Versatile cluster entangled light

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Science  28 Oct 2016:
Vol. 354, Issue 6311, pp. 416-417
DOI: 10.1126/science.aai8675

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Performing a quantum computation may seem complicated, but it can be done with a specially prepared beam of light and good photodetectors. On page 434 of this issue, Schwartz et al. (1) report on a prototype device that uses semiconductor quantum dots that generate long strings of photons in an entangled cluster state of light. Cluster states (2) carry a specific sort of entanglement—that is, the way in which the properties of different photons are correlated. In two dimensions—when the entangling connections form a net or lattice—cluster states become a universal resource; they can be used for all kinds of quantum information processing and are like a fuel for quantum computers. The entanglement can be realized between the polarizations of individual photons—the directions in which their individual electric field vectors point. A quantum calculation (3) can be run by measuring the polarization of each photon, one by one, in a specific order and direction, almost as simple as moving the beads of an abacus.