Probing the intimacies of atom-to-atom contact

A new experiment has mapped the process of two atoms colliding. It’s something that happens every time you touch something, but never before has it been observed in such exquisite detail.

A detailed understanding of the meeting of two atoms is crucial for the development of quantum sensors, qubits for use in quantum computers, or high-density computer memory based on individual atoms.

Kai Yang at the IBM Almaden Research Centre in the US led a team that poked a titanium atom with an iron atom. The former was sitting on a surface, and the latter held onto the tip of an electron microscope. 

The researchers reported in the journal Physical Review Letters how they watched the strength of the magnetic interaction, a measure of the overlap of the edges of the two atoms. From first contact to full collision it grew by a magnitude of 10,000.

Atoms do not have hard edges. Their outer boundary is defined by the roaming of their electrons, within their probability-based orbits, called wave functions. As atoms move toward each other they begin to interact through a force known as the exchange interaction, which grows as the overlap between the wave functions gets stronger.

“The atoms are close enough that one wave function is talking directly to the wave function of the other atom,” says Andreas Heinrich, a team member at the Institute for Basic Science in South Korea. 

“The exchange interaction is responsible for most of chemistry – makes the electronic interactions of molecular bonding, leads to the Pauli exclusion principle, and leads to magnetic interactions, and so on.” 

In this experiment the researchers deposited the titanium atom on a layer of magnesium oxide two atoms thick, which served as an isolating insulator.

They measured the magnetic effect of the approaching iron atom on the titanium atom’s energy levels with two separate methods: electron spin resonance, which gave more detailed measurements for the weaker interaction, and inelastic electron tunnelling spectroscopy, which allowed measurements of the stronger interactions at closer quarters.

The two sets of measurements fell on the same exponential curve, giving the researchers confidence they were measuring the one interaction across a broad scale.

Such precise measurements over such a wide range of energies is a boon for the development of atomic-scale quantum technologies, says Heinrich.

“We can see on the atomic scale, we know where the atoms are,” he explains.

“We can change the placement, so we can build structures, and we can measure the properties and coherently control them for quantum operations.”

An example, Heinrich says, might be to place two or three atoms side by side on a surface, and then to apply a force to one of them and observe how the connections between the atoms vary. 

“We can learn something about the underlying mechanism,” he says.

While Heinrich is proud of the measurements, he said the results were not a surprise.

“It’s a detailed measurement of what we would expect,” he says. 

“That’s what makes quantum mechanics an amazing tool – it just seems to hold for anything you throw at it and be precisely correct. It’s totally amazing.”

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