Entanglement of macroscopic objects

A pair of cesium gas clouds containing 10¹² atoms each, has been entangled by a quantum optics team at the University of Aarhus in Denmark. No previous entanglement with atoms has involved more than four particles. In the present experiment, the physicists sent a single, off-resonant, linearly polarized laser beam through two separated Cs gas samples whose oppositely directed mean spins were transverse to the beam. First, the researchers measured the sum of the two collective spins without knowing the individual collective spin of each sample. A subsequent measurement 0.5 ms later showed that the sum remained the same, which demonstrated that the two gas samples maintained their collective entanglement—as though they were two macroscopic “atoms.” Such a collectively entangled state is unaffected by the decoherence of a few of its constituent atoms. Although the two samples were just millimeters apart, they could, in principle, be much more distant. The researchers think the method might extend to creating entanglement in solid-state samples with long-lived spin states. (B. Julsgaard, A. Kozhekin, E. S. Polzik, Nature 413, 400, 2001 http://dx.doi.org/10.1038/35096524 .)