Sympathetic cooling of trapped atomic and molecular ions is investigated by using ion collisions with a second (ultracold) species of neutral atoms (e.g. sodium) which have previously been laser cooled to less than 1/1000th degree above absolute zero. This research focuses on very low energy ion-neutral atomic collisions as the cooling mechanism, about which very little is known experimentally. The boundaries of classical (Newtonian) and quantum (Schroedinger-Heisenberg) scattering are explored in new ways. The ion-neutral interaction under study is very general, involving the polarization (charge separation) of the neutral partner by the electric field of the ion; the polarized neutral atom then acts back to strongly attract the ion. Molecules (and molecular ions) are very difficult to laser cool directly, so this work should open up applications to general energy exchange processes in the solar system and interstellar space, control of ion-molecule exothermic reactions near zero Kelvin, generation of Wigner crystals of molecular ions in a lattice, precision spectroscopy of ions, ultraprecise atomic and molecular ion clocks, and potentially, quantum computers or quantum memory registers involving arrays of trapped ions. The project uses a special hybrid ion-neutral trap (MOT and Paul trap combination) to overlap clouds of alkali-metal atoms and various kinds of ions (e.g. Ca+, Na2+, Na+) and to allow observation of the rates of cooling and reactions that occur as a result.
This basic research is relevant to atomic physics, molecular physics, frequency standards, space physics and astrophysics, quantum information and memories and their many applications. Atomic, molecular and optical physics (AMO) research at the University of Connecticut currently involves 10 active faculty and ~38 graduate students who receive a broad education involving all these areas, through their individual research, interaction with peers, weekly research seminars and colloquia, visits from scholars, public lectures and community outreach. Research results are frequently presented at conferences. One of the PI's students in undergraduate quantum mechanics for physics majors is now involved in PhD-level research on this project. Three undergraduate summer students, along with two exchange students from the University of Heidelberg have been involved. Undergraduates are encouraged to participate in related independent study projects. Students on the project receive a broad education in basic experimental physics techniques, including ultrahigh vacuum, laser cooling and trapping, optics, laser spectroscopy, ion manipulation and detection, etc.
Collisions between atomic or molecular ions and neutral atoms are basic processes involved in many chemical reactions and interactions that occur in space. This proposal emphasizes measuring the rates of energy and electron exchange involving ions and neutral atomic systems at temperatures that can approach the 3 Kelvin temperature of interstellar space. Working at relatively low temperatures reduces the number of quantum energy levels that are involved and makes the processes easier to understand. For this project we have constructed a hybrid trap for co-trapping atoms and simple ions, consisting of a Paul radio-frequency ion trap overlapping with a magneto-optic atom trap or MOT (concentric), so that ion-neutral collisions can be investigated cleanly and systematically. This kind of experiment was suggested by our group several years ago. The first years of the grant were devoted to constructing and testing the new hybrid trap apparatus and learning how to make measurements with it. Cold ion-neutral interactions are dominated by universal types of long-range "polarization" forces. The rates of reaction from these forces are quite large compared with the rates involved in ordinary chemical reactions between neutral atoms and molecules and they are very important in space. [For a summary and references, see W.W. Smith, O.P. Makarov and J. Lin, 'Cold ion-neutral collisions in a hybrid trap', J.Modern Optics 52, 2253-2260 (2005).] There are a number of potential applications including, but not limited to: ion-molecule reactions in the interstellar medium, trapped-ion quantum computing, quantum information and precision spectroscopy. Our first paper published (in Physical Review) with support from this grant reported simulations of collisional cooling of both Na+ and Ca+ atomic ions by interactions with ultracold (0.001K) sodium (Na) atoms in the hybrid trap. The simulations predicted that for a single atomic ion of Na or Ca, cooling to nearly 0.001K should occur in a few tenths of a second, but for small numbers of ions (more than one), heating effects due to the random motion of the other trapped ions were found to limit the temperature reached to about 1K after a few tenths of a second, which is still quite cold. A second Physical Review paper reported four different indirect experimental measurements that showed evidence of the sympathetic Na+ ion cooling by Na neutrals in the hybrid trap, but for a sample of several thousand ions: increased lifetime of the trapped ions after cooling, measurements of trapped ion number vs. MOT exposure over a fixed time interval showing that an interaction was taking place, MOT-ion-cloud overlap measurements again showing the effect of interactions, and observations showing evaporation of the hottest ions when the Paul trap energy depth was suddenly lowered. The qualitative results of the simulations were confirmed by the experiments for cooling Na+ ions by cold Na atoms. When we tried the same experiment with Ca+ ions refrigerated by cold Na atoms, we found a surprise not accounted for in the simulations: instead of a longer lifetime of the trapped ions, the residence time in the trap was actually shortened. After some consideration, we concluded that a simple chemical reaction was occurring: the Ca+ ions were picking up electrons from the cold Na atoms and becoming neutral Ca atoms which were no longer trapped. From these measurements one can obtain the reaction rate (or rate coefficient) for this electron-transfer or "charge-exchange" reaction. A paper on this experiment appeared as an invited contribution is a special issue of Applied Physics B - Lasers and Optics commemorating the 100th Birthday of Wolfgang Paul, a Nobel Laureate who invented the Paul ion trap used in this research. In order to distinguish between the various species of trapped atomic and molecular ions in the experiments (Ca+, Na+, Na2+ molecular ions, etc.) we had to explore methods of ejecting unwanted ions of a particular mass from the Paul trap (mass-selective resonant quenching, or MSRQ). A paper was published in Reviews of Scientific Instruments exploring these effects as they applied to our particular hybrid trap apparatus and similar kinds of experiments. A Ph.D. student in experimental atomic, molecular and optical physics, Ilamaran Sivarajah, completed all his thesis requirements in December 2012, in addition to the publications mentioned. He then obtained a post-doc with an experimental group working on trapped-ion quantum computing at a university in Australia.
