It is now routine in many laboratories to produce ultracold diatomic molecular gases at temperatures below a microkelvin. Following the success of creating ultracold diatoms over the last two decades, there is also growing interest in forming ultracold polyatomic molecules containing three or more atoms. Owing to their rich internal structure, they offer unique opportunities for studies of cold chemistry, precision measurements, the realization of exotic quantum phases, and quantum information processing. Their increased complexity, however, presents challenges in using conventional cooling methods. Nevertheless, in a recent breakthrough experiment [X.-Y. Chen et al., Nature 626, 283 (2024)], ultracold tetratomic (NaK)2 molecules have been created by associating two fermionic NaK molecules in the presence of microwave fields. These are called field-linked molecules because an external field is required for their formation. This has stimulated immense interest in the field of ultracold research, as their method is near-universal and can be applied to a wide range of ultracold polar molecules.
The observed ultracold tetratomic molecules (“tetramers”) are formed in extremely weakly bound states at a very long range, with distances between the two constituent diatoms of about 100 nm. They have limited lifetimes due to loss induced by the external field. On the other hand, the realization of long-lived samples of ultracold tetramer molecular gas, produced near the global minimum of their ground-state interaction potential energy surface, will allow us to explore the aforementioned new physics. In my Marie Skłodowska-Curie Action, my objectives were to develop new theoretical methods for transferring weakly bound FL tetramers to their absolute ground state using optical fields.
Objective 1: Developing methodology for mitigating loss of ultracold molecules in ground and excited electronic states using external static electric and microwave fields.
Objective 2: Developing a new methodology for stimulated adiabatic transfer of tetramer molecules from the excited state to the ground rovibrational electronic state.
