Laboratoire de Physique Théorique de la Matière Condensée

Séminaires du LPTMC

10.4.2024 - 7.6.2024
  • Guy Bunin (Technion Haïfa)

    Date 27.05.2024 10:45 - 11:45

    Many-species dynamics in space

    Natural ecosystems often harbor individuals of many species, spread out in space. We describe two very different dynamical behaviors (‘phases’) that can be found in such systems, depending on the interactions between the species. In one phase, population sizes undergone huge fluctuations, spanning many orders of magnitude, that persist indefinitely in time. In the other phase, every location in space assumes one of many stable states, where each state is characterized by the combination of species present in that location. These different states may then expand in space, resulting in a self-replication mechanism that competes over space. This leads to selection over ecosystem states, in analogy with Darwinian selection.

  • Weitao Chen (National University of Singapore)

    16.05.2024 14:00 - 15:00

    Multifractality and dynamics at the Anderson transition: From finite dimension to infinite dimension

    Multifractality is an exotic property that emerges at the Anderson transition. Meanwhile, the dynamics are highly influenced by the multifractality of the eigenstates. This presentation will focus on the emergence of multifractality and its dynamic signatures in random-matrix ensembles amenable to analytical treatment. Firstly, I will revisit random-matrix ensembles that capture multifractal properties in finite dimensions, emphasizing the scale-invariant properties of dynamics as a consequence of multifractality. Secondly, I will introduce new random-matrix ensembles featuring critical properties in infinite dimension, the upper critical dimension of the Anderson transition. Through analytical arguments, these models reveal two scenarios of critical properties: logarithmic multifractality and critical localization. These results will help to clarify some elusive problems of Anderson transitions in random graphs.

    References: Physical Review E 108(5) , 054127 (2023); arXiv:2312.17481 (2023).

  • Denis Ullmo (LPTMS)

    13.05.2024 10:45 - 11:45

    Pedestrians in static crowds are not grains, but game players.

    The short-term (‘operational’) dynamics of pedestrian crowds are generally thought to involve no anticipation, except perhaps the avoidance of the most imminent
    collisions. I will show that current models rooted in this belief fail to reproduce essential features observed experimentally by Nicolas et al. [Sci. Rep. 9, 105 (2019).] when a static crowd is crossed by an intruder.

    The missing ingredient can be identified as the pedestrians’ ability to plan ahead far enough beyond the next interaction. On this basis, I will introduce a minimal model based on mean-field game theory which proves remarkably successful in capturing the experimental observations associated with this setting, but also other daily-life situations such as partial metro boarding. These findings are clear evidence that a long term game theoretical approach is key to capturing essential elements of the dynamics of crowds.

    [refs : Phys. Rev. E 107, 024612 (2023), SciPost Phys. 16, 104 (2024)]

  • Séminaire TQM: Dganit Meidan (BGU, Beer-Sheva, Israël)

    02.05.2024 14:00 - 15:00
    Séminaires TQM

    Theory of free fermion dynamics – from monitored to post selected evolution

    Monitored quantum systems undergo Measurement-induced Phase Transitions (MiPTs) stemming from the interplay between measurements and unitary dynamics. When the detector readout is post- selected to match a given value, the dynamics is generated by a Non-Hermitian Hamiltonian with MiPTs characterized by different universal features. Here, we derive a partial post-selected stochastic Schrodinger equation based on a microscopic description of continuous weak measurement. This formalism connects the monitored and post-selected dynamics to a broader family of stochastic evolution. We apply the formalism to a chain of free fermions subject to partial post-selected monitoring of local fermion parities. Within a 2-replica approach, we obtained an effective bosonized Hamiltonian in the strong post-selected limit. Using a renormalization group analysis, we find that the universality of the non-Hermitian MiPT is stable against a finite (weak) amount of stochasticity. We further show that the passage to the monitored universality occurs abruptly at finite partial post-selection, which we confirm from the numerical finite size scaling of the MiPT. Our approach establishes a way to study MiPTs for arbitrary subsets of quantum trajectories and provides a potential route to tackle the experimental post-selected problem.

  • Alessio Lerose (University of Oxford)

    24.04.2024 10:45 - 11:45

    Synthetic quantum matter out of equilibrium: A few recent advances from theory to simulation

    "Synthetic matter" has emerged as a new paradigm of quantum many-body physics, characterized by unprecedented degree of spatiotemporal control and programmability of Hamiltonian interactions. If on the one hand these experimental developments bring us closer to Feynman's vision of a universal quantum simulator for challenging open questions in many-body physics, on the other hand new fundamental theory questions on the behavior of quantum matter far from thermal equilibrium become accessible. Thermalization dynamics of isolated quantum systems and non-thermal states of matter are now at the center of multiple research efforts in theoretical physics. In this talk I will describe recent advances in understanding the mechanism of thermalization as well as long-lived non-equilibrium states of matter. Specifically, I will introduce an influence-functional approach to quantum many-body dynamics and describe preliminary evidence that it helps classifying non-equilibrium universal behavior. Furthermore, I will discuss the synthetic-matter version of the celebrated Coleman's false-vacuum decay scenario, and show that unique dynamical features appear, including emergent quasi-many-body-localized dynamics of interfaces and metastable long-range order. In parallel, I will describe how such theoretical advances led to unforeseen developments in applications, from a numerical method for strongly correlated electrons to a strategy for quantum simulation of real-time phenomena in lattice gauge theories.