arXiv subjects

Pietro Brighi

Publications and source records attributed to Pietro Brighi.

14 recordsLinked to original sources

arXiv2026

Collective Dynamics in Spin Chains with Constrained Dissipation: Classically Fragile but Quantum Robust

Collective non-equilibrium phenomena in many-body systems are strongly influenced by fluctuations, particularly in the vicinity of phase transitions, where the interplay between classical and quantum effects can alter cooperative behavior. Here, we investigate this classical-quantum competition in a kinetically constrained spin chain, the dissipative far-East model. In absence of fluctuations, the system exhibits emergent collective dynamics, resulting in phase coexistence. We find that this phenomenon is robust to quantum fluctuations, which further stabilize it, whereas classical fluctuations suppress phase coexistence at long times. We map out the steady-state phase diagram using a cluster mean-field approach and characterize the dynamics with stochastic tensor-network methods. Within the bistable phase we identify signatures of dynamical heterogeneity in both the local magnetization and the entanglement dynamics. This highlights how competing quantum fluctuations and classical kinetic constraints shape the relaxation dynamics and the phase structure of many-body systems.

quant-ph↗

arXiv2025

Pairing-induced phase transition in the non-reciprocal Kitaev chain

Investigating the robustness of non-reciprocity in the presence of competing interactions is central to understanding non-reciprocal quantum matter. In this work, we use reservoir engineering to induce non-reciprocal hopping and pairing in the fermionic Kitaev chain, and reveal the emergence of a pairing-induced phase transition. The two phases appear in the spectrum of the non-Hermitian Kitaev Hamiltonian describing the dynamics of correlations, separated by an exceptional point. In the non-reciprocal phase, dynamics are characterized by directionality and slow relaxation, and the steady state supports non-reciprocal density and spatial correlations. At strong pairing, we uncover an unexpected density wave phase, featuring short relaxation times, a modulation in particle occupation and strikingly different correlation spreading depending on pairing non-reciprocity. Our work highlights the non-trivial breakdown of non-reciprocity due to superconducting pairing and invites experimental investigation of non-reciprocal fermionic systems.

quant-ph↗

arXiv2025

Dissipative quantum North-East-Center model: steady-state phase diagram, universality and nonergodic dynamics

In this work we study the dissipative quantum North-East-Center (NEC) model: a two-dimensional spin-1/2 lattice subject to chiral, kinetically constrained dissipation and coherent quantum interactions. This model combines kinetic constraints and chirality at the dissipative level, implementing local incoherent spin flips conditioned by an asymmetric majority-vote rule. Through a cluster mean-field approach, we determine the steady-state phase diagram of the NEC model under different Hamiltonians, consistently revealing the emergence of two distinct phases, bistable and normal, across all cases considered. We further investigate the stability of the steady-state with respect to inhomogeneous fluctuations in both phases, showing the emergence of instabilities at finite wavevectors in the proximity of the phase transition. Next, we study the nonergodicity of the model in the bistable phase. We characterize the dynamics of minority islands of spins surrounded by a large background of spins pointing in the opposite direction. We show that in the bistable phase, the minority islands are always reabsorbed by the surrounding at a constant velocity, irrespectively of their size. Finally, we propose and numerically benchmark an equation of motion for the reabsorption velocity of the islands where thermal and quantum fluctuations act independently at linear order.

quant-ph↗

arXiv2025

Finite steady-state current defies non-Hermitian many-body localization

Non-Hermitian many-body localization (NH MBL) has emerged as a possible scenario for stable localization in open systems, as suggested by spectral indicators identifying a putative transition for finite system sizes. In this work, we shift the focus to dynamical probes, specifically the steady-state spin current, to investigate transport properties in a disordered, non-Hermitian XXZ spin chain. Through exact diagonalization for small systems and tensor-network methods for larger chains, we demonstrate that the steady-state current remains finite and decays exponentially with disorder strength, showing no evidence of a transition up to disorder values far beyond the previously claimed critical point. Our results reveal a stark discrepancy between spectral indicators, which suggest localization, and transport behavior, which indicates delocalization. This highlights the importance of dynamical observables in characterizing NH MBL and suggests that traditional spectral measures may not fully capture the physics of non-Hermitian systems. Additionally, we observe a non-commutativity of limits in system size and time, further complicating the interpretation of finite-size studies. These findings challenge the existence of NH MBL in the studied model and underscore the need for alternative approaches to understand localization in non-Hermitian settings.

cond-mat.dis-nn↗

arXiv2025

Probing the many-body localized spin-glass phase through quench dynamics

Eigenstates of quantum many-body systems are often used to define phases of matter in and out of equilibrium; however, experimentally accessing highly excited eigenstates is a challenging task, calling for alternative strategies to dynamically probe nonequilibrium phases. In this work, we characterize the dynamical properties of a disordered spin chain, focusing on the spin-glass regime. Using tensor-network simulations, we observe oscillatory behavior of local expectation values and bipartite entanglement entropy. We explain these oscillations deep in the many-body localized spin glass regime via a simple theoretical model. From perturbation theory, we predict the timescales up to which our analytical description is valid and confirm it with numerical simulations. Finally, we study the correlation length dynamics, which, after a long-time plateau, resumes growing in line with renormalization group (RG) expectations. Our work suggests that RG predictions can be quantitatively tested against numerical simulations and experiments, potentially enabling microscopic descriptions of dynamical phases in large systems.

cond-mat.dis-nn↗

arXiv2025

Restoring thermalization in long-range quantum magnets with staggered magnetic fields

Quantum systems with strong long-range interactions are thought to resist thermalization because of their discrete energy spectra. We show that applying a staggered magnetic field to a strong long-range Heisenberg antiferromagnet restores thermalization for a large class of initial states by breaking permutational symmetry. Using self-consistent mean-field theory and exact diagonalization, we reveal that the energy spectrum, while composed of discrete subspaces, collectively forms a dense spectrum. The equilibration time is independent of system size and depends only on the fluctuations in the initial state. For initial states at low to intermediate energies, the long-time average aligns with the microcanonical ensemble. However, for states in the middle of the spectrum the long-time average depends on the initial state due to quantum scar-like eigenstates localized at unstable points in classical phase space. Our results can be readily tested on a range of experimental platforms, including Rydberg atoms or optical cavities.

quant-ph↗

arXiv2024

Anomalous transport in the kinetically constrained quantum East-West model

We study a chaotic particle-conserving kinetically constrained model, with a single parameter which allows us to break reflection symmetry. Through extensive numerical simulations we find that the domain wall state shows a variety of dynamical behaviors from localization all the way to ballistic transport, depending on the value of the reflection breaking parameter. Surprisingly, such anomalous behavior is not mirrored in infinite-temperature dynamics, which appear to scale diffusively, in line with expectations for generic interacting models. However, studying the particle density gradient, we show that the lack of reflection symmetry affects infinite-temperature dynamics, resulting in an asymmetric dynamical structure factor. This is in disagreement with normal diffusion and suggests that the model may also exhibit anomalous dynamics at infinite temperature in the thermodynamic limit. Finally, we observe low-entangled eigenstates in the spectrum of the model, a telltale sign of quantum many body scars.

quant-ph↗

arXiv2024

Non-reciprocal dynamics and the non-Hermitian skin effect of repulsively bound pairs

We study the dynamics of a Bose-Hubbard model coupled to an engineered environment which in the non-interacting limit is described by the celebrated Hatano-Nelson model. At strong interactions, two bosons occupying the same site form a so-called repulsively bound pair, or doublon. Using tensor-network simulations, we clearly identify a distinct doublon lightcone and show that the doublon inherits non-reciprocity from that of single particles. Applying the idea of reservoir engineering at the level of doublons, we introduce a new set of dissipators and we analytically show that then the doublon dynamics are governed by the Hatano-Nelson model. This brings about an interaction-induced non-Hermitian skin effect and non-reciprocal doublon motion. Combining features of the two models we study, we show that single particles and doublons can be made to spread with opposite directionality, opening intriguing possibilities for the study of dynamics in interacting non-reciprocal models.

quant-ph↗

arXiv2023

Many-body localization proximity effect in two-species bosonic Hubbard model

The many-body localization (MBL) proximity effect is an intriguing phenomenon where a thermal bath localizes due to the interaction with a disordered system. The interplay of thermal and non-ergodic behavior in these systems gives rise to a rich phase diagram, whose exploration is an active field of research. In this work, we study a bosonic Hubbard model featuring two particle species representing the bath and the disordered system. Using state of the art numerical techniques, we investigate the dynamics of the model in different regimes, based on which we obtain a tentative phase diagram as a function of coupling strength and bath size. When the bath is composed of a single particle, we observe clear signatures of a transition from an MBL proximity effect to a delocalized phase. Increasing the bath size, however, its thermalizing effect becomes stronger and eventually the whole system delocalizes in the range of moderate interaction strengths studied. In this regime, we characterize particle transport, revealing diffusive behavior of the originally localized bosons.

cond-mat.dis-nn↗

arXiv2022

Hilbert space fragmentation and slow dynamics in particle-conserving quantum East models

Quantum kinetically constrained models have recently attracted significant attention due to their anomalous dynamics and thermalization. In this work, we introduce a hitherto unexplored family of kinetically constrained models featuring a conserved particle number and strong inversion-symmetry breaking due to facilitated hopping. We demonstrate that these models provide a generic example of so-called quantum Hilbert space fragmentation, that is manifested in disconnected sectors in the Hilbert space that are not apparent in the computational basis. Quantum Hilbert space fragmentation leads to an exponential in system size number of eigenstates with exactly zero entanglement entropy across several bipartite cuts. These eigenstates can be probed dynamically using quenches from simple initial product states. In addition, we study the particle spreading under unitary dynamics launched from the domain wall state, and find faster than diffusive dynamics at high particle densities, that crosses over into logarithmically slow relaxation at smaller densities. Using a classically simulable cellular automaton, we reproduce the logarithmic dynamics observed in the quantum case. Our work suggests that particle conserving constrained models with inversion symmetry breaking realize so far unexplored universality classes of dynamics and invite their further theoretical and experimental studies.

quant-ph↗

arXiv2021

Propagation of Many-body Localization in an Anderson Insulator

Many-body localization (MBL) is an example of a dynamical phase of matter that avoids thermalization. While the MBL phase is robust to weak local perturbations, the fate of an MBL system coupled to a thermalizing quantum system that represents a "heat bath" is an open question that is actively investigated theoretically and experimentally. In this work we consider the stability of an Anderson insulator with a finite density of particles interacting with a single mobile impurity -- a small quantum bath. We give perturbative arguments that support the stability of localization in the strong interaction regime. Large scale tensor network simulations of dynamics are employed to corroborate the presence of the localized phase and give quantitative predictions in the thermodynamic limit. We develop a phenomenological description of the dynamics in the strong interaction regime, and demonstrate that the impurity effectively turns the Anderson insulator into an MBL phase, giving rise to non-trivial entanglement dynamics well captured by our phenomenology.

cond-mat.dis-nn↗

arXiv2021

Localization of a mobile impurity interacting with an Anderson insulator

Thermalizing and localized many-body quantum systems present two distinct dynamical phases of matter. Recently, the fate of a localized system coupled to a thermalizing system viewed as a quantum bath received significant theoretical and experimental attention. In this work, we study a mobile impurity, representing a small quantum bath, that interacts locally with an Anderson insulator with a finite density of localized particles. Using static Hartree approximation to obtain an effective disorder strength, we formulate an analytic criterion for the perturbative stability of the localization. Next, we use an approximate dynamical Hartree method and the quasi-exact time-evolved block decimation (TEBD) algorithm to study the dynamics of the system. We find that the dynamical Hartree approach which completely ignores entanglement between the impurity and localized particles predicts the delocalization of the system. In contrast, the full numerical simulation of the unitary dynamics with TEBD suggests the stability of localization on numerically accessible timescales. Finally, using an extension of the density matrix renormalization group algorithm to excited states (DMRG-X), we approximate the highly excited eigenstates of the system. We find that the impurity remains localized in the eigenstates and entanglement is enhanced in a finite region around the position of the impurity, confirming the dynamical predictions. Dynamics and the DMRG-X results provide compelling evidence for the stability of localization.

cond-mat.dis-nn↗

arXiv2020

Stability of mobility edges in disordered interacting systems

Many-body localization provides a mechanism to avoid thermalization in isolated interacting quantum systems. The breakdown of thermalization may be complete, when all eigenstates in the many-body spectrum become localized, or partial, when the so-called many-body mobility edge separates localized and delocalized parts of the spectrum. Previously, De Roeck \textit{et al.}[arXiv:1506.01505] suggested a possible instability of the many-body mobility edge in energy density. The local ergodic regions -- so called "bubbles" -- resonantly spread throughout the system, leading to delocalization. In order to study such instability mechanism, in this work we design a model featuring many-body mobility edge in \emph{particle density}: the states at small particle density are localized, while increasing the density of particles leads to delocalization. Using numerical simulations with matrix product states we demonstrate the stability of MBL with respect to small bubbles in large dilute systems for experimentally relevant timescales. In addition, we demonstrate that processes where the bubble spreads are favored over processes that lead to resonant tunneling, suggesting a possible mechanism behind the observed stability of many-body mobility edge. We conclude by proposing experiments to probe particle density mobility edge in Bose-Hubbard model.

cond-mat.dis-nn↗

arXiv2019

Effect of the Anomalous Diffusion of Fluctuating Cooper Pairs in the Density of States of Superconducting NbN Thin Films

Recent scanning tunnelling microscopy experiments in NbN thin disordered superconducting films found an emergent inhomogeneity at the scale of tens of nanometers. This inhomogeneity is mirrored by an apparent dimensional crossover in the paraconductivity measured in transport above the superconducting critical temperature $T_c$. This behavior was interpreted in terms of an anomalous diffusion of fluctuating Cooper pairs, that display a {\em quasi-confinement} (i.e., a slowing down of their diffusive dynamics) on length scales shorter than the inhomogeneity identified by tunnelling experiments. Here we assume this anomalous diffusive behavior of fluctuating Cooper pairs and calculate the effect of these fluctuations on the electron density of states above $T_c$. We find that the density of states is substantially suppressed up to temperatures well above $T_c$. This behavior, which is closely reminiscent of a pseudogap, only arises from the anomalous diffusion of fluctuating Cooper pairs in the absence of stable preformed pairs, setting the stage for an intermediate behavior between the two common paradigms in the superconducting-insulator transition, namely the localisation of Cooper pairs (the so-called bosonic scenario) and the breaking of Cooper pairs into unpaired electrons due to strong disorder (the so-called fermionic scenario).

cond-mat.supr-con↗