Talks

Critical gauge theories on the fuzzy sphere: a Monte Carlo perspective

The deconfined quantum critical point (DQCP) is an example of phase transitions beyond the Landau symmetry breaking paradigm that attracts wide interest. However, its nature has not been settled after decades of study. In this talk, we apply the recently proposed fuzzy sphere regularisation to study the SO(5) non-linear sigma model (NL

Quantum Dynamics Seminar: Exact Factorization: Quantum-Classical Approaches and Light-Matter Interaction

The exact factorization (EF) formalism has gained considerable interest within the field of nonadiabatic molecular dynamics simulations, as it resulted in the development of several promising numerical methods (see, e.g., (1)). In the EF formalism, the molecular wavefunction is expressed as a product of a time-dependent marginal nuclear and a time-dependent conditional electronic wavefunction. This decomposition naturally leads to interesting properties, as the emergence of a gauge field, as well as a well-defined separation between nuclear and electronic degrees of freedom, that gives an excellent starting point to derive mixed quantum-classical approaches. In this talk, I will present conceptual and mathematical properties of the EF approach, and the derivation of trajectory based mixed quantum-classical methods, specifically, the coupled-trajectory mixed quantum-classical (CTMQC) algorithm and of the (combined) coupled-trajectory Tully surface hopping ((C)CTTSH) method. I illustrate the methods with numerical studies on fulvene and 4-(dimethylamino)benzonitrile (DMABN) (2). Finally, I will present exact factorization-based approaches to light-matter interactions in both classical, periodic (Floquet) driving fields (3) and cavity quantum electrodynamics settings (4), demonstrating how mixed quantum-classical methods perform in this regime - opening new paths for efficient control studies of larger molecules and predictive modeling in molecular photochemistry. (1) ‎Ibele, L., Sangiogo Gil, E., Villaseco Arribas, E., Agostini, F., Phys. Chem. Chem. Phys., 26, 26693 (2024). (2) Schürger, P., Ibele, L., Lauvergnat, D., Agostini, F., J. Chem. Phys., 162, 104117 (2025). (3) Schirò M., Eich, F. G., Agostini, F., J. Chem. Phys. 154, 114101 (2021). (4) Sangiogo Gil E., Lauvergnat D., Agostini F., J. Chem. Phys. 161, 084112. (2024). (5) Magi, C., Schürger, P., Agostini, F., in preparation (2025).

Tunable Matter -- Many More is More Different

In 1972 Phil Andersen articulated the motto of condensed matter physics as “More is different.” However, for most many-body systems the behavior of a trillion bodies is nearly the same as that of a thousand. Here I argue for a class of condensed matter, “tunable matter," in which many more is different. The ultimate example of tunable matter is the brain, whose cognitive capabilities increase as size increases from 302 neurons (C. Elegans) to a million neurons (honeybees) to 100 billion neurons (humans). I propose that tunable matter provides a unifying conceptual framework for understanding not only a wide range of systems that perform biological functions, but also physical systems capable of being trained to develop special collective behaviors without using a processor.

Effect of interaction softness on the collective properties of active Brownian particles

Quantum algorithms and quantum control

While quantum processors have progressed immensely over the last decade, they still face significant hurdles such as short coherence times and high error rates. As a result, they are not able to compete with classical information processing in solving problems of practical interest unless big advances take place both at the bottom of the stack (hardware, control) and at the top (algorithms). I will discuss our contributions across the quantum information processing stack, from the control of qubits to quantum algorithm development and back.

Specificity and tolerance of the immune T cell repertoire

The adaptive immune system protects the body from an ever-changing landscape of foreign pathogens. The two arms of the adaptive immune system, T cells and B cells, mount specific responses to pathogens by utilizing the diversity of their receptors, generated through hypermutation. T cells recognize and clear infected hosts when their highly variable receptors bind sufficiently strongly to antigen-derived peptides displayed on a cell surface. To avoid auto-immune responses, randomly generated receptors that bind strongly to self-peptides are eliminated in the “central" process of thymic selection, ensuring a mostly self-tolerant repertoire of mature T cells. “Peripheral” tolerance, including a quorum mechanism further protects against self-targeting T cells that escape thymic selection. We discuss how these mechanisms can still fail during persistent infections.

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Subexponential decay of local correlations from diffusion-limited dephasing

Chaotic quantum systems at finite energy density are expected to act as their own heat baths, rapidly dephasing local quantum superpositions. We argue that in fact this dephasing is subexponential for chaotic dynamics with conservation laws in one spatial dimension: all local correlation functions decay as stretched exponentials or slower. The stretched exponential bound is saturated for operators that are orthogonal to all hydrodynamic modes. This anomalous decay is a quantum coherent effect, which lies beyond standard fluctuating hydrodynamics; it vanishes in the presence of extrinsic dephasing. Our arguments are general, subject principally to the assumption that there exist zero-entropy charge sectors (such as the particle vacuum) with no nontrivial dynamics: slow relaxation is due to the persistence of regions resembling these inert vacua, which we term "voids". In systems with energy conservation, this assumption is automatically satisfied because of the third law of thermodynamics.

Colloquium SPEQED25

Exotic metals, fractionalization, and quantum criticality

The quest for novel states of matter is important both on fundamental grounds and in view of possible applications, with superconductivity and the various quantum Hall effects being outstanding examples. This talk will summarize recent developments in the field, with an emphasis on the effects on frustration and intrinsic topological order. I will highlight frustration-based routes to novel forms of order and disorder, non-Fermi liquid metals and exotic superconductivity, and I will discuss aspects on quantum phase transitions between the various phases. Connections to experiments on kagome and pyrochlore metals as well as cuprate high-temperature superconductors will be made.

Colloquium

Colloquium