In collaboration with Jun Ye's group at JILA, we are studying an ultracold gas of polar molecules. The ultracold molecule gas is made by first cooling atoms to ultralow temperature, and then converting atoms pairwise into diatomic molecules. We use a Fano-Feshbach resonance to form very weakly bound KRb molecules from a mixture of K-40 and Rb-87 atoms. We then use lasers to coherently transfer these large, weakly bound molecules into their ground ro-vibrational state.
We discovered that the trapped, ultracold KRb molecules can be lost due to chemical reactions, even at temperatures as low as 200 nanoKelvin. This ultracold chemistry is a fascinating topic, and we have learned how quantum statistics and trapping geometry can be used to suppress the reaction rate.
One of our goals is to explore the many-body quantum behavior of a gas with long-range interactions, as opposed to the usual short-range interactions of ultracold atoms. KRb molecules have an electric dipole moment of about 0.6 Debye in their ground state. In an applied electric field (or when the molecules are in a superposition of rotational states) the molecules have strong, dipole-dipole interactions. We have explored effects of these interactions for ultracold molecules confined in a three-dimensional optical lattice trap.