Laboratoire de Physique Théorique

de la Matière Condensée



Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle  523 du LPTMC - Tour 12-13 


Silvia Pappalardi (ENS)

Quantum bounds and fluctuation dissipation theorem

In recent years, there has been intense attention on the constraints posed by quantum mechanics on the dynamics of the correlation at low temperatures, triggered by the postulation and deriva- tion of quantum bounds on the transport coefficients or on the chaos rate. However, the physical meaning and the mechanism enforcing such bounds is still an open question. In this talk, I will discuss the quantum fluctuation-dissipation theorem (the KMS conditions) as the principle underlying bounds on correlation time scales. By restating the problem in a replicated space, I will show that the quantum bound to chaos is a direct consequence of the KMS condition, as applied to a particular pair of two-time correlation and response functions. Encouraged by this, I will describe how quantum fluctuation-dissipation relations act in general as a blurring of the time-dependence of correlations, which can imply bounds on their decay rates. Thinking in terms of fluctuation-dissipation opens a direct connection between bounds and other thermodynamic properties.

Francesco Mori (LPTMS)

Stochastic resetting: from geometric properties to optimal control

"When in a difficult situation, it is sometimes better to give up and start all over again''. While this empirical truth has been regularly observed in a wide range of circumstances, quantifying the effectiveness of such a heuristic strategy remains an open challenge. In this talk, I will first consider the minimal model of a single diffusive particle that is reset to its starting position with a constant rate. I will present recent results on the geometrical properties of this process, including the convex hull [1] and the number of visited sites [2]. Then, I will introduce a novel framework that allows to optimally control a very general class of dynamical systems through restarts [3]. This approach, analog to the celebrated Hamilton-Jacobi-Bellman equation, is successfully applied to simple settings and provides the basis to investigate realistic restarting strategies across disciplines.

[1] S. N., Majumdar, F. Mori, H. Schawe, and G. Schehr, Phys. Rev. E 103, 022135 (2021).

[2] M. Biroli, F. Mori, and S. N. Majumdar, preprint arXiv:2202.04906 (2022).

[3] B. De Bruyne and F. Mori, preprint arXiv:2112.11416 (2021).

Xhek Turkeshi (Collège de France)

Measurement-induced phase transitions in random circuits

I will discuss what happens to a quantum many-body system when its unitary evolution is interspersed with local measurements. The tension between unitary dynamics (generating entanglement) and measurements (localizing information) resolves in a dynamical phase transition between one phase where dominated by scrambling and another one in which frequent measurements constraint the state on a reduced manifold. In this talk, I will consider the minimal model for the transition, where the unitary dynamics is generated by random circuits. I will discuss how the measurement-induced phase transition is encoded in structural facets of the system wave-function, which is analyzed through the lens of participation entropy. Large-scale numerical simulations and the investigation of a variety of models identify a robust order parameter for the transition.


Kazumasa A. Takeuchi (Univ. Tokyo)

Bacterial glass

An interesting question in active matter physics is what states of matter may arise in active matter and how different they are from thermal systems. In this seminar, I will talk about our recent experimental attempt [1] to realize a glassy state of bacterial populations, using a microfluidic device [2] that can maintain uniform growth conditions even for dense populations. We characterize glassy properties of dense bacterial populations, such as dramatic dynamic slowdown and dynamic heterogeneity. Comparing with thermal systems, we argue a possible role of collective motion and cell shape therein.

[1] H. Lama, M. Yamamoto, Y. Furuta, T. Shimaya, and K. A. Takeuchi, to appear.
[2] T. Shimaya, R. Okura, Y. Wakamoto, and K. A. Takeuchi, Commun. Phys. 4, 238 (2021).


Uniquement par zoom

Meeting ID: 847 7757 6669
Passcode: 597796

Leonid Mirny (MIT)

Chromatin as a memory machine

One of the hallmarks of nuclear organization in eukaryotes is the spatial segregation of transcriptionally active (euchromatin) and inactive (heterochromatin) genomic regions. Recently we found that such compartmentalization is driven by affinity between heterochromatin regions (Falk et al Nature 2019) . Despite the widespread of such compartmentalized organization in nature, its functional roles remain elusive.

Here we examine the role of compartmentalization in the maintenance of epigenetic memory, i.e. maintenance of pattern of histone marks for hundreds of generations. We modeled joint dynamics of chromatin and histone marks: loss and spreading of marks, and refolding of chromosomes through the cell cycle. A surprinting analogy between the spreading of histone marks and the spreading of a disease in a pandemic helped to identify factors that provide robust memory. We further found a parallel between epigenetic memory and an associative memory in the neural network. Our analysis shows that operation of chromatin as a memory device requires enzyme limitation and spatial spreading of the marks in the dense and spatial segregated heterochromatin, suggesting a functional role for this hallmark of nuclear organization.


En version hybride, dans la salle de séminaire du LPTMC (couloir 13-12, 5ème, salle 5-23) et sur zoom:

Meeting ID: 885 2329 4809
Passcode: 613679