China achieves quantum coherent synthesis of triatomic molecules in ultracold atomic-molecular gas mixture for the first time

Pan Jianwei and Zhao Bo of the University of science and technology of China cooperated with Bai Chunli group of the Institute of chemistry of the Chinese Academy of Sciences to realize the coherent synthesis of triatomic molecules for the first time in the mixture of ultracold atoms and diatomic molecules. In this study, near the Feshbach resonance of potassium atom and sodium potassium ground state molecule, they used RF field to coherently synthesize atoms and diatomic molecules into ultracold triatomic molecules, which took an important step towards the research of quantum simulation and ultracold quantum chemistry based on ultracold atoms and molecules. On February 10, this important research achievement was published in the international authoritative academic journal Nature.

Figure: schematic diagram of synthesis of triatomic molecules by RF field from the mixture of ultracold atoms and diatomic molecules

Quantum computing and quantum simulation have powerful parallel computing and simulation capabilities. They can not only solve the computational problems that cannot be handled by classical computers, but also effectively reveal the laws of complex physical systems, so as to provide guidance for the development of new energy and the design of new materials. The ultimate goal of quantum computing research is to build a general-purpose quantum computer, but to achieve this goal requires the preparation of large-scale quantum entanglement and fault-tolerant computing, which still needs long-term unremitting efforts. At present, the short-term goal of quantum computing is to develop special quantum computer, that is, special quantum simulator, which can solve some specific problems that can not be solved by existing classical computers. For example, quantum simulation of ultracold atoms and molecules, which uses highly controllable ultracold quantum gas to simulate complex physical systems that are difficult to calculate, can accurately and comprehensively study complex systems, so it has a wide range of application prospects in chemical reactions and new material design.

Supercooled molecules will open up new ideas for the realization of quantum computing and provide an ideal platform for quantum simulation. However, due to the complexity of vibrational and rotational energy levels in molecules, it is very difficult to prepare supercooled molecules by direct cooling. The development of ultracold atom technology provides a new way to prepare ultracold molecules. People can bypass the difficulty of directly cooling molecules and synthesize molecules from ultracold atomic gas by using laser and electromagnetic field. The study of using light to synthesize molecules from atomic gas can be traced back to the 1980s. The emergence of laser cooling atomic technology makes the rapid development of photosynthesis of diatomic molecules, and has been widely used in high-precision spectral measurement. After the success of photosynthesis of diatomic molecules, people began to think about whether to use quantum control technology to synthesize diatomic molecules from the mixture of atoms and diatomic molecules. In the review article [Rev. Mod. Phys. 78483, (2006)] published in 2006, Professor Paul julienne of the National Bureau of standards and others reviewed the development history of photosynthesis of diatomic molecules in the past two decades, and pointed out that the synthesis of triatomic molecules from the mixture of atoms and diatomic molecules is an important research direction in the field of synthetic molecules in the future. Due to the disadvantages of low density and high temperature, the photosynthesized diatomic molecular gas has not been used to study the synthesis of triatomic molecules. Later, with the development of Feshbach resonance technology in ultracold atomic gas, the synthesis of molecules by magnetic field or RF field became the main technical means to prepare ultracold diatomic molecules. Diatomic molecules prepared from ultracold atoms have the advantages of high phase space density and low temperature, and can be coherently transferred to the vibrational ground state by laser. Since the joint experimental team of Deborah Jin and ye Jun, academicians of the American Academy of Sciences, prepared rubidium potassium ultracold ground state molecules in 2008, diatomic molecules of various alkali metal atoms have been prepared in other laboratories and widely used in the research of ultracold chemistry and quantum simulation.

The successful preparation of ultracold ground state molecules has aroused people's interest in the synthesis of triatomic molecules. In 2015, Professor Olivier dulieu of the national scientific research center of France and others theoretically analyzed the feasibility of synthesizing triatomic molecules from atomic diatomic molecular mixture [Phys. Rev. Lett. 115, 073201 (2015)]. However, because the interaction of three atom molecules is extremely complex and cannot be calculated accurately, it is impossible to predict the energy of the bound state and the coupling strength of the scattered state and the bound state of three atom molecules in theory. The research team of the University of science and technology of China first observed the Feshbach resonance of atoms and diatomic molecules at ultra-low temperature in 2019. The relevant results were published in the journal Science [science 363, 261 (2019)]. Near the Feshbach resonance, the energy of the bound state and the energy of the scattered state of the triatomic molecule tend to be consistent, and the coupling between the scattered state and the bound state is greatly enhanced by resonance. The successful observation of atomic and molecular Feshbach resonance provides a new opportunity for the synthesis of triatomic molecules. However, because the Feshbach resonance of atoms and molecules is very complex and difficult to understand in theory, whether and how to use Feshbach resonance to synthesize triatomic molecules is still a great challenge in experiment.

In this study, the research team of the University of science and technology of China and the research team of the Institute of chemistry of the Chinese Academy of Sciences successfully realized the coherent synthesis of triatomic molecules by RF field for the first time. In the experiment, they prepared sodium potassium ground state molecules in a single hyperfine state from an ultracold atomic mixture near absolute zero. Near the Feshbach resonance of potassium atom and sodium potassium molecule, the scattered state of atom and molecule is coupled with the bound state of triatomic molecule by RF field. They successfully observed the signal of radio frequency synthesis of triatomic molecules on the radio frequency loss spectrum of sodium and potassium molecules, and measured the binding energy of triatomic molecules near the Feshbach resonance. This work opens up a new way for the study of quantum simulation and supercooled chemistry. Ultracold triatomic molecule is an ideal research platform to simulate the three body problem in quantum mechanics. The three body problem is extremely complex. Even the classical three body problem can not be solved accurately due to the existence of chaotic effect. Under the constraints of quantum mechanics, the three body problem becomes more elusive. How to understand and describe the three body problem in quantum mechanics has always been an important problem in few body physics. In addition, ultracold triatomic molecules can be used to achieve ultra-high-precision spectral measurements, which provides an important benchmark for characterizing the complex three body interaction potential energy surface. Because the calculation of potential energy surface needs to solve the multi electron Schrodinger equation with high accuracy, the potential energy surface of ultracold triatomic molecules also provides important information for the problem of electronic structure in quantum chemistry.

The research work has been supported by the Ministry of science and technology, the natural science foundation of China, the Chinese Academy of Sciences, Anhui Province, Shanghai and other units.

Paper link:https://www.nature.com/articles/s41586-021-04297-2