Physicists Film Electrons Tunneling Through Graphene’s Energy Gap in Slow Motion

Physicists from the University of Regensburg, Philipps-Universität Marburg and Forschungszentrum Jülich have succeeded in visualizing, in real time, how intense electric fields drive electrons in graphene across forbidden energy gaps. Using a novel combination of laser technology and electron microscopy, the team captured the ultrafast dynamics of a quantum process known as the Landau-Zener-Majorana transition, in which electrons appear to leap between energy bands without ever occupying the states in between. The study, published in the journal Nature (DOI: 10.1038/s41586-026-11214-y), offers a direct, moving picture of one of the quantum world’s most counterintuitive phenomena.

The behavior of the electrons is roughly analogous to riding an elevator in which the floor indicator suddenly jumps from one, two and three straight to twenty. You never passed through the seventeen floors in between, yet you arrive on the twentieth floor. What sounds impossible in everyday life is routine in the quantum world, and such processes have long been suspected to play an increasingly important role in high-speed semiconductor electronics.

The researchers observed how electrons are accelerated by the electric field of a light pulse and switch between energy bands as they move. In semiconductors and related materials, electrons can only occupy certain energy ranges, known as bands, which are separated by a forbidden energy gap. Under ordinary circumstances, electrons remain firmly trapped within their respective bands. During a Landau-Zener-Majorana transition, however, electrons appear to overcome this forbidden zone with ease, and the new measurements show exactly how and when this happens.

The slow-motion images revealed a surprising result: under the influence of the strong light fields, the electrons overcome the band gap with a tunneling probability of up to one hundred percent. What makes Landau-Zener-Majorana transitions so fascinating is that they can occur with high probability even when the energy states involved are clearly separated from each other. The decisive factor is how fast the electrons approach the band gap. In this respect, these dynamic transitions differ fundamentally from standard quantum mechanical tunneling through a spatial energy barrier.

Observing processes this fast required an entirely new experimental approach. The team developed a combination of laser technology and electron microscopy built around an instrument called a photoemission momentum microscope. Unlike a conventional microscope, it does not show magnified images of objects. Instead, it records the energies and states of motion of the electrons and uses them to create a map of their distribution across the energy landscape of the material.

“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 special challenge was to combine electron spectroscopy with strong, extremely fast oscillating light fields that set the electrons in motion in a targeted manner.

“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. His cooperation partner Ulrich Höfer, one of the pioneers of ultrafast electron spectroscopy, adds: “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.”

Those samples were crucial to the experiment’s success. The measurements were based on tailor-made, atomically thin carbon layers of the highest purity, produced at Forschungszentrum Jülich. “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,” emphasizes Stefan Tautz from Forschungszentrum Jülich. Another important factor, he notes, was the use of “rotated” graphene, a specialty from Jülich that allows the quantum dynamics to emerge particularly clearly.

The new method can be used to investigate not only fundamental quantum mechanical transitions, but also the processes that limit the targeted control of electrons. The measurements on graphene already show how interactions with the crystal lattice disrupt the initially orderly movement of the electrons. Such findings are particularly important for the development of future technologies in which electrons are no longer controlled only by electrical voltages, but directly by the extremely fast oscillations of light.

“Our experiment opens a panoramic window into the quantum world,” says Giacomo Inzani from the University of Regensburg. “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.”

Subject of Research: Real-time visualization of Landau-Zener-Majorana electron transitions in graphene using ultrafast photoemission momentum microscopy

Article Title: A quantum elevator in slow-motion-video

Article References: A quantum elevator in slow-motion-video. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

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Keywords: graphene, Landau-Zener-Majorana transitions, quantum tunneling, ultrafast spectroscopy, photoemission momentum microscopy, electron dynamics, band structure, lightwave electronics, Forschungszentrum Jülich, Nature, quantum, elevator