Researchers from ICFO and Aarhus University report in Nature Communications a novel technique that uses orbital angular momentum to detect entanglement in the attosecond regime.

The Significance of Entanglement in Quantum Physics and Attosecond Physics

Entanglement is one of the sweet words we listen to lately when referring to quantum physics. It has provided staggering results in quantum simulations and computing, proving to optimize time in calculations, or even enhance imaging processes. In addition, attosecond physics is the field of physics that studies processes that occur in matter on the scale of attoseconds, i.e. 10^-18s the time it takes for electrons to move through atoms and molecules. The combination of entanglement techniques and attoscience has led to a new field where attosecond imaging methods exploit quantum phenomena like interference, however, the role of entanglement remains unclear/unexplored.

Recent Advances in Studying Entanglement in Attoscience

Now, the potential for entanglement to optimize or improve attosecond imaging has been unexplored so far because, among other things, scientists never assumed that it would have such a leading role in the process. However, in recent years, there has been a growing interest in entanglement in attoscience, with the main focus on the entanglement between electrons and ions, revealing a connection between them that allows better understanding of coherence. Most of the focus has been on the connection between electrons and ions, but studies on the entanglement between two ionized electrons have received less attention. And those few studies that have tackled the issue have based their work on the entanglement of continuous quantities, which are challenging to compute and interpret, and often impossible to measure.

Demonstrating Entanglement through Electron Vortex States

In a recent study published in Nature Communications, ICFO researchers Andrew S. Maxwell, who then transferred to Aarhus carrying this study, and ICREA Prof. at ICFO Maciej Lewenstein, in collaboration with Lars Madsen from Aarhus University, have shown that the production and measurement of electron vortex states, which are free electrons with helical wavefront that may carry orbital angular momentum (OAM), offer a solution to the above problems.

Quantifying Entanglement and Developing Detection Methods

In their study, they exploit the discrete degree of freedom, the OAM—inherent to all free particles, to clearly demonstrate the manifestation of entanglement in non-sequential double ionization (NSDI), a highly correlated two-electron ionization process. Through known conservation laws and the superposition of intermediate excited states, they demonstrate that the OAM of the two ionized electrons in NSDI is in fact entangled.

They then use the logarithmic negativity as a way to quantify the entanglement for a wide range of targets and parameters. They construct an entanglement witness, which provides a novel way for detecting the entanglement in an experiment in a much simpler way, avoiding full tomographic measurements. The interplay of intermediate excited states allows the photoelectrons to approach maximally entangled states. Furthermore, the entanglement is robust as it survives incoherent averaging over the focal volume of the laser.

Implications of OAM Entanglement for Attosecond Imaging

The use of this new technique involving OAM in attosecond processes provides a new pathway for improving imaging techniques and learning how to control matter on ultrafast times scales. Furthermore, the OAM entanglement of the photoelectrons demonstrates the fundamental non-classical nature of NSDI.

 

Links

Link to paper

Link to the research group led by ICREA Prof. at ICFO Maciej Lewenstein

 

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