Physicists at the MIT-Harvard Center for Ultracold Atoms have pulled off a feat once confined to the blackboards of theorists: they’ve taken the first direct images of atoms freely interacting in space.

The researchers produced striking images of bosons and fermions — the two major families of quantum particles. Here they were caught in the act of clustering and pairing, as predicted by quantum mechanics but never before seen in such stark clarity.

“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,” said Martin Zwierlein, a professor of physics at MIT and senior author of the study.

Peering into the Quantum Fog

Atoms can be slippery. Each one is about a tenth of a nanometer across — millions could fit along a single human hair. But their size isn’t what makes them elusive. It’s their quantum nature.

In the strange world of quantum mechanics, you can’t know exactly where an atom is and how fast it’s moving at the same time. That uncertainty has long stymied efforts to capture atoms in motion, especially as they interact.

Existing techniques, like absorption imaging, can show a shadowy outline of a cloud of atoms. But that’s like taking a picture of a cloud without discerning the individual droplets. What the MIT team wanted was more ambitious than anything attempted before. They wanted to image the individual droplets forming the mist — to see atoms mid-motion, shaping the invisible dynamics of quantum matter.

They achieved this with a new method they call atom-resolved microscopy. The team used atom-resolved fluorescence microscopy to study quantum gases not in a lattice, as is common, but in the continuum — a more natural state where atoms roam freely, unconfined by a crystalline structure.

They studied two classic systems. One was a weakly interacting two-dimensional Bose gas made from sodium atoms. The other was a strongly interacting two-dimensional Fermi gas using lithium atoms.

First, they allowed a cloud of atoms to interact in a loose trap formed by a laser beam. Then they suddenly turned on a lattice of light — effectively freezing the atoms in place. A second laser illuminated them just long enough to take a snapshot.

The team had to tread carefully. Too much light, and the atoms would be knocked from their positions. Too little, and they’d remain invisible.

“The hardest part was to gather the light from the atoms without boiling them out of the optical lattice,” said Zwierlein. “You can imagine if you took a flamethrower to these atoms, they would not like that.”

Bosons Bunch, Fermions Flee

Whether particles tend to cluster or repel depends not only on physical forces, but also on their quantum nature. Bosons — particles like photons or helium-4 atoms — are gregarious. They tend to occupy the same space. Fermions — like electrons or lithium-6 atoms — are loners, barred from occupying the same state by the Pauli exclusion principle.

These differences were stark in the images obtained through the new technique. In images of the thermal Bose gas, bosons crowded together more often than random chance would predict. In contrast, the Fermi gas revealed a void around each atom — a “Fermi hole,” where no other same-spin fermion dared to tread.

“The probability to find two bosons near each other is enhanced above mere chance, while for fermions it is reduced,” the authors wrote in their study published in the journal Physical Review Letters.

Under ultracold conditions, bosons form a curious state of matter known as a Bose-Einstein condensate (BEC), in which all the particles share the same quantum wave. BECs were first predicted in the 1920s by Albert Einstein and the Indian physicist Satyendra Bose.

Zwierlein’s lab was now able to see this bunching in action.

Visualising Physics

For the first time, physicists could directly observe the wave-like nature of bosons. This behavior was first theorized by Louis de Broglie, whose ideas laid the foundation for quantum mechanics.

“We understand so much more about the world from this wave-like nature,” Zwierlein said. “But it’s really tough to observe these quantum, wave-like effects. However, in our new microscope, we can visualize this wave directly.”

The images of the fermions were also intriguing. The physicists saw two types of lithium atoms forming pairs, the fundamental building blocks of phenomena like superconductivity. The team could now see what theory had long suggested: that these elusive pairs form even in free space, far from the confines of a crystal lattice.

“This kind of pairing is the basis of a mathematical construction people came up with to explain experiments,” said MIT physicist Richard Fletcher, a co-author of the study. “But when you see pictures like these, it’s showing in a photograph, an object that was discovered in the mathematical world. So it’s a very nice reminder that physics is about physical things. It’s real.”

Why It Matters

The ability to image quantum gases in the continuum with single-particle resolution is more than a technical feat — it’s a new language for decoding the most complex phases of matter. For decades, researchers have used ultracold atoms to simulate everything from high-temperature superconductors to neutron stars. Now, they can inspect those simulations atom by atom.

The research could also aid in engineering future quantum devices — sensors, simulators, and possibly even computers — that rely on the subtle interplay of quantum particles. As the technology matures, researchers hope to extend it to three-dimensional systems, to Bose-Fermi mixtures, or to gases with imbalanced spins — where theory predicts even stranger behaviors like supersolids.

In the immediate future, Zwierlein’s group wants to push their microscope further. They aim to investigate exotic quantum states such as those seen in the quantum Hall effect — where electrons form strange, highly correlated states under a magnetic field.

“That’s where theory gets really hairy — where people start drawing pictures instead of being able to write down a full-fledged theory because they can’t fully solve it,” said Zwierlein. “Now we can verify whether these cartoons of quantum Hall states are actually real. Because they are pretty bizarre states.”

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