For the first time, scientists have observed solo atoms floating freely and interacting in space. The discovery helps to confirm some of the most basic principles of quantum mechanics that were first predicted more than a century ago but were never directly verified.
Individual atoms are notoriously difficult to observe due to their quantum nature. Researchers cannot, for example, know both an atom's position and its velocity at the same time, due to quantum weirdness. But using certain laser techniques, they have captured images of clouds of atoms.
"It's like seeing a cloud in the sky, but not the individual water molecules that make up the cloud," Martin Zwierlein, a physicist at MIT and co-author of the new research, said in a statement.
The new method goes one step further, allowing scientists to capture images of "free-range" atoms in free space. First, Zwierlein and his colleagues corralled a cloud of sodium atoms in a loose trap at ultracold temperatures. Then, they shot a lattice of laser light through the cloud to temporarily freeze the atoms in place. A second, fluorescent laser then illuminated the individual atoms' positions.
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The observed atoms belong to a group called bosons. These particles share the same quantum mechanical state and, as a result, behave like a wave, bunching together. This concept was first proposed by French physicist Louis de Broglie in 1924 and has subsequently become known as a "de Broglie wave."
Sure enough, the bosons Zwierlein and his team observed displayed de Broglie wave behavior. The researchers also captured images of lithium fermions — a type of particle that repels similar particles rather than bunching together.
The results were published May 5 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.
"We are able to see single atoms in these interesting clouds of atoms and what they are doing in relation to each other, which is beautiful," Zwierlein said.
In the future, the team plans to use the new technique — called "atom-resolved microscopy" — to investigate other quantum mechanical phenomena. For example, they may use it to try observing the "quantum Hall effect," in which electrons sync up under the influence of a strong magnetic field.
