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

**Pierre-François Cohadon (Laboratoire Kastler Brossel and Virgo Collaboration)**

**Gravitational-wave detection and quantum-limited measurements**

Detecting gravitational waves required 4 decades of experimental effort to reach a sensitivity at the h~10-21 level, corresponding to mirror displacements below 10-18 m.

I will review this "noise hunting" effort and give some details about the recent observation of 2 neutron star mergers.

Apart from classical noise (seismic noise, thermal noise...), it was realized as soon as in the late 70s that quantum fluctuations of the light field were responsible for the Standard Quantum Limit, a sensitivity limit that second-generation gravitational-wave interferometers will reach once they operate at their design sensitivity, within a few years. A number of ideas have been considered to beat the SQL: squeezed states of the light field, tailoring the optical response function or taking advantage of the mirror mechanical response to radiation pressure. I will present the first experimental demonstrations of such ideas, either on suspended interferometers or table-top experiments.

Séminaire du LPTMC du mardi 24/10 à 11h à la bibliothèque du LPTHE (couloir 13-14 4ème étage).

**David Carpentier (ENS Lyon)**

**The Chiral Anomaly and Ballistic Transport of Weyl Fermions**

The chiral anomaly is a unique property of massless relativistic particles in three dimensions. While the chiral nature of particles was expected to be a conserved property, the breaking of this symmetry was a bizarre property of the associated field theory. The recent focus on materials in which low energy electrons behave as massless relativistic Weyl particles has opened the possibility to experimentally access consequences on transport of this field theory property. Indeed, an anomalous contribution to the conductance in the presence of a magnetic field was associated to the presence of the chiral anomaly. In this talk, I will focus on the different regime of ballistic transport of Weyl fermions in small mesoscopic conductors.

I will show how the anomalous transport can be traced back to the presence of anomalous Landau Levels in such a junction, and to the chiral anomaly. I will finally comment on the relation between diffusive and ballistic regimes.

**Riccardo Ben Ali Zinati**

Etudiant en thèse à la Sissa de Trieste

**Functional RG approach to the Potts model**

This talk is intended to be a detailed presentation of the nprg approach to the Potts model. I will first introduce the model, its symmetry and a useful representation in terms of the hypertetrahedron (simplex) symmetry group.

I will then exploit this geometric approach to construct the corresponding field theory, i.e. the field theory for an n-component scalar with discrete global symmetry S_(n+1). I finally turn to the implementation of the nprg for the Potts field theory and develop two approximations: an algorithm to compute the beta functions of power interactions for arbitrary n and the LPA for fixed n, both in arbitrary dimension.

I conclude giving some results with particular attention to Percolation and Spanning forest universality classes and draw some perspectives on future possible applications.

reference: https://arxiv.org/pdf/1707.03410.pdf

**Fabien Paillusson**

Enseignant-chercheur à l'université de Lincoln en Angleterre

"**N!, indistinguishability and entropy: the instructive case of polydisperse systems**"

The so-called Gibbs paradox is a paradigmatic narrative illustrating the necessity to account for the N! ways of permuting N identical particles when summing over microstates. Yet, there exist some mixing scenarios for which the expected thermodynamic outcome depends on the interpretation of this combinatorial term one chooses and this is what we wish to investigate in this talk.

In the first part of the talk we will briefly introduce what the Gibbs paradox is about and what is the standard rationale used to justify its resolution. In a second part, we will allow ourself to question from a historical standpoint whether the Gibbs paradox has actually anything to do with Gibbs' work. In so doing, we also aim at shedding a new light with regards to some of the theoretical claims surrounding its resolution. In a third part we will then turn to the statistical thermodynamics of discrete and continuous mixtures and introduce the notion of composition entropy to characterise these systems. This will enable us to address, in a certain sense, a "curiosity" pointed out by Gibbs in a paper published in 1876. Finally, we will finish by proposing a connexion between the results we propose and a recent extension of the Landauer bound regarding the minimum amount of heat to be dissipated to reset one bit of memory.

**Le séminaire aura lieu à la bibliothèque du LPTHE et non au LPTMC- Tour 13 4ème étage**

**Oleg Starykh (University of Utah)**

**Emergent Ising orders of frustrated magnets**

Much of the research in frustrated quantum magnets has focused on the elusive quest for magnetically disordered phases with highly entangled ground states - quantum spin liquids. Somewhat intermediate between these rare states and commonplace magnets are {\em nematic} phases which appear as a result of a two-magnon condensation and are characterized by the presence of a gap for excitations with spin one. As a result, nematic states exhibit no dipolar magnetic order.

In my talk I describe two simple models supporting spin nematic phases. The first of them is provided by the two-magnon instability of the 1/3 magnetization plateau state of the quantum triangular antiferromagnet. I show that the two-magnon instability, which takes place near the end-point of the magnetization plateau, leads to a novel two-dimensional vector chiral phase with alternating spin currents. This interesting state spontaneously breaks inversion symmetry and can be thought of as appearing due to a fluctuation-generated Dzyaloshinskii-Moriya interaction. The second example involves an easy-axis spin-1 antiferromagnet in which transition into nematic state occurs via condensation of spin excitons.

ESSAI