arXiv subjects

A. S. Sanz

Publications and source records attributed to A. S. Sanz.

At least 19 recordsLinked to original sources

arXiv2026

Bohmian mechanics: A legitimate hydrodynamic picture for quantum mechanics, and beyond

Since its inception, Bohmian mechanics has been surrounded by a halo of controversy. Originally proposed to bypass the limitations imposed by von Neumann's theorem on the impossibility of hidden-variable models in quantum mechanics, it faced strong opposition from the outset. Over time, however, its use in tackling specific problems across various branches of physics has led to a gradual shift in attitude, turning the early resistance into a more moderate acceptance. A plausible explanation for this change may be that, since the late 1990s and early 2000s, Bohmian mechanics has been taking on a more operational and practical role. The original hidden-variable idea has gradually faded from its framework, giving way to a more pragmatic approach that treats it as a suitable analytical and computational tool. This discussion explores how and why such a shift in perspective has occurred and, therefore, answers questions such as whether Bohmian mechanics should be considered once and for all a legitimate quantum representation (i.e., worth being taught in elementary quantum mechanics courses) or, by extension, whether these ideas can be transferred to and benefit other fields. Here, the Schrödinger equation and several specific numerical examples are re-examined in the light of a less restrictive view than the standard one usually adopted in quantum mechanics.

quant-ph↗

arXiv2025

Engineering of self-bending surface plasmon polaritons through Hermite-Gaussian mode expansion

Surface plasmon polaritons have received much attention over the last decades in photonics or nanotechnology due to their inherent high sensitivity to metal surface variations (e.g., presence of adsorbates or changes in the roughness). It is thus expected that they will find promising major applications in widely cross-disciplinary areas, from material science to medicine. Here we introduce a novel theoretical framework suitable for designing new types of structured paraxial surface plasmon beams and controlling their propagation. More specifically, this method relies on a convenient Hermite--Gaussian mode expansion, which constitutes a complete basis set upon which new types of structured paraxial plasmon beams can be generated. The family of beams generated in this way presents a rather peculiar feature: they exhibit local intensity maxima at different propagation distances, which enables the control over where to place the beam energy. This, thus, opens up worthwhile pathways to manipulate light propagation along metal surfaces at the nanoscale. As a proof-of-concept, we provide numerical evidence of the feasibility of the method by analyzing the propagation of Airy-based surface plasmon polaritons along an air--silver interface.

physics.optics↗

arXiv2025

Generalized flux trajectories: New insights into partially coherent Airy beams

The propagation of Airy beams in free space is characterized by being non dispersive, which warrants the shape invariance of their intensity distribution, and self-accelerating along the transverse direction. These distinctive traits are still present in partially coherent Airy beams as long as the reach of their back tail (and hence their energy content) is not importantly reduced. To investigate the effects associated with the decrease of the beam coherence and its power content (by smoothly reducing the reach of their back tails), here we introduce a novel and insightful methodology based on a generalization of the concept of flux trajectory for paraxial partially coherent beams. This methodologies emphasizes the role of phase relations, thus helping to clarify why and how the beam smears out spatially along its propagation. This formalism, though, is general enough to tackle other types of structured light beams with whatever degree of partial coherence, from full coherence to total incoherence.

physics.optics↗

arXiv2024

Symmetries and singular behaviors with Bohmian trajectories

Quantum mechanics is able to predict challenging behaviors even in the simplest physical scenarios. These behaviors are possible because of the important dynamical role that phase plays in the evolution of quantum systems, and are very similar, on the other hand, to effects observable in analogous optical systems. This work focuses on how Bohmian mechanics proves to be a rather convenient theoretical framework to analyze phase-based phenomena, since the phase constitutes the central element in this hydrodynamical formulation of quantum mechanics. More specifically, it allows us to understand how spatial phase variations give rise to velocity fields that eventually rule the dynamical behavior of quantum systems, and that, when integrated in time locally (i.e., taking into account specific positions), they provide us with a neat local (point by point) description of the system evolution in the configuration space. Indeed, it will also be seen that this idea transcends the quantum realm and can be profitably used to describe the behavior of optical analogs with rather singular behaviors. With this purpose, two interesting phenomena that take place in free space are considered, namely, the self-acceleration and shape-invariance of Airy beams, and spontaneous self-focusing.

quant-ph↗

arXiv2024

Bohmian analysis of dark solutions in interfering Bose-Einstein condensates: The dynamical role of underlying velocity fields

In the last decades, the experimental research on Bose-Einstein interferometry has received much attention due to promising technological implications. This has thus motivated the development of numerical simulations aimed at solving the time-dependent Gross-Pitaevskii equation and its reduced one-dimensional version to better understand the development of interference-type features and the subsequent soliton dynamics. In this work, Bohmian mechanics is considered as an additional tool to further explore and analyze the formation and evolution in real time of the soliton arrays that follow the merging of two condensates. An alternative explanation is thus provided in terms of an underlying dynamical velocity field, directly linked to the local phase variations undergone by the condensate along its evolution. Although the reduced one-dimensional model is considered here, it still captures the essence of the phenomenon, rendering a neat picture of the full evolution without diminishing the generality of the description. To better appreciate the subtleties of free versus bound dynamics, two cases are discussed. First, the soliton dynamics exhibited by a coherent superposition of two freely released condensates is studied, discussing the peculiarities of the underlying velocity field and the corresponding flux trajectories in terms of both the peak-to-peak distance between the two initial clouds and the addition of a phase difference between them. In the latter case, an interesting correspondence with the well-known Aharonov-Bohm effect is found. Then, the recurrence dynamics displayed by the more general case of two condensates released from the two opposite turning points of a harmonic trap is considered in terms of the distance between such turning points. [...]

quant-ph↗

arXiv2024

Exploring the dynamics of finite-energy Airy beams: A trajectory analysis perspective

In practice, Airy beams can only be reproduced in an approximate manner, with a limited spatial extension and hence a finite energy content. To this end, different procedures have been reported in the literature, based on a convenient tuning of the transmission properties of aperture functions. In order to investigate the effects generated by the truncation and hence the propagation properties displayed by the designed beams, here we resort to a new perspective based on a trajectory methodology, complementary to the density plots more commonly used to study the intensity distribution propagation. We consider three different aperture functions, which are convoluted with an ideal Airy beam. As it is shown, the corresponding trajectories reveals a deeper physical insight about the propagation dynamics exhibited by the beams analyzed due to their direct connection with the local phase variations undergone by the beams, which is in contrast with the global information provided by the usual standard tools. Furthermore, we introduce a new parameter, namely, the escape rate, which allow us to perform piecewise analyses of the intensity distribution without producing any change on it, e.g., determining unambiguously how much energy flux contributes to the leading maximum at each stage of the propagation, or for how long self-accelerating transverse propagation survives. The analysis presented in this work thus provides an insight into the behavior of finite-energy Airy beams, and therefore is expected to contribute to the design and applications exploiting this singular type of beams.

physics.optics↗

arXiv2023

Young's experiment with entangled bipartite systems: The role of underlying quantum velocity fields

We consider the concept of velocity fields, taken from Bohmian mechanics, to investigate the dynamical effects of entanglement in bipartite realizations of Young's two-slit experiment. In particular, by comparing the behavior exhibited by factorizable two-slit states (cat-type state analogs in the position representation) with the dynamics exhibited by a continuous-variable Bell-type maximally entangled state, we find that, while the velocity fields associated with each particle in the separable scenario are well-defined and act separately on each subspace, in the entangled case there is a strong deformation in the total space that prevents this behavior. Consequently, the trajectories for each subsystem are not constrained any longer to remain confined within the corresponding subspace; rather, they exhibit seemingly wandering behavior across the total space. In this way, within the subspace associated with each particle (that is, when we trace over the other subsystem), not only interference features are washed out, but also the so-called Bohmian non-crossing rule\linebreak (i.e., particle trajectories are allowed to get across the same point at the same time).

quant-ph↗

arXiv2023

A quantum trajectory analysis of singular wave functions

The Schrödinger equation admits smooth and finite solutions that spontaneously evolve into a singularity, even for a free particle. This blowup is generally ascribed to the intrinsic dispersive character of the associated time evolution. We resort to the notion of quantum trajectories to reinterpret this singular behavior. We show that the blowup can be directly related to local phase variations, which generate an underlying velocity field responsible for driving the quantum flux toward the singular region.

quant-ph↗

arXiv2022

Partially coherent Airy beams: A cross-spectral density approach

Airy beams are known for displaying shape invariance and self-acceleration along the transverse direction while they propagate forwards. Although these properties could be associated with the beam coherence, it has been revealed that they also manifest in the case of partially coherent Airy-type. Here, these properties are further investigated by introducing and analyzing a class of partially coherent Airy beams under both infinite and finite energy conditions. The key element within the present approach is the so-called cross-spectral density, which enables a direct connection with the quantum density matrix, making the analysis exportable to the quantum realm to study the dynamics of Airy wave packets acted by both incoherence and decoherence. As it is shown, in the case of infinite energy beams both properties are preserved even under the circumstance of total incoherence provided the underlying structure of the beam remains equal to that of an Airy beam. In the case of finite energy beams, a situation closer to a realistic scenario, as experimental beams cannot have an infinite extension, it is shown that a propagation range along which both properties are preserved can be warranted. This is controlled by a critical distance, which depends on the spread range determined by the parameters ruling the extension of random field spatial fluctuations. Such a distance is determined by defining a position-dependent parameter that quantifies the degree of overlapping between the propagated beam and the input one displaced by an amount equivalent to the propagation distance.

quant-ph↗

arXiv2022

Flux trajectory analysis of Airy-type beams

Airy beams are solutions to the paraxial Helmholtz equation known for exhibiting shape invariance along their self-accelerated propagation in free space. These two properties are associated with the fact that they are not square integrable, that is, they carry infinite energy. To circumvent this drawback, families of so-called finite-energy Airy-type beams have been proposed in the literature and, in some cases, also implemented in the laboratory. Here an analysis of the propagation of this type of structured light beams is presented from a flux trajectory perspective with the purpose to better understand the mechanisms that make infinite and finite energy beams to exhibit different behaviors. As it is shown, while the foremost part of the beam can be clearly and unambiguously associated with the well-known accelerating term, the rear part of the beam corresponds to a nearly homogeneous distribution of flow trajectories, particularly for large propagation distances. This is shown to be related with an effective transfer of trajectories between adjacent lobes (gradually, from the fore part of the beam to its rear part), which leads to smearing out the transverse flow along the rear part of the beam. This is sharp contrast with the situation found in ideal Airy beams, where trajectories belonging to a given lobe of the intensity distribution remain the same all along the propagation. The analysis is supplemented with an also trajectory-based description of Young's experiment performed with finite-energy Airy beams in order to provide a dynamical understanding of the autofocusing phenomenon observed with circular Airy beams.

physics.optics↗

arXiv2022

Quantum-classical entropy analysis for nonlinearly-coupled continuous-variable bipartite systems

The correspondence principle plays a fundamental role in quantum mechanics, which naturally leads us to inquire whether it is possible to find or determine close classical analogs of quantum states in phase space -- a common meeting point to both classical and quantum density statistical descriptors. Here, this issue is tackled by investigating the behavior of classical analogs arising upon the removal of all interference traits displayed by the Wigner distribution functions associated with a given pure quantum state. Accordingly, the dynamical evolution of the linear and von Neumann entropies is numerically computed for a continuous-variable bipartite system, and compared with the corresponding classical counterparts, in the case of two quartic oscillators nonlinearly coupled under regular and chaos conditions. Three quantum states for the full system are considered: a Gaussian state, a cat state, and a Bell-type state. By comparing the quantum and classical entropy values, and particularly their trends, it is shown that, instead of entanglement production, such entropies rather provide us with information on the system (either quantum or classical) delocalization. This gradual loss of information translates into an increase in both the quantum and the classical realms, directly connected to the increase in the correlations between both parties' degrees of freedom which, in the quantum case, is commonly related to the production of entanglement.

quant-ph↗

arXiv2021

Decoherence in quantum cavities: Environmental erasure of carpet-type structures

Carpet-type structures constitute an ideal laboratory to study and analyze the robustness of the interference process that underlies this phenomenon against the harmful effects of decoherence. Here, without losing any generality, for simplicity, the case of a particle with a mass m is considered and described by a localized state corresponding to the ground state of a square box of width w, which is released inside a wider cavity (with a width L > w). The effects of decoherence are then numerically investigated by means of a simple dynamical model that captures the essential features of the phenomenon under Markovian conditions, leaving aside extra complications associated with a more detailed dynamical description of the system-environment interaction. As it is shown, this model takes into account and reproduces the fact that decoherence effects are stronger as energy levels become more separated (in energy), which translates into a progressive collapse of the energy density matrix to its main diagonal. However, because energy dissipation is not considered, an analogous behavior is not observed in the position representation, where a proper spatial localization of the probability density does not take place, but rather a delocalized distribution. This result emphasizes the fact that classicality is reached only if both decoherence and dissipation coexist; otherwise, non-classical traits might still persist. Actually, as it is also shown, in the position representation some off-diagonal correlations indeed survive unless an additional spatial-type factor is included in the model. This makes evident the rather complex nature of the decoherence phenomenon and hence the importance to have a familiarity with how it manifests in different representations, particularly with the purpose to determine and design reliable control mechanisms.

quant-ph↗

arXiv2021

Bohm's quantum "non-mechanics": An alternative quantum theory with its own ontology?

The ontological aspect of Bohmian mechanics, as a hidden-variable theory that provides us with an objective description of a quantum world without observers, is widely known. Yet its practicality is getting more and more acceptance and relevance, for it has proven to be an efficient and useful resource to tackle, explore, describe and explain such phenomena. This practical aspect emerges precisely when the pragmatic application of the formalism prevails over any other interpretational question, still a matter of debate and controversy. In this regard, the purpose here is to show and discuss how Bohmian mechanics emphasizes in a natural manner a series of dynamical features difficult to find out through other quantum approaches. This arises from the fact that Bohmian mechanics allows us to establish a direct link between the dynamics exhibited by quantum systems and the local variations of the quantum phase associated with their state. To illustrate these facts, simple models of two physically insightful quantum phenomena have been chosen, namely, the dispersion of a free Gaussian wave packet and Young-type two-slit interference. As it is shown, the outcomes from their analysis render a novel, alternative understanding of the dynamics displayed by these quantum phenomena in terms of the underlying local velocity field that connects the probability density with the quantum flux. This field, nothing but the so-called guidance condition in standard Bohmian mechanics, thus acquires a prominent role to understand quantum dynamics, as the mechanism responsible for such dynamics. This goes beyond the passive role typically assigned to this field in Bohmian mechanics, where traditionally trajectories and quantum potentials have received more attention instead.

quant-ph↗

arXiv2020

Quantum Zermelo problem for general energy resource bounds

A solution to the quantum Zermelo problem for control Hamiltonians with general energy resource bounds is provided. Interestingly, the energy resource of the control Hamiltonian and the control time define a pair of conjugate variables that minimize the energy-time uncertainty relation. The resulting control protocol is applied to a single qubit as well as to a two-interacting qubit system represented by a Heisenberg spin dimer. For these low-dimensional systems, it is found that physically realizable control Hamiltonians exist only for certain, quantized, energy resources.

quant-ph↗

arXiv2020

Uncertainty principle for axial power content of highly focused fields

In the analysis of the on-axis intensity for a highly focused optical field it is highly desirable to deal with effective relations aimed at characterizing the field behavior in a rather simple fashion. Here, a novel and adequate measure for the size of the region where the axial power content mainly concentrates is proposed on the basis of an uncertainty principle. Accordingly, a meaningful relationship is provided for both the spread of the incident beam at the entrance of the highly focused optical system and the size of the region where the on-axis power mainly concentrates.

physics.optics↗

arXiv2020

Graph approach to quantum teleportation dynamics

Quantum teleportation plays a key role in modern quantum technologies. Thus, it is of much interest to generate alternative approaches or representations aimed at allowing us a better understanding of the physics involved in the process from different perspectives. With this purpose, here an approach based on graph theory is introduced and discussed in the context of some applications. Its main goal is to provide a fully symbolic framework for quantum teleportation from a dynamical viewpoint, which makes explicit at each stage of the process how entanglement and information swap among the qubits involved in it. In order to construct this dynamical perspective, it has been necessary to define some auxiliary elements, namely virtual nodes and edges, as well as an additional notation for nodes describing potential states (against nodes accounting for actual states). With these elements, not only the flow of the process can be followed step by step, but they allow us to establish a direct correspondence between this graph-based approach and the usual state vector description. To show the suitability and versatility of this graph-based approach, several particular teleportation examples are examined, which include bipartite, tripartite and tetrapartite maximally entangled states as quantum channels. From the analysis of these cases, a general protocol is discussed in the case of sharing a maximally entangled multi-qubit system.

quant-ph↗

arXiv2020

Bohmian-based approach to Gauss-Maxwell beams

Usual Gaussian beams are particular scalar solutions to the paraxial Helmholtz equation, which neglect the vector nature of light. In order to overcome this inconvenience, Simon et al. (J. Opt. Soc. Am. A 1986, 3, 536-540) found a paraxial solution to Maxwell's equation in vacuum, which includes polarization in a natural way, though still preserving the spatial Gaussianity of the beams. In this regard, it seems that these solutions, known as Gauss-Maxwell beams, are particularly appropriate and a natural tool in optical problems dealing with Gaussian beams acted or manipulated by polarizers. In this work, inspired in the Bohmian picture of quantum mechanics, a hydrodynamic-type extension of such a formulation is provided and discussed, complementing the notion of electromagnetic field with that of (electromagnetic) flow or streamline. In this regard, the method proposed has the advantage that the rays obtained from it render a bona fide description of the spatial distribution of electromagnetic energy, since they are in compliance with the local space changes undergone by the time-averaged Poynting vector. This feature confers the approach a potential interest in the analysis and description of single-photon experiments, because of the direct connection between these rays and the average flow exhibited by swarms of identical photons (regardless of the particular motion, if any, that these entities might have), at least in the case of Gaussian input beams. In order to illustrate the approach, here it is applied to two common scenarios, namely the diffraction undergone by a single Gauss-Maxwell beam and the interference produced by a coherent superposition of two of such beams.

physics.optics↗

arXiv2019

A simple projective setup to study optical cloaking in the classroom

Optical cloaking consists in hiding from sight an object by properly deviating the light that comes from it. An optical cloaking device (OCD) is an artifact that hides the object and, at the same time, its presence is not (or should not be) noticeable for the observer, who will have the impression of being looking through it. At the level of paraxial geometrical optics, suitable for undergraduate courses, simple OCDs can be built by combining a series of lenses. With this motivation, here we present an analysis of a simple projective OCD arrangement. First, a simple theoretical account in terms of the transfer matrix method is provided, and then the outcomes from a series of teaching experiments carried out with this device, easy to conduct in the classroom, are discussed. In particular, the performance of such an OCD is investigated by determining the effect of the hidden object, role here played by the opaque zone of an iris-type diaphragm, on the projected image of an illuminated transparent slide (test object). That is, cloaking is analyzed in terms of the optimal position and opening diameter of a diaphragm that still warrants an almost unaffected projected image. Because the lenses are not high-quality ones, the OCD is not aberration-free, which is advantageously considered to determine acceptable cloaking conditions (i.e., the tolerance of the device).

physics.ed-ph↗