Julian Hofmann and colleagues have demonstrated quantum entanglement for two atoms 20 meters apart and devised a way for the atoms to signal their entanglement. The feat is a major step forward for the study of quantum entanglement and the development of practical applications such as quantum computing and communications networks. Quantum entanglement is sometimes known as “spooky action at a distance”; two particles’ quantum properties are so tightly linked, even when the particles are far apart, that one particle’s quantum state changes when its partner’s quantum state is measured. But for entanglement to have any practical applications, the scientists need to know when the entangled state first occurs. To create this “heralded entanglement,” the researchers excited two single rubidium atoms in different laboratory rooms 20 meters apart. The procedure produced an entangled atom-photon pair in each room, which was used to entangle the atoms. Detection of the photons in a specific state was the “announcement” that allowed the researchers to know the two atoms were entangled. In a related Perspective, Jürgen Volz and Arno Rauschenbeutel discuss the experiment’s implications for probing fundamental questions in quantum mechanics.
Article #8: "Heralded Entanglement Between Widely Separated Atoms," by J. Hofmann; M. Krug; N. Ortegel; L. Gérard; M. Weber; W. Rosenfeld; H. Weinfurter at Ludwig-Maximilians Universität München in München, Germany; W. Rosenfeld; H. Weinfurter at Max-Planck Institut für Quantenoptik in Garching, Germany.