The quantum phenomenon known as entanglement keeps spreading its arms to hold ever more particles in its spooky embrace.
Quantum entanglement is an effect through which multiple particles share correlated properties—across arbitrarily large distances—that snap into place instantaneously. For instance, a pair of entangled photons in different locations might be joined by their polarizations, a property that describes the orientation of a light wave’s oscillation. Measure one photon’s polarization, and the polarization of the other instantly assumes the same value. In other words, the photons are either both horizontally polarized or both vertically polarized, but neither assumes a definite value until one or the other is measured.
If that strikes you as more than a little counterintuitive, you’re in good company. Albert Einstein once disparaged quantum entanglement as “spooky action at a distance.” As he and his colleagues wrote in 1935, “No reasonable definition of reality could be expected to permit this.” Reasonable or no, entanglement indeed appears to be a part of reality, as numerous experiments have demonstrated.
Now, experimenters at the University of Science and Technology of China (USTC) in Shanghai have entangled not one but four pairs of photons, linking the polarizations of eight photons. The achievement, described in a study published online February 12 in Nature Photonics, extends the range of previous experiments that had entangled up to six photons. (Scientific American is part of Nature Publishing Group.) Even larger ensembles have been entangled using individual atoms as the particle of choice, but entangled photons hold much promise for quantum communication schemes, since they can carry messages across large
distances.
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