The Fermionic Cousin of Bose–Einstein Condensate Makes its Debut

Experimenters have been romping in the BEC playground ever since a gaseous Bose–Einstein condensate was first produced four years ago. It was almost inevitable, then, that researchers would also want to explore its fermionic cousin—the quantum degenerate state one expects to get by cooling a gas of atoms with half‐integer spins. Whereas the integer‐spin atoms (bosons) in a Bose–Einstein condensate all pile into the lowest energy state, the half‐integer spin atoms (fermions) are prevented from doing so by the Pauli exclusion principle, which prohibits two identical fermions from occupying the same quantum state. Instead, the atoms fill all the lowest energy levels up to the Fermi energy, just as electrons do in a solid. The crossover into this quantum degenerate state is gradual, compared to the abrupt phase transition into a Bose condensate. But that crossover has now been convincingly detected by Deborah Jin, a physicist at the National Institute of Standards and Technology, and Brian DeMarco, a physics graduate student at the University of Colorado at Boulder, both working at JILA.