Atom-Molecule Dark States

Physicists at the University of Innsbruck have demonstrated that atom pairing (molecule formation) in Bose-Einstein condensates (BECs) using photoassociation is coherent. Coherent pairing of atoms (locking them into a particular quantum relationship) has been observed before using a tuned magnetic condition---a Feshbach resonance---between the atoms. But molecules made that way are only feebly attached. By contrast the process of photoassociation---i.e. using light to fuse two atoms into one molecule---allows more deeply bound molecule states to be established. The trouble is that the same laser light can also be absorbed to dissociate the molecules rather than only perform its associative task. The counter measure used by the Innsbruck researchers (contact Johannes Hecker Denschlag, 43-512-507-6340, johannes.denschlag@uibk.ac.at) is to create a "dark state" in which the light cannot be absorbed. A dark state is a special quantum condition: it consists of three quantum energy levels, two stable ground states and one excited level. If laser light at the two frequencies needed for the transitions from both the ground states to the excited state are present simultaneously, the two excitations (from the two lower energy states) can destructively interfere with each other if there is phase coherence between the ground states. (Homely example: offer one cookie to two children and, if they fall into the right kind of arguing, the cookie goes uneaten.) The consequence is that no light gets absorbed and the molecules are stable. Such "electromagnetically induced transparency" has been observed before for transitions within atoms (PNU 343) but the Innsbruck scientists are the first to use it for a transition between a BEC of atoms and molecules. In their experiments, the same (two-color) laser light that creates the dark state is also the light that photoassociates rubidium atoms into molecules. Johannes Hecker Denschlag says that atom-molecule dark states are a convenient tool to analyze the atom-molecule system and to optimize the conversion of atomic into molecular BECs. BECs of ultracold molecules represent, because of their many internal degree of freedom (vibrational and rotational), a new field of research beyond atomic BECs. (Winkler et al., Physical Review Letters, 5 August 2005; lab website, www.dark.ultracold.at)