Sympathetic cooling of molecular ions with ultracold atoms

Introduction The ability to produce atoms and atomic ions in a single quantum state has revolutionized atomic physics [1] and the technologies it provides [2–5]. To produce this quantum matter, the temperature of the atoms and ions are lowered such that the Gibbs factor is appreciable for only the quantum ground state. The techniques for reaching these, socalled, ultracold temperatures, which are the subject of several recent Nobel prizes, are centered around laser cooling [6]. In laser cooling, the simple electronic structure of an atomic system is exploited to remove thermal energy from a particle by scattering a large number of photons from it. If the ability to prepare single quantum state matter, i.e. ultracold matter, could be extended to more complicated systems, e.g. molecules, a new era in both physics and chemistry, and the technologies they provide, will begin [7, 8]. Unfortunately, the same complexity that makes molecules attractive renders them unamenable, with few exceptions [9–11], to laser cooling. An interesting alternative to laser cooling for the production of a single-quantum-state molecular sample, is to use a reservoir of ultracold atoms to sympathetically cool the molecules to ultracold temperatures [12–14] – in much the same way that cryogenic systems use a helium buffer gas to cool other objects to cryogenic temperatures. In principle, this technique of ultracold-atom sympathetic cooling is completely general since it relies only on the second law of thermodynamics. In reality, however, there are many practical issues that must be considered before efficient sympathetic cooling of molecules by ultracold atoms can occur. These issues and the means to understand and control them are the subject of this review. This manuscript is an attempt to concisely describe the current