A Bose–Einstein condensate inside an atom

Impurity-induced scattering plays a part in some of the most extraordinary phenomena in condensed-matter physics: Anderson localization , giant magnetoresistance , and the Kondo effect , to name just a few. Theorists suspect that in the ultracold regime, where scatterers must be described as quantum mechanical waves, impurities might yield even richer physics. To that end, a long-standing goal has been to implant ions in a Bose–Einstein condensate (BEC) of neutral atoms. That goal has eluded experimenters, in large part due to ions’ extreme sensitivity to electric fields: Even in carefully engineered traps, stray electric fields impart a jiggling motion that typically warms the system to millikelvin temperatures, out of the quantum regime.

Now researchers led by Florian Meinert and Tilman Pfau , of the University of Stuttgart in Germany, have devised a clever workaround to that problem: They implant in their BEC not an ion but a neutral atom masquerading as one. They start with a dense, cigar-shaped BEC of rubidium atoms (green in the illustration), held in a tightly focused optical trap (pink). They then excite one of the atoms to a Rydberg state, a highly excited state in which the outermost electron roams far from the nucleus in a loosely bound orbit (purple). For large principal quantum numbers n, the orbital radius far exceeds the width of the BEC. In that limit, neighboring atoms in the BEC effectively see only the Rydberg atom’s positively charged core. Crucially, although the loosely bound electron exerts little sway over the BEC, it acts as a Faraday cage that shields the ionic core from stray electric fields.

The setup allowed the researchers to achieve submicrokelvin temperatures, orders of magnitude cooler than previous ion–atom mixtures. But the recoil energy deposited by the excitation pulse nevertheless prevents the system from reaching the quantum-scattering regime. Meinert and his colleagues speculate that they can overcome that hurdle using more sophisticated excitation schemes. (K. S. Kleinbach et al., Phys. Rev. Lett. 120, 193401, 2018 .)