Scientists have succeeded in unraveling the mystery of the ‘electron tunneling’ process for the first time. The feat, a core concept in quantum mechanics, was achieved by researchers from Pohang University of Science and Technology.

With their new experiments, scientists could pave the way to unlock the 100-year-old mystery of electron tunneling.

“Through this study, we were able to find clues about how electrons behave when they pass through the atomic wall,” said Professor Dong Eon Kim from POSTECH’s Department of Physics.

Quantum tunneling involves electrons passing through energy barriers

Researchers revealed that 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.

In their experiment, researchers used 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).

So far, researchers 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.

Under-the-barrier-recollision dynamics

Published in Physics Review Letters, the study unravels under-the-barrier-recollision dynamics leading to Freeman resonances (FR). The under-the-barrier-recollision model, which goes beyond the traditional direct multiphoton transition description, predicts distinct features of FR phenomena that cannot be explained by the existing direct multiphoton transition scenario.

“The model predicts the dominance of high-order FR over above-threshold ionization in the photoelectron energy spectra and the flat dependence of the FR signal on the laser intensity, both in the nonadiabatic tunneling regime,” said researchers in the study.

Electrons gain energy inside the barrier and collide again with the nucleus

During the experiments, 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, according to a press release.

This is a completely new discovery that could not be predicted by existing theories.

The study is the first of its kind 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, according to researchers.

In the study, researchers stressed that the newly found Freeman resonances (FR) characteristics further emphasize the need for improved theoretical frameworks to fully elucidate in more detail the underlying mechanisms at play in the mid- and high-intensity regimes of strong-field ionization, including the exact atomic potential, and multiple electron correlations.

In conclusion, researchers highlighted that the strong-field approximation (SFA) model incorporating under-the-barrier recollisions (UBR) reveals intriguing FR features: in the mid- and high-intensity regimes, FRs dominate above-threshold ionization (ATI), higher-order FRs persist, and the FR signal is nearly constant over the laser intensity.