3600 Atoms in Two Places at Once

Bose Einstein condensates (BEC), clouds of ultracold atoms which fall together into a single coherent state, continue to be a marvelous working material for studying subtle quantum effects. Last year physicists at the Max Planck Institute for Quantum Optics (MPQ) managed to load a BEC of rubidium atoms into a three-dimensional optical lattice, an artificial crystalline environment in which crossing laser beams provide the forces needed to pinion atoms in the 3D equivalent of an egg crate. Moreover, by a delicate modification of the laser light the resident atoms could be made to undergo a quantum transition between two phases. In one phase the atoms constitute a superfluid: all the atoms have a coordinated wave function, but the number of atoms in any one "well" in the egg carton is unknown. In the other phase the atoms constitute an insulator: the number of atoms in each well is known exactly to be equal to one, but the atoms are all uncoordinated with respect to each other (that is, they no longer can be considered a coherent quantum material). These were the results as of a year ago (see Greiner et al., Nature, 3 January 2003.) Speaking at last week's meeting of the American Association for the Advancement of Science (AAAS) in Denver, Immanuel Bloch reported that he and his MPQ colleagues have exploited the fact that the Rb atoms possess two magnetic substates and have succeeded, by a further adjustment of the confining laser beams, to separate each atom into two entangled spatially separated parts. The researchers are also attempting to get the different diploid atoms (an average of 3600 per plane in the lattice) to interact; one aim is to engineer an unprecedented degree of quantum entanglement, possibly for computational purposes.