LPTMC Seminars

Salle 523, couloir 12-13, 5è étage

Hybrid between biologically inspired and quantum inspired many-body states

Deep neural networks can represent very different sorts of functions, including complex quantum many-body states. Tensor networks can also represent these states, have more structure and are easier to optimize. However, they can be prohibitively costly computationally in two or higher dimensions. In this seminar I will propose a hybrid network [1] which borrows features from the two different formalisms. I will showcase the ansatz by obtaining the representation of a transverse field quantum Ising model with a long range 1/r^6 antiferromagnetic interaction on a 10×10 square lattice. The model corresponds to the Rydberg (cold) atoms platform proposed for quantum annealing.

[1] Srdinsek, Waintal, arXiv:2506.05050 (2025)

Salle 523, couloir 12-13, 5è étage

Exceptional Stationary State in a Dephasing Many-Body Open Quantum System

The late-time dynamics of many-body systems is one of the central problems in statistical mechanics. The eventual emergence of Gibbs ensembles at late times for closed systems is usually explained using the Eigenstate Thermalization Hypothesis (ETH): it postulates, among other things, local indistinguishability of the energy eigenstates and proper statistical ensembles. Rare eigenstates that violate ETH are known as many-body scars and can affect the dynamics of the system in a non-trivial way.

We investigate a related mechanism for an open quantum many-body system. In particular, we focus on a model that hosts, together with the infinite-temperature state, another additional stationary state. The latter is exceptional in many respects and plays the role of a quantum scar. We discuss the properties of the model, focusing on the fate of interfaces between the two states. We find that at late times an effective description is based on stochastic fluctuations of the interface; in particular, the scar is progressively eroded at a finite velocity, and the interface broadens diffusively. While this mechanism resembles hydrodynamics of local conserved charges, important differences are pointed out.

This is a joint work with Alice Marché, Gianluca Morettini, Leonardo Mazza, and Lorenzo Gotta [Phys. Rev. Lett. 135, 020406 (2025)]

Salle 523, couloir 12-13, 5è étage

A perturbative approach to the macroscopic fluctuation theory

The typical behavior of a large class of microscopic diffusive dynamics can be described by macroscopic PDEs and the fluctuations are also well encoded by the dynamical large deviations. This macroscopic description is fully determined by 2 transport coefficients, namely the diffusion coefficient and the conductivity. A great achievement of the macroscopic fluctuation theory is to represent the density large deviations of the corresponding stationary states in terms of a macroscopic variational principle (known as the quasi-potential). For general transport coefficients, this dynamical variational principle is not explicit and a closed form has been only obtained for a restricted class of models.

In a small forcing regime, we will explain how the large deviation functional of the density can be computed perturbatively by using the macroscopic fluctuation theory. This applies to general domains in any dimension and to diffusive dynamics with arbitrary transport coefficients. [Joint work with B. Derrida]