The University of Science and Technology of China prepared ultracold three-atom molecular ensembles with high phase spatial density

Pan Jianwei, Zhao Bo and others of the University of Science and Technology of China (hereinafter referred to as USTC) prepared ultracold triatomic molecular ensembles with high phase spatial density for the first time in the world by coherent synthesis. In this study, they used magnetic association technology to prepare ultracold three-atom molecular ensembles from degenerate sodium-potassium-potassium atomic mixtures near the ground state diatomic molecules and atomic Feshbach resonance, which was an important step towards the study of ultracold quantum chemistry and quantum simulation based on ultracold molecules. On December 2, the research results were published in the international authoritative academic journal Science.

The reviewers agreed that this work is a milestone in the field of ultracold molecular research, opening up a new direction for the study of ultracold chemistry and quantum simulation.

Schematic .jpeg of magnetic association to prepare ultracold triatomic molecular ensemble

Schematic diagram of magnetic association to prepare ultracold triatomic molecular ensemble Courtesy of China University of Science and Technology

The use of highly controllable ultracold molecules to simulate complex and difficult to calculate chemical reaction processes can accurately and comprehensively study complex systems, so it has a wide range of application prospects in ultracold chemistry and new material design. However, due to the complexity of the oscillation level inside the molecule and the lack of cyclic transitions required for laser cooling, it is very difficult to prepare ultracold molecules by direct cooling. With the development of cold atom technology, the coherent synthesis of ultracold molecules from ultracold atoms provides a new way for the preparation of ultracold molecular ensembles.

In 1998, the Wolfgang Ketterle group at MIT observed the Feshbach resonance in atoms. In 2003, Deborah Jin’s research group at the University of Colorado developed magnetic association techniques to prepare potassium diatomic molecules using the Feshbach resonance of atoms. Diatomic molecules prepared from ultracold atoms have the advantages of high phase space density and low temperature, and can be coherently transferred to the vibration-to-ground state with a laser. In recent years, diatomic molecules of a variety of alkali metal atoms have been prepared in other laboratories and widely used in the research of ultra-cold chemistry and quantum simulation.

With the great success of diatomic molecule research, people began to study how to prepare ultracold triatomic molecules. However, due to the extreme complexity of triatomic molecules, theoretical calculations cannot be carried out, and whether it is possible to prepare triatomic molecular ensembles by coherent synthesis has always been an open question. In 2019, a research team from the University of Science and Technology of China observed the Feshbach resonance between sodium-potassium molecules and potassium atoms at ultra-low temperatures, laying the foundation for the synthesis of three-atom molecules. On this basis, the joint research group of the University of Science and Technology of China and the Institute of Chemistry of the Chinese Academy of Sciences adopted radio frequency synthesis technology in early 2022 to achieve radiofrequency synthesis of ultra-cold three-atom molecules near the Feshbach resonance of sodium-potassium ground state molecules and potassium atoms. However, due to the short lifetime and low synthesis efficiency of triatomic molecules, indirect evidence of synthesizing triatomic molecules can only be obtained through the loss of diatomic molecules or atoms, and it is still a great experimental challenge to directly probe triatomic molecules and prepare ultracold triatomic molecular ensembles.

In this study, the team started from the quantum degenerate sodium-potassium molecule and potassium atomic mixture, and slowly scanned the magnetic field to insolatically transfer the sodium-potassium molecule-potassium atom scattering state to the three-atom molecular bound state by slowly scanning the magnetic field, thus successfully preparing an ultracold three-atom molecular ensemble with high phase spatial density coherently by magnetic association technology for the first time. The research team used radio frequency dissociation technology to dissociate triatomic molecules into free sodium-potassium molecules and atoms, and obtained the dissociation spectrum of triatomic molecules, thereby realizing the direct detection of triatomic molecules.

The experimental results show that the phase space density of the obtained triatomic molecular gas is increased by about 10 orders of magnitude compared to other methods. The preparation of ultracold triatomic molecular ensembles paves the way for simulating the three-body problem under quantum mechanics, and the obtained high phase space density also makes it possible to prepare Bose–Einstein condensation of three-atom molecules. (Source: Wang Min, China Science News)

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