A Quantum Mystery Unveiled

Tunneling is one of the most intriguing phenomena in quantum mechanics, with no equivalent in classical physics. It plays a critical role in the interactions between intense lasers and atoms or molecules, driving processes like high-order harmonic generation through the dynamics of electrons following tunnel ionization.

Delving Beneath the Barrier

While the process of tunneling has been extensively studied, the behavior of electrons dwelling under the tunneling barrier has remained largely a mystery. In laser-induced strong field ionization, two scenarios are typically identified: the multiphoton regime at lower intensities, and the tunneling regime at high intensities.

Yet, many investigations have been conducted in a middle ground where multiphoton effects are visible, while tunneling is still the predominant mechanism.

A Groundbreaking Discovery

Recent research from Michael Klaiber and Karen Hatsagortsyan of the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg has introduced an exciting new model predicting a unique electron excitation mechanism. Their work, published in the journal *Physical Review Letters*, suggests that electrons could be reflected at the end of the tunnel—a purely quantum effect.

As they move backwards under the barrier, electrons can gain enough energy to reach an excited state of the atom, leading to possible ionization through the absorption of just a few photons.

From Theory to Reality

The researchers applied this innovative model to the strong-field ionization of xenon atoms, encompassing both direct multiphoton ionization and under-the-barrier recollision in a comprehensive three-dimensional calculation. Remarkably, similar results were observed for krypton atoms, both theoretically and experimentally, underscoring the broader significance of under-barrier dynamics in strong laser fields.

Expanding Horizons in Quantum Control

Klaiber commented on the implications of these findings: "These new insights enhance our understanding of tunneling dynamics, which could revolutionize laser spectroscopy and attosecond physics."

Additionally, the under-barrier resonances may lead to notable alterations and attosecond tunneling time delays in similar contexts, potentially impacting strong-field molecular, solid-state, and even high-energy tunneling quantum dynamics.