CORDIS - EU research results

Ultracold negative ions by laser cooling

Final Report Summary - UNIC (Ultracold negative ions by laser cooling)

The cooling of single atoms and ions to ultracold temperatures is a key prerequisite to investigations of their structure and properties. In particular, the technique of laser cooling has enabled the discovery of new phenomena such as Bose-Einstein condensation and engendered various technical applications, from quantum information to atomic clocks. It is based on the excitation of an atomic system's valence electron by a laser photon, causing the particle to rebound, followed by emission in an arbitrary direction. Although laser cooling was first demonstrated more than four decades ago, it has so far only been applied to neutral and positively charged atoms and molecules. This is due to the fact that negative ions are fragile systems mainly bound due to quantum-mechanical correlation effects rather than by Coulomb attraction to a positive core. Almost none of them have the types of excited electronic states required for efficient laser excitation and cooling.

The aim of the UNIC project was to overcome this limitation by developing a novel technique for the laser cooling of negatively charged particles. The method will be applied to atomic anions stored in an electromagnetic trap. Once they are cooled, they can be used to cool any other species of negative ions by collisions. The excitation process requires the availability of a strong transition between the ground state and an excited state. There is a very small number of atomic anions that meet this criterion. The first candidate ion to be discovered was Os^-, in 2000. This ion was extensively studied by our group, using high-precision laser spectroscopy. Os^- was found to be a very challenging ion for laser cooling, mainly due to a rather low transition rate, which causes the cooling to take very long (many minutes).

In the course of the UNIC project, another anion laser cooling candidate, La^-, was investigated using similar but more advanced techniques. The different wavelength region (mid-infrared) required significant modifications of our apparatus, including a new state-of-the-art laser system and a new detector for neutral particles. In addition, the production of a La^- ion beam has proven challenging, and several technical advances were required to make available a stable and reliable source of ions. The laser spectroscopy experiments were completed successfully and yielded crucial information on the potential laser cooling transition in this ion. The transition rate (and hece the cooling) rate was found to be about 100 times higher than in Os^-, allowing the cooling of an ensemble to ultracold temperatures within seconds rather than minutes. Based on the resolved hyperfine structure, an optimal cooling scheme requiring only one fundamental laser wavelength and two sidebands created by electro-optical modulation was developed. Finally, the capture of atomic anions from an energetic beam into an electromagnetic trap was demonstrated and investigated. The actual realization of laser cooling of atomic anions is forthcoming.