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Physical Review Letters

Polarization Correlation of the Two Photons Emitted by Metastable Atomic Deuterium: A Test of Bell's Inequality

Walter Perrie, Alan J. Duncan, H. J. Beyer, Hans Kleinpoppen

Published Apr 22, 1985• 152 citations
Publisher

Abstract

The polarization correlation of the two photons emitted simultaneously by metastable atomic deuterium in a true second-order decay process has been measured for the first time. The results are in agreement with quantum mechanics and violate Bell's inequality by nearly 2 standard deviations in an experiment where the process of absorption and reemission is not important.

DOI
10.1103/physrevlett.54.1790

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Measurements of the polarization correlation of the two-photons emitted simultaneously by metastable atomic deuterium in a true second order radiative process, are described. Two experiments, one involving the use of a half-wave plate and two linear polarizers, and the other three linear polarizers. have been carried out to investigate the validity of quantum mechanics in hitherto unexplored situations and to test local realistic theories of the type recently proposed by Garucclo and Selleri. The result, of the experiment. provide clear evidence against local realistic theories in which the probability of detection is postulated to be dependent on the angle between a polarization vector I. and a detection vector λ- The quantum mechanical predictions for such experiments, in which non-ideal analysers, were employed, have also been established and have been shown to be in agreement with the experimental results. In addition. the results of circular polarization \nmeasurements confirm the conservation of angular momentum along the common axis of detection and give further verification of the correctness of the form of the state vector describing the state of polarization of the two-photons. In another experiment, the effect of an electric field on the polarization properties of the two-photons has been measured for the first time. The Stoke's parameters characterizing the two-photon beam have been determined and the results suggest that the two-photon radiation is partially polarized at 22.5o with respect to the electric field vector. It is suggested that the affect is due to the removal of the degeneracy of the virtual Intermediate P states of the deuterium atom.

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Polarization correlation in the two-photon decay of atomic hydrogen: nonlocality versus entanglement

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From the work by Perrie et al. [Phys. Rev. Lett. 54, 1790 (1985)], photon pairs from the 2s 1/2 → 1s 1/2 (two-photon) decay of atomic hydrogen are known to be quantum mechanically correlated. In these experiments, the polarization states of the photons emitted in back-to-back geometry were shown to violate the Bell inequality as a qualitative sign of nonlocality and entanglement. In the present contribution, we analyze how these nonlocal quantum correlations, as given by the violation of the Bell inequality, differ from the concurrence as a true entanglement measure. Results are shown for both quantifiers in dependence of the decay geometry and the initial polarization of the atoms for the 2s 1/2 → 1s 1/2 and 3d 5/2 → 1s 1/2 two-photon decay of atomic hydrogen. These results display the difference between nonlocality and entanglement and, hence, may stimulate further experiments on nonlocal quantum correlations in atomic systems.

The European Physical Journal D•Jul 11, 2008•5 citations
The polarization correlation of the two photons emitted simultaneously by metastable atomic deuterium in a true second-order decay process has been measured for the first time. The results are in agreement with quantum mechanics and violate Bell's inequality by nearly 2 standard deviations in an experiment where the process of absorption and reemission is not important.
A 1-keV metastable atomic deuterium [D(2S)] beam of density about 104 atoms cm-3 has been produced by charge-exchange, in caesium vapour, of deuterons extracted from a radio-frequency ion source. The linear and circular polarisation correlation of the two photons emitted spontaneously by metastable atomic deuterium in a true second order radiative process have been measured for the first time. While the circular polarisation results confirm the conservation of angular momentum along the common axis of detection, the correlation in linear polarisation has been applied in a test of Bell’s Inequality which allows a quantitative distinction to be made between the predictions of Quantum Mechanics and local realistic ("hidden variable") theories. The results are in agreement with Quantum Mechanics and violate Bell’s Inequality by almost two standard deviations.
Measurements have been made of the polarization properties of the two photons emitted in the decay of metastable atomic deuterium in an experiment in which three linear polarizers, instead of the normal two, are used. The quantum-mechanical predictions for these imperfect polarizers have been calculated and shown to be in good agreement with the experimental results. The results have also been used to test local realistic models which are capable of explaining all existing two-polarizer-type experiments, and have provided conclusive evidence against these theories.
Measurements of the polarization correlation of the two-photons emitted simultaneously by metastable atomic deuterium in a true second order radiative process, are described. Two experiments, one involving the use of a half-wave plate and two linear polarizers, and the other three linear polarizers. have been carried out to investigate the validity of quantum mechanics in hitherto unexplored situations and to test local realistic theories of the type recently proposed by Garucclo and Selleri. The result, of the experiment. provide clear evidence against local realistic theories in which the probability of detection is postulated to be dependent on the angle between a polarization vector I. and a detection vector λ- The quantum mechanical predictions for such experiments, in which non-ideal analysers, were employed, have also been established and have been shown to be in agreement with the experimental results. In addition. the results of circular polarization \nmeasurements confirm the conservation of angular momentum along the common axis of detection and give further verification of the correctness of the form of the state vector describing the state of polarization of the two-photons. In another experiment, the effect of an electric field on the polarization properties of the two-photons has been measured for the first time. The Stoke's parameters characterizing the two-photon beam have been determined and the results suggest that the two-photon radiation is partially polarized at 22.5o with respect to the electric field vector. It is suggested that the affect is due to the removal of the degeneracy of the virtual Intermediate P states of the deuterium atom.
From the work by Perrie et al. [Phys. Rev. Lett. 54, 1790 (1985)], photon pairs from the 2s 1/2 → 1s 1/2 (two-photon) decay of atomic hydrogen are known to be quantum mechanically correlated. In these experiments, the polarization states of the photons emitted in back-to-back geometry were shown to violate the Bell inequality as a qualitative sign of nonlocality and entanglement. In the present contribution, we analyze how these nonlocal quantum correlations, as given by the violation of the Bell inequality, differ from the concurrence as a true entanglement measure. Results are shown for both quantifiers in dependence of the decay geometry and the initial polarization of the atoms for the 2s 1/2 → 1s 1/2 and 3d 5/2 → 1s 1/2 two-photon decay of atomic hydrogen. These results display the difference between nonlocality and entanglement and, hence, may stimulate further experiments on nonlocal quantum correlations in atomic systems.