“`html
MIT physicists have captured the initial images of solitary atoms freely interacting in space. These visuals unveil correlations among the “free-range” particles that had been theorized but never directly witnessed until now. Their results, published today in the journal Physical Review Letters, will facilitate scientists in visualizing previously unseen quantum phenomena in actual space.
The images were acquired using a technique pioneered by the team that enables a cloud of atoms to move and engage interactively. The researchers subsequently activate a lattice of light that briefly immobilizes the atoms, then employ finely adjusted lasers to rapidly illuminate the suspended atoms, capturing an image of their locations before the atoms naturally disperse.
The physicists utilized the method to visualize clouds of various types of atoms, achieving several imaging milestones. The researchers directly observed atoms referred to as “bosons,” which clustered in a quantum phenomenon to form a wave. They also recorded atoms identified as “fermions” as they paired up in free space — a crucial mechanism that facilitates superconductivity.
“We can observe single atoms within these fascinating clusters of atoms and their interactions with one another, which is exquisite,” states Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT.
In the same journal issue, two additional groups report using analogous imaging techniques, including a team led by Nobel laureate Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT. Ketterle’s team visualized enhanced pair correlations among bosons, while the other group, from École Normale Supérieure in Paris, led by Tarik Yefsah, a former postdoc in Zwierlein’s lab, imaged a cloud of noninteracting fermions.
The study by Zwierlein and his colleagues is co-authored by MIT graduate students Ruixiao Yao, Sungjae Chi, and Mingxuan Wang, along with MIT assistant professor of physics Richard Fletcher.
Inside the cloud
A solitary atom measures approximately one-tenth of a nanometer in diameter, which is one-millionth the thickness of a human hair. Unlike hair, atoms behave and interact according to the principles of quantum mechanics; it is this quantum nature that complicates our understanding of atoms. For instance, we cannot simultaneously determine exactly where an atom is located and how quickly it is moving.
Scientists can employ various techniques to image individual atoms, including absorption imaging, where laser light is directed toward the atom cloud, casting its shadow onto a camera screen.
“These methodologies permit you to visualize the overall shape and structure of an atom cloud, but not the individual atoms themselves,” Zwierlein points out. “It’s akin to observing a cloud in the sky, yet not the individual water molecules that constitute the cloud.”
He and his associates opted for a markedly different approach to directly image atoms interacting in free space. Their method, termed “atom-resolved microscopy,” involves first confining a cloud of atoms in a loose trap formed by a laser beam. This trap holds the atoms in one location, allowing them to interact freely. The researchers then activate a lattice of light, momentarily freezing the atoms in their positions. Subsequently, a second laser illuminates the stationed atoms, whose fluorescence reveals their individual locations.
“The most challenging aspect was collecting light from the atoms without overwhelming them in the optical lattice,” Zwierlein explains. “Imagine employing a flamethrower on these atoms; they would certainly dislike that. Over the years, we’ve acquired various techniques on how to manage this. This is the first time we’ve achieved this in situ, allowing us to suddenly freeze the atoms’ motion when they’re strongly interacting, and observe them one by one. That’s what renders this technique more powerful than previous methods.”
Clusters and pairs
The team employed the imaging technique to directly observe interactions among both bosons and fermions. Photons serve as an example of a boson, whereas electrons are a variety of fermion. Atoms can be classified as bosons or fermions based on their total spin, which is determined by whether their count of protons, neutrons, and electrons is even or odd. Generally, bosons attract one another, while fermions repel.
Zwierlein and his group initially imaged a cloud of bosons composed of sodium atoms. At low temperatures, a cloud of bosons transitions into what is referred to as a Bose-Einstein condensate — a state of matter wherein all bosons share the same quantum state. MIT’s Ketterle was among the first to create a Bose-Einstein condensate of sodium atoms, for which he received the 2001 Nobel Prize in Physics.
Zwierlein’s group can now image the individual sodium atoms within the cloud, enabling them to observe their quantum interactions. It has long been hypothesized that bosons should “cluster” together, possessing an elevated probability of proximity. This clustering is a direct consequence of their ability to share a common quantum mechanical wave. This wave-like characteristic was initially predicted by physicist Louis de Broglie, and it is the “de Broglie wave” hypothesis that partially sparked the inception of modern quantum mechanics.
“We discern significantly more about the world due to this wave-like nature,” Zwierlein asserts. “Yet, it’s genuinely challenging to observe these quantum, wave-like effects. Nevertheless, with our innovative microscope, we can visualize this wave directly.”
During their imaging experiments, the MIT team managed to observe, for the first time in situ, bosons clustering together while sharing one quantum, correlated de Broglie wave. The team also imaged a cloud of two types of lithium atoms. Both types are fermions that naturally repel their own kind but can strongly interact with specific other fermion types. As they imaged the cloud, the researchers confirmed that the opposite fermion types indeed interacted and formed fermion pairs — a coupling they could directly observe for the first time.
“This type of pairing corresponds to a mathematical construction that was formulated to elucidate experiments. However, when you view images such as these, they showcase in a photograph an entity that was discovered in the mathematical realm,” remarks study co-author Richard Fletcher. “Thus, it serves as a lovely reminder that physics is about tangible things. It’s real.”
Looking ahead, the team plans to utilize their imaging technique to visualize more exotic and less understood phenomena, such as “quantum Hall physics” — scenarios where interacting electrons demonstrate novel correlated behaviors under a magnetic field.
“That’s where theoretical models become truly intricate — where researchers resort to sketches instead of being able to articulate a comprehensive theory because they cannot fully resolve it,” Zwierlein states. “Now we can ascertain whether these conceptual models of quantum Hall states are indeed tangible. Because they represent quite bizarre states.”
This work was partially supported by the National Science Foundation through the MIT-Harvard Center for Ultracold Atoms, along with contributions from the Air Force Office of Scientific Research, the Army Research Office, the Department of Energy, the Defense Advanced Research Projects Agency, a Vannevar Bush Faculty Fellowship, and the David and Lucile Packard Foundation.
“`