Reporting in the May 20 issue of Nature, physicists from Max Planck Institut für Quantenoptik, Ludwig Maximilians Universität, Johannes Gutenberg Universität, Université Paris Sud and the University of Amsterdam describe the successful realization of a Tonks-Girardeau gas. Theoretically proposed approximately 40 years ago, a Tonks-Girardeau gas is a system of correlated bosonic particles whose repulsive interactions are so dominant that the particles display fermionic properties akin to the Pauli exclusion principle and are prevented from occupying the same position in space.To achieve this regime, the researchers loaded a Bose-Einstein condensate of 30,000 to 40,000 rubidium-87 atoms into a two-dimensional optical lattice potential created by two orthogonal standing waves of 823-nm laser radiation, which confined the atoms in an array of "tubes" of laser radiation. After 10 ms, they applied another, 854-nm standing wave along the third dimension, which acted to increase the effective mass of the ultracold atoms and which thereby enhanced the effects of the repulsive interactions between them.Momentum distributions of the atoms obtained after the trapping potentials were turned off confirmed the "fermionization" of the system.