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.

Spatiotemporal Characterization of Active Dynamics in Channels: Theory and Experiments

Swimming microorganisms often live in confined, complex environments, where they transition between bulk and near-surface dynamics. Their dynamics can be quantified in terms of first-passage statistics. In this talk, I will first consider run-and-tumble bacteria confined in a channel. Combining theoretical predictions based on a renewal framework with experimental observations of Escherichia coli, we study the statistics of the time required, after leaving one wall, to encounter either wall. I will discuss how incorporating heterogeneity in tumbling rates or non-exponential run-duration distributions affects the survival probability. In the second part of the talk, I will consider active Brownian dynamics between two walls. Using a systematic expansion, we compute first-passage properties. Exploiting Siegmund duality, we infer the corresponding spatial properties for active Brownian particles confined between hard walls and reveal a transition towards a wall-accumulated state, reminiscent of experimental observations.

Superconducting dome and incipient modulate phase in SrTiO3

SrTiO₃ is a “quantum paraelectric” in which dipolar fluctuations grow upon cooling, yet long-range ferroelectric order never develops. In this seminar, I will discuss the evolution of these dipolar fluctuations, as measured by inelastic neutron scattering, as the system is tuned toward superconducting and ferroelectric phases.

First, I will show that the superconducting dome of SrTiO₃ is driven by the competition between the increase in the density of states and the inevitable collapse of the quantum paraelectric phase under electron doping. Second, I will demonstrate that these dipolar fluctuations couple to a transverse acoustic mode (elastic constant c₄₄), with this coupling being most pronounced at small q-vectors. I will further show that SrTiO₃ lies near a modulated phase, as evidenced by significant softening of its transverse acoustic branch.

Both the amplitude of the coupling and the modulation vector are strongly influenced by the enhancement of the ferroelectric and antiferrodistortive (AFD) phase transitions. These findings suggest that SrTiO₃ is not only an incipient ferroelectric but also an incipient modulated material, with the modulated phase cooperating, rather than competing, with ferroelectricity and the AFD transition.

If time permits, I will also present the electric-field dependence of the thermal conductivity—another probe of acoustic phonons—in SrTiO₃. This will provide further evidence of TO–TA hybridization in SrTiO₃.

Rare events and single big jump effects in stochastic processes

Rare events in stochastic processes are typically described within large deviation theory (LDT), where atypical fluctuations arise from the accumulation of many small contributions. In systems with sub-exponential statistics, however, rare events can instead be dominated by a single large fluctuation, as prescribed by the big jump principle (BJP). In this talk, I will discuss this alternative mechanism and its interplay with standard large deviation behavior across different classes of stochastic processes. I will first focus on power-law dynamics, such as Lévy processes, where single big jump effects control extreme value statistics and first-passage properties, including the behavior of the fastest trajectories in multi-particle settings, in particular their mean exit time from a bounded domain. I will then turn to processes with stretched-exponential statistics, with particular emphasis on the Ornstein–Uhlenbeck process. Using a renewal representation, one can identify a crossover between a regime of typical fluctuations described by LDT and a rare-event regime governed by the BJP, providing a physical interpretation of previously observed anomalous solutions in terms of single big jump effects. Finally, I will briefly discuss a perturbative approach that allows one to access intermediate regimes of moderate deviations, interpolating between these two limits.