LPTMC Seminars

Kirone Mallick (IPhT, Saclay)
08.10.2024 10:45  11:45SéminairesMacroscopic ﬂuctuations out of equilibrium
A system, subject to continuous exchanges of matter, energy or information with its surroundings, may reach a nonequilibrium steady state in which various currents break timereversal invariance and continuously generate entropy. Such a state can not be accounted for by the Principles of Thermodynamics or the GibbsBoltzmann laws of statistical physics. Besides, linear response theory and the OnsagerMachlup functional provide useful descriptions of large scale fluctuations in such driven systems only at first order, in the vicinity of equilibrium.
In the last two decades, important advances in our understanding of processes far from equilibrium have been achieved, for which rare events, large deviations and ﬂuctuations relations provide a uniﬁed framework. The emergence of universal features can be studied thanks to a variational principle, proposed by G. JonaLasinio and his collaborators, known as the Macroscopic Fluctuation Theory (MFT). In this theory, optimal ﬂuctuations far from equilibrium are determined at a coarsegrained scale by two coupled nonlinear hydrodynamic equations. The objective of this talk is to present these concepts and to illustrate them with some exact solutions of the MFT equations.

Andrea Tononi (ICFO, Barcelona, Spain)
01.10.2024 12:45  13:45SéminairesTemporal Bell inequalities in a manybody system
We formulate a temporal ClauserHorne inequality by considering two parties choosing two observables to measure at different consecutive times. For two entangled antipodal spins joined by a spin chain, we show that the inequality is violated during a small finite time interval between the measurements. This fact contrasts with the time evolution in vacuum, which is describable in terms of a hiddenvariable theory. Our result demonstrates that the finite velocity for quantum information spreading in the chain prevents signaling and therefore the immediate vanishing of quantumness. 
[Séminaire exceptionnel] Sofyan Iblisdir
30.09.2024 11:00  12:00SéminairesCollective Monte Carlo updates through tensor network renormalization
We introduce a MetropolisHastings Markov chain for Boltzmann distributions of classical spin systems. It relies on approximate tensor network contractions to propose correlated collective updates at each step of the evolution. We present benchmarks for a variety of instances of the twodimensional Ising model, including ferromagnetic, antiferromagnetic, (fully) frustrated and EdwardsAnderson spin glass cases. With modest computational effort, our Markov chain achieves sizeable acceptance rates, even in the vicinity of critical points. It compares well with other Monte Carlo schemes such as the Metropolis or Wolff algorithm: equilibration times appear to be reduced by a factor that varies between 40 and 2000, depending on the model and the observable being monitored. The scheme can be adapted to three dimensions, matrix models, or a confined gas of hard spheres.