For the first time in the laboratory, a special bonding state has been created between atoms: A laser beam allows atoms to be polarized so that they are positively charged on one side and negative on the other. This causes them to attract each other and create a very special bonding state – much weaker than the bond between two atoms in a regular molecule, but still measurable. The attraction comes from the polarized atoms themselves, but it is the laser beam that enables them to do so – in a sense it is a “molecule” of light and matter.
Theoretically, this effect has long been predicted, but now scientists at the Vienna Center for Quantum Science and Technology (VCQ) at TU Wien, in collaboration with the University of Innsbruck, have managed to measure this exotic atomic bond for the first time. time. This interaction is useful for manipulating extremely cold atoms, and the effect could also play a role in the formation of molecules in space. The results have now been published in the scientific journal Physical Assessment X.
Positive and Negative Charge
In an electrically neutral atom, a positively charged nucleus is surrounded by negatively charged electrons, which surround the atomic nucleus just like a cloud. “Now if you switch on an external electric field, this charge distribution shifts a bit,” explains Prof. Philipp Haslinger, whose research at the Atominstitut of TU Wien is supported by the FWF START program. “The positive charge shifted slightly in one direction, the negative charge something the other way, the atom suddenly has a positive side and a negative side, it’s polarized.”
Light is just a electromagnetic field that changes very quickly, so it is also possible to create this polarization effect with laser light. When several atoms are next to each other, the laser light polarizes them all in exactly the same way: positive on the left and negative on the right, or vice versa. In either case, two neighboring atoms spin different charges towards each other, leading to an attractive force.
Experiments with the atomic trap
“This is a very weak pull, so you have to do the experiment very carefully to be able to measure it,” said Mira Maiwöger of TU Wien, the first author of the publication. “If atoms have a lot of energy and are moving quickly, the attraction immediately disappears. That’s why a cloud of ultra-cold atoms was used.”
The atoms are first collected and cooled in a magnetic trap on an atom chip, a technique that was developed at the Atominstitut in the group of Prof. Jörg Schmiedmayer. Then the trap is turned off and the atoms are released in free fall. The atomic cloud is “ultra-cold” at less than a millionth Kelvin, but it has enough energy to expand during the fall. However, if the atoms are polarized with a laser beam during this phase and thus an attraction is created between them, this expansion of the atomic cloud is slowed down – and thus the attraction is measured.
Quantum lab and space
“Polarizing individual atoms with laser beams is actually nothing new,” said Matthias Sonnleitner, who laid the theoretical foundation for the experiment. “However, the crucial thing about our experiment is that for the first time we have succeeded in polarizing different atoms together in a controlled manner, creating a measurable attraction between them.”
This one attractive force is an additional tool for checking cold atoms. But it can also be important in astrophysics: “In the vastness of space, small forces can play an important role,” says Philipp Haslinger. “Here we were able to show for the first time that electromagnetic radiation can generate a force between atomswhich may help shed new light on astrophysical scenarios that have not yet been explained.”
Mira Maiwöger et al, Observation of light-induced dipole-dipole forces in ultracold atomic gases, Physical Assessment X (2022). DOI: 10.1103/PhysRevX.12.031018
Vienna University of Technology
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