Séminaires du LPTMC

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)

Competing orders in many-electron systems: a renormalization group perspective

The renormalization group is an established approach to study quantum many-body systems, and this applies especially to one of its modern implementations known as the functional renormalization group (FRG). In particular, the FRG constitutes a flexible and unbiased tool for the study of competing orders. In this talk, I will outline recent progress in this direction for correlated electron systems. To this end, I will first discuss the competition between antiferromagnetism, charge density waves and superconductivity in the 2D Hubbard model, thus making a connection with high-temperature superconductors. The special role of bosonization methods will be emphasized along the way. I will also show how the FRG can be combined with dynamical mean-field theory to treat strongly interacting regimes, with a focus on d-wave superconductivity. As a next step, I will increase the complexity of the model by including non-local interactions and discuss unconventional superconductivity in an extended Hubbard model with a connection to moiré materials. Special consideration will also be given to the treatment of retarded interactions with electron-phonon couplings. Finally, I will highlight recent FRG studies of quantum criticality in Dirac materials, with a connection to graphene.

Equilibrium phase transition between a fluid and an amorphous solid

When a liquid is cooled, it can form a glass: a mechanically rigid but structurally disordered solid. Experimentally, this transformation occurs when the system falls out of equilibrium and no longer explores all accessible configurations on experimental timescales. A central open question, dating back more than a century, is whether this dynamical arrest reflects an underlying equilibrium phase transition. While theory predicts such a transition in idealized models (with deep connections to spin glass physics), its existence in realistic finite-dimensional systems remains unsettled. I will review this problem and present numerical results for a two-dimensional glass-forming liquid. By combining complementary Monte Carlo techniques, we equilibrate the system down to zero temperature over a range of system sizes and directly measure its equilibrium thermodynamic and structural properties. These results provide evidence for an equilibrium phase transition between a fluid and an amorphous solid. I will conclude by discussing the implications and open questions raised by this finding.

Collective dynamics in binary superfluids: From dissipationless flow to dispersive shock waves

Binary superfluids are typically characterized by two distinct internal states, giving rise to a spin mode in addition to the conventional density mode. These systems can be realized, for example, using Bose-Einstein condensation into two hyperfine atomic states, or the propagation of a two-polarization laser in a birefringent nonlinear medium. In this talk, I will mainly present two theoretical investigations into collective phenomena in these systems. First, inspired by a recent experiment at LKB [1], I will discuss the critical speed for dissipationless flow of a two-dimensional binary superfluid of light past an impurity [2]. For a weak impurity, the drag is determined within linear-response theory and aligns with Landau’s criterion. For an impurity of arbitrary strength, the critical speed is obtained from the conditions for strong ellipticity of the stationary flow equations. We identify the emission of linear waves and vortex structures in the density and spin sectors as primary mechanisms for dissipative flow. Second, I will examine a coherently driven binary Bose-Einstein condensate, inspired by an experiment at Institut d’optique [3]. In its ground state, this system can be effectively described by a single coherent field satisfying the cubic-quintic nonlinear Schrodinger equation. In a one-dimensional geometry, we investigate nonlinear periodic solutions arising from steplike initial conditions. Using modulation theory, we analyze contact dispersive shock waves [4], nonlinear structures that are fundamentally absent in the standard, cubic nonlinear Schrodinger framework. To conclude, I will briefly complement these results by looking toward simpler, single-component systems, to highlight a recent experimental result at LKB [5, 6]: the observation that a finite-mass impurity can self-propel against a two-dimensional superfluid flow via vortex-antivortex shedding. Reducing the impurity to its center of mass and using a point-vortex model, I will describe how quantized vortices can serve as momentum-transfer agents, effectively bridging the physics of quantum fluids with the field of active matter.

[1] C. Piekarski, N. Cherroret, T. Aladjidi, and Q. Glorieux, Spin and density modes in a binary fluid of light, Phys. Rev. Lett. 134, 223403 (2025).
[2] P.-E. Larré, C. Michel, and N. Cherroret, Critical speed of a binary superfluid of light (2026).
[3] A. Hammond, L. Lavoine, and T. Bourdel, Tunable three-body interactions in driven two-component Bose-Einstein condensates, Phys. Rev. Lett. 128, 083401 (2022).
[4] T. Congy, P.-E. Larré, and P. Sprenger, Modulation theory for cubic-quintic nonlinear Schrodinger equations (2026).
[5] M. Baker-Rasooli, T. Aladjidi, T. D. Ferreira, A. Bramati, M. Albert, P.-E. Larré, and Q. Glorieux, Swimming against a superfluid flow: Self-propulsion via vortex-antivortex shedding in a quantum fluid of light, arXiv:2512.09028 (2025).
[6] T. Aladjidi, M. Baker-Rasooli, T. D. Ferreira, A. Bramati, M. Albert, P.-E. Larré, and Q. Glorieux, Critical velocity of a flow of superfluid light past a finite-mass impurity of tunable width (2026).