Quantum tunneling has traditionally been understood as a counterintuitive process in which particles traverse a classically insurmountable potential barrier. Recent research has, however, revealed a previously unknown aspect of this mechanism: under-the-barrier recollision (UBR), in which electrons can reflect and collide with the nucleus while still within the barrier. This discovery fundamentally reshapes the conventional view of tunneling, revealing dynamic, multi-step interactions within the barrier.
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Quantum Tunneling: A Quick Recap
Quantum tunneling is a fundamental quantum phenomenon in which particles traverse a potential energy barrier despite lacking the classical energy required to overcome it. This effect arises from the wave-like nature of matter, as described by the Schrödinger equation, which permits a finite probability of detecting a particle in regions that are classically forbidden.
To illustrate this, imagine a ball rolling toward a hill that it cannot classically climb. In classical mechanics, the ball simply rolls back. Quantum mechanics, however, describes the ball as a wave that spreads over space, giving it a small but finite chance to appear on the other side of the hill. The wider the wave, the higher the probability of “tunneling” through the barrier.1
The phenomenon gained historical significance through George Gamow’s 1928 explanation of alpha decay, showing that alpha particles, composed of two protons and two neutrons, escape heavy nuclei via tunneling rather than thermal activation. The correlation between alpha particle energies and nuclear half-lives, ranging from microseconds to billions of years, highlighted the probabilistic nature of tunneling, where higher-energy particles face narrower effective barriers and exhibit sharply increased escape probabilities.
One of the most prominent practical applications of quantum tunneling is the scanning tunneling microscope (STM), invented by Gerd Binnig and Heinrich Rohrer in 1981. The STM exploits the exponential dependence of tunneling current on tip-sample separation, achieving spatial precision of approximately 0.001 nm, sufficient to visualize individual atoms.
Traditional theoretical approaches, particularly the Wentzel-Kramers-Brillouin (WKB) approximation, have largely analyzed tunneling as a simple transmission process. These models treat tunneling as a passive, unidirectional event, in which particles either cross or reflect from the barrier, without accounting for dynamic interactions occurring within the barrier itself.2,3
However, recent studies have revealed that tunneling involves more complex dynamics than previously thought.
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What Is Under-the-Barrier Recollision?
Physicists from POSTECH and the Max Planck Institute recently discovered a new quantum phenomenon while investigating electron tunneling in atoms under intense laser pulses.
Contrary to the conventional view that electrons interact with the nucleus only after emerging from the tunneling barrier, the researchers demonstrated that such collisions can also occur within the barrier, with electrons sometimes moving backward to interact with the nucleus before emerging. They termed this phenomenon under-the-barrier recollision (UBR).
During this backward propagation, electrons gain additional energy, enabling excitation to higher atomic states, which can then be ionized through the absorption of a few photons. This mechanism enhances Freeman resonance, producing ionization levels far greater than those of conventional above-threshold ionization (ATI) and showing weak dependence on laser intensity.
This concept goes beyond the scope of existing theories, fundamentally expanding our understanding of tunneling dynamics and revealing interactions within the barrier that were previously deemed impossible.
To explain these findings, the researchers developed a novel strong-field approximation (SFA) model that incorporates both direct multiphoton transitions (DMT) and UBR. The model predicts unique photoelectron spectra, characterized by prominent Freeman resonance peaks and intensity-independent features absent in conventional ATI processes.
UBR challenges the traditional paradigm of tunneling as a passive, one-way process by demonstrating that the barrier region itself becomes an active arena for electron-nucleus interactions. This revelation necessitates a more sophisticated understanding of tunneling dynamics that incorporates the full complexity of particle motion within potential barriers.4
Methodological Innovations
The researchers employed advanced experimental and computational methods to investigate ultrafast electron dynamics in UBR with high temporal and energy precision.
Central to their approach was a refined velocity-map imaging (VMI) setup, featuring carefully engineered electrostatic lenses and finely tuned electrode voltages. This configuration significantly improved energy resolution while preserving momentum information more effectively than standard setups.
Using this system, they captured two-dimensional photoelectron momentum distributions from xenon and krypton atoms. These measurements enabled the reconstruction of detailed energy and angular spectra, allowing them to identify distinct Freeman resonance features.
To ensure highly reproducible measurements across multiphoton and tunneling regimes, they integrated an active laser power stabilization (ALPS) system with feedback-controlled optics, allowing fine control of laser intensity and enabling systematic intensity scans that revealed subtle under-the-barrier recollision effects.
The researchers also proposed a Coulomb-corrected SFA model to provide a more precise description of electron dynamics in intense laser fields. Unlike conventional SFA, this model incorporates the long-range Coulomb interaction between the electron and the atomic nucleus, enabling it to capture phenomena such as under-the-barrier recollision that standard SFA cannot predict.
By combining semiclassical trajectory calculations with Coulomb effects, the model successfully reproduced detailed features in photoelectron spectra, including Freeman resonance lines, and offers a complementary analytical framework for interpreting experimental data.
These findings were further validated through time-dependent Schrödinger equation (TDSE) simulations, which modeled the full quantum evolution of electron wave packets to isolate and analyze the contributions of under-barrier recollision.4
Similar Research Highlights
Over the past few years, several experimental efforts have explored phenomena related to UBR, providing initial evidence of sub-barrier electron–nucleus interactions and their impact on tunneling dynamics.
For example, researchers at the Max Planck Institute for Nuclear Physics investigated how sub-barrier recollisions influence tunneling time delays at the barrier exit, showing that interference between direct and recolliding trajectories reduces the measured time delay by an amount equal to the asymptotic delay while maintaining a substantial positive value. Their theoretical analysis highlighted the need to introduce an additional temporal characteristic describing the initiation of the tunneling wave packet. 5
In a separate study, Max Planck researchers explored the role of under-barrier recollisions in shaping the phase of photoelectron wave packets, establishing a general analytical relationship between wave packet phase and tunneling rate. They demonstrated that the Coulomb field of the atomic potential enhances both the amplitude of recolliding trajectories and phase shifts, counteracting lateral spreading of the wave packet during sub-barrier propagation.6
These findings provide critical insights for developing free electron wave packets with tailored properties and enhance the understanding of tunneling dynamics and quantum trajectory reconstruction.
Implications for Quantum Science
UBR redefines the conventional understanding of tunneling by revealing that electrons can undergo complex, multi-step interactions within the barrier rather than simply transmitting through it. This expands the concept of tunneling time, showing that electrons may spend extended durations inside the barrier, experiencing multiple interactions before emerging or reflecting.
The ability to control these interactions by adjusting phase relationships and laser parameters offers new possibilities for quantum control, enabling precise shaping of electron wave packets and the generation of attosecond pulses. These insights carry broad implications for technologies that rely on controlled electron dynamics, including semiconductors, quantum computing, and ultrafast laser systems. They also lay important groundwork for advancing time-resolved spectroscopy used to explore ultrafast physical and chemical processes.4,7
Explore the interpretations of tunneling and other quantum phenomena
References and Further Reading
- Paul Flowers, & Richard Langley. (2013). Tunneling. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Quantum_Mechanics/02._Fundamental_Concepts_of_Quantum_Mechanics/Tunneling
- Hendricks, J. (2016). Quantum Physics. https://books.google.com.pk/books/about/Quantum_Physics.html?id=oGPdCwAAQBAJ&redir_esc=y
- The University of Illinois Urbana-Champaign. (2015). Quantum Tunneling. https://courses.physics.illinois.edu/phys485/fa2015/web/tunneling.pdf
- Khurelbaatar, T., Klaiber, M., Sukiasyan, S., Hatsagortsyan, K. Z., Keitel, C. H., & Kim, D. E. (2025). Unveiling Under-the-Barrier Electron Dynamics in Strong Field Tunneling. Physical Review Letters, 134(21). https://doi.org/10.1103/physrevlett.134.213201
- Klaiber, M., Bakucz Canário, D., & Hatsagortsyan, K. Z. (2023). Sub-barrier recollisions and the three classes of tunneling time delays in strong-field ionization. Physical Review A, 107(5). https://doi.org/10.1103/physreva.107.053103
- Klaiber, M., Hatsagortsyan, K. Z., & Keitel, C. H. (2024). Signatures of under-the-barrier dynamics in a tunneling electron wavepacket. Communications Physics, 7(1), 1-9. https://doi.org/10.1038/s42005-024-01868-3
- Pohang University of Science & Technology (POSTECH). (2025). Quantum tunneling mystery solved after 100 years—and it involves a surprise collision. https://www.sciencedaily.com/releases/2025/07/250727235835.htm
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