Novel approach for scattering resonances of ultracold molecules

Scattering resonances occur in collisions between atoms or molecules when a bound state of the collision complex occurs at the collision energy. In atomic quantum gases, resonances are used to control the effective contact interactions between atoms, enabling applications like quantum simulation. However, for most ultracold molecules, scattering resonances do not occur due to the fast loss which takes place when two molecules collide. In an earlier study , it was demonstrated that ultracold molecules can be protected from these losses in mutual collisions using microwaves. Researchers from the Institute for Molecules and Materials (IMM) of Radboud University and MPQ in Garching now demonstrate that the microwaves also lead to a new type of scattering resonance for polar molecules which opens new possibilities for investigating quantum many-body physics using molecules.

Field-linked resonances

Ultracold molecules undergo fast loss in collisions when two molecules come close together. These losses are prevented by “microwave shielding”, where the molecules are dressed with microwaves that create repulsive interactions between the molecules.

Outside the repulsive shield, the molecules interact via dipole-dipole interactions. These interactions can be attractive, creating a potential well that can support bound states. The positions of these “field-linked” bound states can then be controlled at will using the microwaves.

By pushing the field-linked state around, we can create a resonance that can be used to control the effective ‘contact interaction’ between molecules.

Illustration of interaction potentials for different microwave polarization
Figure: Illustration of interaction potentials for different microwave polarization that support zero, one, and two field-linked states, which results in zero, one, and two resonance peaks when scanning the microwave frequency.

Controllable interactions

The work has been done in collaboration between theory by Tijs Karman at the IMM and the experimental group of Xinyu Luo and Immanuel Bloch at the Max Planck institute for Quantum optics (MPQ) in Garching, Germany. The research team measured interactions between “shielded” molecules, and how these depend on the microwave frequency and polarization. Using the microwaves, the researchers could  independently control both the dipole-dipole interaction and the ‘contact interaction’ at shorter distances. The ‘contact interaction’ is controlled using the “field-linked states”.

This realizes a stable quantum gas of molecules with controllable interactions; an ideal platform to investigate novel quantum many-body physics and to produce ultracold four-atomic molecules. The results have been published in a recent Nature publication entitled ‘Field-linked resonances of polar molecules’.

Theoretical Chemistry

Tijs Karman is Assistant Professor in the Theoretical and Computational Chemistry department. The group is part of IMM. They aim to explain and predict properties of molecules, clusters, and molecular solids. Part of the research is to study quantum phenomena in molecular collisions. Karman focuses on theoretical research into collisions between ultracold molecules, which are promising for quantum computing and simulation. Ultimately realizing these applications requires understanding and controlling collisions and interactions between molecules.

Article information

Field-linked resonances of polar molecules
Xing-Yan Chen, Andreas Schindewolf, Sebastian Eppelt, Roman Bause, Marcel Duda, Shrestha Biswas, Tijs Karman, Timon Hilker, Immanuel Bloch, and Xin-Yu Luo
Nature (2023)

More information?

For more information, please contact
Tijs Karman: t.karman [at]
IMM Communications: imm-communication [at]

Link Nature publication Evaporation of microwave-shielded polar molecules to quantum degeneracy | Nature

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