An ion in a sea of ultracold neutral atoms

We propose to investigate a new line of fascinating experiments in which a single, trapped, Ca+ ion is immersed in a Bose-Einstein condensate of neutral Rb-atoms. These experiments will for the first time combine the two advanced research areas and state-of-the-art technologies of single ion trapping and neutral quantum gases. This combination will open the door to a new generation of experiments that are interdisciplinary in nature and have considerable potential for future applications. We will focus on the interaction between the ion and atomic gas in order to control and manipulate them on the quantum level. A number of intriguing physical systems can be realized ranging from collision studies between atoms and ions to quantum computation. The atom-ion interaction gives rise to an interesting singular 1/r4 potential, in which from a classical point of view no stable orbits exist. Within the three years of this application we want to study the particle dynamics in this potential, determine the elastic and inelastic scattering processes, identify the various reaction channels, and search for scattering (Feshbach) resonances. This should allow us to experimentally discover a predicted mesoscopic atom-ion bound state. Also, it should be possible for the first time to carry out true ultracold chemical reactions between an ion and a neutral atom. We will test whether we can use the ultracold neutral atoms to sympathetically cool atomic and molecular ions externally and internally to their ground state. If successful this would represent a valuable tool for future research with ion traps. There are many perspectives for interesting experiments with this set-up beyond the three year funding period. Charge hopping and mobility in an ultracold gas of atoms can be studied. Furthermore, working with an atomic Mott insulator state it should be possible to entangle individual atoms with ions. Using ions as addressable markers could present a unique solution for the known `addressing problem in quantum information processing schemes with neutral atoms in optical lattices. Due to the remarkable progress in the fields of ultracold trapped atoms and ions, these experiments have now become possible. Atoms and ions can be fully controlled and manipulated down to the quantum level. Quantum mechanical problems can now be experimentally analyzed in an essentially perfect environment. Fortunately the trapping techniques for ions and atoms are also wonderfully compatible with each other so that both traps can be operated without essentially disturbing each other. The ion will be held in a tight radio-frequency Paul trap in the end-cap design which offers large optical access from all sides. The ultracold atoms will be trapped in an optical dipole trap which overlaps the ion trap. Atom- and ion trapping technologies complement each other, and the combination will even further enhance the possibilities in the fields of quantum optics and ultracold quantum gases. The Institut für Experimentalphysik at the Universität Innsbruck is an ideal place to pursue this endeavor due to the expertise in ion trapping and in neutral atomic and molecular quantum gases accumulated here.

We set up an experiment to study the fundamental interactions of a few, trapped, laser-cooled ions with ultracold neutral atoms. This experimentally unexplored terrain offers a multitude of fascinating experiments on the quantum level. In recent years, the production of neutral quantum gases has triggered a revolution of new insights and phenomena. Charged quantum gases, consisting of cold atoms and ions, with their strong interaction between the particles will even more extend the possibilities for research in novel regimes. Interestingly, even though ion trapping and atom optics use similar technologies, e.g. laser cooling and ultra high vacuum, the two fields have been segregated until now. Our experimental work brought together for the first time the field of ultracold neutral atoms and of trapped laser cooled ions. It is a first step to open this research area and to explore the scientific potential of the ultracold atom-ion system. As a result of our work we developed and constructed an apparatus that allows combining a few trapped Barium ions with ultracold neutral Rb atoms and Rb Bose-Einstein condensates. In brief, this technology includes an optical transport of a cloud of cooled and trapped Rb atoms from one vacuum chamber to another chamber where laser cooled ions are trapped in a Paul trap. The cloud is then moved onto the trapped ions such that the ions are immersed in the atom cloud in precisely controlled conditions. The dynamics of the neutral atoms and Ba ions can be observed via highly sensitive CCD cameras. We then investigated the cold collisions between the atoms and ions. As expected, elastic collisions cross sections were found to be very large due to the long range potential between the particles. As an important inelastic collision we observed charge- exchange between a neutral Rb atom and a Ba+ ion. Fortunately, this process is quite rare, as it takes place only once in ten thousand collisions. This is good news for future experiments because it limits unwanted losses and opens the door for a large range of future experiments, e.g. where we want to demonstrate the existence of a strange multi-atom ion bound state or where we want to study the properties of a charged quantum gas.