Entanglement of Macroscopic Objects

Entanglement of macroscopic objects, a pair of gas clouds containing a trillion atoms each, has been achieved by a research team in Denmark (Eugene Polzik, University of Aarhus, 011-45-89423745, polzik@ifa.au.dk), constituting by far the largest material objects entangled on demand and paving the way for quantum teleportation between macroscopic objects. The accomplishment, published in this week's issue of Nature (Julsgaard et al., 27 September 2001), was announced in preliminary form this June at the first International Conference on Quantum Information, sponsored in part by the Optical Society of America and the American Physical Society.

One of the most profound features of quantum mechanics, entanglement is a special interrelationship between objects in which measuring one object instantly influences the other, even if the two are completely isolated from one another. No previous entanglement with atoms has involved more than four particles. Furthermore, atoms have only been entangled at close proximity, either as ions spaced microns apart in a tiny trap (Update 475), or atoms flying over a short range through narrowly spaced cavities (Hagley et al., Phys. Rev. Lett., 7 July 1997).

In the present experiment, researchers sent a light beam through two cesium gas samples, each held in a special paraffin-coated cell. The beam changed each sample's "collective spin," which describes, in a sense, the net direction in which all of the atoms' tiny magnets add up. First, the researchers measured the sum of the two collective spins without knowing the individual collective spin of each sample. A subsequent measurement, nearly a millisecond later, showed that the sum remained the same. This demonstrated that the two gas samples maintained their special interrelationship and were entangled. Although the two samples were just millimeters apart, they could in principle be separated, and thereby entangled, at much longer distances. Entanglement of such large objects enables "bulk" properties, like collective spin, to be "teleported," or transferred, from one gas cloud to another.