Atomic ‘Scars’ Reveal How Quantum Systems Become Classical

Simon His and colleagues at the Sorbonne University have demonstrated how disorder fundamentally alters the behaviour of electrons ejected from atoms during high-order harmonic generation. Their one-dimensional simulations reveal that scattering from a disordered environment causes a shift from quantum to classical behaviour in photoelectron dynamics, evidenced by the localisation of probability density around unstable periodic orbits. This process, mirroring the phenomenon of quantum scars, uniquely occurs within a time-dependent framework, directly observable in real-time dynamics and offering new insights into strong-field physics.

Photoelectron dynamics reveal active quantum scars and stochastic decoherence in disordered systems

Simulations demonstrate a transition from quantum to classical behaviour in photoelectrons, with harmonic peak intensities reducing by up to 25% compared to gas-phase models. This reduction in harmonic intensity signifies a diminished efficiency in attosecond pulse generation, as high-order harmonic generation (HHG) is the primary method for creating these extremely short pulses of light. The shift, previously unobservable, arises from stochastic dephasing caused by elastic scattering within a disordered environment. Elastic scattering, in this context, refers to collisions where the photoelectron’s kinetic energy is conserved, but its direction is altered by the surrounding scattering potential. Employing an open quantum systems’ perspective, analysis of the photoelectron dynamics revealed that local disturbances induce global decoherence and subsequent localisation of probability density around unstable periodic orbits. Open quantum systems theory is crucial here, as it allows for the modelling of the atom not as an isolated entity, but as a system interacting with, and therefore influenced by, its environment.

Analogous to ‘quantum scars’ typically seen in static systems, this process now occurs dynamically within time-dependent frameworks, offering a new understanding of strong-field physics and the behaviour of electrons in complex materials. Quantum scars are remnants of quantum behaviour persisting in classically chaotic systems, appearing as enhanced probability density along unstable periodic orbits. In traditional studies, these scars are observed in the energy spectrum of static systems. However, this research demonstrates their dynamic emergence in the time domain, directly linked to the evolution of the photoelectron wavefunction. Detailed simulations revealed the stochastic nature of the surrounding environment introduces a purity loss in the photoelectron state, quantified by examining the statistical mixture of initial states. This purity loss is a direct measure of decoherence, the destruction of quantum superposition and entanglement. The team confirmed dynamic scarring emerges directly from real-time electron dynamics, not solely from spectral analysis of the harmonic signal. This is significant because it validates the observation of dynamic scarring as a genuine physical phenomenon, rather than an artefact of signal processing. Further investigation focused on the implications of this dynamic behaviour for attosecond pulse generation and the broader field of ultrafast dynamics. The ability to control and understand decoherence is paramount for optimising HHG efficiency and achieving shorter, more intense attosecond pulses.

Decoherence in disordered systems impacts attosecond pulse generation

Increasingly, scientists focus on understanding how disorder impacts quantum systems, a challenge with implications ranging from materials science to biological processes. Disorder, whether structural imperfections in a solid or fluctuations in a biological environment, inevitably introduces decoherence and alters quantum behaviour. A key limitation lies in extrapolating findings from simplified, one-dimensional models to the complex, three-dimensional environments found in real-world scenarios. While one-dimensional models offer computational tractability and conceptual clarity, they necessarily omit many degrees of freedom present in realistic systems. Previous stochastic models attempted to bridge this gap, but relied on averaging techniques that may introduce artificial effects rather than reflecting genuine physical ensembles. Averaging over many realisations of the disorder can obscure the underlying dynamics and mask the true impact of individual scattering events.

Acknowledging the simplification inherent in modelling complex systems, these models offer valuable insight into the transition between quantum and classical behaviour in scattering environments. The transition from quantum to classical behaviour is not abrupt, but rather a gradual process driven by increasing decoherence. This research provides a framework for understanding the factors that accelerate this transition and the resulting impact on quantum phenomena. High-order harmonic generation, a technique used to create attosecond light pulses, is profoundly affected by the loss of quantum coherence. The efficiency of HHG relies on the coherent superposition of multiple photon interactions with the atom; decoherence disrupts this coherence, reducing the harmonic signal. Consequently, understanding this process allows scientists to better predict and control electron behaviour, potentially improving laser-based technologies and furthering investigations into ultrafast dynamics within atoms and molecules. Attosecond pulses are used to probe the dynamics of electrons in matter, offering unprecedented temporal resolution. Optimising HHG efficiency is therefore crucial for advancing this field.

Structural disorder fundamentally alters photoelectron behaviour during strong-field physics. A model system, combined with open quantum systems theory, a framework describing how quantum systems interact with their surroundings, linked local disturbances to a widespread loss of quantum coherence. The open quantum systems approach allows for the inclusion of environmental effects, such as scattering, in a rigorous and physically meaningful way. This manifests as a transition from predictable quantum motion to classical chaotic behaviour, evidenced by the localisation of electron probability around unstable periodic orbits. The localisation of probability density around unstable orbits is a hallmark of classical chaos, indicating that the electron’s motion is no longer governed by the laws of quantum mechanics. The observed 25% reduction in harmonic peak intensities highlights the significant impact of disorder on HHG efficiency and underscores the importance of considering environmental effects in strong-field physics.

The research demonstrated that structural disorder surrounding an atom causes a loss of quantum coherence in photoelectrons, driving a transition towards classical behaviour. This decoherence is linked to local disturbances and manifests as electron probability localising around unstable periodic orbits, mirroring quantum scars seen in other systems. The study revealed a 25% reduction in high-order harmonic generation intensities due to this effect, highlighting how environmental factors impact the process. These findings provide a framework for understanding how quantum systems interact with their surroundings and influence electron behaviour during strong-field interactions.