Slow-motion electron camera reveals light-driven quantum jumps in graphene

Introduction

A collaborative team from the University of Regensburg, Philipps‑Universität Marburg and the Forschungszentrum Jülich has visualized, for the first time, the ultrafast quantum jumps of electrons in graphene driven by intense light pulses. By pairing a high‑field laser with a photo‑emission momentum microscope—a kind of "slow‑motion camera" for electrons—the researchers recorded real‑time maps of electrons as they traversed graphene’s band structure, revealing Landau‑Zener‑Majorana (LZM) transitions with tunneling probabilities approaching 100%.

How the experiment works

The key instrument is a photo‑emission momentum microscope, which differs from a conventional optical microscope in that it does not form a spatial image. Instead, it measures the kinetic energy and momentum of electrons emitted from a material when illuminated by a short laser pulse. By synchronising the laser’s electric field with the electron detection, the team could watch electrons accelerate through the valence band, approach the conduction band, and, if their velocity was high enough, tunnel across the forbidden energy gap.

A crucial innovation was the integration of an ultra‑intense, sub‑cycle laser field that oscillates on the order of a few femtoseconds. This field provides the rapid acceleration needed for LZM transitions: the faster an electron reaches the band edge, the higher the probability it will tunnel rather than reflect. The graphene samples themselves were atomically thin, high‑purity layers grown at Jülich, with a controlled “twist” angle that enhances the visibility of quantum dynamics.

What the images reveal

The momentum‑resolved images show electrons moving along yellow‑striped trajectories that represent the band structure of graphene. When the laser field peaks, electrons in the valence band acquire enough momentum to cross the Dirac point and appear in the conduction band almost instantaneously. Remarkably, the measured tunneling probability reaches near‑unity, indicating that the transition is governed more by the electron’s speed than by the static height of the energy gap.

These observations differentiate LZM transitions from conventional quantum tunnelling, which depends primarily on barrier width and height. In the LZM case, the dynamic electric field reshapes the energy landscape on a sub‑cycle timescale, allowing electrons to “jump” across the gap without occupying intermediate states—much like an elevator that skips several floors in a single instant.

Implications for future technologies

Understanding and controlling LZM transitions opens several avenues for next‑generation electronics and photonics:

• Light‑controlled circuitry: Devices could be switched on and off by tailoring ultrafast light fields rather than static voltages, potentially reaching terahertz operation speeds.
• Quantum material engineering: The technique can be applied to other two‑dimensional systems (e.g., transition‑metal dichalcogenides) to explore light‑induced topological phases and exotic quasiparticles.
• Reduced energy loss: By steering electrons directly with light, it may become possible to minimize resistive heating, a major bottleneck in conventional semiconductor scaling.

The study also highlights the role of lattice interactions: as electrons propagate, scattering with phonons gradually disrupts their coherent motion, setting limits on how far the light‑driven control can extend. Quantifying these decoherence pathways will be essential for designing practical devices.

Looking ahead

The authors stress that this is only the beginning. The momentum‑microscopy platform can be adapted to probe a wide range of ultrafast phenomena, from charge‑density wave dynamics to light‑induced superconductivity. By providing a panoramic, real‑time view of electron motion in solids, the method promises to transform our ability to manipulate matter at its most fundamental level.

--- The findings are detailed in the paper “Sub‑cycle videography of Landau‑Zener‑Majorana transitions in graphene,” published in Nature (2026).