arXiv subjects

Horacio M. Pastawski

Publications and source records attributed to Horacio M. Pastawski.

At least 19 recordsLinked to original sources

arXiv2025

Multiple Quantum Many-Body Clustering Probed by Dynamical Decoupling

The manipulation of quantum information in large systems requires precise control of quantum systems that are out-of-equilibrium. As the size of the system increases, its fragility in response to external perturbations and intrinsic decoherence processes also increases. The degradation of the system response makes accurate measurements a challenging and time-consuming task. However, quantum information lifetime enhancement can be achieved by dynamical decoupling techniques (DD), where an external drive with a frequency much higher than the system's internal evolution renders signal acquisition with decay times greater than 1000-fold. In this study, we demonstrate that the system response during a prethermal period, subject to Floquet control, can be utilized to probe the multiple quantum evolution of dense and highly connected spin systems. This approach exhibits an enhanced sensitivity at a reduced experimental time. The enhanced signal-to-noise ratio achieved enabled the use of numerical inversion strategies to model the evolution of the excited multiple quantum coherences, which describe the number of correlated spins within a cluster. We observed for the first time, to the best of our knowledge, that the increase in the number of correlated spins with multiple quantum evolution is accompanied by an increase in the distribution of spin cluster sizes, which follows a quadratic law.

quant-ph↗

arXiv2024

Global Out of Time Order Correlators as a Signature of Scrambling Dynamics of Local Observables

Out-of-Time-Order Correlators (OTOCs) serve as a proxy for quantum information scrambling, which refers to the process where information stored locally disperses across the many-body degrees of freedom in a quantum system, rendering it inaccessible to local probes. Most experimental implementations of OTOCs to probe information scrambling rely on indirect measurements based on global observables, using techniques such as Loschmidt echoes and Multiple Quantum Coherences, via time reversal evolutions. In this article, we establish a direct connection between OTOCs with global and local observables in the context of NMR experiments, where the observable is the total magnetization of the system. We conduct a numerical analysis to quantify the differences in the evolution of both magnitudes, evaluating the excitation dynamics in spin ring systems with 8 to 16 spins, using a many-body Hamiltonian and long-range interactions. Our analysis decomposes the global echo into a sum of local echoes and cross-contributions, leading to local and global OTOCs. The results indicate that, after an initial transient period, local OTOCs determine the global ones. We observe that the difference between the average of local OTOCs and the global one, as well as their fluctuations, becomes negligible as the system size increases. Thus, for large homogeneous systems, global and local OTOCs become equivalent. This behavior aligns with that observed in highly interacting or chaotic systems in several experiments.

quant-ph↗

arXiv2022

Simulating a Catalyst induced Quantum Dynamical Phase Transition of a Heyrovsky reaction with different models for the environment

Through an appropriate election of the molecular orbital basis, we show analytically that the molecular dissociation occurring in a Heyrovsky reaction can be interpreted as a Quantum Dynamical Phase Transition, i.e., an analytical discontinuity in the molecular energy spectrum induced by the catalyst. The metallic substrate plays the role of an environment that produces an energy uncertainty on the adatom. This broadening induces a critical behavior not possible in a quantum closed system. We use suitable approximations on symmetry, together with both Lanczos and canonical transformations, to give analytical estimates for the critical parameters of molecular dissociation. This occurs when the bonding to the surface is (\sqrt{2}) times the molecular bonding. This value is slightly weakened for less symmetric situations. However simple, this conclusion involves a high order perturbative solution of the molecule-catalyst system. This model is further simplified to discuss how an environment-induced critical phenomenon can be evaluated through an idealized perturbative tunneling microscopy set-up. In this case, the energy uncertainties in one or both atoms are either Lorentzian or Gaussian. The former results from the Fermi Golden Rule, i.e., a Markovian approximation. The Gaussian uncertainty, associated with non-Markovian decoherent processes, requires the introduction of a particular model of a spin bath. The partially coherent tunneling current is obtained from the Generalized Landauer-Büttiker Equations. The resonances observed in these transport parameters reflect, in many cases, the critical properties of the resonances in the molecular spectrum.

physics.atm-clus↗

arXiv2021

Emergent decoherence induced by quantum chaos in a many-body system: A Loschmidt echo observation through NMR

In the long quest to identify and compensate the sources of decoherence in many-body systems far from the ground state, the varied family of Loschmidt echoes (LEs) became an invaluable tool in several experimental techniques. A LE involves a time-reversal procedure to assess the effect of perturbations in a quantum excitation dynamics. However, when addressing macroscopic systems one is repeatedly confronted with limitations that seem insurmountable. This led to formulate the \textit{central hypothesis of irreversibility} stating that the time-scale of decoherence, $T_3$, is proportional to the time-scale of the many-body interactions we reversed, $T_2$. We test this by implementing two experimental schemes based on Floquet Hamiltonians where the effective strength of the dipolar spin-spin coupling, i.e. $1/T_2$, is reduced by a variable scale factor $k$. This extends the perturbations time scale, $T_Σ$, in relation to $T_2$. Strikingly, we observe the superposition of the normalized Loschmidt echoes for the bigger values of $k$. This manifests the dominance of the intrinsic dynamics over the perturbation factors, even when the Loschmidt echo is devised to reverse that intrinsic dynamics. Thus, in the limit where the reversible interactions dominate over perturbations, the LE decays within a time-scale, $T_3\approx T_2/R$ with $R=(0.15 \pm 0.01)$, confirming the emergence of a perturbation independent regime. These results support the central hypothesis of irreversibility.

quant-ph↗

arXiv2021

Ergodicity breaking in an incommensurate system observed by OTOCs and Loschmidt Echoes: From quantum diffusion to sub-diffusion

The metal-insulator transition (MIT), which includes Anderson localization and Mott insulators as extreme regimes, has received renewed interest as the many-body effects often constitute a limitation for the handling of quantum interference. This resulted in the field dubbed many-body localization (MBL), intensively studied theoretically and experimentally as understanding the appearance of equilibration and thermalization becomes relevant in dealing with finite systems. Here, we propose a new observable to study this transition in a spin chain under the ``disorder'' of a Harper-Hofstadter-Aubry-André on-site potential. This quantity, which we call zeroth-order gradient entanglement (ZOGE) is extracted from the fundamental Fourier mode of a family of out-of-time-ordered correlators (OTOCs). These are just Loschmidt Echoes, where a field gradient is applied before the time reversal. In the absence of many-body interactions, the ZOGE coincides with the inverse participation ratio of a Fermionic wave function. By adding an Ising interaction to an XY Hamiltonian, one can explore the MBL phase diagram of the system. Close to the critical region, the excitation dynamics is consistent with a diffusion law. However, for weak disorder, quantum diffusion prevails while for strong disorder the excitation dynamics is sub-diffusive.

quant-ph↗

arXiv2020

Current-induced forces in single-resonance systems

In recent years, there has been an increasing interest in nanoelectromechanical devices, current-driven quantum machines, and the mechanical effects of electric currents on nanoscale conductors. Here, we carry out a thorough study of the current-induced forces and the electronic friction of systems whose electronic effective Hamiltonian can be described by an archetypal model, a single energy level coupled to two reservoirs. Our results can help better understand the general conditions that maximize the performance of different devices modeled as a quantum dot coupled to two electronic reservoirs. Additionally, they can be useful to rationalize the role of current-induced forces in the mechanical deformation of one-dimensional conductors.

cond-mat.mes-hall↗

arXiv2019

Emergent perturbation independent decay of the Loschmidt echo in a many-spin system studied through scaled dipolar dynamics

Evaluating the role of perturbations versus the intrinsic coherent dynamics in driving to equilibrium is of fundamental interest to understand quantum many-body thermalization, in the quest to build ever complex quantum devices. Here we introduce a protocol that scales down the coupling strength in a quantum simulator based on a solid-state nuclear spin system, leading to a longer decay time T2, while keeping perturbations associated to control error constant. We can monitor quantum information scrambling by measuring two powerful metrics, out-of-time-ordered correlators (OTOCs) and Loschmidt Echoes (LEs). While OTOCs reveal quantum information scrambling involving hundreds of spins, the LE decay quantifies, via the time scale T3, how well the scrambled information can be recovered through time reversal. We find that when the interactions dominate the perturbation, the LE decay rate only depends on the interactions themselves, T3 ~ T2, and not on the perturbation. Then, in an unbounded many-spin system, decoherence can achieve a perturbation-independent regime, with a rate only related to the local second moment of the Hamiltonian.

quant-ph↗

arXiv2018

Nonequilibrium current-induced forces caused by quantum localization: Anderson Adiabatic Quantum Motors

In recent years there has been an increasing interest in nanomachines. Among them, current-driven ones deserve special attention as quantum effects can play a significant role there. Examples of the latter are the so-called adiabatic quantum motors. In this work, we propose using Anderson's localization to induce nonequilibrium forces in adiabatic quantum motors. We study the nonequilibrium current-induced forces and the maximum efficiency of these nanomotors in terms of their respective probability distribution functions. Expressions for these distribution functions are obtained in two characteristic regimes: the steady-state and the short-time regimes. Even though both regimes have distinctive expressions for their efficiencies, we find that, under certain conditions, the probability distribution functions of their maximum efficiency are approximately the same. Finally, we provide a simple relation to estimate the minimal disorder strength that should ensure efficient nanomotors.

cond-mat.mes-hall↗

arXiv2017

Dynamics and decoherence in nonideal Thouless quantum motors

Different proposals for adiabatic quantum motors (AQMs) driven by DC currents have recently attracted considerable interest. However, the systems studied are often based on simplified models with highly ideal conditions where the environment is neglected. Here, we investigate the performance (dynamics, efficiency, and output power) of a prototypical AQM, the Thouless motor. To include the effect of the surroundings on this type of AQMs, we extended our previous theory of decoherence in current-induced forces (CIFs) to account for spatially distributed decoherent processes. We provide analytical expressions that account for decoherence in CIFs, friction coefficients and the self-correlation functions of the CIFs. We prove that the model is thermodynamically consistent and we find that decoherence drastically reduces the efficiency of the motor mainly due to the increase in conductance, while its effect on the output power is not much relevant. The effect of decoherence on the current-induced friction depends on the length of the system, reducing the friction for small systems while increasing it for long ones. Finally, we find that reflections of the electrons at the boundary of the system induce additional conservative forces that affect the dynamics of the motor. In particular, this results in the hysteresis of the system and a voltage dependent switching.

cond-mat.mes-hall↗

arXiv2017

Loschmidt echo in many-spin systems: a quest for intrinsic decoherence and emergent irreversibility

If a magnetic polarization excess is locally injected in a crystal of interacting spins, this "excitation" would spread as consequence of spin-spin interactions. Such an apparently irreversible process is known as spin diffusion and it can lead the system back to "equilibrium". Even so, a unitary quantum dynamics would ensure a precise memory of the non-equilibrium initial condition. Then, if at certain time, say $t/2$, an experimental protocol reverses the many-body dynamics, it would drive the system back to the initial non-equilibrium state at time $t$. As a matter of fact, the reversal is always perturbed by small experimental imperfections and/or uncontrolled internal or environmental degrees of freedom. This limits the amount of signal $M(t)$ recovered locally at time $t$. The degradation of $M(t)$ accounts for these perturbations, which can also be seen as the sources of decoherence. This idea defines the Loschmidt echo (LE), which embodies the various time-reversal procedures implemented in nuclear magnetic resonance. Here, we present an invitation to the study of the LE following the pathway induced by the experiments. With such a purpose, we provide a historical and conceptual overview that briefly revisits selected phenomena that underlie the LE dynamics, ultimately leading to the discussion of irreversibility as an emergent phenomenon. In addition, we introduce the LE formalism by means of spin-spin correlation functions and we present new results that could trigger new experiments and theoretical ideas. In particular, we propose to transform an initially localized excitation into a more complex initial state, enabling a dynamically prepared LE. This induces a global definition of the LE in terms of the raw overlap between many-body wave functions. Our results show that as the complexity of the prepared state increases, it becomes more fragile towards small perturbations.

quant-ph↗

arXiv2016

Loschmidt echo in many-spin systems: contrasting time-scales of local and global measurements

A local excitation in a quantum many-spin system evolves deterministically. A time-reversal procedure, involving the inversion of the signs of every energy and interaction, should produce the excitation revival. This idea, experimentally coined in NMR, embodies the concept of the Loschmidt echo (LE). While such an implementation involves a single spin autocorrelation $M_{1,1}$, i.e. a local LE, theoretical efforts have focused on the study of the recovery probability of a complete many-body state, referred here as global or many-body LE $M_{MB}$. Here, we analyze the relation between these magnitudes, in what concerns to their characteristic time scales and their dependence on the number of spins $N$. We show that the global LE can be understood, to some extent, as the simultaneous occurrence of $N$ independent local LEs, i.e. $M_{MB}\sim \left( M_{1,1}\right) ^{N/4}$. This extensive hypothesis is exact for very short times and confirmed numerically beyond such a regime. Furthermore, we discuss a general picture of the decay of $M_{1,1}$ as a consequence of the interplay between the time scale that characterizes the reversible interactions ($T_{2}$) and that of the perturbation ($τ_{Σ}$). Our analysis suggests that the short time decay, characterized by the time scale $τ_{Σ}$, is greatly enhanced by the complex processes that occur beyond $T_{2}$ . This would ultimately lead to the experimentally observed $T_{3},$ which was found to be roughly independent of $τ_{Σ}$ but closely tied to $T_{2}$.

quant-ph↗

arXiv2015

Role of energy uncertainties in ergodicity breaking induced by competing interactions and disorder. A dynamical assessment through the Loschmidt echo

A local excitation in a quantum many-particle system evolves deterministically. A time-reversal procedure, involving the invertion of the signs of every energy and interaction, should produce an excitation revival: the Loschmidt echo (LE). If somewhat imperfect, only a fraction of the excitation will refocus. We use such a procedure to show how non-inverted weak disorder and interactions, when assisted by the natural reversible dynamics, fully degrade the LE. These perturbations enhance diffusion and evenly distribute the excitation throughout the system. Such a dynamical paradigm, called ergodicity, breaks down when either the disorder or the interactions are too strong. These extreme regimes give rise to the well known Anderson localization and Mott insulating phases, where quantum diffusion becomes restricted. Accordingly, regardless of the kinetic energy terms, the excitation remains mainly localized and out-of-equilibrium, and the system behaves non-ergodically. The LE constitutes a fair dynamical witness for the whole phase diagram since it evidences a surprising topography in which ergodic and non-ergodic phases interpenetrate each other. Furthermore, we provide an estimation for the critical lines separating the ergodic and non-ergodic phases around the Mott and Anderson transitions. The energy uncertainties introduced by disorder and interaction shift these thresholds towards stronger perturbations. Remarkably, the estimations of the critical lines are in good agreement with the phase diagram derived from the LE dynamics.

cond-mat.stat-mech↗

arXiv2015

Decoherence in current induced forces: Application to adiabatic quantum motors

Current induced forces are not only related with the discrete nature of electrons but also with its quantum character. It is natural then to wonder about the effect of decoherence. Here, we develop the theory of current induced forces including dephasing processes and we apply it to study adiabatic quantum motors (AQMs). The theory is based on Büttiker's fictitious probe model which here is reformulated for this particular case. We prove that it accomplishes fluctuation-dissipation theorem. We also show that, in spite of decoherence, the total work performed by the current induced forces remains equal to the pumped charge per cycle times the voltage. We find that decoherence affects not only the current induced forces of the system but also its intrinsic friction and noise, modifying in a non trivial way the efficiency of AQMs. We apply the theory to study an AQM inspired by a classical peristaltic pump where we surprisingly find that decoherence can play a crucial role by triggering its operation. Our results can help to understand how environmentally induced dephasing affects the quantum behavior of nano-mechanical devices.

cond-mat.mes-hall↗

arXiv2015

Decoherent time-dependent transport beyond the Landauer-Büttiker formulation: a quantum-drift alternative to quantum jumps

We present a model for decoherence in time-dependent transport. It boils down into a form of wave function that undergoes a smooth stochastic drift of the phase in a local basis, the Quantum Drift (QD) model. This drift is nothing else but a local energy fluctuation. Unlike Quantum Jumps (QJ) models, no jumps are present in the density as the evolution is unitary. As a first application, we address the transport through a resonant state $\left\vert 0\right\rangle $ that undergoes decoherence. We show the equivalence with the decoherent steady state transport in presence of a Büttiker's voltage probe. In order to test the dynamics, we consider two many-spin systems whith a local energy fluctuation. A two-spin system is reduced to a two level system (TLS) that oscillates among $\left\vert 0\right\rangle $ $\equiv $ $ \left\vert \uparrow \downarrow \right\rangle $ and $\left\vert 1\right\rangle \equiv $ $\left\vert \downarrow \uparrow \right\rangle $. We show that QD model recovers not only the exponential damping of the oscillations in the low perturbation regime, but also the non-trivial bifurcation of the damping rates at a critical point, i.e. the quantum dynamical phase transition. We also address the spin-wave like dynamics of local polarization in a spin chain. The QD average solution has about half the dispersion respect to the mean dynamics than QJ. By evaluating the Loschmidt Echo (LE), we find that the pure states $\left\vert 0\right\rangle $ and $\left\vert 1\right \rangle $ are quite robust against the local decoherence. In contrast, the LE, and hence coherence, decays faster when the system is in a superposition state. Because its simple implementation, the method is well suited to assess decoherent transport problems as well as to include decoherence in both one-body and many-body dynamics.

quant-ph↗

arXiv2015

Proliferation of effective interactions: decoherence-induced equilibration in a closed many-body system

We address the question on how weak perturbations, that are quite ineffective in small many-body systems, can lead to decoherence and hence to irreversibility when they proliferate as the system size increases. This question is at the heart of solid state NMR. There, an initially local polarization spreads all over due to spin-spin interactions that conserve the total spin projection, leading to an equilibration of the polarization. In principle, this quantum dynamics can be reversed by changing the sign of the Hamiltonian. However, the reversal is usually perturbed by non reversible interactions that act as a decoherence source. The fraction of the local excitation recovered defines the Loschmidt echo (LE), here evaluated in a series of closed $N$ spin systems with all-to-all interactions. The most remarkable regime of the LE decay occurs when the perturbation induces proliferated effective interactions. We show that if this perturbation exceeds some lower bound, the decay is ruled by an effective Fermi golden rule (FGR). Such a lower bound shrinks as $ N $ increases, becoming the leading mechanism for LE decay in the thermodynamic limit. Once the polarization stayed equilibrated longer than the FGR time, it remains equilibrated in spite of the reversal procedure.

quant-ph↗

arXiv2015

Plasmonic Graded-Chains as Deep-Subwavelength Light Concentrators

We have studied the plasmonic properties of aperiodic arrays of identical nanoparticles (NPs) formed by two opposite and equal graded-chains (a chain where interactions change gradually). We found that these arrays concentrate the external electromagnetic fields even in the long wavelength limit. The phenomenon was understood by identifying the system with an effective cavity where plasmonics excitations are trapped between effective band edges, resulting from the change of passband with NP's position. Dependence of excitation concentration on several system's parameter was also assessed. This includes, different gradings as well as NP's couplings, damping, and resonant frequencies. In the spirit of the scaling laws in condensed matter physics, we developed a theory that allows us to rationalize all these system's parameters into universal curves. The theory is quite general and can also be used on many other situations (different arrays for example). Additionally, we also provided an analytical solution, in the tight-binding limit, for the plasmonic response of homogeneous linear chains of NPs illuminated by a plane wave. Our results can find applications on sensing, near field imaging, plasmon-enhanced photodetectors, as well as to increase solar cell efficiency.

cond-mat.mes-hall↗

arXiv2014

Landau-Zener and Rabi oscillations in the spin-dependent conductance

We describe the spin-dependent quantum conductance in a wire where a magnetic field is spatially modulated. The change in direction and intensity of the magnetic field acts as a perturbation that mixes spin projections. This is exemplified by a ferromagnetic nanowire. There the local field varies smoothly its direction generating a domain wall (DW) as described by the well-known Cabrera-Falicov model. Here, we generalize this model to include also a strength modulation. We identify two striking diabatic regimes that appear when such magnetic inhogeneity occurs. 1) If the field strength at the DW is weak enough, the local Zeeman energies result in an avoided crossing. Thus, the spin-flip probability follows the Landau-Zener formula. 2) For strong fields, the spin-dependent conductance shows oscillations as a function of the DW width. We interpret them in terms of Rabi oscillations. Time and length scales obtained from this simplified view show an excellent agreement with the exact dynamical solution of the spin-dependent transport. These results remain valid for other situations involving modulated magnetic structures and thus they open new prospects for the use of quantum interferences in spin-based devices. This paper is dedicated to the memory of the lifelong collaborator Patricia Rebeca Levstein.

cond-mat.mes-hall↗

arXiv2014

Generalized multi-terminal decoherent transport: Recursive algorithms and applications to SASER and giant magnetoresistance

Decoherent transport in mesoscopic and nanoscopic systems can be formulated in terms of the D'Amato-Pastawski (DP) model. This generalizes the Landauer-Büttiker picture by considering a distribution of local decoherent processes. However, its generalization for multi-terminal setups is lacking. We first review the original two-terminal DP model for decoherent transport. Then, we extend it to a matrix formulation capable of dealing with multi-terminal problems. We also introduce recursive algorithms to evaluate the Green's functions for general banded Hamiltonians as well as local density of states, effective conductances and voltage profiles. We finally illustrate the method by analyzing two problems of current relevance. 1) Assessing the role of decoherence in a model for phonon lasers (SASER). 2) Obtaining the classical limit of Giant Magnetoresistance from a spin-dependent Hamiltonian. The presented methods should pave the way for computationally demanding calculations of transport through nanodevices, bridging the gap between fully coherent quantum schemes and semiclassical ones.

cond-mat.mes-hall↗