Two Atoms Announce Their Long-Distance Relationship

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Science  06 Jul 2012:
Vol. 337, Issue 6090, pp. 40-41
DOI: 10.1126/science.1224202

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Entanglement is a counterintuitive property of quantum mechanics that puts it at odds with our classical view of the world. For example, an entangled state of two photons can be created such that each single photon is unpolarized—it will always pass through a polarization analyzer (e.g., a filter) with 50% probability no matter how the analyzer is oriented. However, once one of the two photons has passed through its analyzer, the second photon becomes polarized, and its orientation will then be orthogonal to the first. Such effects are not only interesting from a fundamental point of view: If the two entangled particles are shared between two distant parties, the perfect quantum correlations can be used to realize a so-called quantum channel over which quantum information can be transmitted. However, to exploit these schemes in long-distance quantum communication, the generation of entanglement over macroscopic distances must be “heralded”—a separate signal must verify that the entangled state was created. On page 72 of this issue, Hofmann et al. (1) describe the accomplishment of this goal with two individual atoms held in laser traps and separated by a distance of 20 m.