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arXiv:quant-ph/0209158 · Physical Review A

Physical limitations on quantum nonlocality in the detection of photons emitted from positron-electron annihilation

Victor D. Irby

Published Mar 17, 2003• 13 citations
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Abstract

Recent experimental measurements of the time interval between detection of the two photons emitted in positron-electron annihilation have indicated that collapse of the spatial part of the photon's wave function, due to detection of the other photon, does not occur. Although quantum nonlocality actually occurs in photons produced through parametric down-conversion, the recent experiments give strong evidence against measurement-induced instantaneous spatial localization of high-energy $\ensuremath{\gamma}$ photons. A quantum-mechanical analysis of the Einstein-Podolsky-Rosen problem is presented, which may help to explain the observed differences between photons produced through parametric down-conversion and photons produced through positron-electron annihilation. The results are found to concur with the recent experiments involving $\ensuremath{\gamma}$ photons.

DOI
10.1103/physreva.67.034102

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Recent experimental measurements of the time interval between detection of the two photons emitted in positron-electron annihilation have indicated that collapse of the spatial part of the photon's wave function, due to detection of the other photon, does not occur. Although quantum nonlocality actually occurs in photons produced through parametric down-conversion, the recent experiments give strong evidence against measurement-induced instantaneous spatial localization of high-energy \ensuremathγ\ensuremath{\gamma}\ensuremathγ photons. A quantum-mechanical analysis of the Einstein-Podolsky-Rosen problem is presented, which may help to explain the observed differences between photons produced through parametric down-conversion and photons produced through positron-electron annihilation. The results are found to concur with the recent experiments involving \ensuremathγ\ensuremath{\gamma}\ensuremathγ photons.
Recent experimental measurements of the time interval between detection of the two photons emitted in positron-electron annihilation have indicated that collapse of the spatial part of the photon's wave function, due to detection of the other photon, does not occur. Although quantum nonlocality actually occurs in photons produced through parametric down-conversion, the recent experiments give strong evidence against measurement-induced instantaneous spatial localization of high-energy $\ensuremath{\gamma}$ photons. A quantum-mechanical analysis of the Einstein-Podolsky-Rosen problem is presented, which may help to explain the observed differences between photons produced through parametric down-conversion and photons produced through positron-electron annihilation. The results are found to concur with the recent experiments involving $\ensuremath{\gamma}$ photons.
We discuss violation of Bell inequalities by the regularized Einstein-Podolsky-Rosen (EPR) state, which can be produced in a quantum optical parametric down-conversion process. We propose an experimental photodetection scheme to probe nonlocal quantum correlations exhibited by this state. Furthermore, we show that the correlation functions measured in two versions of the experiment are given directly by the Wigner function and the Q function of the EPR state. Thus, the measurement of these two quasidistribution functions yields a novel scheme for testing quantum nonlocality.
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.
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.
In recent years parametric-down-conversion photon sources have been proposed and used to test local realism via Bell-type inequalities. This study shows that this technique fails to discriminate between quantum mechanics and local realism in all those tests in which polarization correlations are measured, since dichotomic observables cannot be associated satisfactorily with the state produced in these experiments. A more adequate description of these photon sources leads to an alternative inequality for Einstein locality, which is always satisfied by quantum-mechanical predictions. The result can be extended to all the experiments in which only one-half of the emitted pairs are detected.
Einstein, Podolsky, and Rosen pointed out that the quantum-mechanical description of “physical reality” implies an unphysical, instantaneous action between distant measurements. To avoid such an action at a distance, these three concluded that quantum mechanics had to be incomplete. However, its extensions involving additional “hidden variables,” allowing for the recovery of determinism and locality, have been disproved experimentally (Bell’s theorem). In this paper, an opposite solution of the paradox is presented, based on the greater indeterminism of the modern quantum field theory (QFT) description of particle physics, which prevents the preparation of any state having a definite number of particles. The resulting uncertainty in photon radiation has interesting consequences in quantum information theory (e.g., cryptography and teleportation). Moreover, since it allows for fewer elements of Einstein-Podolsky-Rosen (EPR) physical reality than the old non-relativistic quantum mechanics, QFT satisfies the EPR condition of completeness without the need for hidden variables. The residual physical reality never violates locality; thus, the unique objective proof of “quantum nonlocality” is removed in an interpretation-independent way. At the same time, the supposed nonlocality of the EPR correlations turns out to be a problem in interpretation of the measurement process. If we do not rely on hidden variables or new physics beyond QFT, the viable interpretation is a minimal statistical one, which preserves locality and Lorentz symmetry.