Physicists usually build entanglement from the ground up, by forcing multiple atoms, ions, or photons into close contact. Now, following up on a 2003 proposal by Anders Sørensen and Klaus Mølmer, researchers at the Max Planck Institute of Quantum Optics in Germany have sculpted their way to entanglement by removing undesired components of the wavefunction that describes two atoms.

Stephan Welte and colleagues loaded a pair of rubidium atoms into a 0.5-mm-long cavity and optically pumped both atoms into a given state—for example, spin down. A laser pulse then rotated the atoms, creating a superposition of up and down states in each atom but still no special relationship between the atoms. To initiate entanglement, the researchers injected linearly polarized photons to bounce around the cavity. (In the illustration here, photons are shown in blue and atoms in red; the inset is of a fluorescence image from a trial.) A measured...

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