Physicists have successfully observed ultracold atoms in a unique ‘edge state’ where they flow along a boundary without any resistance. This groundbreaking research may assist scientists in manipulating electrons, allowing them to move without friction in materials, potentially leading to the ultra-efficient transmission of energy and data.
Electrons usually have the freedom to move in any direction within metals. When they hit an obstacle, they encounter friction and scatter unpredictably, similar to billiard balls colliding.
However, in specific exotic materials, electrons can exhibit a focused flow. In these cases, electrons may become restricted to the edges of the material, moving in a single direction, much like ants in a line along the edge of a blanket. This rare “edge state” allows electrons to flow without friction, smoothly navigating around obstacles while adhering to their path along the perimeter. Unlike superconductors, where all electrons move without resistance, edge states facilitate current only at the material’s boundary.
Recently, researchers at MIT have directly visualized these edge states within a cloud of ultracold atoms. For the first time, they captured images of atoms traveling along a boundary without experiencing resistance, even when obstacles were introduced. This study, published in Nature Physics, could pave the way for manipulating electrons in materials to achieve efficient, lossless energy and data transmission.
“Imagine creating small segments of a suitable material and placing them in future devices so electrons can travel along the edges, connecting various parts of your circuit without any loss,” explains Richard Fletcher, a co-author and physics assistant professor at MIT. “But importantly, the charm of this work is seeing firsthand physics that is typically hidden in materials and not directly observable.”
The research team at MIT also includes graduate students Ruixiao Yao and Sungjae Chi, former graduate students Biswaroop Mukherjee PhD ’20 and Airlia Shaffer PhD ’23, along with Martin Zwierlein, the Thomas A. Frank Professor of Physics. All co-authors are affiliated with MIT’s Research Laboratory of Electronics and the MIT-Harvard Center for Ultracold Atoms.
Always on the edge
The notion of edge states was first proposed to explain an intriguing effect called the Quantum Hall effect, which was observed for the first time in 1980 during experiments with layered materials where electrons were confined to two dimensions, carried out under ultracold temperatures and a magnetic field. In these studies, researchers found that when a current was sent through these materials, electrons did not pass straight through. Instead, they accumulated on one side in discrete quantum amounts.
To clarify this unusual behavior, scientists hypothesized that edge states are responsible for these Hall currents. They suggested that under the influence of a magnetic field, electrons in a current could be diverted to the edges of the material, where they would flow and accumulate, explaining the initial observations.
“The manner in which charge travels under a magnetic field indicates that edge modes must exist,” states Fletcher. “However, witnessing them is a remarkable achievement, as these states operate over femtosecond intervals and across minuscule fractions of a nanometer, making them incredibly challenging to capture.”
Rather than trying to catch electrons in their edge state, Fletcher and the team realized they could recreate similar physics in a larger and more observable setting. They studied how ultracold atoms behaved in a specially designed environment that mimics the behavior of electrons subjected to a magnetic field.
“In our configuration, the same physics happens with atoms but over milliseconds and micrometers,” Zwierlein elaborates. “This allows us to take images and observe the atoms essentially moving indefinitely along the edge of the system.”
A swirling dynamic
In their latest experiment, the team manipulated a cloud of approximately 1 million sodium atoms, maintaining the atoms in a laser-controlled trap and cooling them to extremely low temperatures (nanokelvin range). They then rotated the trap, mimicking the motion found in rides like a Gravitron.
“The trap aims to draw the atoms inward, but there’s a centrifugal force pushing them outward,” Fletcher explains. “These forces counterbalance, giving the atoms the sensation of residing in flat space, despite their spinning environment. Additionally, there’s the Coriolis effect, which deflects them if they attempt to travel in a straight line. Thus, these comparatively massive atoms behave like electrons in a magnetic field.”
Into this setup, the researchers imposed a “boundary” created by a ring of laser light, which formed a circular barrier around the rotating atoms. As they captured images, they witnessed the atoms flowing along the edge of the laser ring and moving in one direction.
“You can think of these atoms as marbles spun rapidly in a bowl, endlessly travelling around the rim without friction, slowing down, or scattering into the surrounding system,” Zwierlein suggests. “The flow is coherent and beautiful.”
“These atoms maintain a frictionless flow for hundreds of micrometers,” adds Fletcher. “Such uninterrupted motion, without scattering, is an uncommon occurrence in ultracold atom systems.”
This smooth flow persisted even when the researchers introduced an obstacle, akin to a speed bump, using a point of light placed along the edge of the laser ring. When the atoms encountered this new obstacle, they continued flowing without interruption or scattering, effortlessly bypassing the barrier.
“We deliberately introduced a large green repulsive blob, expecting the atoms to bounce back,” Fletcher notes. “Instead, they effortlessly navigated around it, returning to the boundary and continuing their smooth flow.”
The behaviors observed in the atoms mirror the predictions for how electrons would act in edge states. Their findings indicate that the atomic setup effectively models electron behavior in such states.
“This is a pristine illustration of an essential and elegant concept in physics, allowing us to directly confirm the significance of edge states,” Fletcher concludes. “The logical next step is to introduce more obstacles and interactions into the system, which may lead to less predictable outcomes.”
This research received partial support from the National Science Foundation.
