Heralded Entanglement Between Widely Separated Atoms

Quantum entanglement between particles lies at the core of quantum mechanics. For practical applications such as long-distance quantum communications, cryptography, and quantum networks, entanglement must be heralded. That is, a signal flags the existence of the entangled state so that it can be manipulated and transferred. Hofmann et al. (p. [72][1]; see the Perspective by [ Volz and Rauschenbeutel ][2]) excited trapped single atoms in different laboratory rooms 20 meters apart and were able to show that manipulation, interference, and detection of the photons from the excited atoms could signal when the atoms are entangled. The extent of the observed entanglement may be sufficient to start to probe fundamental questions relating to the entanglement process and thus address the foundations of quantum mechanics. [1]: /lookup/doi/10.1126/science.1221856 [2]: /lookup/doi/10.1126/science.1224202