The Advantages of Extra Entanglement

The development of quantum physics has led to a revolution in modern technology and lies at the heart of applications in communication, computation, medicine, and navigation. Fragile quantum correlations, known from Einstein's infamous wording as “spooky action at a distance,” point to novel ways to compute even faster or communicate more securely. Although the original two-particle entangled states following the seminal paper by Einstein, Podolsky, and Rosen ( 1 ) are well understood and can be routinely prepared in experiments, many-particle entangled states are in general hard to describe theoretically or to produce experimentally. The increasing fragility of quantum states with increasing numbers of atoms makes macroscopic entanglement rare. On page 180 of this issue, Haas et al. ( 2 ) have demonstrated the creation of a robust entangled state of more than 40 ultracold atoms within a single operation. Their method relies on a simultaneous, coherent interaction with the light field of a high-finesse optical cavity to reveal the presence of only a single atom with a different internal state.