ICTP-SAIFR Activities

In this presentation, we review historical and modern results on the linear response of dynamical systems to external perturbations and the celebrated relationship between fluctuations and dissipation expressed by the Fluctuation-Dissipation Theorem (FDT). The conceptual derivation of FDT originated from the definition of the equilibrium state and Onsager’s regression hypothesis. Over time the fluctuation-dissipation relation was vividly investigated for systems away from equilibrium, frequently exhibiting wild fluctuations of measured parameters. In this talk, we recall the major formulations of the FDT, such as those proposed by Langevin, Onsager, and Kubo [1]. We discuss as well the importance of fluctuations in a wide class of growth phenomena. We point out challenges and opportunities where the relationship breaks down and cannot be adapted.[1] F. A. Oliveira, R. M. S. Ferreira, L. C. Lapas and M.H. Vainstein, Review article Anomalous Diffusion: A Basic Mechanism for the Evolution of Inhomogeneous Systems Frontiers in Physics, 7, (18) (2019).  

Disordered elastic systemsare ubiquitous and encompass a rich variety of systems ranging from amorphous solids, colloidal suspensions and network glasses to dislocation systems, biopolymer fiber networks, confluent cell tissues and even strange metals and machine learning. TheJamming transition characterizes the loss of rigidity of a large class of amorphous materials and provides one of the most paradigmatic frameworks to describe the critical behavior that is exhibited by several classes of disordered viscoelastic systems. In this talk, I will discuss the fascinating statistical physics of systems near jamming and show results of a new theory that is both analytically tractable, and that includes an interesting crossover scaling behavior. I will then combine this theory with phenomenological scaling arguments to derive the singular behavior of the dynamic response near this transition, which I will use to determine critical exponents, invariant scaling combinations and analytical formulas for universal scaling functions of a variety of quantities. Finally, I will show how these results can shed light into the anomalous charge-density fluctuations of strange metals and discuss how the incorporation of active elements (self-propulsion) affects the universal scaling framework of a class of disordered elastic systems.

Music has long filled a uniquely important role in bridging human culture and biology, stretching back over millennia, and of course today provides respite and remedy in an increasingly stressful world. We do not sing alone. On land, four kinds of animals produce songs or calls: birds, frogs, mammals, and insects. Some of these animals (and fish) also do so underwater. The principal sounds such animal species make are signaling behaviors directly related to mating success and social cohesion and their ranges are molded by their forms and by their particular forest, savannah or seaside habitat.Human music also has origins, motivations and mechanisms in common with other animals. Traces of a long and ongoing history comes from archaeology, anthropology and brain studies. Evidence of a measurable impact on human biology comes from neurobiology, social psychology, and public health.

Gravitational waves (GWs) are probes of the cosmological model since, as electromagnetic waves, they travel across the largest distances in our Universe. These signals can also track generic modifications of General Relativity on large scales, especially when combined with other cosmological probes. There are several methods for using such fascinating signals to learn more about cosmology. For instance, GWs are distance indicators, and similarly to supernovae, they allow reconstructions of a distance-redshift relation when the latter can be inferred from electromagnetic counterparts emitted by the same source or known indirectly via binary population studies or galaxy catalogs. In this colloquium I will discuss some of the current methods for doing cosmological parameter estimation and tests of gravity with GWs, focusing on studies of future multi-messenger observations and on combinations of GW data with large-scale structure surveys.

Quantum systems bring unprecedented advantages to the realization of classical cryptographic tasks. A remarkable example is quantum key distribution, which allows two parties, Alice and Bob, to establish a secret key that can be used for unconditionally secure communication. Furthermore, the strong correlations exhibited by quantum systems can be explored to construct cryptographic protocols that are secure even when the parties are completely ignorant about the internal working of their devices. This is the device-independent paradigm. In this talk I will introduce the device-independent scenario and review recent results on device-independent quantum key distribution. Then I will move to the case of many parties and discuss extensions to multipartite cryptographic tasks. Finally, I will present new results where we introduce the paradigm of self-testing with dishonest parties and design a protocol for device-independent certification of the quantum state distributed in a network when some of the nodes may collude and act dishonestly.