Séminaires du LPTMC

Dissipative Quantum Chaos — from Theory to the Lab

Dissipative quantum chaos is an emerging field that investigates how chaotic behavior arises in quantum systems coupled to their environments. Understanding the dynamics of such open quantum systems is crucial, particularly for describing quantum matter in realistic scenarios where environmental interactions cannot be neglected. In this talk, I will outline recent theoretical advances in characterizing universal features of dissipative quantum systems using non-Hermitian random matrix theory. Key spectral signatures of dissipative chaos, such as complex spacing ratios (CSRs), universal ring-to-disk transitions, and a systematic symmetry-based classification of many-body superoperators under antiunitary involutions, will be discussed. Turning from theory to practice, I will present our recent experimental achievements demonstrating dissipative quantum chaos and integrability transitions. These results were obtained by measuring CSRs in the superoperator spectra of open many-body quantum systems implemented on a state-of-the-art superconducting quantum processor, providing the first direct laboratory observation of these theoretical predictions.

A Josephson junction in a multimode environment: emergent quantum phase transition and exact low-energy duality

The physics of a single Josephson junction coupled to a resistive environment is a long-standing fundamental problem at the center of an intense debate about the existence and properties of the superconducting-to-insulating Schmid-Bulgadaev transition. To circumvent the potential subtleties in the original derivation, we investigate the emergent criticality of a junction coupled to a multimode resonator when the number of modes is increased [1]. By solving the system via exact diagonalization, we show that at the transition point the spectrum displays universality (scale invariance) not only at low frequencies. This reflects in finite-frequency spectral signatures of the phase transition, in agreement with recent experiments. The spectrum at the critical point is successfully compared with analytical and numerical results obtained in the past [2]. Finally, we prove a low-energy exact self-duality of the model, that emerges from two different finite-size circuits with different conserved quantities [3]. This confirms and generalizes the approximate self duality that is usually invoked, and proves the independence of the transition point on the ratio of Josephson to charging energy.

[1] Giacomelli L., Ciuti C., Nature Communications, 15(1), 5455 (2024)

[2] Paris, Giacomelli, Daviet, Ciuti, Dupuis, Mora, Phys. Rev. B, 111(6), 064509 (2025)

[3] Giacomelli L., Devoret M. H., Ciuti C., Phys. Rev. Lett., 136(13), 130401 (2026)

Slimming down through frustration

In many disease, proteins aggregate into fibers. Why? One could think of molecular reasons, but here we try something more general. We propose that when particles with complex shapes aggregate, geometrical frustration builds up and fibers generically appear. Such a rule could be very useful in designing artificial self-assembling systems.

Replica Symmetry breaking for Ulam's problem

The description of complex system by the concept of Replica Symmetry Breaking (RSB) was shaped by Giorgio Parisi in the 1980s to solve the mean-field spin glass, as honored by the Nobel price in 2021. RSB has been used to analyze systems such as spin glasses, neural networks, optimization problems, or machine learning. Unfortunaley, numerically these well know RSB-exhibiting problems are difficult since only exponential-time exact algorithms are available.

Here two models are considered, directed polymers in random media and increasing subsequences, called Ulam's problem for the ground states, i.e. longest subsequences. The distributions of free energies or sequence lengths, respectively, exhibit complex large-deviation behavior, which can be numerically addressed by rare-event sampling algorithms.

Furthermore, for both models it is possible to sample exactly in perfect thermal equilibrium with polynomial-time algorithms. This means, large system sizes are accessible, in contrast to, e.g., the case of spin glasses. The results from perfect sampling of some problem disorder ensembles indicate
the presence of RSB with complex structured landscapes. Thus, the study of complex RSB behavior is conveniently accessible numerically for some models.

Finally, for partially presorting random sequences, obe obtains a transition similar to a ferromagnet-spin glass transition.