arXiv subjects

Maurizio Fagotti

Publications and source records attributed to Maurizio Fagotti.

At least 19 recordsLinked to original sources

arXiv2026

Temperature Beyond Equilibrium in Isolated Quantum Many-Body Systems and Their Subsystems

Temperature is one of the central concepts of thermodynamics, yet its meaning far from equilibrium remains unclear. The problem is especially challenging in isolated quantum many-body systems, whose states evolve unitarily, may remain far from equilibrium, and retain energy coherence, a genuinely quantum feature with no direct classical counterpart. Specifically, energy fluctuations in a nonstationary quantum state have two distinct origins. Part of them comes from uncertainty in the energy populations and has the usual thermodynamic meaning. The rest comes from quantum coherence between energy sectors and is responsible for the state's time dependence. We propose that, even away from equilibrium, temperature identifies the state within the family of regular states sharing the same energy-coherence structure. This provides a natural definition of temperature for a broad class of nonequilibrium quantum states. The resulting inverse temperature is not, in general, obtained by differentiating entropy with respect to energy. The usual maximum-entropy principle is instead replaced by a principle of minimum discrimination information, which selects the least distinguishable state compatible with the prescribed energy and coherence structure. We also extend the construction to subsystems and show that, although their inverse temperature is not determined by the reduced state alone, its instantaneous rate of change is a local quantity, determined by the thermodynamic structure induced on the subsystem at that time.

quant-ph↗

arXiv2026

Emergent integrable dynamics in a non-integrable Rydberg-atom chain

We show that integrable and non-integrable dynamics can coexist in the same Rydberg-atom chain, depending on the initial state in which the system is prepared. In the setting we consider, Rydberg atoms mainly experience an effective dipolar interaction and, within the nearest-neighbor approximation, their dynamics can be mapped onto an effective integrable Fermi gas with ballistic transport. The inclusion of longer-range couplings, however, is essential for the theory to be predictive; those terms break the conservation laws associated with integrability, enabling, in particular, the emergence of genuine diffusive transport. We study the dynamics generated by a bipartition protocol and reveal a sharp qualitative change in behavior as the particle density is varied, suggesting the possibility of accessing weaker and stronger integrability breaking dynamics in the same Hamiltonian depending on the relevance of local interactions. We propose a theoretical mechanism that accounts for the differences. Our findings provide clear and concrete evidence that integrability breaking is not solely a property of the Hamiltonian and of the magnitude of the couplings that break integrability, but also of the state in which the system is prepared.

cond-mat.quant-gas↗

arXiv2026

Quantum-Coherent Thermodynamics: Leaf Typicality via Minimum-Variance Foliation

Equilibrium statistical ensembles commute with the Hamiltonian and thus carry no coherence in the energy eigenbasis. We develop a framework in which energy fluctuations can retain genuinely quantum-coherent contributions. We foliate state space into ``minimum-variance leaves,'' defined by minimizing the average energy variance over all pure-state decompositions, with the minimum set by the quantum Fisher information. On each leaf we construct the least-biased state compatible with normalization and mean energy, defining a leaf-canonical ensemble. The Gibbs ensemble is recovered on the distinguished commuting leaf, while generic states are organized by their leaf label. This structure provides a natural setting to extend eigenstate thermalization beyond equilibrium via a ``leaf typicality'' hypothesis. According to that hypothesis, local observables depend only on the leaf and energy and are reproduced by a representative pure state drawn from the optimal ensemble, whose minimized energy spread reduces the complexity of time evolution.

quant-ph↗

arXiv2026

Observing weakly broken conservation laws in a dipolar Rydberg quantum spin chain

Integrable quantum many-body systems host families of extensive conservation laws, some of which are fragile: even infinitesimal perturbations can qualitatively alter their dynamical constraints. Here we show that this fragility leaves a clear experimental fingerprint in a one-dimensional quantum spin chain of as few as 14 Rydberg atoms. Weak integrability breaking from interatomic dipolar couplings is directly detectable within experimentally accessible times in the dynamics of non-local observables. In particular, magnetization fluctuations are highly sensitive to the breaking of fragile conservation laws and exhibit anomalous growth, which we observe experimentally; similar signatures appear in a semilocal string observable. Numerical simulations on substantially longer chains and a simplified classical stochastic model reproduce those features. We establish non-local observables as a sensitive probe of fragile conservation laws in quantum spin chains and Rydberg-atom arrays as a platform to test perturbative descriptions of quantum many-body dynamics with weak integrability breaking.

cond-mat.quant-gas↗

arXiv2025

Transport in a System with a Tower of Quantum Many-Body Scars

We report the observation of unconventional transport phenomena in a spin-1 model that supports a tower of quantum many-body scars, and we discuss their properties uncovering their peculiar nature. In quantum many-body systems, the late-time dynamics of local observables are typically governed by conserved operators with local densities, such as energy and magnetization. In the model under investigation, however, there is an additional dynamical symmetry restricted to the subspace of the Hilbert space spanned by the quantum many-body scars. The latter significantly slows the decay of autocorrelation functions of certain coherent states of quantum many-body scars and is responsible for the unconventional form of transport that we detect numerically. We show that excited states with energy close to that of the quantum many-body scars play a crucial role in sustaining the transport. Finally, we propose a generalized eigenstate thermalization hypothesis to describe specific properties of states with energy close to the scars.

quant-ph↗

arXiv2025

Asymptotic behaviour of determinants through the expansion of the Moyal star product

We work out a generalization of the Szegö limit theorems on the determinant of large matrices. We focus on matrices with nonzero leading principal minors and elements that decay to zero exponentially fast with the distance from the main diagonal, but we relax the constraint of the Toeplitz structure. We obtain an expression for the asymptotic behaviour of the determinant written in terms of the factors of a left and right Wiener-Hopf type factorization of an appropriately defined symbol. For matrices with elements varying slowly along the diagonals (e.g., in locally Toeplitz sequences), we propose to apply the analogue of the semiclassical expansion of the Moyal star product in phase-space quantum mechanics. This is a systematic method that provides approximations up to any order in the typical scale of the inhomogeneity and allows us to obtain explicit asymptotic formulas.

math-ph↗

arXiv2025

Macroscopic Quantum States and Universal Correlations in a Disorder-Order Interface Propagating over a 1D Ground State

We consider translationally invariant quantum spin-$\frac{1}{2}$ chains with local interactions and a discrete symmetry that is spontaneously broken at zero temperature. We envision experimenters switching off the couplings between two parts of the system and preparing them in independent equilibrium states. One side of the chain is prepared in a disordered phase, and the other in a symmetry-breaking ground state. When the couplings are switched back on, time evolution ensues. We argue that in integrable systems the front separating the ordered region recedes at the maximal velocity of quasiparticle excitations over the ground state. We infer that, generically, the order parameters should vary on a subdiffusive scale of order $t^{1/3}$, where $t$ is time, and their fluctuations should exhibit the same scaling. This interfacial region exhibits full range correlations, indicating that it cannot be decomposed into nearly uncorrelated subsystems. Using the transverse-field Ising chain as a case study, we demonstrate that all order parameters follow the same universal scaling functions. Through an analysis of the skew information, we uncover that the breakdown of cluster decomposition has a quantum contribution: each subsystem within the interfacial region, with extent comparable to the region, exists in a macroscopic quantum state.

cond-mat.stat-mech↗

arXiv2025

Disorder-Order Interface Propagating over the Ferromagnetic Ground State in the Transverse Field Ising Chain

We consider time evolution of order parameters and entanglement asymmetries in the ferromagnetic phase of the transverse-field Ising chain. One side of the system is prepared in a ferromagnetic ground state and the other side either in equilibrium at higher temperature or out of equilibrium. We focus on the disorder-order interface in which the order parameter attains a nonzero value, different from the ground state one. In that region, correlations follow a universal behaviour. We analytically compute the asymptotic scaling functions of the one- and two-point equal time correlations of the order parameter and provide numerical evidence that also the non-equal time correlations are universal. We analyze the Rényi entanglement asymmetries of subsystems and obtain a prediction that is expected to hold also in the von Neumann limit. Finally, we show that the Wigner-Yanase skew information of the order paramerter in subsystems within the interfacial region scales as their length squared. We propose a semiclassical approximation that is particularly effective close to the edge of the lightcone.

cond-mat.stat-mech↗

arXiv2025

Kicking Quantum Fisher Information out of Equilibrium

Quantum Fisher Information (QFI) is a ubiquitous quantity with applications ranging from quantum metrology and resource theories to condensed matter physics. In equilibrium local quantum many-body systems, the QFI of a subsystem with respect to an extensive observable is typically proportional to the subsystem's volume. Specifically, in large subsystems at equilibrium, the QFI per unit volume squared becomes negligible. We reveal a natural mechanism that amplifies the QFI in a quantum spin chain with a zero-temperature ordered phase. At zero or sufficiently low temperatures, a transient localized perturbation enhances the QFI, causing it to scale quadratically with the subsystem's length. Furthermore, this enhancement can be controlled through more general localized kicking protocols. We also revisit the behavior of the quantum Fisher information after a global quench in the thermodynamic limit, focusing on the generation of localized -- confined within compact subsystems -- multipartite entanglement. We show that the density of localized multipartite entanglement approaches zero at late times, but there is an optimal time frame proportional to the length in which subsystems fall into macroscopic quantum states. We test our predictions against numerical data obtained using a novel technique, based on a remarkable identity between quantum Fisher information and Wigner-Yanase-Dyson skew information, that allows one to compute the quantum Fisher information with respect to the order parameter in noninteracting spin chains.

quant-ph↗

arXiv2025

Energy-filtered quantum states and the emergence of non-local correlations

Energy-filtered quantum states are promising candidates for efficiently simulating thermal states. We explore a protocol designed to transition a product state into an eigenstate located in the middle of the spectrum; this is achieved by gradually reducing its energy variance, which allows us to comprehensively understand the crossover phenomenon and the subsequent convergence towards thermal behavior. We introduce and discuss three energy-filtering regimes (short, medium and long), and we interpret them as stages of thermalization. We show that the properties of the filtered states are locally indistinguishable from those of time-averaged density matrices, routinely employed in the theory of thermalization. On the other hand, unexpected non-local quantum correlations are generated in the medium regimes and are witnessed by the Rényi entanglement entropies of subsystems, which we compute via replica methods. Specifically, two-point correlation functions break cluster decomposition and the entanglement entropy of large regions scales as the logarithm of the volume during the medium filter time.

quant-ph↗

arXiv2024

Nonequilibrium symmetry-protected topological order: emergence of semilocal Gibbs ensembles

We consider nonequilibrium time evolution in quantum spin chains after a global quench. Usually a nonequilibium quantum many-body system locally relaxes to a (generalised) Gibbs ensemble built from conserved operators with quasilocal densities. Here we exhibit explicit examples of local Hamiltonians that possess conservation laws with densities that are not quasilocal but act as such in the symmetry-restricted space where time evolution occurs. Because of them, the stationary state emerging at infinite time can exhibit exceptional features. We focus on a specific example with a spin-flip symmetry, which is the commonest global symmetry encountered in spin-$1/2$ chains. Among the exceptional properties, we find that, at late times, the excess of entropy of a spin block triggered by a local perturbation in the initial state grows logarithmically with the subsystem's length. We establish a connection with symmetry-protected topological order in equilibrium at zero temperature and study the melting of the order induced either by a (symmetry-breaking) rotation of the initial state or by an increase of the temperature.

cond-mat.stat-mech↗

arXiv2024

Entanglement entropy of two disjoint intervals and spin structures in interacting chains in and out of equilibrium

We take the paradigm of interacting spin chains, the Heisenberg spin-$\frac{1}{2}$ XXZ model, as a reference system and consider interacting models that are related to it by Jordan-Wigner transformations and restrictions to sub-chains. An example is the fermionic analogue of the gapless XXZ Hamiltonian, which, in a continuum scaling limit, is described by the massless Thirring model. We work out the Rényi-$α$ entropies of disjoint blocks in the ground state and extract the universal scaling functions describing the Rényi-$α$ tripartite information in the limit of infinite lengths. We consider also the von Neumann entropy, but only in the limit of large distance. We show how to use the entropies of spin blocks to unveil the spin structures of the underlying massless Thirring model. Finally, we speculate about the tripartite information after global quenches and conjecture its asymptotic behaviour in the limit of infinite time and small quench. The resulting conjecture for the ``residual tripartite information'', which corresponds to the limit in which the intervals' lengths are infinitely larger than their (large) distance, supports the claim of universality recently made studying noninteracting spin chains. Our mild assumptions imply that the residual tripartite information after a small quench of the anisotropy in the gapless phase of XXZ is equal to $-\log 2$.

cond-mat.stat-mech↗

arXiv2023

Universality in the tripartite information after global quenches

We consider macroscopically large 3-partitions $(A,B,C)$ of connected subsystems $A\cup B \cup C$ in infinite quantum spin chains and study the Rényi-$α$ tripartite information $I_3^{(α)}(A,B,C)$. At equilibrium in clean 1D systems with local Hamiltonians it generally vanishes. A notable exception is the ground state of conformal critical systems, in which $I_3^{(α)}(A,B,C)$ is known to be a universal function of the cross ratio $x=|A||C|/[(|A|+|B|)(|C|+|B|)]$, where $|A|$ denotes $A$'s length. We identify different classes of states that, under time evolution with translationally invariant Hamiltonians, locally relax to states with a nonzero (Rényi) tripartite information, which furthermore exhibits a universal dependency on $x$. We report a numerical study of $I_3^{(α)}$ in systems that are dual to free fermions, propose a field-theory description, and work out their asymptotic behaviour for $α=2$ in general and for generic $α$ in a subclass of systems. This allows us to infer the value of $I_3^{(α)}$ in the scaling limit $x\rightarrow 1^-$, which we call ``residual tripartite information''. If nonzero, our analysis points to a universal residual value $-\log 2$ independently of the Rényi index $α$, and hence applies also to the genuine (von Neumann) tripartite information.

cond-mat.stat-mech↗

arXiv2023

Quantum jamming brings quantum mechanics to macroscopic scales

A quantum spin-$\frac{1}{2}$ chain with an axial symmetry is normally described by quasiparticles associated with the spins oriented along the axis of rotation. Kinetic constraints can enrich such a description by setting apart different species of quasiparticles, which can get stuck at high enough density, realising the quantum analogue of jamming. We identify a family of interactions satisfying simple kinetic constraints and consider generic translationally invariant models built up from them. We study dynamics following a local unjamming perturbation in a jammed state. We show that they can be mapped into dynamics of ordinary unconstrained systems, but the nonlocality of the mapping changes the scales at which the phenomena manifest themselves. Scattering of quasiparticles, formation of bound states, eigenstate localisation become all visible at macroscopic scales. Depending on whether a symmetry is present or not, the microscopic details of the jammed state turn out to have either a marginal or a strong effect. In the former case or when the initial state is almost homogeneous, we show that even a product state is turned into a macroscopic quantum state.

quant-ph↗

arXiv2023

Universality in the tripartite information after global quenches: (generalised) quantum XY models

We consider the Rényi-$α$ tripartite information $I_3^{(α)}$ of three adjacent subsystems in the stationary state emerging after global quenches in noninteracting spin chains from both homogeneous and bipartite states. We identify settings in which $I_3^{(α)}$ remains nonzero also in the limit of infinite lengths and develop an effective quantum field theory description of free fermionic fields on a ladder. We map the calculation into a Riemann-Hilbert problem with a piecewise constant matrix for a doubly connected domain. We find an explicit solution for $α=2$ and an implicit one for $α>2$. In the latter case, we develop a rapidly convergent perturbation theory that we use to derive analytic formulae approximating $I_3^{(α)}$ with outstanding accuracy.

cond-mat.stat-mech↗

arXiv2023

Growing Schrödinger's cat states by local unitary time evolution of product states

We envisage many-body systems that can be described by quantum spin-chain Hamiltonians with a trivial separable eigenstate. For generic Hamiltonians, such a state represents a quantum scar. We show that, typically, a macroscopically-entangled state naturally grows after a single projective measurement of just one spin in the trivial eigenstate; moreover, we identify a condition under which what is growing is a "Schrödinger's cat state". Our analysis does not reveal any particular requirement for the entangled state to develop, provided that the trivial eigenstate does not minimise/maximise a local conservation law. We study two examples explicitly: systems described by generic Hamiltonians and a model that exhibits a $U(1)$ hidden symmetry. The latter can be reinterpreted as a 2-leg ladder in which the interactions along the legs are controlled by the local state on the other leg through transistor-like building blocks.

quant-ph↗

arXiv2023

No eigenstate of the critical transverse-field Ising chain satisfies the area law

We argue that, in a basis common to all one-site shift invariant conserved charges, there is no eigenstate of a noninteracting local spin-1/2 chain Hamiltonian that satisfies the area law if the ground state has half-integer central charge. That is to say, in those models all (quasi)local one-site shift invariant conserved operators are gapless. From the standpoint of bipartite entanglement properties, we show indeed that there are three distinct one-site shift invariant noninteracting models, two of which are equivalent to the XX model (for one of them the transformation breaks one-site shift invariance) and the other to the critical Ising model. The former class has two locally distinct one-site shift invariant excited states satisfying the area law; the latter two classes have none.

cond-mat.stat-mech↗

arXiv2022

Global Quenches after Localised Perturbations

We investigate the effect of a single spin flip preceding a global quench between translationally invariant local Hamiltonians in spin-$\tfrac{1}{2}$ chains. The effect of the localised perturbation does not fade away however large the distance from the perturbation is. In particular, translational invariance is not restored and the infinite time limit depends on whether the spin was flipped or not. We argue that this phenomenon is more general than the particular example considered and we conjecture that it is triggered by topological properties, specifically, the existence of "semilocal charges".

cond-mat.str-el↗