MIT Physicists Capture First Images of Fermion Pairs, Revealing Quantum Realm Secrets

MIT physicists have achieved a groundbreaking feat by visualizing particle pairings within a cloud of atoms, unlocking fresh insights into the behavior of electrons in superconducting materials. The discovery, published in the esteemed journal Science, holds the potential to advance our understanding of superconductivity and revolutionize the development of heat-free electronics. The remarkable images offer a glimpse into how electrons form superconducting pairs that effortlessly navigate materials without encountering friction.

When electronic devices like laptops or smartphones heat up, it is a consequence of energy lost during transmission. The same holds true for power lines that transmit electricity across cities, where approximately 10 percent of the generated energy dissipates during transmission due to electrons moving independently, colliding with one another. These collisions generate friction and, consequently, heat.

However, when electrons pair up, they can transcend these challenges and traverse materials without friction. This phenomenon, known as “superconductivity,” can occur in various materials, albeit at extremely low temperatures. If scientists can induce superconductivity at higher temperatures, closer to room temperature, it could pave the way for revolutionary devices such as heat-free laptops, phones, and ultra-efficient power lines. However, unraveling the process through which electrons form pairs is crucial.

In a significant breakthrough, MIT physicists have captured snapshots of particle pairings within a cloud of atoms, shedding light on the mechanism behind electron pairing in superconducting materials. These snapshots, the first of their kind, were taken by MIT researchers and directly illustrate the pairing of fermions, a major class of particles encompassing electrons, protons, neutrons, and specific types of atoms.

The MIT team focused on fermions represented by potassium-40 atoms and created conditions that mimic the behavior of electrons in specific superconducting materials. They developed a technique to image a supercooled cloud of potassium-40 atoms, enabling them to observe particle pairings, even when the particles were separated by a small distance. This approach revealed intriguing patterns and behaviors, such as the formation of checkerboard-like structures disrupted by solitary particles passing through.

The findings, reported in the journal Science on July 6, serve as a visual blueprint for understanding how electrons might pair up in superconducting materials. Moreover, the results may shed light on how neutrons form a densely packed and dynamic superfluid within neutron stars.

“Fermion pairing is fundamental to superconductivity and numerous phenomena in nuclear physics,” says study author Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT. “But no one had witnessed this pairing directly. Thus, seeing these images onscreen for the first time was simply awe-inspiring.”

Co-authors of the study include Thomas Hartke, Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, all affiliated with MIT’s Department of Physics, the MIT-Harvard Center for Ultracold Atoms, and the Research Laboratory of Electronics.

Overcoming challenges, gaining insights

Directly observing the pairing of electrons is an arduous task due to their minuscule size and incredible speed, which surpasses the capabilities of existing imaging techniques. To comprehend their behavior, physicists like Zwierlein turn to analogous atomic systems. Despite their disparate sizes, electrons and certain atoms share similarities as fermions—particles exhibiting a property known as “half-integer spin.” When fermions with opposite spins interact, they can form pairs, much like electrons in superconductors or specific atoms in a gas cloud.

Zwierlein’s research group has been studying the behavior of fermions represented by potassium-40 atoms. These atoms, classified as fermions, can be prepared in two distinct spin states. When atoms with different spins interact, they have the potential to form pairs similar to those observed in superconducting electrons. However, under normal room-temperature conditions, the interactions between atoms occur rapidly and are challenging to capture.

To obtain a more comprehensive view of their behavior, Zwierlein and his colleagues examine the particles as an extremely dilute gas of approximately 1,000 atoms. They subject the gas to ultracold nanokelvin temperatures, effectively slowing down the atoms’ motion. Additionally, they confine the gas within an optical lattice—an arrangement of laser light that allows atoms to move within a grid—and leverage this lattice as a map to precisely locate the atoms.

In their latest study, the team refined their existing technique for imaging fermions. They devised a method to temporarily freeze the atoms in place, allowing them to capture separate snapshots of potassium-40 atoms with a specific spin. By overlaying these images, they could identify where the two types of atoms formed pairs and explore their interactions.

“Reaching a stage where we could capture these images was exceptionally challenging,” Zwierlein explains. “We encountered imaging issues, with atoms escaping and technical complexities to resolve in the lab over the years. The perseverance of our students was remarkable, and finally being able to witness these images was truly exhilarating.”

A mesmerizing dance

The observations made by the team corroborated the pairing behavior predicted by the Hubbard model, a widely accepted theory that is believed to hold the key to understanding electron behavior in high-temperature superconductors. Although predictions about electron pairings in these materials have been tested through this model, direct observation remained elusive until now.

The team created and imaged different atom clouds thousands of times, translating each image into a digitized grid representation. Each grid displayed the locations of both types of atoms, represented as red and blue. By examining these maps, the researchers discerned squares containing either a lone red or blue atom, squares with paired red and blue atoms (depicted as white), and empty squares devoid of either atom type (black).

Individual images already exhibited numerous local pairs and red and blue atoms in close proximity. Analyzing sets of hundreds of images enabled the team to demonstrate that atoms frequently formed pairs, occasionally binding tightly within one square, while at other times, establishing looser pairs separated by one or several grid spacings. This physical separation, known as “nonlocal pairing,” was predicted by the Hubbard model but never directly observed.

The researchers also observed that pairs appeared to organize into a larger checkerboard pattern that periodically distorted as a member of one pair ventured outside its square, momentarily disrupting other pairings. This phenomenon, termed a “polaron,” had been predicted but never directly observed.

“In this dynamic milieu, particles constantly hop onto each other, moving apart and yet never straying too far,” Zwierlein remarks.

The pairing behavior observed in these atoms likely occurs in superconducting electrons as well. Zwierlein believes that the new snapshots obtained by the team will contribute to scientists’ comprehension of high-temperature superconductors and potentially offer insights into methods for achieving superconductivity at higher, more practical temperatures.

“This is an exciting new study,” says Immanuel Bloch, a professor of experimental physics at Ludwig-Maximilians University in Munich, who was not involved in the research. “It provides a splendid example of directly observing intricate correlations through highly controlled quantum simulation experiments and will inspire thinking about more complex correlation patterns that can be directly captured in experiments.”

Reference: “Direct observation of nonlocal fermion pairing in an attractive Fermi-Hubbard gas” by Thomas Hartke, Botond Oreg, Carter Turnbaugh, Ningyuan Jia and Martin Zwierlein, 6 July 2023, Science.
DOI: 10.1126/science.ade4245

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• MIT News: MIT Physicists Generate the First Snapshots of Fermion Pairs
• Science Journal: Direct observation of nonlocal fermion pairing in an attractive Fermi-Hubbard gas
• U.S. National Science Foundation: NSF Award Abstract – Imaging Fermions in Superfluids
• MIT-Harvard Center for Ultracold Atoms: Homepage