“Through this study, we were able to find clues about how electrons behave when they pass through the atomic wall,” stated Professor Dong Eon Kim. Physicists have been mystified for more than a century about what precisely happens to an electron that tunnels through a potential barrier a process at the heart of quantum mechanics, but so far, not fully understood. The most recent breakthrough experiment not only shed light on this quantum ride but also upended fundamental expectations on electron dynamics within the tunnel.
Quantum tunneling is often likened to a particle somehow flying across a wall that it is unable to climb over, an effect on which all from semiconductor operation to nuclear fusion hinges. While the tunneling entrance and exit sites had been charted, the electron’s passage through the forbidden region was a “black box.” Using intense laser pulses in an interdisciplinary research partnership between POSTECH and the Max Planck Institute for Nuclear Physics, researchers investigated this mysterious phase and uncovered a phenomenon that challenges existing models at its core.
The team’s experiment revealed that electrons don’t simply move through the barrier without interference. Instead, they undergo what the researchers have termed as “under-the-barrier recollision” a process where the electron, having not yet moved outside the barrier, collides with the atomic nucleus again. This finding overturns the conventional belief that any electron-nucleus interaction was not possible unless after the electron had already exited the tunnel. The observation of this recollision was made possible by leveraging attosecond pulse generation and ultrafast measurement techniques, which are now de facto tools in strong-field physics. These techniques, employing waveform-synthesised laser pulses with pulse widths below three femtoseconds, allow reliable monitoring of electron motion on timescales where quantum effects prevail.
More striking is that electrons gain energy in their under-the-barrier path, reaching a rise of Freeman resonance a resonance-driven ionisation process. This process creates ionization rates higher than theoretical predictions and, most surprisingly, with comparatively little dependence on the intensity of the driving laser. Such independence from laser intensity is unprecedented and suggests a novel class of ionization processes. The researchers described it as follows: “This is a completely new discovery that could not be predicted by existing theories.”
The implications of these findings resonate far beyond academic curiosity. In semiconductor physics, quantum tunneling is both a blessing and a curse. With decreasing device size to nanoscales, tunneling currents are unavoidable, leading to leakage and power loss in conventional transistors. At the same time, the phenomenon is also used in high-end devices such as tunnel field-effect transistors (TFETs) and resonant tunneling diodes, where control of electron flow through barriers needs to be extremely accurate to provide low-power, high-speed switching. The new technological freedom to engineer tunneling processes to unprecedented precision opens up the field to realize quantum-class devices. Through control of the coupling of the electron to the nucleus in the barrier, it may be possible to design the energy distribution and coherence of tunneling electrons, enhancing device performance and reliability.
The technological reach extends to quantum computing as well. Quantum bits, or qubits, often rely on tunneling effects for manipulation and readout of the state. The ability to control tunneling dynamics at the sub-femtosecond level could provide new paths to quantum error correction and suppression of decoherence, as quantum information is notoriously immune to uncontrolled interactions and energy noise. As a recent article pointed out, addressing the issue of quantum tunneling is critical to further advance semiconductor technologies and realize even more powerful and efficient electronic devices in the era of quantum technology.
Moreover, the experimental approaches adopted to achieve this breakthrough attosecond streaking and waveform synthesis are themselves pushing the boundaries of ultrafast science. By precision-engineering the electric field of laser pulses to attosecond scales, researchers can now generate and characterize isolated attosecond pulses across a broad range of energies and shed light on the most extreme phenomena in nature. Such control is necessary to investigate and ultimately manage electron dynamics in materials, molecules, and devices.
Its broader importance was brought out by Professor Kim: “Now, we can finally understand tunneling more deeply and control it as we wish.” This portends a paradigm shift not only in the underlying physics of quantum mechanics, but also in the engineering of next-generation technologies that depend on the nuanced ballet of electrons beneath the barrier.
