Physicists have captured an extraordinary quantum phenomenon in which electrons appear to leap across energy gaps that would normally be off limits. Using a new type of slow-motion electron camera, researchers were able to watch these incredibly fast movements in graphene, revealing how powerful electric fields can drive electrons between different energy states.
The effect is similar to stepping into an elevator and watching the floor display change from “1,” “2,” and “3” directly to “20.” Instead of traveling through the seventeen floors in between, you somehow arrive at the twentieth floor. Although this sounds impossible in everyday life, electrons can make comparable jumps in the quantum world.
These unusual events are known as Landau-Zener-Majorana transitions. Physicists have long suspected that they could become increasingly important in high-speed semiconductor electronics, but observing exactly how they unfold has been a major challenge.
Now, researchers from the University of Regensburg, Philipps-Universität Marburg, and Forschungszentrum Jülich have directly tracked these transitions in graphene, a material made from an extremely thin layer of carbon atoms. Their findings, published in Nature, provide a detailed look at how electrons respond to intense electric fields generated by light.
How Electrons Cross Forbidden Energy Gaps
In semiconductors and similar materials, electrons are restricted to particular ranges of energy called energy bands. Between these bands are regions known as forbidden energy gaps, where electrons cannot ordinarily occupy stable states.
Under normal conditions, electrons tend to remain within their existing energy bands. However, the researchers observed something very different when they exposed graphene to intense pulses of light. The electric field accelerated the electrons and caused some of them to switch from one energy band to another, crossing the intervening gap through a quantum mechanical transition.
To capture these movements, the scientists developed an experimental technique that combines advanced laser technology with electron microscopy. This allowed them to examine electron behavior on timescales far too short for conventional imaging methods.
A Slow-Motion Camera Reveals Quantum Jumps
“If you want to know who has won a race, you have to watch all participants,” explains Vincent Eggers, first author of the study. “Likewise, we must observe as many electrons as possible at one time to catch those that undergo a quantum mechanical transition while moving.”
The key instrument is a photoemission momentum microscope. Rather than producing an enlarged picture of a material’s surface, as a traditional microscope would, it measures the energies of electrons and how they are moving. Researchers then use this information to create detailed maps showing how electrons are distributed across different energy states.
One of the biggest technical obstacles was combining these measurements with powerful light fields that oscillate at extremely high speeds. The team needed to drive the electrons into motion while simultaneously recording how their energies and movements changed.
“Our team worked on this experiment for many years. Now, we can track electrons moving across the entire energy landscape of a solid in real time,” says Rupert Huber.
Ulrich Höfer, a collaborator on the project and a pioneer in ultrafast electron spectroscopy, describes the excitement of witnessing the effect firsthand.
“Being able to observe such a fundamental quantum mechanical process during our first measurement campaign was a very special moment for us – it was also made possible by the samples we used.”
Ultra-Pure Graphene Makes the Discovery Possible
The experiment depended on exceptionally pure graphene samples manufactured at Forschungszentrum Jülich. These specially prepared carbon layers were only one atom thick and had to be nearly flawless for the measurements to work.
“These precision experiments require material that is nearly perfect at the atomic level. This ensures that impurities do not obscure the delicate quantum dynamics of the electrons. Another important factor for the study was the use of “rotated” graphene, a specialty from Jülich, which allows the quantum dynamics to emerge particularly clearly,” emphasizes Stefan Tautz from Forschungszentrum Jülich.
The carefully engineered material made it possible to distinguish the delicate quantum effects from interference caused by imperfections in the graphene.
What the researchers saw was remarkable. When exposed to sufficiently strong light fields, electrons crossed the forbidden energy gap with a transition probability reaching 100 percent.
This result highlights one of the most surprising features of Landau-Zener-Majorana transitions. Electrons can move between energy states with very high probability even when those states are separated by a substantial energy gap.
The crucial factor is how quickly the electrons approach the gap under the influence of the electric field. This makes the process fundamentally different from conventional quantum tunneling, in which particles pass through a barrier in physical space.
A New Path Toward Light-Controlled Electronics
Beyond revealing a fundamental quantum phenomenon, the new imaging technique could help scientists understand what prevents electrons from moving in a controlled and predictable way.
The graphene experiments have already provided an important clue. Although the electrons initially move in an orderly fashion, their interactions with the material’s crystal lattice gradually disrupt that motion. Understanding these effects could help researchers develop more precise ways to control electron behavior.
This knowledge is particularly valuable for future electronic technologies that could use the extremely rapid oscillations of light to direct electrons instead of relying entirely on electrical voltages. Such approaches could open new possibilities for manipulating electronic properties at exceptionally high speeds.
The researchers also expect their method to work with many other quantum materials, allowing scientists to investigate previously inaccessible electron dynamics and potentially discover new ways to control them.
As Giacomo Inzani from the University of Regensburg explains, the achievement marks the beginning of a much broader exploration.
“Our experiment opens a panoramic window into the quantum world. Now, we can follow in real time how electrons move through the energy landscape of a material. And this is just the beginning. Our method can be applied to many other quantum materials, opening up completely new possibilities for controlling their electronic properties selectively with light.”