Regensburg-led physicists directly track electrons making forbidden quantum jumps in graphene

Researchers from University of Regensburg, Philipps-Universität Marburg and Forschungszentrum Jülich reported in Nature that they watched electrons in graphene cross energy gaps that are normally forbidden to stable electron states. The team used intense light pulses to accelerate electrons and a photoemission momentum microscope to map their energy and motion. The jumps are Landau-Zener-Majorana transitions, quantum events in which a particle moves between states that classical physics would treat as separated by a barrier.

The evidence comes from momentum-space maps of electrons emitted from ultra-pure graphene after laser excitation. Instead of seeing only smooth motion within an energy band, the measurements showed electrons appearing in another band while preserving signatures of their earlier motion, the hallmark of a sudden quantum transition. The instrument measured electron energies and momenta simultaneously, allowing the team to reconstruct occupation of bands in real time. The study's methods note: it is a single-material spectroscopic experiment on ultra-pure one-atom-thick graphene, not a device demonstration, and the observed jumps depend on strong light fields.

Landau-Zener-Majorana transitions have been discussed for decades in quantum physics, but they are hard to watch because the crossings happen on femtosecond timescales and can be masked by heating or scattering. Graphene is useful because its electrons move in a simple, controllable energy structure and its clean samples reduce noise. The Regensburg-Marburg-Jülich work connects ultrafast laser spectroscopy with electron imaging, a method suited to other quantum materials. The result does not overturn quantum mechanics; it sharpens a known prediction by showing how field-driven band crossings unfold.

The finding matters because high-speed electronics and optoelectronics operate near limits where light and electric fields can push electrons between bands, a regime relevant to ultrafast switches, detectors and quantum-material devices. The immediate advance is measurement: researchers can now see where electrons go when a strong field forces them into normally inaccessible states. That capability is practical for device physics, not a ready-made computer component.