Scientists create cesium-based 'artificial atoms' in a quantum simulator

In a new study, scientists use a solid-state quantum simulator to create artificial atoms to probe the behavior of molecules and atomic structure.
Tejasri Gururaj
artificial atom
the science of artificial atoms


Studying the quantum properties of matter is challenging due to many factors, such as cost, technical complexity, the need for specialized equipment, and the delicate nature of quantum systems. Classical computers and classical systems do not have the capacity to simulate the intricate world of quantum mechanics. 

Quantum simulators allow scientists to study quantum systems and discover new materials, properties, and phenomena previously unknown. Quantum simulators can be quantum computers or physical systems which are used to probe matter at the tiniest scales. 

A team of scientists has now developed a solid-state quantum simulator to emulate molecular orbitals. The research group, led by Alexander Khajetoorians from Radboud University in the Netherlands, used the quantum simulator to create synthetic benzene using artificial atoms.

Creating artificial atoms

The team chose to create artificial cesium atoms. 

The first step was choosing indium antimonide (InSb) as the substrate. InSb is a narrow band gap semiconductor that provides the necessary conductivity, surface confinement, and decoupling from the bulk material to create artificial atoms and study their behavior effectively.

Then the team used scanning tunneling microscopy and spectroscopy techniques to create surface confinements of electrons on the semiconductor surface. In simple terms, they confined the electrons to specific energy levels or states on the semiconductor surface.

The next step was to localize the cesium atoms. They used highly electropositive, or electron-releasing, cesium atoms on the semiconductor surface. The cesium atoms released electrons into a bound state in the semiconductor bandgap, forming a 2D electron gas. 

The 2D electron gas did not couple strongly with the bulk material, implying that the behavior and properties of the 2D electron gas were relatively independent and distinct from the properties of the bulk material. This allowed them to study and manipulate the artificial atoms and molecular structures formed by the cesium atoms.

Finally, they arranged the cesium atoms into localized states such that the structure looked like an atom without a nucleus or a cesium ring. These cesium rings exhibited electromagnetic potentials similar to those between real atoms. 

Scientists create cesium-based 'artificial atoms' in a quantum simulator
Scanning tunnelling microscopy image of artificial atoms, each built using eight caesium atoms

From artificial atoms to molecules

The team continued their work by arranging the artificial atoms into artificial molecules, including trans and cis butadienes and artificial benzene. Butadienes are a group of organic compounds commonly used in the production of synthetic rubber, and benzene is also an organic compound used in the production of various chemicals and as a solvent in industries such as pharmaceuticals, paints, and adhesives.

The researchers observed proof of sp2 hybridization in their 2D models, which refers to mixing atomic orbitals to form molecular orbitals. The sp2 hybridization is significant as it demonstrates the artificial atoms' ability to mimic chemical bonding, which can be used to study new materials and properties. 

In a press release, co-author Daniel Wegner said, "We can now play mind games that previously only a theoretician could with DFT (density functional theory) code and just say what if? What if I change the bond order of a molecule? A chemist cannot do that: a chemist can only synthesize what nature determines is the ground state. But there is no relaxation of artificial atomic sites – they cannot stabilize and move by themselves: we move them."

This finding provides valuable insights into the behavior of molecular orbitals and the interplay between atomic structures and resulting electronic properties, contributing to a deeper understanding of chemical bonding and potentially paving the way for new discoveries in quantum chemistry and material science.

The findings of the study are published in the journal Science.

Study abstract:

Bottom-up quantum simulators have been developed to quantify the role of various interactions, dimensionality, and structure in creating electronic states of matter. Here, we demonstrated a solid-state quantum simulator emulating molecular orbitals, based solely on positioning individual cesium atoms on an indium antimonide surface. Using scanning tunneling microscopy and spectroscopy, combined with ab initio calculations, we showed that artificial atoms could be made from localized states created from patterned cesium rings. These artificial atoms served as building blocks to realize artificial molecular structures with different orbital symmetries. These corresponding molecular orbitals allowed us to simulate two-dimensional structures reminiscent of well-known organic molecules. This platform could further be used to monitor the interplay between atomic structures and the resulting molecular orbital landscape with submolecular precision.

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