Precision experiments with stored ions and antimatterMax Planck Institute for Nuclear PhysicsUniversity of HeidelbergEuropean Research Council
Ultracold Ions and Antimatter Research
Contakt Contact
Priv.-Doz. Dr. Alban Kellerbauer

Tel.: +49-6221-516-138
Fax: +41-22-7669185 (efax)

Max Planck Institute for Nuclear Physics
Room Bo-164
Saupfercheckweg 1
69117 Heidelberg


Laser cooling of anions

Laser cooling is an incredibly powerful technique that has enabled exciting physics research ranging from condensates to ion crystals and atomic clocks. Until today, only neutral atoms or positively charged ions can be laser-cooled because negative ions are fragile systems that usually don't have bound excited states. Since 2000, a very small number of atomic anions (negative ions) with suitable transitions between bound states have been discovered. If they can be successfully laser-cooled, they can be used to cool sympathetically (by collisions) any other negatively charged particle species. Some ten years ago, we proposed this technique of indirect laser cooling, initially based on the negative osmium ion, which was the first atomic anion discovered to have a bound excited state with the right properties. Ultimately, it is planned to employ this cooling technique to antiprotons in order to improve the precision of antimatter experiments at CERN.

Indirect laser cooling principle
Principle of laser-cooling atomic anions in a Penning trap

At first we experimentally studied Os by high-resolution laser spectroscopy, funded by an Emmy Noether Grant of the German DFG funding agency. The first milestone of the project was the full characterization of the bound–bound transition. In addition to the transition frequency, we also measured the cross-section to high precision. While we were able to learn a lot about the relevant transition in all stable isotopes of Os (hyperfine structure, isotope shift, Zeeman splitting), it turned out that the decay rate of the excited state was too low to allow for efficient laser cooling. In the meantime, theoretical calculations on several lanthanide anions suggested that a bound-bound transition in Os would have a 100 times higher decay rate and hence be more suited for laser cooling.

Resonance in Os anion
Resonance of the bound-bound electric-dipole transition in 192Os

Since 2011, funded by an ERC Starting/Consolidator Grant, we have been investigating La by laser spectroscopy. The only stable lanthanum isotope 139La has a non-zero nuclear spin, therefore the studied transition has a hyperfine structure. We resolved seven out of the nine hyperfine transitions and identified them. Our estimate of the transition cross-section confirmed the theoretical prediction that the decay rate is about 4.5 kHz (for a single hyperfine transition) and hence sufficient for laser cooling. Two additional laser wavelengths will be required to repump neighboring hyperfine levels. We have developed a sideband generation system based on electro-optical modulation to produce these wavelengths from our main laser.

Resonance in La anion
Resonance of the potential laser cooling transition in 139La, with the hyperfine structure (mostly) resolved.

We are currently commissioning a newly-built Paul trap for the confinement and laser cooling of La ions in the absence of a magnetic field. Ions from our cesium sputter ion source will be either trapped directly in the Paul trap or first confined in the Penning trap and pre-cooled with electrons.

Paul trap
Paul trap for the confinement and laser-cooling of La ions

Further reading: