Exploring Topological Quantum Critical Points

A recent study published in Physical Review Letters predicted the existence of a new state of matter: quantum critical points with topological properties occurring in the paradigmatic strongly-correlated extended Bose-Hubbard model.

Condensed Matter Physics and Topological Phases

Condensed matter physics deals with systems of many interacting particles which can be in different phases of matter. One of the main achievements of the last century was the development of the Ginzburg-Landau theory, which allows to the classification of distinctive phases by means of local order parameters. Nevertheless, in the last decades, a new class of states of matter escaping this paradigm has been discovered: The topological phases.

Research and Challenges in Topological Insulators

The research on topological insulators reached its first peak in 1997 when Altland and Zirnbauern classified all the possible topological phases of non-interacting systems by means of symmetry arguments. Despite this seminal classification shedding light on many novel features of quantum matter, it is essential to acknowledge that the comprehensive understanding of topological phases remains an ongoing endeavor, with further investigations poised to refine and expand upon the existing framework.

Quantum Critical Points: Recent Discoveries and Proposals

In a recent article published in Physical Review Letters, a team of researchers revealed that interacting processes can lead to topological properties persisting at the specific points delineating the boundaries between two distinct phases, thus providing compelling evidence for the existence of topological quantum critical points. This seminal discovery not only enriches our understanding of quantum dynamics but also underscores the profound interconnectedness between topology and the intricate fabric of quantum phenomena, offering tantalizing prospects for future research avenues to explore.

An international team of theoretical researchers provides a thorough overview of quantum physics and its introduction to particle physics.

Advancements in Quantum Simulation: Exploring Many-Body Phenomena

In recent decades, researchers have made amazing progress in isolating and manipulating individual quantum systems and studying quantum many-body phenomena in depth, achieving outstanding results and achievements in the field.

The Significance of Quantum Simulators

Quantum simulators, one of the pillars of quantum technologies, constitute a specific branch of quantum computing. While quantum computing deals with more generalized computing processing problems, quantum simulators tend to tackle specific problems to find particular solutions.

Exploring Complex Quantum Systems

Moreover, quantum simulators now offer the possibility of gathering information to understand and simulate many-body systems in condensed matter physics and even high-energy physics. Furthermore, by learning about complex quantum systems that are either inaccessible to experiments or cannot be approached using standard analytical or numerical methods, quantum simulators have opened new avenues of exploration. To study these systems, simulators use superconducting circuits, ultracold atoms, trapped ions, Rydberg atoms, and photonic systems, among others, to mimic these systems and understand, describe, and model them.

A Collaborative Endeavor: Quantum Simulations of Lattice Gauge Theory

In a recent review paper published in Philosophical Transactions A, a team of researchers from ICFO, Ludwig Maximilians University, Universidad Complutense de Madrid, Jagiellonian University, Adam Mickiewicz University, Swansea University, Universität Heidelberg, Johannes Gutenberg-Universität, Vilnius University, Capital Normal University, Forschungszentrum Jülich, University of Cologne, UAM/CSIC, ICCUB-University of Barcelona, SISSA, University of Innsbruck, IQOQI, and the Hebrew University of Jerusalem has collaborated to provide an overview of quantum simulations of lattice gauge theory. They aim to enrich the understanding of quantum many-body physics in general and delve into the world of particle and nuclear physics in particular, showing the state-of-the-art in these fields and future perspectives for applications.

Novel Approaches in Lattice Field Theories

More importantly, the researchers introduce a novel approach to lattice field theories by taking the most commonly used theoretical models in gauge theory and replacing their fermionic matter (electrons, protons, neutrons, etc.) with bosonic matter (photons, mesons, etc.) since theorists have realized that these latter elements are more accessible and easier to manipulate for experimentalists.

Recent Advances and Future Directions

Moreover, within the paper, they highlight recent achievements in physics by reviewing the bosonic model of Schwinger. Additionally, they delve into how ultra-cold atoms can explore interesting strongly correlated phenomena related to condensed matter and high-energy physics. Furthermore, they also focus on recent advances in the field of quantum simulators and the different platforms used in tabletop experiments. These platforms include trapped ions, ultracold atoms, or superconducting qubits, allowing the study of isolated many-particle dynamics in real-time. Moreover, they emphasize measuring higher-order correlations and entanglement as key elements in understanding these systems.

As ICREA Prof. at ICFO Maciej Lewenstein points out,

“By employing atomic systems such as ultracold atoms in optical lattices, an enormous range of paradigmatic models from condensed-matter and high-energy physics are being currently studied using table-top experiments. This is turning Feynman’s idea of a quantum simulator into a reality”.

Cited article: Aidelsburger Monika, Barbiero Luca, Bermudez Alejandro, Chanda Titas, Dauphin Alexandre, González-Cuadra Daniel, Grzybowski Przemysław R., Hands Simon, Jendrzejewski Fred, Jünemann Johannes, Juzeliūnas Gediminas, Kasper Valentin, Piga Angelo, Ran Shi-Ju, Rizzi Matteo, Sierra Germán, Tagliacozzo Luca, Tirrito Emanuele, Zache Torsten V., Zakrzewski Jakub, Zohar Erez and Lewenstein Maciej. Cold atoms meet lattice gauge theory 2021, Phil. Trans. R. Soc. A. 380: 2021006420210064

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