arXiv subjects

Jesús Casado-Pascual

Publications and source records attributed to Jesús Casado-Pascual.

At least 19 recordsLinked to original sources

arXiv2026

Controlling quantum transport by measurement-rate modulation

Temporal modulation of the measurement rate provides a powerful mechanism for controlling open-system dynamics through measurement backaction. We demonstrate this mechanism in a minimal exactly solvable model of a continuously monitored quantum particle on a switching lattice. We derive exact analytical expressions for the long-time current under both periodic and stochastic switching, revealing a common slow-switching limit and identifying a measurement-induced crossover between Zeno- and anti-Zeno-dominated transport regimes that controls the direction of the current.

quant-ph↗

arXiv2026

Exploring fixed points and eigenstates of quantum systems with reinforcement learning

We introduce a reinforcement learning algorithm designed to identify the fixed points of a given quantum operation. The method iteratively constructs the unitary transformation that maps the computational basis onto the basis of fixed points through a reward-penalty scheme based on quantum measurements. In cases where the operation corresponds to a Hamiltonian evolution, this task reduces to determining the Hamiltonian eigenstates. The algorithm is first benchmarked on random Hamiltonians acting on two and three qubits and then applied to many-body systems of up to six qubits, including the transverse-field Ising model and the all-to-all pairing Hamiltonian. In both cases, the algorithm is demonstrated to perform successfully; in the pairing model, it can also reveal hidden symmetries, which can be exploited to restrict learning to specific symmetry sectors. Finally, we discuss the possibility of post-selecting high-fidelity states even when full convergence has not been reached.

quant-ph↗

arXiv2026

Impact of Amplitude and Phase Damping Noise on Quantum Reinforcement Learning: Challenges and Opportunities

Quantum machine learning (QML) is an emerging field with significant potential, yet it remains highly susceptible to noise, which poses a major challenge to its practical implementation. While various noise mitigation strategies have been proposed to enhance algorithmic performance, the impact of noise is not fully understood. In this work, we investigate the effects of amplitude and phase damping noise on a quantum reinforcement learning algorithm. Through analytical and numerical analysis, we assess how these noise sources influence the learning process and overall performance. Our findings contribute to a deeper understanding of the role of noise in quantum learning algorithms and suggest that, rather than being purely detrimental, unavoidable noise may present opportunities to enhance QML processes.

quant-ph↗

arXiv2026

Hidden time-nonlocal Floquet symmetries

We investigate the Floquet spectrum of a detuned, driven two-level system and show that it exhibits exact quasienergy crossings when the detuning is an integer multiple of the energy quantum of the driving field. This behavior can be explained by a hidden time-nonlocal parity, which allows the Floquet modes to be classified as even or odd. Then a generic feature is the emergence of exact crossings between quasienergies of different parity. A constructive proof of the existence of the symmetry is based on a scalar recurrence relation. Moreover, we present a general scheme for its numerical computation, which can be applied to models beyond the two-level system. Analytical results are illustrated with numerical data.

quant-ph↗

arXiv2024

Quantum ratchet with Lindblad rate equations

A quantum random walk model is established on a one-dimensional periodic lattice that fluctuates between two possible states. This model is defined by Lindblad rate equations that incorporate the transition rates between the two lattice states. Leveraging the system's symmetries, the particle velocity can be described using a finite set of equations, even though the state space is of infinite dimension. These equations yield an analytical expression for the velocity in the long-time limit, which is employed to analyze the characteristics of directed motion. Notably, the velocity can exhibit multiple inversions, and to achieve directed motion, distinct, nonzero transition rates between lattice states are required.

quant-ph↗

arXiv2023

Benefits of Open Quantum Systems for Quantum Machine Learning

Quantum machine learning is a discipline that holds the promise of revolutionizing data processing and problem-solving. However, dissipation and noise arising from the coupling with the environment are commonly perceived as major obstacles to its practical exploitation, as they impact the coherence and performance of the utilized quantum devices. Significant efforts have been dedicated to mitigate and control their negative effects on these devices. This Perspective takes a different approach, aiming to harness the potential of noise and dissipation instead of combatting them. Surprisingly, it is shown that these seemingly detrimental factors can provide substantial advantages in the operation of quantum machine learning algorithms under certain circumstances. Exploring and understanding the implications of adapting quantum machine learning algorithms to open quantum systems opens up pathways for devising strategies that effectively leverage noise and dissipation. The recent works analyzed in this Perspective represent only initial steps towards uncovering other potential hidden benefits that dissipation and noise may offer. As exploration in this field continues, significant discoveries are anticipated that could reshape the future of quantum computing.

quant-ph↗

arXiv2021

Reply to "Comment on `Directed motion of spheres by unbiased driving forces in viscous fluids beyond the Stokes' law regime' "

We refute the criticism expressed in a Comment by P. J. Mart\'ınez and R. Chacón [arXiv:2105.09308] on our paper Phys. Rev. E \textbf{97}, 032219 (2018). We first show that our paper is free of any mistakes and that the criticism expressed in the Comment is based on a misunderstanding or misrepresentation of what is actually claimed in our paper. We also show that the argument presented against the general formalism mentioned in the Comment is invalid because it is based on a confusion by the authors of the Comment between the general formalism per se and its lowest order approximation. In fact, the arguments contained in the Comment do not show that such a formalism is in general incorrect, as the authors of the Comment suggest, but rather that the supposedly universal predictions of the theory of ratchet universality are in general incorrect. In addition, we show that other arguments presented in Comment are invalid because they are based on incorrect equations.

physics.flu-dyn↗

arXiv2021

System size synchronization

In this work we bring out the existence of a novel kind of synchronization associated to the size of a complex system. A dichotomic random jump process associated to the dynamics of an externally driven stochastic system with $N$ coupled units is constructed. We define an output frequency and phase diffusion coefficient. System size synchronization occurs when the average output frequency is locked to the external one and the average phase diffusion coefficient shows a very deep minimum for a range of system sizes. Analytical and numerical procedures are introduced to study the phenomenon, and the results describe successfully the existence of system size synchronization.

cond-mat.stat-mech↗

arXiv2021

Spin dynamics under the influence of elliptically rotating fields: Extracting the field topology from time-averaged quantities

We focus on quantum systems that can be effectively described as a localized spin-$s$ particle subject to a static magnetic field coplanar to a coexisting elliptically rotating time-periodic field. Depending on the values taken on by the static and rotating components, the total magnetic field shows two regimes with different topological properties. Along the boundary that separates these two regimes, the total magnetic field vanishes periodically in time and the system dynamics becomes highly nonadiabatic. We derive a relation between two time-averaged quantities of the system which is linked to the topology of the applied magnetic field. Based on this finding, we propose a measurable quantity that has the ability to indicate the topology of the total magnetic field without knowing a priori the value of the static component. We also propose a possible implementation of our approach by a trapped-ion quantum system. The results presented here are independent of the initial state of the system. In particular, when the system is initialized in a Floquet state, we find some interesting properties of the quasienergy spectrum which are linked to the topological change of the total magnetic field. Throughout the paper, the theoretical results are illustrated with numerical simulations for the case of a two-level quantum system.

cond-mat.stat-mech↗

arXiv2019

Generic shape of multichromatic resonance peaks

We investigate dissipative dynamical systems under the influence of an external driving with two or more frequencies. Our main quantities of interest are long-time averages of expectation values which turn out to exhibit universal features. In particular, resonance peaks in the vicinity of commensurable frequencies possess a generic enveloping function whose width is inversely proportional to the averaging time. While the universal features can be derived analytically, the transition from the specific short-time behavior to the long-time limit is illustrated for the examples of a classical random walk and a dissipative two-level system both with biharmonic driving. In these models, the dependence of the time-averaged response on the relative phase between the two driving frequencies changes with increasing integration time. For short times, it exhibits the $2π$ periodicity of the dynamic equations, while in the long-time limit, the period becomes a fraction of this value.

cond-mat.mes-hall↗

arXiv2015

Universal asymptotic behavior in nonlinear systems driven by a two-frequency forcing

We examine the time-dependent behavior of a nonlinear system driven by a two-frequency forcing. By using a non-perturbative approach, we are able to derive an asymptotic expression, valid in the long-time limit, for the time average of the output variable which describes the response of the system. We identify several universal features of the asymptotic response of the system, which are independent of the details of the model. In particular, we determine an asymptotic expression for the width of the resonance observed by keeping one frequency fixed, and varying the other one. We show that this width is smaller than the usually assumed Fourier width by a factor determined by the two driving frequencies, and independent of the model system parameters. Additional general features can also be identified depending on the specific symmetry properties of the system. Our results find direct application in the study of sub-Fourier signal processing with nonlinear systems.

cond-mat.stat-mech↗

arXiv2015

General approach for dealing with dynamical systems with spatiotemporal periodicities

Dynamical systems often contain oscillatory forces or depend on periodic potentials. Time or space periodicity is reflected in the properties of these systems through a dependence on the parameters of their periodic terms. In this paper we provide a general theoretical framework for dealing with these kinds of systems, regardless of whether they are classical or quantum, stochastic or deterministic, dissipative or nondissipative, linear or nonlinear, etc. In particular, we are able to show that simple symmetry considerations determine, to a large extent, how their properties depend functionally on some of the parameters of the periodic terms. For the sake of illustration, we apply this formalism to find the functional dependence of the expectation value of the momentum of a Bose-Einstein condensate, described by the Gross-Pitaewskii equation, when it is exposed to a sawtooth potential whose amplitude is periodically modulated in time. We show that, by using this formalism, a small set of measurements is enough to obtain the functional form for a wide range of parameters. This can be very helpful when characterizing experimentally the response of systems for which performing measurements is costly or difficult.

cond-mat.stat-mech↗

arXiv2010

Effect of a high-frequency magnetic field on the resonant behavior displayed by a spin-$1/2$ particle under the influence of a rotating magnetic field

In this paper, we investigate the role of a high-frequency magnetic field in the resonant behavior displayed by a spin-$1/2$ particle under the influence of a rotating magnetic field. We propose two alternative methods for analyzing the system dynamics, namely, the averaging method and the multiple scale method. The analytical results achieved by applying these two methods are compared with those obtained from the numerical solution of the Schrödinger equation. This comparison leads to the conclusion that the multiple scale method provides a better understanding of the system dynamics than the averaging method. In particular, the averaging method predicts the complete destruction of the resonant behavior by an appropriate choice of the parameter values of the high-frequency magnetic field. This conclusion is disproved both by the numerical results, and also by the results obtained from the multiple scale method.

quant-ph↗

arXiv2009

Stochastic resonance in finite arrays of bistable elements with local coupling

In this article, we investigate the stochastic resonance (SR) effect in a finite array of noisy bistable systems with nearest-neighbor coupling driven by a weak time-periodic driving force. The array is characterized by a collective variable. By means of numerical simulations, the signal-to-noise ratio (SNR) and the gain are estimated as functions of the noise and the interaction coupling strength. A strong enhancement of the SR phenomenon for this collective variable in comparison with SR in single unit bistable systems is observed. Gains larger than unity are obtained for some parameter values and multi- frequency driving forces, indicating that the system is operating in a non-linear regime albeit the smallness of the driving amplitude. The large SNR values observed are basically due to the fact that the output fluctuations are small and short lived, in comparison with their typical values in a linear regime. A non-monotonic behavior of the SNR with the coupling strength is also obtained.

cond-mat.stat-mech↗

arXiv2007

Thermal equilibrium and statistical thermometers in special relativity

There is an intense debate in the recent literature about the correct generalization of Maxwell's velocity distribution in special relativity. The most frequently discussed candidate distributions include the Juettner function as well as modifications thereof. Here, we report results from fully relativistic one-dimensional (1D) molecular dynamics (MD) simulations that resolve the ambiguity. The numerical evidence unequivocally favors the Juettner distribution. Moreover, our simulations illustrate that the concept of 'thermal equilibrium' extends naturally to special relativity only if a many-particle system is spatially confined. They make evident that 'temperature' can be statistically defined and measured in an observer frame independent way.

cond-mat.stat-mech↗

arXiv2007

Very large stochastic resonance gains in finite sets of interacting identical subsystems driven by subthreshold rectangular pulses

We study the phenomenon of nonlinear stochastic resonance (SR) in a complex noisy system formed by a finite number of interacting subunits driven by rectangular pulsed time periodic forces. We find that very large SR gains are obtained for subthreshold driving forces with frequencies much larger than the values observed in simpler one-dimensional systems. These effects are explained using simple considerations.

cond-mat.stat-mech↗

arXiv2007

Stochastic resonance with weak monochromatic driving: gains above unity induced by high-frequency signals

We study the effects of a high-frequency (HF) signal on the response of a noisy bistable system to a low-frequency subthreshold sinusoidal signal. We show that, by conveniently choosing the ratio of the amplitude of the HF signal to its frequency, stochastic resonance gains greater than unity can be measured at the low-frequency value. Thus, the addition of the HF signal can entail an improvement in the detection of weak monochromatic signals. The results are explained in terms of an effective model and illustrated by means of numerical simulations.

cond-mat.stat-mech↗

arXiv2006

Flux reversal in a simple random walk model on a fluctuating symmetric lattice

A rather simple random walk model on a one-dimensional lattice is put forward. The lattice as a whole switches randomly between two possible states which are spatially symmetric. Both lattice states are identical, but translated by one site with respect to each other, and consist of infinite arrays of absorbing sites separated by two non-absorbing sites. Exact explicit expressions for the long-time velocity and the effective diffusion coefficient are obtained and discussed. In particular, it is shown that the direction of the steady motion can be reversed by conveniently varying the values of either the mean residence times in the lattice states or the transition rates to the absorbing and non-absorbing sites.

cond-mat.stat-mech↗