Physicists have just captured light acting as part of the “glue” between atoms, in a kind of loosely bound molecule.
“We have succeeded for the first time in polarizing several atoms together in a controlled manner, creating a measurable attractive force between them,” says Matthias Sonnleitner, a physicist at the University of Innsbruck.
Atoms connect to form molecules in a variety of ways, all involving a trade of charges as a kind of “superglue”.
Some share their negatively charged electrons, forming relatively strong bonds, like the simpler gases of two bonded oxygen atoms that we constantly breathe, with the complex hydrocarbons found floating in space. Some atoms are attracted by virtue of differences in their overall charge.
Electromagnetic fields can alter the arrangement of charges around the atom. Since light is a rapidly changing electromagnetic field, a shower of properly directed photons can drive electrons into positions that could, in theory, see them bond.
“If you now activate an external electric field, this charge distribution changes a little,” explains physicist Philipp Haslinger of the Technical University of Vienna (TU Wien).
“The positive charge moves slightly in one direction, the negative charge slightly in the other direction, the atom suddenly has a positive side and a negative side, it’s polarized.”
Haslinger, TU Wien atomic physicist Mira Maiwöger and colleagues used ultracold rubidium atoms to show that light can polarize atoms in the same way, which in turn causes neutral atoms to become slightly sticky
“This is a very weak attractive force, so the experiment needs to be carried out very carefully to be able to measure it,” says Maiwöger.
“If the atoms have a lot of energy and are moving fast, the attractive force disappears immediately. That’s why a cloud of ultracold atoms was used.”
The team trapped a cloud of about 5,000 atoms below a gold-coated chip, in a single plane, using a magnetic field.
This is where they cooled the atoms to temperatures close to absolute zero (−273 °C or −460 °F), forming a quasicondensate, so that the rubidium particles begin to act collectively and share properties as if they were in the fifth state of matter, but not to the same extent.
Hit with a laser, the atoms experienced a variety of forces. For example, the radiation pressure of incoming photons can push them along the light beam. Meanwhile, the responses of the electrons can attract the atom to the more intense part of the beam.
To detect the subtle attraction thought to arise between atoms in this stream of electromagnetism, the researchers had to do some careful calculations.
When they turned off the magnetic field, the atoms fell freely 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 expanded naturally, so the researchers were able to take measurements at different densities.
At high densities, Maiwöger and his colleagues found that up to 18 percent of the atoms were missing from the observational images they were taking. They believe these absences were caused by light-assisted collisions that ejected rubidium atoms from their cloud.
This demonstrated part of what was happening: not only was the incoming light influencing the atoms, but also the scattering of light from other atoms. When the light hit the atoms it gave them a polarity.
Depending on the type of light used, the atoms were attracted or repelled by a greater intensity of light. Thus, they were either dragged into the lower light or higher light region, in each case they ended up accumulating together.
“An essential difference between ordinary radiation forces and the [light triggered] The interaction is that the latter is an effective particle-particle interaction, mediated by scattered light,” Maiwöger and colleagues write in their paper.
“It doesn’t trap the atoms in a fixed position (for example, the focus of a laser beam), but instead attracts them to regions of maximum particle density.”
Although this force that holds atoms together is much weaker than the molecular forces we are more familiar with, on large scales it can add up. This can change 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 can help shed new light on as-yet-unexplained astrophysical scenarios.”
This research was published in Physical Review X.