Molecules can form in a variety of ways – and researchers have just found another one. They used a laser to create special bonding between ultracold atoms. The light from the laser can shift the distribution of electrons within the atoms, polarizing them. This makes one side of the atom positive and the other negatively charged. This polarization allows the atom to weakly bond as long as the laser keeps it going, making this a “molecule” made of matter and light.
This concept was predicted a long time ago and has been finally created, as reported in Physical Review X. While this bond is much weaker than a regular molecular bond, it is measurable.
"This is a very weak attractive force, so you have to conduct the experiment very carefully to be able to measure it," lead author Mira Maiwöger said in a statement. "If atoms have a lot of energy and are moving quickly, the attractive force is gone immediately. This is why a cloud of ultracold atoms was used."
The rubidium atoms were cooled to less than a millionth of a Kelvin, just a fraction of a degree above absolute zero. The atoms were in a cloud in free fall. It had enough energy to expand, and measuring it allowed the team to know if their approach worked.
The laser polarized the atoms in the cloud at the same time and in the same way, so two neighboring atoms were facing each other with opposite charges and did not have to move. As the expansion of the cloud slowed down after the laser was involved, the team was able to measure the attractive force.
"Polarising individual atoms with laser beams is basically nothing new," added Matthias Sonnleitner, who laid the theoretical foundation for the experiment. "The crucial thing about our experiment, however, is that we have succeeded for the first time in polarising several atoms together in a controlled way, creating a measurable attractive force between them."
The approach will certainly be used to control ultracold atoms in the lab, but researchers believe that it might explain interactions between atoms much further afield. In the depth of space, at very low temperatures, polarized atoms might attract each other.
"In the vastness of space, small forces can play a significant role," explained Philipp Haslinger. "Here, we were able to show for the first time that electromagnetic radiation can generate a force between atoms, which may help to shed new light on astrophysical scenarios that have not yet been explained."