Two atoms 20 m apart have signaled their entanglement, a significant step forward for the study of quantum entanglement and the development of practical applications such as quantum computing and communications networks. In quantum entanglement – sometimes called “spooky action at a distance” – the quantum properties of two particles are so tightly linked, even when great distances apart, that one particle’s quantum state changes when its partner’s quantum state is measured. For entanglements to have any practical applications, scientists must understand how the entangled state first occurs. Artist’s view of the experiment: Two photons emitted by two single atoms are brought to interference on a beamsplitter; joint detection of the photons creates entanglement of the distant atoms. Courtesy of W. Rosenfeld. Now, Julian Hofmann and colleagues at Ludwig Maximilian University believe that they have devised a way for the atoms to signal their entanglement. To create this “heralded entanglement,” the team independently excited two single rubidium atoms trapped in laboratory rooms 20 m apart. Excited via short optical laser pulses, the atoms emitted single photons. Optical fibers brought the photons together at an intermediate location, where they were brought to interference on a beamsplitter. The twofold detection of the photons behind the beamsplitter projected the atoms, yielding an entangled atom-photon pair in each room. The registration of the two photons in their specific state reported that the two photons were entangled. This is important because verification at every attempt could destroy the quantum state. The findings appeared in Science (doi: 10.1126/science.1221856).