Direct observation of swap cooling in atom–ion collisions

Abstract

Abstract Collisions with cold particles can dissipate a hot particle’s energy and therefore can be exploited as a cooling mechanism. Kinetics teach us that cooling a particle down by several orders of magnitude typically takes many elastic collisions as each one only carries away a fraction of the collision energy. Recently, for a system comprising hot ions and cold atoms, a very fast cooling process has been suggested (Ravi et al 2012 Nat. Commun. 3 1126) where cooling over several orders of magnitude can occur in a single step. Namely, in a homo-nuclear atom–ion collision, an electron can resonantly hop from an ultracold atom onto the hot ion, converting the cold atom into a cold ion. Here, we demonstrate such swap cooling in a direct way as we experimentally observe how a single energetic ion loses energy in a cold atom cloud. In order to contrast swap cooling with sympathetic cooling, we perform the same measurements with a hetero-nuclear atom–ion system, for which swap cooling cannot take place, and indeed observe very different cooling dynamics. Ab initio numerical model calculations agree well with our measured data and corroborate our interpretations.