Quantum Gas of
Deeply Bound Ground State Molecules

Institut für Experimentalphysik,
University of Innsbruck, and
Austrian Academy of Sciences,
Innsbruck, Austria

Danzl et al., "Quantum Gas of Deeply Bound Ground State Molecules", Science, Vol. 321., no. 5892, pp. 1062 - 1066 DOI: 10.1126/science.1159909, 22 August 2008, originally published in Science Express on 10 July 2008.

Success Story: Laser Cooling of Atoms

The capability to cool atoms with the aid of laser beams to temperatures of a few billionth of a degree above absolute zero temperature (-273.150C) has revolutionized our understanding of the fundamental properties of matter, as described by quantum mechanics. From a technological point of view, laser cooled atoms are at the heart of modern atomic clocks and provide the definition of our unit of time, the second, as well as time standards for modern communication. When particles are cooled to extremely low temperatures at high densities in a trap, the quantum mechanical wave functions that describe the particles begin to overlap and a so-called "quantum gas" is created. If all these wave functions oscillate in perfect synchrony, a Bose-Einstein condensate (BEC) emerges. In a BEC, hundreds of thousands or even millions of individual particles coalesce to form one single quantum mechanical "super"-wave function.

What about Molecules?

In comparison to atoms, molecules exhibit a much richer internal structure. In a diatomic molecule the two nuclei can vibrate against each other and exhibit rotational motion. This is what makes them interesting for a series of fundamental studies. Many research groups around the world are actively working on reaching the same degree of quantum mechanical control over molecules as one currently has over atoms. But at the same time their rich internal structure makes it difficult to cool molecules. Bose-Einstein condensation of molecules by directly cooling them from room temperature seems to date completely out of reach.

Can we benefit from the fact that atoms are so well controlled?

The crucial trick is to first cool atoms and create an atomic BEC. Only afterwards are individual atoms associated to very weakly bound atom pairs by a magnetic field ramp. This technique has been mastered since 2003. From a molecular physicist's point of view, these atom pairs are molecules that reside in the most highly excited of all vibrational quantum states of the molecule. But such a loosely bound pair is not a stable molecule as would be needed for many experimental studies. When two of these loosely bound atom pairs collide, large amounts of internal energy can be converted to external motion, which leads to a change of the internal state and loss of the particles from the trap. Bose Einstein condensation of these atom pairs is therefore precluded.

Transfer to Deeply Bound Molecular States

The present work shows for the first time that it is possible in the quantum gas regime to efficiently transfer atom pairs from the highest vibrational state to deeply bound molecular states. This is achieved with the aid of two lasers and is done in a coherent way, i.e. with full quantum mechanical control. Since two light particles or photons are involved in this process, it is a called a two-photon transition. The first laser lifts an electron up to an electronically excited state, whereas the second laser brings the electron down to the original electronic state but in addition takes away a large fraction of the vibrational energy of the molecule. The resulting molecular state does no longer represent a weakly bound atom pair but instead a chemically tightly bound molecule. For the transfer, the STIRAP (Stimulated Raman Adiabatic Passage) technique is employed which allows extremely high transfer efficiencies to the deeply bound state by avoiding actually populating the potentially lossy electronically excited state. The transfer does not heat the molecular sample and hence does not destroy its quantum gas character.

Production of a quantum gas of deeply bound molecules. Weakly bound atom pairs are formed from an atomic BEC. In the crucial step, these atom pairs are transformed to deeply bound molecules that form a quantum gas. (copyright: AAAS)


On the basis of these experiments it will now be possible to reach the absolute ground state of the molecule with a second two-photon transition. This so called "rovibronic" ground state is the lowest internal quantum state of the molecule in terms of rotational, vibrational and electronic energy. Since upon a collision of two molecules in the absolute ground state no internal energy can be converted to external motion, these molecules are stable against collisions and should allow the formation of a BEC of molecules.

Deeply bound molecules in the quantum gas regime allow a series of fundamental experiments that could revolutionize our understanding of the structure of matter. For chemical processes an exceedingly high degree of control would be possible, and in the quantum gas regime they could be driven by bosonic stimulation. Spectroscopic studies would benefit from the extremely well defined conditions that characterize a quantum gas at ultralow temperatures. A precise determination of the binding energies in the molecule allows for example an evaluation of a possible change of the ratio between the electron and the proton mass. In case it was found that natural constants are not quite as constant as we like to believe, we would have to rethink our picture of the universe.

Our results were published online on the 10th of July 2008 on ScienceExpress, the ahead of print publication platform of Science Magazine.

The Team

Johann G. Danzl, Elmar Haller, Mattias Gustavsson, Manfred J. Mark, Russell Hart, Nadia Bouloufa, Oliver Dulieu, Helmut Ritsch, Hanns-Christoph Nägerl

These experiments were carried out by the CsIII team headed by Hanns-Christoph Nägerl.Theory support was provided by Nadia Bouloufa and Olivier Dulieu from Orsay (France) and Helmut Ritsch from Innsbruck.

The Innsbruck team (top, left to right) Elmar Haller, Johann G. Danzl, Russel Hart, Manfred J. Mark
(bottom, left to right) Mattias Gustavsson, Hanns-Christoph Nägerl

Helmut Ritsch

Olivier Dulieu



Article: Science Article

Preprint server: arXiv:0806.2284

German press release: Press release (pdf)

Photos for download: ultracold media photos


The experiments are supported by the START-price of the Bundesministerium f|r Wissenschaft und Forschung (BMWF) and the Austrian Science Fund (FWF) .

last change: 03-09-08 by JGD