S. Trotzky, P. Cheinet, S. Foelling, M. Feld, U. Schnorrberger, A. M. Rey, A. Polkovnikov, E. Demler, M. D. Lukin, and I. Bloch, Time-Resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices, Science 319, 295-299 (2008).