Century-Old Quantum Mystery Solved: Scientists Finally See Inside the Electron’s “Tunnel”

A team of physicists has experimentally uncovered what electrons do during quantum tunneling, revealing a surprising internal collision process that challenges long-held beliefs.

Professor Dong Eon Kim of POSTECH’s Department of Physics, in collaboration with the Max Planck Korea-POSTECH Initiative, has made a major breakthrough in understanding a fundamental concept of quantum mechanics known as electron tunneling. For the first time, his research team has successfully revealed what occurs during this mysterious process and confirmed their findings through experimental observation. Their work was published in the journal Physical Review Letters and is drawing widespread attention for addressing a puzzle that has remained unsolved for more than a century.

Although it may sound like science fiction, particles in the quantum world can actually perform feats that resemble teleportation. One such phenomenon is called quantum tunneling, where electrons manage to pass through energy barriers (often compared to walls) that they do not have enough energy to overcome, as if carving a hidden path straight through.

This effect plays a critical role in how semiconductors function, vital components found in smartphones and computers, and is also key to nuclear fusion, the reaction that powers the sun. While scientists have long known what happens before and after tunneling takes place, the exact behavior of electrons as they move through the barrier itself has remained a persistent mystery. Researchers could identify the start and end points of this process, but what occurs in between had never been fully understood.

Inside the Tunnel: A New Discovery

Professor Kim Dong Eon’s team, along with Professor C. H. Keitel’s team at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, conducted an experiment using intense laser pulses to induce electron tunneling in atoms.

The results revealed a surprising phenomenon: electrons do not simply pass through the barrier but collide again with the atomic nucleus inside the tunnel. The research team named this process ‘under-the-barrier recollision’ (UBR). Until now, it was believed that electrons could only interact with the nucleus after exiting the tunnel, but this study confirmed for the first time that such interaction can occur inside the tunnel.

Even more intriguingly, during this process, electrons gain energy inside the barrier and collide again with the nucleus, thereby strengthening what is known as ‘Freeman resonance.’ This ionization was significantly greater than that observed in previously known ionization processes and was hardly affected by changes in laser intensity. This is a completely new discovery that could not be predicted by existing theories.

A New Era of Control in Quantum Technology

This research is significant as it is the first in the world to elucidate the dynamics of electrons during tunneling. It is expected to provide an important scientific foundation for more precise control of electron behavior and increased efficiency in advanced technologies such as semiconductors, quantum computers, and ultrafast lasers that rely on tunneling.

Professor Kim Dong Eon stated, “Through this study, we were able to find clues about how electrons behave when they pass through the atomic wall,” and added, “Now, we can finally understand tunneling more deeply and control it as we wish.”

Reference: “Unveiling Under-the-Barrier Electron Dynamics in Strong Field Tunneling” by Tsendsuren Khurelbaatar, Michael Klaiber, Suren Sukiasyan, Karen Z. Hatsagortsyan, Christoph H. Keitel and Dong Eon Kim, 27 May 2025, Physical Review Letters.
DOI: 10.1103/PhysRevLett.134.213201

This research was supported by the National Research Foundation of Korea and the Capacity Development Project of the Korea Institute for Advancement of Technology.

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