Albert Einstein’s description of Bose–Einstein condensation is based on a statistical argument. In a gas of identical bosons, the statistical weighting of each state is such that the total occupation of the excited states is capped at an upper bound N c. If the number of identical particles exceeds N c, all additional particles must occupy the ground state. That textbook picture doesn’t include interparticle interactions, but it does assume thermal equilibrium, which cannot exist without some form of interaction, so it’s no surprise that the picture doesn’t describe real systems exactly. Now, Zoran Hadzibabic (Cambridge University, UK) and colleagues have taken a closer look at how the role of interactions in a Bose–Einstein condensate relates to the textbook picture. The researchers found, as shown in the figure, that for a gas of potassium-39 atoms in an optical trap, the thermal component (excited-state atoms) consistently exceeded the textbook upper bound. Qualitatively, they attribute the difference to repulsion between the thermal and condensed components, which turns the harmonic trap into a Mexican hat potential and thereby increases the number of thermally accessible excited states. Quantitatively, they found that when they tuned the interparticle interaction strength, by applying a magnetic field near a Feshbach resonance, an extrapolation to the zero-interaction limit recovered the textbook picture. (N. Tammuz et al., Phys. Rev. Lett., in press.)—Johanna Miller
