Pairs of trapped atoms undergo superexchange

Manganese oxide is an archetypal antiferromagnet: Below a critical temperature, the unpaired electrons on the Mn atoms arrange themselves between the O atoms in an alternating spin-up/spin-down configuration. Direct spin-spin interaction is not responsible for the correlation. Rather, a virtual process called superexchange prevails. By hopping to virtual states on the O atoms, the electrons can correlate their spins and lower their energy. Superexchange has now been observed in pairs of optically trapped atoms. Immanuel Bloch of the University of Mainz, Germany, and his collaborators loaded rubidium-87 atoms into an array of double-well potentials formed by interfering laser beams. Bloch’s team could ensure that each double well started out with a spin-up atom in one well and a spin-down atom in the other. Superexchange occurred when one atom virtually hopped to join its neighbor. By adjusting the barrier between the wells, the Mainz researchers could make tunneling between the wells—and therefore superexchange—more or less likely. The observable consequence was an oscillation in time of the net spin polarization about perfect antialignment. To detect the oscillation, the researchers raised the left wells to dump their contents into the right wells. The energy acquired by the transferred atoms depended on their spin. When the trap was turned off, the atoms fell through a Stern–Gerlach filter. Where the atoms ended up revealed their spin and whether they originated from a left or right well. Repeating the procedure for different release times traced the oscillation, which matched textbook theory. (S. Trotzky et al., Science 319 , 295, 2008 http://dx.doi.org/10.1126/science.1150841 .)