Phys. Rev. Lett. (in the press); preprint at

Ultracold atomic gases have long served as a textbook paradigm of many-body physics, allowing studies of, say, Bose–Einstein condensation and superfluidity, in clean and well-controlled environments. However, the spatial inhomogeneity caused by the external harmonic trap can be problematic. It can smear or even qualitatively change the expected experimental signatures, such as the divergence of the correlation length close to phase transitions. This has motivated the search for homogeneous gases, a goal that has been achieved previously for fermions in three dimensions.

Now, Klaus Hueck and colleagues have reduced the dimension to two, by making a homogeneous sheet of Fermi gas well suited to investigating the interplay between reduced dimensionality and strong interactions in quantum many-body systems. As a first benchmark experiment, Hueck et al. measured the two-dimensional equation of state and the momentum distribution in the non-interacting case, showing a textbook example of statistical physics. Introducing an attractive interaction, atoms form tightly bound dimers and a macroscopic occupation of low-momentum modes compatible with Berezinskii–Kosterlitz–Thouless superfluidity.