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arXiv:quant-ph/0607139 · Physical Review Letters

Photon Polarization Entanglement Induced by Biexciton: Experimental Evidence for Violation of Bell’s Inequality

Goro Oohata, Ryōsuke Shimizu, Keiichi Edamatsu

Published Apr 3, 2007• 53 citations
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Abstract

We have investigated the polarization entanglement between photon pairs generated from a biexciton in a CuCl single crystal via resonant hyperparametric scattering. The pulses of a high repetition pump are seen to provide improved statistical accuracy and the ability to test Bell's inequality. Our results clearly violate the inequality and thus manifest the quantum entanglement and nonlocality of the photon pairs. We also analyzed the quantum state of our photon pairs using quantum state tomography.

DOI
10.1103/physrevlett.98.140503

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We have investigated the polarization entanglement between photon pairs generated from a biexciton in a CuCl single crystal via resonant hyperparametric scattering. The pulses of a high repetition pump are seen to provide improved statistical accuracy and the ability to test Bell's inequality. Our results clearly violate the inequality and thus manifest the quantum entanglement and nonlocality of the photon pairs. We also analyzed the quantum state of our photon pairs using quantum state tomography.
We have investigated the excitation-power dependence of polarization-entangled photon pairs generated from a CuCl single crystal using biexciton resonant hyper-parametric scattering. The two-photon polarization density matrix of photon pairs generated from a CuCl single crystal maintained at 10 K was measured under different excitation powers. The measured two-photon polarization density matrix, which corresponds to the two-photon polarization state, changes with an increase in the excitation power. The evaluations of the tangle and the linear entropy obtained from the measured density matrix indicate that the two-photon polarization density matrix can be expressed as the mixed state of an ideal state and a totally mixed state, and the mixture ratio of the totally mixed state increases as the excitation density increases. The variations of the density matrix and the mixture ratio with the excitation power originate from the generation of the multiple photon pairs. Furthermore, a numerical calculation considering simultaneous detection efficiency was able to reproduce the experimental results.
Conventional tests of Bell’s inequality rely on entangled photon pairs. Here, we replace entangled pairs with two independent photons of orthogonal polarization, and demonstrate that Bell’s inequality is still violated. Given the inherent local realism of independent photons, this experiment proves that Bell’s inequality cannot falsify the local realism of photons. We thus conjecture that the violation of Bell’s inequality by entangled photon pairs originates from their orthogonal polarizations, rather than the breakdown of local realism. To interpret this unexpected violation with independent photons, we further substitute the two photons with two monochromatic light beams, and calculate the transmittance correlation through polarizers via Malus’s law and Karl Pearson’s correlation formula. We show that this correlation also defies Bell’s inequality. Retracing the derivation of Bell’s inequality reveals its validity is restricted to binary events, which accounts for the observed violation with light beams. Finally, we propose a thought experiment involving gradual attenuation of light intensity down to the single-photon regime, and hypothesize that single-photon transmission through a polarizer does not constitute a binary event. This hypothesis provides a unified interpretation for both our experimental findings and all canonical Bell inequality tests reported to date.