of the polarization correlations of initial and pre-scattered annihilation photons has been carried out. The angular distributions of scattered in polarimeters photons turned out to be the same for both types of events. Moreover, the correlation function in the Bell's inequality is also the same for both states. We discuss the implications of our results for quantum measurement theory and for the quantum-entangled positron emission tomography.
We present a theoretical study of the evolution of polarization entanglement and quantum coherence in 511 keV photon pairs produced by para-positronium decay during successive Compton scattering events. We start with a maximally entangled Bell state and employ the generalized Stokes-Mueller formalism to derive the two-photon density matrix following single-, double-, and triple-Compton scattering, explicitly considering both polar and azimuthal scattering geometries. Using this framework, we quantify the degradation of quantum correlations through concurrence (as a measure of entanglement) and the l1l_1l1​-norm (as a measure of coherence). Our results demonstrate that entanglement is highly sensitive to the scattering geometry and disappears near right-angle scattering, while quantum coherence remains finite even in regimes where entanglement vanishes completely. These findings provide a unified description of polarization-dependent decoherence in annihilation photon pairs and clarify the distinct roles of entanglement and coherence in realistic two-photon interactions. These results are relevant for quantum-enhanced positron emission tomography and highlight the persistence of quantum resources in scattering-dominated media.
Abstract The experimental setup for study of Compton scattering of entangled and decoherent annihilation gammas is described. These gammas are born in two-photon positron-electron annihilation. Polarizations of annihilation gammas are mutually orthogonal, while the polarization of each photon is indefinite. If one of the photons of the pair interacts with matter, the pair becomes decoherent and photons acquire the determined polarization states. The kinematics of Compton scattering depends on the polarization states and can be different for entangled and decoherent pairs of annihilation photons. The angular correlations of scattered gammas for both, entangled and decoherent states are measured and compared. It is shown, that the kinematics of Compton scattering are similar for both cases.
Abstract An experimental setup for studying the Compton scattering of annihilation photons in various (entangled and decoherent) quantum states is presented. Two entangled γ-quanta with an energy of 511 keV and mutually orthogonal polarizations are produced by positron-electron annihilation in a thin aluminum plate and are emitted in opposite directions. To measure both photons, the setup provides two equivalent arms of Compton polarimeters. A Compton polarimeter consists of a plastic scintillation scatterer and an array of NaI(Tl) detectors for measuring photons deflected at an angle of 90°. The intermediate scatterer of the GAGG scintillator with SiPM readout is inserted into one of the arms to create a tagged decoherence process prior to the measurement of annihilation photons in polarimeters. The performance of Compton scatterers and NaI(Tl) counters is discussed. The polarization modulation factor and the analyzing power of Compton polarimeters are evaluated from the angular distributions of scattered gammas. The Compton scattering of photons in entangled and decoherent states is compared reliably for the first time.
The entanglement of photons originating from electron-positron annihilation has not been experimentally proven.owever, the independent experiments performed so far unanimously confirm that the correlation between the linear polarizations of back-to-back photons from electron-positron annihilation is consistent with the assumption that these photons are entangled in the polarization.Yet, unexpectedly, recent experiments differ as regards the correlation of polarization direction of back-to-back photons after the decoherence induced by the 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.Decoherent states were ensured provided by forcing one of the annihilation photons to scatter earlier before measuring the polarization correlation based on Compton kinematics.The comparison is made between the experimental setups used for the different measurements, and the results obtained are briefly discussed, highlighting the parameters that are important in performing such measurements.Finally, the main features of the J-PET detector are presented, along with the schemes for performing similar studies, so that the conclusive results can be used as remarks to solve the puzzle in question.Solving the decoherence puzzle will have crucial consequences for basic studies of entanglement, as well as for the proposed application of the photon polarization in positron emission tomography.If the correlation of the polarization 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 scatter fraction in positron emission tomography diagnostics.
We present the concept and development of a novel accelerator-based source of high-intensity quantum-entangled 511 keV gamma-ray pairs produced through positron-electron annihilation. The source leverages the unique capabilities of the proposed Jefferson Lab positron facility to generate polarized, high-current positron beams with a well-defined time structure. These beams enable the production of entangled annihilation photons at intensities far exceeding those available from conventional radioactive sources. The resulting gamma-ray pairs can be characterized using Compton polarimetry techniques, providing a powerful platform for precision studies of quantum entanglement. The combination of high intensity, controllable polarization, and precise timing also opens new opportunities in medical imaging, materials science, quantum information science, and spintronics. Compared with traditional radioactive sources, the proposed system offers orders-of-magnitude higher photon flux, tunable beam parameters, and unprecedented control of the annihilation process. We discuss the source design, methods for entanglement characterization, potential applications, and future development directions.