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Decoherence leads to the loss of quantum correlations in entangled systems due to interactions with surrounding matter. While entanglement and its decoherence is well established in the optical photonic regime, its behaviour at the MeV energy scale, such as $\gamma$-ray pairs produced in positron annihilation, is far less well established and with only limited experimental evidence to date.
This question of the fundamental nature of entanglement and decoherence is also foundational for developing the next generation of Positron Emission Tomography (PET), where annihilation photons are used for image reconstruction. Image quality is degraded by scattering and random coincidences, reducing spatial resolution and accuracy [1].
Evidence that entanglement of annihilation gamma is robust, at least for single Compton scattering processes [2], challenges the previous expectation of rapid decoherence.
This motivates the use of Compton polarimetry to access polarisation dependent scattering correlations. Advances in highly segmented $\gamma$-ray detector systems [3, 4] have demonstrated sensitivity to photon polarisation via azimuthal scattering distributions, opening a path for potential applications in PET imaging.
The current study aims to investigate the evolution of entanglement in MeV-scale $\gamma$-ray pairs, focusing on double and triple Compton scattering of annihilation photons with potential to progress to higher order scattering. The data were collected using the GRIFFIN spectrometer at TRIUMF-ISAC [5], combined with an array of Double-sided Silicon Strip Detectors (DSSD) for coincidence measurements. Compton polarimetry is used to probe the effects of entanglement on polarisation dependent correlations following prior sequential scattering events. Preliminary results will be presented and the prospects for the project to better constrain the nature of decoherence over a broad kinematic range will be outlined.
References:
[1] D.P. Watts, J. Bordes, J.R. Brown, A. Cherlin, R. Newton, J. Allison, M. Bashkanov, N. Efthimiou and N. A. Zachariou, Photon quantum entanglement in the MeV regime and its application in PET imaging, Nat. Commun., 12, 2646 (2021)
[2] J. Bordes et al, First Detailed Study of the Quantum Decoherence of Entangled Gamma Photons, Phys. Rev. Lett. 133, 132502 (2024)
[3] B. Alikhani, A. Givechev, A. Heinz, P.R. John, J. Leske, M. Lettmann, O.Möller, N. Pietralla, C. Röder, Compton polarimetry with a 36-fold segmented HPGe-detector of the AGATA-type,
Nucl. Instrum. Methods Phys. Res. A, 675 (2012)
[4] C. Morse, H.L. Crawford, A.O. Macchiavelli, A. Wiens et al., The polarization sensitivity of GRETINA, Nucl. Instrum. Methods Phys. Res. A, 1025, 166155 (2022)
[5] A.B. Garnsworthy, C.E. Svensson, M. Bowry, R. Dunlop, A.D. MacLean, B. Olaizola, J.K. Smith et al., The GRIFFIN facility for Decay-Spectroscopy studies at TRIUMF-ISAC, Nucl. Instrum. Methods Phys. Res. A, 918 (2019)