Electrons, which we tend to imagine as orderly travelers confined to their lanes, have been seen doing something stranger: leaping across a forbidden gap in a single sheet of carbon atoms, as though skipping the floors of an elevator. A team from several German institutions, working with graphene engineered to be nearly flawless, has now caught this quantum leap in real time under intense pulses of light. The observation matters because it hints at a future in which light, rather than voltage alone, steers the behavior of matter at speeds our current electronics cannot reach.
Electrons Leap Across Forbidden Energy Gaps in Graphene, Tracked in Real Time
Electrons crossed the forbidden gap with a probability of 100 percent
Mark
So what actually happened in the graphene?
Mimi
Electrons jumped from one energy band to another, across a gap where they normally can't sit. Intense light pulses drove them, and the team watched it live.
Mark
Why does the 100 percent figure stand out?
Mimi
You might expect a jump across a big gap to be rare. Here, with strong enough light fields, the transition probability reached 100 percent.
Luke
Worth being precise: that was under sufficiently strong light fields, in very pure graphene. It's not a claim that every electron in every material does this.
Mark
And this isn't ordinary tunneling?
Mimi
Right. Tunneling is a particle passing through a barrier in space. This depends on how quickly the electrons approach the gap under the electric field.
Mark
Why did the sample matter so much?
Mimi
Impurities would blur the quantum dynamics. Jülich made nearly flawless, one-atom-thick graphene, including a rotated form that makes the dynamics show up clearly.
Luke
The electronics promise is still a hope. The source says the knowledge is valuable for light-driven devices and that the method may work on other materials. No device has been shown.
Mark
Is there anything beyond the headline result?
Mimi
Yes. The electrons first move in an orderly way, then interactions with the crystal lattice gradually disrupt that motion. Understanding that could help control them.
Luke
How much control that buys is unknown. That part hasn't been answered yet.
Der Puls
- Electrons are jumping between energy bands in graphene, skipping over forbidden gaps that they should never cross under normal conditions.
- Intense light pulses are driving the electrons so hard that some transitions reach a 100 percent probability, a striking result for a process once thought rare.
- Researchers used a photoemission momentum microscope to record electron energies and motion while the light fields oscillated, a race against extremely short timescales.
- Pristine, engineered graphene has allowed the team to separate true quantum dynamics from distortions caused by impurities.
- The method is expected to extend to other quantum materials, opening a path toward ultrafast, light-driven electronics.
Electrons, which we tend to imagine as orderly travelers confined to their lanes, have been seen doing something stranger: leaping across a forbidden gap in a single sheet of carbon atoms, as though skipping the floors of an elevator. A team from several German institutions, working with graphene engineered to be nearly flawless, has now caught this quantum leap in real time under intense pulses of light. The observation matters because it hints at a future in which light, rather than voltage alone, steers the behavior of matter at speeds our current electronics cannot reach.
Imagine watching an elevator display jump from 1 to 20 without ever seeing the floors in between. Electrons can make a similar move in the quantum world, and now a team of physicists has watched it happen in graphene, a layer of carbon only one atom thick.
The researchers, drawn from the University of Regensburg, Philipps-Universität Marburg, and Forschungszentrum Jülich, published their findings in Nature. Their subject is a class of events called Landau-Zener-Majorana transitions, which physicists have long suspected would grow in importance for high-speed semiconductor electronics but had never been seen clearly. Normally electrons remain within the energy bands they occupy. When the team struck graphene with intense light, its electric field accelerated the electrons, and some leapt across the forbidden gap between bands.
Capturing this required pairing advanced laser technology with a photoemission momentum microscope, an instrument that measures how electrons carry energy and move rather than magnifying a surface. First author Vincent Eggers likened the challenge to a race: to know who wins, you must watch every runner, so the team tracked as many electrons as possible at once. The graphene itself had to be nearly flawless, and Jülich's rotated graphene allowed the quantum effects to emerge without interference from imperfections.
The decisive factor turned out to be how quickly electrons approach the gap under the field, which sets this process apart from conventional tunneling through a barrier. The work also sheds light on why electrons lose orderly motion as their interactions with the crystal lattice gradually disrupt it, knowledge that could one day enable more precise control. Giacomo Inzani called the experiment a panoramic window into the quantum world, and the team expects the method to reach many other quantum materials, where the next question is whether light can selectively tune their electronic properties.
Picture stepping into an elevator, watching the display read 1, then 2, then 3, and then 20. You never passed through the seventeen floors in between. Electrons can make a comparable move in the quantum world, and a team of physicists has now watched it happen in graphene, in real time.
The researchers come from the University of Regensburg, Philipps-Universität Marburg, and Forschungszentrum Jülich. Their results, published in Nature, concern events called Landau-Zener-Majorana transitions. Physicists have long suspected these transitions could matter more and more in high-speed semiconductor electronics, but seeing exactly how they unfold has been a major challenge. Graphene is a layer of carbon atoms only one atom thick.
The background is this. In semiconductors and similar materials, electrons are confined to certain ranges of energy called bands. Between the bands lie forbidden gaps, where electrons cannot ordinarily settle into stable states. Normally electrons stay in the band they already occupy. When the team hit graphene with intense pulses of light, the light's electric field accelerated the electrons, and some of them switched bands, crossing the gap through a quantum mechanical transition.
To catch this, the group built a technique that pairs advanced laser technology with electron microscopy, letting them examine electron behavior on timescales far too short for conventional imaging. The central instrument is a photoemission momentum microscope. It does not magnify a surface the way an ordinary microscope does. It measures the energies of electrons and how they are moving, and researchers use that to build maps of how electrons are spread across energy states.
Combining those measurements with powerful light fields that oscillate at extreme speeds was one of the largest technical obstacles. The electrons had to be set in motion while their energies and movements were being recorded. First author Vincent Eggers explained the logic with a race: to know who won, you must watch all the participants, so the team had to observe as many electrons as possible at once to catch the ones making a transition mid-motion. Rupert Huber said the team had worked on the experiment for many years and can now track electrons moving across the entire energy landscape of a solid in real time. Ulrich Höfer, a collaborator and pioneer of ultrafast electron spectroscopy, called it a very special moment to see such a fundamental process in the first measurement campaign, something the samples also made possible.
Those samples came from Jülich: graphene that had to be nearly flawless at the atomic level so that impurities would not obscure the delicate dynamics. Stefan Tautz of Jülich also pointed to the use of "rotated" graphene, a Jülich specialty, which lets the quantum dynamics emerge particularly clearly. The engineered material let the researchers separate the quantum effects from interference caused by imperfections.
What they saw was striking: with sufficiently strong light fields, electrons crossed the forbidden gap with a transition probability of 100 percent. The deciding factor is how quickly the electrons approach the gap under the electric field. That sets the process apart from conventional quantum tunneling, in which particles pass through a barrier in physical space.
The technique also gives a clue about what stops electrons from moving in a controlled, predictable way. In the graphene, the electrons start out moving in an orderly fashion, but their interactions with the crystal lattice gradually disrupt that motion. Understanding those effects could lead to more precise control, which matters for future electronics that use the rapid oscillations of light, rather than only electrical voltages, to steer electrons.
Giacomo Inzani of the University of Regensburg called the experiment a panoramic window into the quantum world and said it is just the beginning. The team expects the method to work on many other quantum materials, which is the next thing to watch: whether it can be used to control their electronic properties selectively with light.
Bemerkenswerte Zitate
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.— Giacomo Inzani, University of Regensburg
To know who won a race, you have to watch all participants, so many electrons must be observed at once to catch those making a transition.— Vincent Eggers, first author (paraphrased)
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