Physicists have used a new method for observing individual atoms interacting in free space for the first time. The new method confirms a century-old theory of quantum mechanics. For the first time, scientists have observed atoms floating and interacting freely in space. The discovery helps confirm some of the most basic principles of quantum mechanics, which were first predicted more than a century ago but had never been directly tested.
Individual atoms are notoriously difficult to observe because of their quantum nature. Researchers can’t, for example, know both an atom’s position and its speed at the same time because of quantum weirdness. But using certain laser techniques, they have produced images of clouds of atoms.
Top: Two illustrations show how atoms in an atom trap (red) are suddenly frozen in place by an optical lattice. Bottom: Three microscope images show (from left to right) bosonic 23Na forming a Bose-Einstein condensate; one spin state in the weakly interacting Fermi mixture of 6Li; and both spin states of the strongly interacting Fermi mixture, directly showing pair formation. Yao et al.
“It’s like seeing a cloud in the sky but not the individual water molecules that make it up,” Martin Zwierlein, a physicist at the Massachusetts Institute of Technology and a co-author of the new study, said in a statement.
The new method goes a step further, allowing scientists to take pictures of atoms “free-roaming” in free space. First, Zwierlein and his colleagues trapped a cloud of sodium atoms in a free-roaming trap at ultra-cold temperatures. They then beamed a lattice of laser light through the cloud to temporarily freeze the atoms in place. Then a second, fluorescent laser illuminated the positions of individual atoms.
The observed particles belong to a group called bosons. They have the same quantum mechanical state and, as a result, behave like a wave, huddling together. This concept was first proposed by the French physicist Louis de Broglie in 1924 and later became known as the “de Broglie wave.”
Illustration of atoms floating freely in the air. Stanislav Pytel
Sure enough, the bosons that Zwierlein and his team observed exhibited de Broglie wave behavior. The researchers also captured images of lithium fermions, a type of particle that repel similar particles rather than clump together.
The results were published May 5, 2025, in the journal Physical Review Letters. Two other groups reported using a similar technique to observe pairs of bosons and fermions in the same issue of the journal.
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“We can see individual atoms in these interesting clouds of atoms and how they interact with each other, and that’s beautiful,” Zwierlein said.
In the future, the team plans to use the new technique — called “atom-resolved microscopy” — to study other quantum mechanical phenomena. For example, they could use it to observe the “quantum Hall effect,” in which electrons sync up when exposed to a strong magnetic field.
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