For the first time, two atoms 20 meters 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 referred to as “spooky action at a distance” — two particles’ quantum properties are so tightly linked, even when they are 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 need to 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 beam splitter. Joint detection of the photons creates entanglement of the distant atoms. (Image: W. Rosenfeld) Now, Julian Hofmann and colleagues at Ludwig Maximilian University believe 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 different laboratory rooms 20 m apart. The atoms were excited via short optical laser pulses, which emitted single photons. Optical fibers brought these photons together at an intermediate location, where they are interfered 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 (heralded) to the researchers that the two atoms were entangled. This is important because verification at every attempt could destroy the quantum state. The findings appeared in today’s issue of Science. For more information, visit: www.uni-muenchen.de