Measuring Entanglement Among Many Particles

Entanglement of multiparticle systems is desired for precision interferometry, quantum computing, and fundamental tests of quantum mechanics. However, characterizing the level of entanglement in such large systems is far from straightforward. A group of European physicists has devised a new measure of entanglement based on the collective spin. In Physical Review Letters, they demonstrate their method on an atomic cloud, showing that it contains inseparable clusters of at least 28 entangled atoms.