Quantum tunneling mystery solved after 100 years—and it involves a surprise collision

While the idea of teleporting through walls may sound like something out of a movie, such phenomena actually occur in the atomic world. This phenomenon, called ‘quantum tunneling,’ involves electrons passing through energy barriers (walls) that they seemingly cannot surmount with their energy, as if digging a tunnel through them.

This phenomenon is the principle by which semiconductors, i. e., core components of smartphones and computers, operate, and is also essential for nuclear fusion, the process that produces light and energy in the sun. However, until now, while some understanding existed about what happens before and after an electron passes through a tunnel, the exact behavior of the electron as it traverses the barrier remained unclear. We know the entrance and exit of the tunnel, but what happens inside has remained a mystery.

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.

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.”

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