Raman researchers set a trap to cool ions

Trapping ions with cold atoms cools the ions and could lead to experiments that generate molecular ions at interstellar space temperatures. The traps also can store ions in a stable condition for longer periods than previously demonstrated.

Methods for storing and cooling atoms and ions at temperatures just above absolute zero are among the most important atomic physics methods to be developed over the past few years. Recently, scientists at the Raman Research Institute in India and the Institute of Physics at Johannes Gutenberg University Mainz (JGU) developed a technique that counters the predictions that ions would actually be heated from their collisions with cold atoms.

In the experiment, the researchers captured neutral atoms in a magneto-optic trap, cooled them with laser light to -273.15 °C, and stored charged particles in an ion trap. To achieve this, Dr. Günter Werth of JGU set a Paul trap combined with a magneto-optic trap. The two traps made it possible to capture ions and cold atoms in the same location simultaneously.

“The question was whether it would work at all,” Werth said.

Experiments with rubidium ions and rubidium atoms showed that the particles did, in fact, exchange energy. The ions were effectively cooled during a collision with the cold atoms. The outcome was determined by two fundamental processes: During continuous cooling, the atoms indirectly extract energy from the trapped ions; the collision between ions and atoms causes both to exchange charges, transforming a “hot” ion into a “cold” ion.

The system was able to cool a larger number of ions without immediate exhaustion of the atom reservoir because the magneto-optic trap makes it possible to maintain a constant concentration of atoms in the reservoir.

This interaction between ions and atoms is interesting to physicists because it is similar to interactions that might occur in the low temperatures of outer space.

“The expectation is that the interaction of ions and atoms at very low temperatures will result in the formation of molecular ions,” Werth said. “This is a process that we believe also occurs in interstellar space.”

The research was published in Nature Communications (doi: 10.1038/ncomms2131).