Association of Molecules
To date the coldest polar molecules with a temperature below 1μK have been created by magnetic-field Feshbach resonances, photo-association and, most recently, radio-frequency magnetic-dipole transition. In the majority of cases these molecules are formed in highly-excited vibrational levels of the electronic ground state. However, many proposals to use polar molecules, either in simulation of many-body systems or as qubits in a quantum processor, require these molecules to be vibrationally cold.
From 2003 we have been interested in formation of ultracold molecules from laser-cooled atoms. We developed several detailed mechanisms to convert trapped ultracold atoms of the same or different species into vibrationally-cold molecules.
In our work (Eur. Phys. J. D 31, 189 (2004)) on photoassociative formation of ultracold polar KRb molecules we discussed the possibility of creating molecules in the lowest ro-vibrational state from doubly spin-polarized K and Rb atoms, using intermediate excited states of 0+ symmetry. In addition, we investigated the stability of polar KRb molecules in the presence of thermal black-body radiation.
Later in (Phys. Rev. Lett. 99, 073003 (2007)) we proposed a new mechanism to produce ultracold polar molecules with microwave fields. As opposed to production of molecules by photoassociation or magnetic-field Feshbach resonances our method does not rely on the structure and lifetime of electronically excited states or the existence of Feshbach resonances.
Finally, we theoretically analyzed the creation of a high phase-space-density gas of KRb molecules starting from weakly-bound near-threshold vibrational states formed by a magnetic Feshbach resonance (Science 322, 231 (2008) and Nature Physics 4, 622 - 626 (2008)). Our analysis included multi-channel bound-state calculations of the ground- and intermediate excited-state ro-vibrational levels. For the excited states this takes into account the strong spin-orbit interaction. More details have been published in a special issue on cold molecules in New J. Phys. 11, 055043 (2009).
Figure 1. Two-photon Raman process to associate two alkali-metal atoms into a molecule