We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows us to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.
A single ion in an ultracold gas is a versatile experimental platform to study interactions between charged and neutral particles in a controllable manner. When the gas density is large enough, a single ion can be viewed as an impurity in a sea of ultracold atoms or molecules. On the other hand, that single ion can also undergo a chemical reaction with atoms or molecules in the gas. This article discusses the dynamics of a charged impurity in an ultracold bath and the interplay between cold chemistry and impurity physics.
We provide an introduction to the experimental physics of quantum gases. At the low densities of ultracold quantum gases, confinement can be understood from single-particle physics, and interactions can be understood from two-body physics. The structure of atoms provides resonances both in the optical domain and in the radio-frequency domain. Atomic structure data is given for the 27 atomic isotopes that had been brought to quantum degeneracy at the time this chapter was written. We discuss the motivations behind choosing among these species. We review how static and oscillatory fields are treated mathematically. An electric dipole moment can be induced in a neutral atom, and is the basis for optical manipulation as well as short-range interactions. Many atoms have permanent magnetic dipole moments, which can be used for trapping or long-range interactions. The Toronto
40K/
87Rb lattice experiment provides an illustration of how these tools are combined to create an ultracold, quantum-degenerate gas.