The state of a two-particle system is said to be entangled when its quantum-mechanical wavefunction cannot be factorized into two single-particle wavefunctions. This leads to one of the strongest counter-intuitive features of quantum mechanics, namely non-locality1,2. Experimental realization of quantum entanglement is relatively easy for photons; a starting photon can spontaneously split into a pair of entangled photons inside a nonlinear crystal. Here we investigate the effects of nanostructured metal optical elements3 on the properties of entangled photons. To this end, we place optically thick metal films perforated with a periodic array of subwavelength holes in the paths of the two entangled photons. Such arrays convert photons into surface-plasmon waves—optically excited compressive charge density waves—which tunnel through the holes before reradiating as photons at the far side4,5,6,7. We address the question of whether the entanglement survives such a conversion process. Our coincidence counting measurements show that it does, so demonstrating that the surface plasmons have a true quantum nature. Focusing one of the photon beams on its array reduces the quality of the entanglement. The propagation of the surface plasmons makes the array effectively act as a ‘which way’ detector.
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We thank A. van Zuuk and E. van der Drift for the production of the hole arrays, and G. Nienhuis for theoretical discussions. This work was supported by the Stichting voor Fundamenteel Onderzoek der Materie (FOM), and the European Union under the IST-ATESIT contract.
The authors declare that they have no competing financial interests.
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Altewischer, E., van Exter, M. & Woerdman, J. Plasmon-assisted transmission of entangled photons. Nature 418, 304–306 (2002). https://doi.org/10.1038/nature00869
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