Transport of a Single Cold Ion Immersed in a Bose-Einstein Condensate

Transport processes are ubiquitous in nature but their understanding on a microscopic level still forms a prime challenge to explain macroscopic phenomena in complex many body systems.

Here, we report on transport measurements of a single low-energy ionic impurity immersed into a Bose-Einstein condensate. The ionic impurity is created from a single Rydberg excitation by using a field ionization method consisting of an electric field pulse sequence designed such that the initial kinetic energy of the ion is minimized. Subsequently a small electric bias field is applied to drag the ion though the Bose-Einstein condensate. Due to the large ion-atom scattering cross-section, the ion undergoes frequent collisions with the ground state atoms on its way through the dense atomic cloud. We compare our experimental results with stochastic trajectory simulations based on sequential Langevin collisions, which indicate diffusive transport behavior. This allows us to measure the mobility of such a free ion in a Bose-Einstein condensate.

Our results pave the way to reach a collision energy regime where only few partial waves contribute to the scattering and quantum processes start to dictate the transport.