arXiv subjects

Lea Sirota

Publications and source records attributed to Lea Sirota.

16 recordsLinked to original sources

arXiv2025

Solution of Wave Acceleration and Non-Hermitian Jump in Nonreciprocal Lattices

The time evolution of initially localized wavepackets in the discrete Hatano-Nelson lattice displays a rich dynamical structure shaped by the interplay between dispersion and nonreciprocity. Our analysis reveals a characteristic evolution of the wave-packet center of mass, which undergoes an initial acceleration, subsequently slows down, and ultimately enters a regime of uniform motion, accompanied throughout by exponential amplification of the wave-packet amplitude. To capture this behavior, we develop a continuum approximation that incorporates higher-order dispersive and nonreciprocal effects and provides accurate analytical predictions across all relevant time scales. Building on this framework, we then demonstrate the existence of a non-Hermiticity-induced jump - an abrupt spatial shift of the wave-packet center even in the absence of disorder - and derive its underlying analytical foundation. The analytical predictions are in excellent agreement with direct numerical simulations of the Hatano-Nelson chain. Our results elucidate the interplay between dispersion and nonreciprocity in generating unconventional transport phenomena, and pave the way for controlling wave dynamics in nonreciprocal and non-Hermitian metamaterials.

cond-mat.mes-hall↗

arXiv2025

Fast wave transport in two-dimensional $\mathcal{PT}$-symmetric lattices

We present a theoretical investigation of wave dynamics in two-dimensional non-Hermitian $\mathcal{PT}$-symmetric lattices, where onsite, as well as inter-site control couplings are employed. Our analysis shows that these couplings can be tuned to achieve a direction-sensitive group velocity enhancement beyond what is possible in the uncontrolled (Hermitian) counterpart, while ensuring that the wave packet evolution remains bounded and dynamically stable. We derive a dedicated relation between the control parameters, providing a systematic condition under which stability is guaranteed. We then study the topological properties of the non-Hermitian system at hand, and use an experimental-ready topoelecric metamaterial platform to demonstrate the non-Hermitian couplings realization, and the resulting wave dynamics. This framework paves the way to designing stable and fast wave transport in planar non-Hermitian media.

cond-mat.mes-hall↗

arXiv2025

Realizing non-Hermitian tunneling phenomena using non-reciprocal active acoustic metamaterials

Non-reciprocal systems have been shown to exhibit various interesting wave phenomena, such as the non-Hermitian skin effect, which causes accumulation of modes at boundaries. Recent research on discrete systems showed that this effect can pose a barrier for waves hitting an interface between reciprocal and non-reciprocal systems. Under certain conditions, however, waves can tunnel through this barrier, similar to the tunneling of particles in quantum mechanics. This work proposes and investigates an active acoustic metamaterial design to realize this tunneling phenomenon in the acoustical wave domain. The metamaterial consists of an acoustic waveguide with microphones and loudspeakers embedded in its wall. Starting from a purely discrete non-Hermitian lattice model of the system, a hybrid continuous-discrete acoustic model is derived, resulting in distributed feedback control laws to realize the desired behavior for acoustic waves. The proposed control laws are validated using frequency and time domain finite element method simulations, which include lumped electro-acoustic loudspeaker models. Additionally, an experimental demonstration is performed using a waveguide with embedded active unit cells and a digital implementation of the control laws. In both the simulations and experiments the tunneling phenomenon is successfully observed.

physics.app-ph↗

arXiv2025

Supersonic wave propagation in active non-Hermitian acoustic metamaterials

Obtaining a group velocity higher than the speed of sound in a waveguide is a challenging task in acoustic wave engineering. Even more challenging is to achieve this velocity increase without any intervention with the waveguide profile, such as narrowing or widening, and particularly without interfering with the passage by flexible inclusions, either passive or active. Here, we approach this problem by invoking concepts from non- Hermitian physics, and imposing them using active elements that are smoothly sealed within the waveguide wall. In a real-time feedback operation, the elements induce local pressure gain and loss, as well as non-local pressure integration couplings. We employ a dedicated balancing between the control parameters, derived from lattice theory and adjusted to the waveguide system, to drive the dynamics into a stable parity-time-symmetric regime. We demonstrate the accelerated propagation of a wave packet both numerically and experimentally in an air-filled waveguide and discuss the trade-off between stabilization and the achievable velocity increase. Our work prepares the grounds for advanced forms of wave transmission in continuous media, enabled by short and long range active couplings, created via embedded real-time feedback control.

physics.app-ph↗

arXiv2025

Harnessing Nonlinearity to Tame Wave Dynamics in Nonreciprocal Active Systems

We present a mechanism to generate unidirectional pulse-shaped propagating waves, tamed to exponential growth and dispersion, in active systems with nonreciprocal and nonlinear couplings. In particular, when all bulk modes are exponentially localized at one side of the lattice (skin effect), it is expected that wave dynamics is governed by amplification or decay until reaching the boundaries, even in the presence of dissipation. Our analytical results, and experimental demonstrations in an active electrical transmission line metamaterial, reveal that nonlinearity is a crucial tuning parameter in mediating a delicate interplay between nonreciprocity, dispersion, and dissipation. Consequently, undistorted unidirectional solitonic pulses are supported both for low and high nonreciprocity and pulse amplitude strength. The proposed mechanism facilitates robust pulse propagation in signal processing and energy transmission applications.

cond-mat.mes-hall↗

arXiv2025

Noise avalanche and its quantum quenching in bosonic chains with random off-diagonal disorder

Here we discuss a phenomenon of sharp increase in the photon number noise at initial stages of propagation in tight-binding bosonic chains with off-diagonal disorder. Such a "noise avalanche" occurs under classical coherent excitation of waveguides and leads to high super-thermal photon bunching. Additional classical excitation slows but cannot quench this noise avalanche. However, an additional single-photon excitation stops the avalanche.

quant-ph↗

arXiv2024

Controlled fast wavepackets in non-Hermitian lattices

We report the propagation of fast wavepackets in classical non-Hermitian lattices, where the group velocity is controlled by the non-Hermiticity parameters, and can be made higher than in the Hermitian counterpart. Specifically, we obtain a square root dependence of the group velocity on the gain/loss parameter, similarly to the dependence of quantum wavepackets in stretched graphene-like lattices subjected to gain and loss. We derive a targeted mapping from the quantum to the classical Hamiltonian to realize this phenomenon in a dynamically stable form. As a result, fast wavepackets of any frequency supported by the lattice are propagating in time domain with a non-growing amplitude. We demonstrate the system experimentally in a topoelectrical metamaterial, where the non-Hermiticity is generated by embedded operational amplifiers in a feedback setup. Our design paves the way to realize increased group velocities, and other wave phenomena inspired by quantum systems in a form that preserves the original system properties, while supporting an inherently stable dynamics.

cond-mat.mes-hall↗

arXiv2023

Tunneling-like wave transmission in non-Hermitian lattices with mirrored nonreciprocity

We report a peculiar tunneling phenomenon that occurs in lattices with nonreciprocal couplings. The nonreciprocity holds for an inner portion of the lattice, constituting a non-Hermitian interface between outer Hermitian sections. The couplings are mirrored about the interface center. As a standalone system that was widely studied in recent years, each section of the interface supports the non-Hermitian skin effect, in which modes are accumulated at one boundary. Here, we investigate what happens to a wave that propagates along the lattice and hits the interface. The skin mode accumulation, which effectively constitutes a barrier, forbids wave penetration into the interface, but surprisingly, under certain conditions the wave is transmitted to the other side, keeping the interface dark, as if the wave invisibly tunneled through it. Remarkably, the tunneling is independent of the interface length, and a perfect transmission can be achieved independently of frequency and nonreciprocity strength. We derive the phenomenon both for quantum and classical systems, and realize it experimentally in an active topoelectric metamaterial. Our study fosters the research of wave tunneling through other types of non-Hermitian interfaces, which may also include nonlinearities, time-dependence and more.

cond-mat.mes-hall↗

arXiv2023

Gravitational lensing and tunneling of mechanical waves in synthetic curved spacetime

Black holes are considered among the most fascinating objects that exist in our universe, since in the classical formalism nothing, even no light, can escape from their vicinity due to gravity. The gravitational potential causes the light to bend towards the hole, which is known by gravitational lensing. Here we present a synthetic realization of this phenomenon in a lab-scale two-dimensional network of mechanical circuits, based on analogous condensed matter formalism of Weyl semimetals with inhomogeneous nodal tilt profiles. Some of the underlying network couplings turn out as unstable and non-reciprocal, and are implemented by embedded active feedback interactions in an overall stabilized structure. We demonstrate the lensing by propagating mechanical wavepackets through the network with a programmed funnel-like potential, achieving wave bending towards the circle center. We then demonstrate the versatility of our platform by reprogramming it to mimic quantum tunneling of particles through the event horizon, known by Hawking radiation, achieving an exceptional correspondence to the original mass loss rate within the hole. The network couplings and the potential can be further reprogrammed to realize other curvatures and associated relativistic phenomena.

quant-ph↗

arXiv2023

Active Control Approach to Temporal Acoustic Cloaking

We propose a realization of a transformation-based acoustic temporal cloak using an active closed-loop control approach to an equivalent electromagnetic problem. Unlike the more common spatial cloaks the goal of which is hiding fixed objects from detection, the goal of the temporal cloak is hiding the occurrence of events during a finite period of time. In electromagnetic systems, in which events represent, for example, leakage of signals from transmission lines or optical fibers, temporal cloaking solutions usually rely on nonlinear phenomena related to the fibers properties, or on modulating the properties of the propagation medium itself. In particular, the transformation-based solution requires modulating the constitutive parameters of the medium both in space and time. Our control approach is fully linear, where the required change in the medium parameters is programmed into the controllers and created by external actuators in real-time. This cloaking system keeps the physical medium unchanged, and enables to reprogram the cloaking parameters upon request. We demonstrate our solution in a simulation of a one-dimensional water channel.

physics.app-ph↗

arXiv2023

Emerging exceptional point with breakdown of skin effect in non-Hermitian systems

We study the interplay of two distinct non-Hermitian parameters: directional coupling and onsite gain-loss, together with topology, in coupled one-dimensional (1D) non-Hermitian Su-Schrieffer-Heeger (SSH) chains. The SSH model represents one of the simplest two-band models that features boundary localized topological modes. Our study shows how the merging of two topological modes can lead to a striking spectral feature of non-Hermitian systems, namely exceptional point (EP). We reveal the existence EP as a singularity in the parameter space of non-Hermitian couplings carrying a half-integer topological charge. We also demonstrate two different localization behaviors observed in the bulk and hybridized topological modes. While the bulk states and individual topological modes remain localized at the boundaries due to skin effect, the competition between the constituent non-Hermitian parameters can overcome the strength of skin effect and lead to the complete \textit{delocalization} of these hybridized modes. We obtain explicit analytic solutions for the eigenfunction and the eigenenergy of the hybridized modes, which exactly match the numerical results and successfully reveal the underlying cause of delocalization and the emergence of EP.

cond-mat.mes-hall↗

arXiv2023

Real-time-controlled artificial quiet channel for acoustic cloaking under varying detection conditions

We consider the problem of hiding non-stationary objects from acoustic detection in a two-dimensional environment, where both the object's impedance and the properties of the detection signal may vary during operation. The detection signal is assumed to be an acoustic beam created by an array of emitters, which scans the area at different angles and different frequencies. We propose an active control-based solution that creates an effective moving dead zone around the object, and results in an artificial quiet channel for the object to pass through undetected. The control principle is based on mid-domain generation of near uni-directional beams using only monopole actuators. Based on real-time response prediction, these beams open and close the dead zone with a minimal perturbation backwards, which is crucial due to detector observers being located on both sides of the object's route. The back action wave determines the cloak efficiency, and is traded-off with the control effort; the higher is the effort the quieter is the cloaking channel. We validate our control algorithm via numerical experiments in a two-dimensional acoustic waveguide, testing variation in frequency and incidence angle of the detection source. Our cloak successfully intercepts the source by steering the control beams and adjusting their wavelength accordingly.

physics.app-ph↗

arXiv2022

Temporal negative refraction

Negative refraction is a peculiar wave propagation phenomenon that occurs when a wave crosses a boundary between a regular medium and a medium with both constitutive parameters negative at the given frequency. The phase and group velocities of the transmitted wave then turn anti-parallel. Here we propose a temporal analogue of the negative refraction phenomenon using time-dependent media. Instead of transmitting the wave through a spatial boundary we transmit it through an artificial temporal boundary, created by switching both parameters from constant to dispersive with frequency. We show that the resulting dynamics is sharply different from the spatial case, featuring both reflection and refraction in positive and negative regimes simultaneously. We demonstrate our results analytically and numerically using electromagnetic medium. In addition, we show that by a targeted dispersion tuning the temporal boundary can be made nonreflecting, while preserving both positive and negative refraction.

physics.optics↗

arXiv2022

Klein-like tunneling of sound via negative index metamaterials

Klein tunneling is a counterintuitive quantum-mechanical phenomenon, predicting perfect transmission of relativistic particles through higher energy barriers. This phenomenon was shown to be supported at normal incidence in graphene due to pseudospin conservation. Here I show that Klein tunneling analogue can occur in classical systems, and remarkably, not relying on mimicking graphene's spinor wavefunction structure. Instead, the mechanism requires a particular form of constitutive parameters of the penetrated medium, yielding transmission properties identical to the quantum tunneling in graphene. I demonstrate this result by simulating tunneling of sound in a two-dimensional acoustic metamaterial. More strikingly, I show that by introducing a certain form of anisotropy, the tunneling can be made unimpeded for any incidence angle, while keeping most of its original Klein dispersion properties. This phenomenon may be denoted by the omnidirectional Klein-like tunneling. The new tunneling mechanism and its omnidirectional variant may be useful for applications requiring lossless and direction-independent transmission of classical waves.

cond-mat.mes-hall↗

arXiv2020

Real-Time Steering of Curved Sound Beams in a Feedback-based Topological Acoustic Metamaterial

We present the concept of a feedback-based topological acoustic metamaterial as a tool for realizing autonomous and active guiding of sound beams along arbitrary curved paths in free two-dimensional space. The metamaterial building blocks are acoustic transducers, embedded in a slab waveguide. The transducers generate a desired dispersion profile in closed-loop by processing real-time pressure field measurements through preprogrammed controllers. In particular, the metamaterial can be programmed to exhibit analogies of quantum topological wave phenomena, which enables unconventional and exceptionally robust sound beam guiding. As an example, we realize the quantum valley Hall effect by creating, using a collocated pressure feedback, an alternating acoustic impedance pattern across the waveguide. The pattern is traversed by artificial trajectories of different shapes, which are reconfigurable in real-time. Due to topological protection, the sound waves between the plates remain localized on the trajectories, and do not back-scatter by the sharp corners or imperfections in the design. The feedback-based design can be used to realize arbitrary physical interactions in the metamaterial, including non-local, nonlinear, time-dependent, or non-reciprocal couplings, paving the way to new unconventional acoustic wave guiding on the same reprogrammable platform. We then present a non-collocated control algorithm, which mimics another quantum effect, rendering the sound beams uni-directional.

physics.app-ph↗

arXiv2020

Non-Newtonian Topological Mechanical Metamaterials Using Feedback Control

We introduce a method to design topological mechanical metamaterials that are not constrained by Newtonian dynamics. The unit cells in a mechanical lattice are subjected to active feedback forces that are processed through autonomous controllers, pre-programmed to generate the desired local response in real-time. As an example, we focus on the quantum Haldane model, which is a two-band system with directional complex coupling terms, violating Newton's third law. We demonstrate that the required topological phase, characterized by chiral edge modes, can be achieved in an analogous mechanical system only with closed-loop control. We then show that our approach enables to realize, for the first time, a modified version of the Haldane model in a mechanical metamaterial. Here, the complex-valued couplings are polarized in a way that modes on opposite edges of a lattice propagate in the same direction, and are balanced by counter-propagating diffusive bulk modes. The proposed method is general and flexible, and could be used to realize arbitrary lattice parameters, such as non-local or nonlinear couplings, time dependent potentials, non-Hermitian dynamics, and more, on a single platform.

cond-mat.mes-hall↗