Physicists finally measure a long-theorized molecule made of light and matter

Physicists have just seen light playing the role of “glue” between atoms, in a kind of weakly bound molecule.

“We succeeded for the first time in polarizing several atoms together in a controlled way, creating a measurable force of attraction between them”, explains Matthias Sonnleitner, physicist from the University of Innsbruck.

Atoms connect to form molecules in a variety of ways, all involving an exchange of charges like a kind of “superglue.”

Some share their negatively charged electrons, forming relatively strong bonds, like the simpler gases of two conjoined oxygen atoms that we constantly breathe, with the complex hydrocarbons floating in space. Some atoms attract each other due to differences in their overall charge.

Electromagnetic fields can alter the charge arrangements around the atom. Since light is a rapidly changing electromagnetic field, a shower of appropriately directed photons can push electrons into positions that, in theory, could see them bond.

“If you now turn on an external electric field, this charge distribution shifts a bit,” says physicist Philipp Haslinger from the Technical University of Vienna (TU Wien).

“Positive charge is slightly shifted in one direction, negative charge slightly in the other direction, the atom suddenly has a positive side and a negative side, it is polarized.”

Haslinger, TU Wien atomic physicist Mira Maiwöger and her colleagues used ultracold rubidium atoms to demonstrate that light can indeed polarize atoms in the same way, making otherwise neutral atoms a bit sticky.

“It’s a very weak force of attraction, so you have to conduct the experiment very carefully to be able to measure it,” says Maiwöger.

“If the atoms have a lot of energy and move quickly, the force of attraction disappears immediately. That’s why an ultracold atom cloud was used.”

The team trapped a cloud of about 5,000 atoms under a gold-coated chip, in a single plane, using a magnetic field.

This is where they cooled the atoms to temperatures near absolute zero (-273°C or -460°F), forming a quasi-condensate – so the rubidium particles begin to act collectively and share properties as if they were in the fifth state of matter, but not quite to the same extent.

Struck by a laser, the atoms experienced various forces. For example, the radiation pressure of incoming photons can push them along the light beam. During this time, electron responses can pull the atom back toward the most intense part of the beam.

To detect the subtle attraction thought to occur between atoms in this torrent of electromagnetism, researchers had to perform careful calculations.

When they turned off the magnetic field, the atoms fell in free fall for about 44 milliseconds before reaching the laser light field where they were also imaged using light sheet fluorescence microscopy.

During the fall, the cloud naturally expanded, allowing researchers to take measurements at different densities.

At high densities, Maiwöger and his colleagues found that up to 18% of atoms were missing from the observational images they took. They think these absences were caused by light-assisted collisions that knocked the rubidium atoms out of their cloud.

This demonstrated part of what was happening – it was not just incoming light that influenced the atoms, but also the light scattered by the other atoms. When light hit atoms, it gave them polarity.

Depending on the type of light used, atoms were either attracted or repelled by greater light intensity. Thus, they were either drawn to the region of lower light or higher light – in each case, they ended up accumulating together.

“An essential difference between the usual radiation forces and the [light triggered] is that the latter is an efficient particle-particle interaction, mediated by scattered light,” Maiwöger and colleagues write in their paper.

“It does not trap atoms at a fixed position (eg the focus of a laser beam) but pulls them to regions of maximum particle density.”

Although this force pulling the atoms together is much weaker than the molecular forces we are more familiar with, on a large scale it can add up. This can alter emission patterns and resonance lines – features that astronomers use to inform our understanding of celestial objects.

It could also help explain how molecules form in space.

“In the vastness of space, small forces can play a big role,” says Haslinger.

“Here we were able to show for the first time that electromagnetic radiation can generate a force between atoms, which may help shed new light on astrophysical scenarios that have yet to be explained.”

This research was published in Physical examination X.

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