Ultracold Molecular Bose Gases

Ultracold molecular Bose gases, where the gas consists of diatomic molecules of fermionic atoms (atoms with an overall half-integral spin value), provide two important opportunities---the chance to do high-precision spectroscopy of molecules and the chance to study the process by which fermions (normally unable to form into coherent quantum condensates) amalgamate into pairs. The pairs are bosons (entities with a whole-number valued spin) and can form condensates. Randy Hulet and his colleagues at Rice University, the first to engineer a Bose Einstein condensation (BEC) in lithium-7 atoms (see PNU #237), have gotten a batch of Li-6 atoms to pair up (at least 50% of them at a time) at micro-kelvin temperatures by manipulating an external magnetic field. Although the group does not yet have evidence that the pairs, or molecules, have taken the final plunge by forming a BEC, the atoms have held together (in an optical trap) in their paired state for as long as 1 second, compared to millisecond times for previous experiments of this type. Hulet hopes that as the molecular gas hangs together long enough, it will cool off sufficiently through the evaporative process to form a BEC. Having a true BEC of molecules would give researchers the chance to study the Cooper pairing mechanism at work in superconductivity and in superfluidity of liquid helium-3. In ordinary molecules (joined by chemical forces) the constituents (atoms) are very close together. In the Cooper pairs characterizing superconductivity, the constituents (electrons) are only weakly coupled and are far apart from each other. Hulet and his group hope to dissociate the molecular condensate in order to produce Cooper pairs that fall in between these two cases, both as to the size and in the strength of the force holding the pairs together. One might even be able to simulate high-temperature superconductivity by loading ultracold fermion gases into an "optical lattice" consisting of crossed laser beams. (Strecker et al., Physical Review Letters, 22 August 2003; see figure and lab website)