All constituents of ordinary matter fall in one of two categories: bosons or fermions, with, respectively, integer or half-integer values for their spins. The exclusion principle formulated by Wolfgang Pauli in 1925 states that no two identical fermions can occupy the same quantum state simultaneously. Atomic gases of fermions can be used to understand widely different systems, including neutron stars, normal metals, heavy atomic nuclei, metallic hydrogen, superfluidity, and even dense quark matter in the very early universe.

Hot Paper #8 is from the MIT-Harvard Center for Ultracold Atoms (directed by Nobel laureate Wolfgang Ketterle [see also|see also), where the production of nanokelvin temperatures is now routine. The paper reports observation of Bose-Einstein condensation (BEC) of pairs of fermionic atoms in a ⁶Li gas. In other words, the Center has created a Bose-Fermi mixture in which ⁶Li atoms are fermions, while ⁶Li₂ molecules are bosons, which form the condensate, stabilized by the existence of the Fermi sea. About a dozen groups have worked on fermionic atoms, with the goal of finding new forms of superfluidity. Paper #8 is the landmark in this quest because the technical breakthrough it reports gives access to new experimental regimes.

It is already clear that #8 is a crucial step on the road to a far larger discovery, the actual achievement of superfluidity in a Fermi gas. For Science Watch, Martin Zwierlein, a young member of Ketterle’s team, explains that #8 has "triggered enormous experimental and theoretical activities on this novel system." In April 2005 the group sighted superfluidity for the first time, a sensational result (see M.W. Zwierlein, et al., Nature, 435[7045]: 1047-51, 2005.) To achieve this, Zwierlein says, "Our group set the cloud of atoms in rotation, using a laser beam as a spoon to stir things up. We succeeded in observing vortices in our rotating fermionic cloud, which is an unambiguous signature of superfluidity in a strongly interacting gas."

If this result is set in the context of the electron densities involved, the discoveries are the equivalent of finding a metallic superconductor operating above room temperature. So the physics is clearly exotic, or as Zwierlein puts it, "This is the most exciting topic in physics." That’s because researchers now have the unique possibility of studying strongly interacting matter in a clean and highly controlled way, which contrasts sharply with the condensed matter physics of a neutron star or a high-temperature superconductor. The history of physics tells us that when a simple model is clearly superior to its complicated predecessor, great discoveries lie ahead. Which is why #8 is highly cited.

The theme of manipulating the social behavior of bosons and fermions continues in Hot Paper #4, a newcomer. Belén Parades (Max Planck Institute for Quantum Physics, Garching, Germany) led an international team who have blurred the distinction between the two states of matter. They created BEC from ⁸⁷Rb atoms, and then held the gas in a two-dimensional optical lattice formed by the interference of multiple laser beams. The lattice effectively constrained the movement of atoms along one-dimensional light tubes orthogonal to the lattice. In these tubes the atoms experience strong mutual repulsion, and like beads on a wire they cannot change places. Therefore they should behave somewhat like fermions, but not entirely so. Marvin Girardeau and Lewi Tonks proposed in 1960 that a gas of correlated bosonic particles whose repulsive interactions are dominant would display fermionic properties akin to the Pauli exclusion principle, if prevented from occupying the same position in space. Paper #4 marks the first production of this puzzling form of matter, in which bosons are "fermionized." A striking aspect of the paper is its demonstration of quantum-state engineering. For the realization of quantum computation, control and engineering will be required skills.

Finally, the results of Hot Paper #10 should contribute to the eventual realization of new quantum devices, such as interferometers and Josephson junctions. The paper explores fundamental limitations on the stability of BEC at small distances, down to 0.5 m m, from dielectric and metal surfaces. The results suggest that local manipulation of BEC will be possible using thin conductors on dielectric surfaces.