LPTMC Seminars

How Pauli's principle becomes a theorem in relativity

The Pauli principle ceases to be a principle and becomes a theorem when we move from Galilean quantum mechanics to Lorentz-invariant quantum theory, i.e. quantum field theory. The fascinating aspect of this transition is that even though this principle/theorem plays a major role in the Galilean limit (its consequences do not become smaller and smaller as we consider speeds that are small compared to the speed of light), no one has ever been able to prove it by restricting themselves to Galilean invariance.
Much to the chagrin of some, this seminar will only address historical and conceptual aspects of the problem. The Pauli principle will be explained and its demonstration in the context of quantum field theory will be sketched. Some of its most striking consequences will also be reviewed. The role of the four-dimensional space-time will be briefly discussed.

Non-thermal fixed points in far-from-equilibrium 3D Bose gases

Résumé: 

Following a quantum quench, local observables in many-body systems typically thermal-
ize. In certain cases, however, this thermalization occurs via a two-stage process: the system
first exhibits universal dynamical scaling laws with strongly non-thermal properties, before
eventually reaching thermal equilibrium on a longer time scale. This phenomenon is referred
to as a non-thermal fixed point.
In this talk, I will discuss the non-thermal fixed point that emerges when a 3D Bose gas is
quenched across the condensation transition. I will show that it generally involves a transient
"weak turbulence" regime, followed at later times by a coarsening dynamics associated with
the slow recombination of vortex lines. Quenches performed exactly at the critical point, in
contrast, display a distinct coarsening dynamics, presumably without vortices but involving
the diffusion of critical fluctuations. If time permits, I will also discuss the robustness of
this universal dynamics against external perturbations, typically disorder and drive.

The Nobel Prize in Physics 2025: Quantum physics with electrical circuits

The Nobel Prize in Physics 2025 was awarded to John Clarke, Michel Devoret and John Martinis “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit”. I will describe their experiments, which gave birth to the now flourishing domain of quantum electronics.

Vers une phyllotaxie tridimensionnelle

Résumé : Inspirés par l'observation de certaines croissances spirales de plantes, les arrangements phyllotactiques bidimensionnels sont des exemples très intéressants de structures homogènes non périodiques engendrées par des règles simples. En séparant les parties radiales et angulaires , ils peuvent par ailleurs être généralisés à des surfaces de courbure positive ou négative. Nous décrirons ici plusieurs essais de généralisation à trois dimensions de ce type d'arrangements. Un premier exemple  reprend la modalité de construction des réseaux périodiques compacts à 3D par empilement itérés de réseaux triangulaires sur les espaces interstitiels des couches précédentes. Une seconde approche procède différemment,  par croissance radiale, soit de façon automatique en suivant une règle simple, ou bien de façon numérique en minimisant un potentiel d'interaction. Deux autres modèles, pouvant également donner lieu à des structures intéressantes dans R3 seront présentés : un ensemble phyllotactique sur la sphere S3 construit autour d'une fibration de Hopf discrète, et un autre à 4 dimensions obtenu comme produit de deux structures phyllotactiques 2d.

Reference : Some attempts toward 3-dimensional phyllotaxy, Rémy Mosseri and Jean-François Sadoc, Structural chemistry, vol 36, pages 1963–1972 (2025)