The study analyzes the results of a community effort and determines that machine learning greatly improves the estimation of the properties of diffusing particles.

Extending Brownian Motion and Connecting to Complex Systems

Since Albert Einstein provided a theoretical foundation for Robert Brown’s observation of the erratic or unpredictable movement of microscopic particles suspended within pollen grains, researchers have uncovered significant new findings that deviate quite a bit from the laws of Brownian motion in a variety of animate and inanimate systems, from biology to the stock market.

Anomalous diffusion, as scientists call it, extends the concept of Brownian motion and connects to disordered systems, non-equilibrium phenomena, flows of energy and information, and transport in living systems.

Researchers have developed several methods for detecting the occurrence of anomalous diffusion using classical statistics. However, in the last years, the booming of machine learning has boosted the development of data-based methods to characterize anomalous diffusion from single trajectories, providing more refined tools for this problem.

Now, a group of scientists led by researchers from the University of Vic – Central University of Catalunya (Uvic-UCC) together with Optologic researcher Maciej Lewenstein in collaboration with colleagues from ICFO, the University of Gothenburg, the University of Potsdam, and the Universitat Politècnica de València, has provided the first assessment of conventional and novel methods for quantifying anomalous diffusion in a variety of realistic conditions through a community-based effort. The results of the assessment have been recently published in Nature Communications.

AnDi Challenge Spurs Research on Anomalous Diffusion

During the past year, the researchers launched an open competition to benchmark existing methods and to spur the invention of new approaches. The Anomalous Diffusion (AnDi) Challenge brought together a vibrating and multidisciplinary community of scientists working on this problem, involving more than 30 participants from 22 institutions and 11 countries. Ultimately, the analysis of the results obtained on a reference dataset provided an objective assessment of the performance of methods to characterize anomalous diffusion from single trajectories for three specific tasks: anomalous exponent inference, model classification, and trajectory segmentation.

In conclusion, this research not only definitively contributes to the definition of a diverse palette of tools and measures, but it also has the potential for these tools to become standard methods for the analysis of trajectories across a wide range of experiments, from the intricacies of atomic physics to the complexities of ecology. Furthermore, the outcome of this study reinforces the fundamental importance of community-based efforts in the pursuit of scientific advancement. Overall, this collaborative approach is instrumental in fostering progress and innovation within the scientific community.

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Cited article: Muñoz-Gil et al. Objective comparison of methods to decode anomalous diffusion. Nature Communications on October 29, 2021. doi: 10.1038/s41467-021-26320-w

In a theoretical and experimental study published in Nature Physics, an international team of researchers demonstrates the generation of highly non-classical states of light in intense laser–atom interactions.

Advances in Laser Technology and Light-Matter Interaction

Over the past four decades, astounding advances have been made in the field of laser technologies and the understanding of light-matter interactions in the non-linear regime. Consequently, scientists have been able to carry out extremely complex experiments related to, for example, ultra-fast light pulses in the visible and infrared range. Furthermore, they have accomplished crucial milestones such as using a molecule’s own electrons to image its structure, to see how it rearranges and vibrates or breaks apart during a chemical reaction.

The development of high-power lasers allowed scientists to study the physics of ultra-intense laser-matter interactions which, in its standard version, treats ultra-strong ultra-short driving laser pulses only from a classical point of view. The famous theory coined as the “simple man’s model” or the “three-step model” – which had its 25th anniversary in 2019 – dealt with the interaction of an electron with its parent nucleus sitting in a strong laser field environment, and elegantly described it according to classical and quantum processes. However, due to the fact that these laser pulses are highly coherent and contain huge numbers of photons, the description of the interaction in the strong field has so far been incomplete, because it treated the atomic system in a quantum way but the electromagnetic field in a classical way.

Generation of Non-Classical States of Light through Intense Laser-Atom Interactions

In the current landscape, the description of the most relevant processes in ultra-intense laser-matter physics, such as high-harmonic generation, above-threshold ionization, laser-induced electron diffraction, and sequential and non-sequential multi-electron ionization, often overlooks the quantum-fluctuation effects of the laser electric field, let alone the magnetic fields. Nevertheless, the quantum nature of the entire electromagnetic field is inherently present in these processes. Consequently, a natural question arises: does this quantum nature manifest itself in observable ways? Furthermore, under what specific circumstances does this manifestation occur?

In the recent study published in Nature Physics, OPTOlogic researchers at ICFO led by ICREA Prof. Maciej Lewenstein, Max Born Institute in Berlin, in collaboration with the PhD researcher Theocharis Lamprou, led by the Research Director Paraskevas Tzallas, from FORTH, have reported on the theoretical and experimental demonstration that intense laser–atom interactions may lead to the generation of highly non-classical states of light.

Researchers obtained such results by using high-harmonic generation in atoms, where they up-convert large numbers of photons from an infrared driving laser pulse into higher-frequency photons in the extreme ultraviolet spectral range. The quantum electrodynamical theory formulated in this study predicts that a coherent initial state of the driving laser remains coherent but experiences an amplitude shift after interacting with the atomic medium.

Similarly, the harmonic modes’ quantum states become coherent with small coherent amplitudes. However, researchers can condition the driving laser pulse’s quantum state to account for this interaction, transforming it into an optical Schrödinger cat state. This state represents a quantum superposition of two distinct coherent states of light: the initial laser state and the amplitude-reduced coherent state resulting from the interaction with the atoms.

The researchers experimentally accessed the full quantum state of this laser pulse using quantum state tomography. This process requires a coherent reduction of the light’s amplitude to only a few photons on average before measuring all quantum properties of the state.

The results of this study pave the way for investigations into controlling non-classical states of ultra-intense light and exploiting conditioning approaches on physical processes relevant to high-harmonic generation. This research will hopefully create a novel and unexpected link between ultra-intense laser-matter physics, attoscience, quantum information science, and quantum technologies.

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Cited article: M. Lewenstein, M. F. Ciappina, E. Pisanty, J. Rivera-Dean, P. Stammer, Th. Lamprou and P. Tzallas.  Generation of optical Schrödinger “cat” states in intense laser-matter interactions, Nature Physics, 2021.

Researchers from the Physical Chemistry Department of the Fritz Haber Institute and the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have found out that ultrafast switches in material properties can be prompted by laser pulses – and why. This knowledge may enable new transistor concepts.

Advancing Transistor Technology with Light

Making the speed of electronic technology as fast as possible is a central aim of contemporary materials research. The key components of fast computing technologies are transistors: switching devices that turn electrical currents on and off very quickly as basic steps of logic operations. In order to improve our knowledge about ideal transistor materials, physicists are constantly trying to determine new methods to accomplish such extremely fast switches. Researchers from the Fritz Haber Institute of the Max Planck Society in Berlin and the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have now figured out that a novel type of ultrafast switch can be accomplished with light.

Exploring Material Property Control through Light

The physicists involved in the project, are studying how to best get materials to change their properties – to make magnetic metals non-magnetic, for example, or to change the electric conductivity of a crystal. A material’s electrical properties are strongly related to the arrangement of the electrons in the crystal. Controlling the electrons’ arrangement has been a key topic for decades. Most control methods, however, are fairly slow. “We knew that external influences like temperature or pressure variations work”, says Dr. Ralph Ernstorfer, Group Leader at the Department of Physical Chemistry at the Fritz Haber Institute and partner at the Optologic project, “but that takes time, at least a few seconds.” Those who regularly use a smartphone or computer know that a few seconds can feel like an eternity. So Dr. Ernstorfer’s group explored how to switch material properties much faster by means of light.

Ultrafast Switching with Light Pulses

Using brand new equipment at the Fritz Haber Institute, the researchers have massively cut down the switching time to only 100 femtoseconds – 0,000 000 000 000 1 of a second – by shooting ultrashort optical laser pulses at their chosen material, a semi-metallic crystal composed of tungsten and tellurium atoms. Shining light on the crystal encourages it to re-organize its internal electronic structure, which also changes the conductivity of the crystal. In addition, the scientists were able to observe exactly how its electronic structure changed. “We used a new instrument to take pictures of the transition every step of the way”, explains Dr. Samuel Beaulieu, who worked as a postdoctoral fellow with Ralph Ernstorfer at the Fritz-Haber-Institut (2018-2020) and who is now a permanent researcher at the Centre Lasers Intenses et Applications (CELIA) at CNRS-Bordeaux University.

Insight into Ultrafast Electronic Transitions

“This is amazing progress – we used to only know what the electronic structure of the material looked like after, but never during the transition,” he adds. Moreover, state-of-the-art modelling of this new process by Dr. Nicolas Tancogne-Dejean, Dr. Michael Sentef, and Prof. Dr. Angel Rubio from Max the Planck Institute for the Structure and Dynamics of Matter has revealed the origin of this novel type of ultrafast electronic transition. The laser pulse impinging on the materials changes the way electrons interact with each other, which is the driving force of this exotic transition, known as a Lifshitz transition.

Future Prospects for Ultrafast Technology

This method is bound to generate a great deal of knowledge about possible future transistor materials. The fact alone that light can drive ultrafast electronic transitions is the first step towards even quicker and more efficient technology.

Implications for Quantum Device Development

The results of this study also contribute to OPTOlogic, a project aiming to develop a new class of dissipation-less quantum devices. In this project, we aim to artificially induce and control topologically protected states using ultrafast pulses of light. We intend to move and store information with minimal energy expense, while dramatically increasing computing power. Thus, the recent achievement indicates a clear progression toward realizing this goal.

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Cited article: Gierster, L., Vempati, S. & Stähler, J. Ultrafast generation and decay of a surface metal. Nat Commun 12, 978 (2021). https://doi.org/10.1038/s41467-021-21203-6

Researchers from Optologic publish an article in Nature communications. They demonstrate how laboratory-frame photoelectron spectra can be used directly to determine molecular structure with atomic precision through laser-induced electron diffraction (LIED). This innovative approach not only advances our understanding of molecular dynamics but also opens new avenues for precise structural analysis in various scientific fields.

Light microscopes have revolutionized our understanding of microcosmos; however, their resolution is limited to about 100 nanometers. To see how molecules bond, break, or change their structure, we need at least 1000 times better resolution.

Laser-induced electron diffraction (LIED) is a technique that allows one to pinpoint the individual atoms inside a single molecule and to see where each atom moves when the molecule undergoes a reaction. This technique proved to be an amazing tool for the imaging molecules, such as water, carbonyl sulfide or carbon disulfide. However, using a strong laser field to generate the electron diffraction presented challenges in retrieving the exact structure, since the structural resolution depended on the exact knowledge of the laser field itself.

In a study recently published in Nature Communication, ICFO researchers Aurelien Sanchez, Kasra Amini, Tobias Steinle, Xinyao Liu, led by ICREA Prof. at ICFO Jens Biegert, in collaboration with researchers from Kansas State University, Max-Planck-Institut für Kernphysik, Physikalisch-Technische Bundesanstalt, and Friedrich-Schiller-Universität Jena, have reported on an alternative and novel approach that retrieves accurate and precise information about the atomic structure without exact knowledge over the laser field. They successfully applied the method to imaging gas-phase molecule Carbonyl Sulfide (OCS), in particular on the bond lengths between the constituent atoms, showing a significant bent and asymmetrically stretched configuration of the ionized OCS⁺ structure.

Determining the atomic bonds of Carbonyl Sulfide

In their experiment, the scientists took a gas mixture of 1% OCS in helium and expanded it supersonically to create a molecular beam of the gas with a temperature below 90K. They then took a 3.2mm laser and exposed the molecule to the strong laser field. The interaction between the laser and the molecule produced an accelerated electron, which was released from the molecule, accelerated into the laser field and returned back to the target ion by the electric field of the laser; the re-collision of the electron with the ion structure generated a molecular imprint of the structure and, by extracting this information from the electron interference pattern and the scattering angle analysis, the scientists were capable of determining the proper structure of the molecule.

The novelty of the approach

Named ZCP-LIED, the novelty of this approach resides in the fact that the scientists came up with a very clever way to retrieve the atomic information by using the full 2D electron scattering information, mainly the energy and scattering angle spectra of the electron in the laboratory frame instead of the laser frame, which drastically improved the statistics of the results. Alongside using 2D data instead of 1D information, they also identified a distinctive feature in spectra related to what they called the zero crossing point (ZCP) positions (where the interference signal showed a null value). By carrying out the analysis over these critical points, the scientists were able to obtain from a much smaller data set more precise information on the bond lengths of the atoms that make up the molecule, reducing quite considerably the calculation time.

Validation through Quantum Chemistry Comparisons

For validation of their approach, they used various methods, compared them to quantum chemistry theoretical simulations and prove that their ZCP-LIED technique could obtain inter-nuclear distances with much higher precision, and could measure bond distances of similar length (something rather impossible to do with previous methods), that it avoided converting frames of reference, and was able to determine the molecular structure in environments where the background noise could be considerable. Taking all this into account, they reported obtaining the molecular information of 10-atom molecules, and in particular, for the carbonyl sulfide, where they saw that the molecule OCS⁺ had a significantly bent and asymmetrically stretched structure, different to what previous studies had determined for this molecule.

The results obtained by this study have demonstrated that the ZCP-LIED technique could be a very powerful tool to determine the molecular structure of large and more complex molecules. It could also be extended to ultrafast electron diffraction (UED) and even ultrafast X-ray diffraction (UXD) to track the geometric structure molecules in a transient phase.

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Cited article:  A. Sanchez, K. Amini, S.-J. Wang, T. Steinle, B. Belsa, J. Danek, A.T. Le, X. Liu, R. Moshammer, T. Pfeifer, M. Richter, J. Ullrich, S. Gräfe, C.D. Lin, J. Biegert. Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction. Nature Communications, 10.1038/s41467-021-21855-4.