arXiv subjects

Stefan Rotter

Publications and source records attributed to Stefan Rotter.

At least 19 recordsLinked to original sources

arXiv2026

Geometric Pumping of Optical Work in Coherent Wave Scattering

Optical work exchanged between light and time-varying matter underlies a wide range of emerging photonic technologies, yet a general geometric theory describing its generation and control has been lacking. Here we establish such a theory for coherent electromagnetic scattering. We show that the net optical work accumulated during an adiabatic modulation cycle is a geometric quantity determined by the Berry curvature of the outgoing field in parameter space. Nonzero optical work can thus be generated by cyclically modulating two or more independent system parameters. In contrast to conventional adiabatic pumping between incoherent reservoirs, optical work pumping vanishes under maximally mixed illumination. Greater illumination coherence increases the maximum work attainable through wavefront shaping. For fully coherent illumination, we derive the optimal incident wavefront and modulation trajectory that maximize the optical work per cycle. Our framework unifies time-varying photonics, optomechanics, and geometric transport within a common scattering formalism.

physics.optics↗

arXiv2026

Rewiring of adult neural networks - learning from structural plasticity across multiple scales

Synaptic connections are continuously formed and eliminated even in adult neuronal networks. This process - called structural plasticity - allows networks to change their topology to learn and adapt to their environment or compensate for lesions. Yet, in contrast to synaptic plasticity, structural plasticity is less well studied and understood. We here present the current state of knowledge as well as open problems in the field along three guiding questions: (1) What regulates structural plasticity in biological neural networks? (2) What are the functional consequences and advantages of structural plasticity in adult neural networks? (3) How can our understanding of structural plasticity be translated into clinical and technological applications? We argue that understanding structural plasticity is a problem involving multiple temporal and spatial scales. Solving this problem thus requires collaborative efforts of experimentalists, theoreticians as well as computational modelers, and has the potential for comprehensive neuroscientific insights that can guide the development of medical applications, artificial neural networks as well as neuromorphic computing.

q-bio.NC↗

arXiv2026

Observation of symmetry breaking in time-varying scattering systems

From the Higgs mechanism to magnetic ordering, spontaneous symmetry breaking organizes physics across scales. Whether an analogous transition governs waves scattered by periodically driven structures has remained an open experimental challenge. Here we demonstrate a universal symmetry-breaking transition in a periodically driven finite scattering system. By measuring the multispectral Floquet scattering matrix of a strongly modulated microwave resonator, we observe a transition from an unbroken symmetry regime with scattering eigenvalues of equal modulus to a broken symmetry regime where their moduli split apart, through an exceptional point. At parametric resonance, this transition culminates in coherent perfect absorption, where a tailored multispectral input state is completely absorbed without outgoing radiation, the singular limit of the underlying transition. Because it arises from the general algebraic structure of the Floquet scattering matrix rather than from implementation-specific details, this transition should occur broadly in such systems. Unlike instability phenomena in bulk driven media, it emerges in the eigenstructure of a finite scattering operator that couples waves across multiple frequency channels. Our results establish Floquet scattering as an experimentally accessible platform for the observation of symmetry breaking and for dynamic wave control in driven photonic systems.

physics.optics↗

arXiv2026

Zero-forward Kerker scattering via synthesized complex-frequency excitation

All objects illuminated with light inevitably cast a shadow -- a universal phenomenon encapsulated in the fundamental property that all passive systems with plane-wave illumination feature a non-zero forward scattering amplitude. Since Kerker's landmark 1983 paper, considerable effort has been devoted to overcoming this limitation and achieving the elimination of forward scattering -- an objective now widely known as the zero-forward Kerker scattering. However, this objective is fundamentally restricted to active systems, requiring either physical gain in materials or virtual gain in the excitation source. Despite advances in active materials science and non-Hermitian photonics, the experimental realization of zero-forward Kerker scattering still remains an open challenge. Here, we show, both theoretically and experimentally, how zero-forward Kerker scattering can be effectively synthesized by a weighted superposition of readily accessible real-frequency responses. Our synthetic recipe builds on creating an artificial pole at a complex frequency that prevails over inherent scattering poles. These findings not only unlock a realistic experimental framework for non-Hermitian light-matter interactions, but also hold technological relevance for non-invasive sensing and imaging.

physics.optics↗

arXiv2026

Exceptional Points, Lasing, and Coherent Perfect Absorption in Floquet Scattering Systems

Periodically time-varying media, known as photonic time crystals (PTCs), provide a promising platform for observing unconventional wave phenomena. We analyze the scattering of electromagnetic waves from spatially finite PTCs using the multispectral Floquet scattering matrix, which naturally incorporates the frequency-mixing processes intrinsic to such systems. For dispersionless, real, and time-periodic permittivities, this matrix is pseudounitary. Here we demonstrate that this property leads to multiple symmetry-breaking transitions: for increasing driving strength, scattering matrix eigenvalues lying on the unit circle (unbroken symmetry regime) meet at exceptional points (EPs), where they break up into inverse complex conjugate pairs (broken symmetry regime). We identify the symmetry operator associated with these transitions and show that, in time-symmetric systems, it corresponds to the time-reversal operator. Remarkably, at the parametric resonance condition, one eigenvalue vanishes while its partner diverges, signifying simultaneous coherent perfect absorption (CPA) and lasing. Since our approach relies solely on the Floquet scattering matrix, it is not restricted to a specific geometry but instead applies to any periodically time-varying scattering system. To illustrate this universality, we apply our method to a variety of periodically time-modulated structures, including slabs, spheres, and metasurfaces. In particular, we show that using quasi-bound states in the continuum resonances sustained by a metasurface, the CPA and lasing conditions can be attained for a minimal modulation strength of the permittivity. Our results pave the way for engineering time-modulated photonic systems with tailored scattering properties, opening new avenues for dynamic control of light in next-generation optical devices.

physics.optics↗

arXiv2026

Enabling Deterministic Passive Quantum State Transfer with Giant Atoms

Achieving quantum state transfer in passive ways can become a powerful asset for scalable quantum networks. Here, we demonstrate how giant atoms coupled to 1D waveguides provide a platform for such a passive, deterministic transfer. Specifically, we show that when the giant atom's extent is comparable to the width of the emitted wavepacket, this wavepacket can be designed to be time-reversal-symmetric by engineering the positions and strengths of atom-waveguide coupling points. We first derive general analytical conditions under which arbitrary qubit decays can be mapped to wavevector-dependent couplings that guarantee perfect state transfer in the continuum limit of infinitely many coupling points. Then, for experimentally relevant configurations with a finite number of coupling points, we optimize the coupling positions and values. We demonstrate that the transfer fidelity can be boosted well beyond the value for a single coupling point (54%), reaching 87% with only two coupling points and exceeding 99% with ten or more. We further analyze the robustness of the protocol against disorder in the coupling points as well as frequency variations of the qubits and extend the formalism to environments with nonlinear dispersion, showing that dispersion-induced distortions can be fully compensated by judiciously chosen setups. Our results establish giant atoms as a powerful platform for realizing high-fidelity quantum state transfer and demonstrate how time-dependent control can be instead encoded into the device design.

quant-ph↗

arXiv2026

T-matrix representation of optical scattering response: Suggestion for a data format

The transition matrix, frequently abbreviated as T-matrix, contains the complete information in a linear approximation of how a spatially localized object scatters an incident field. The T-matrix is used to study the scattering response of an isolated object and describes the optical response of complex photonic materials made from ensembles of individual objects. T-matrices of certain common structures, potentially, have been repeatedly calculated all over the world again and again. This is not necessary and constitutes a major challenge for various reasons. First, the resources spent on their computation represent an unsustainable financial and ecological burden. Second, with the onset of machine learning, data is the gold of our era, and it should be freely available to everybody to address novel scientific challenges. Finally, the possibility of reproducing simulations could tremendously improve if the considered T-matrices could be shared. To address these challenges, we found it important to agree on a common data format for T-matrices and to enable their collection from different sources and distribution. This document aims to develop the specifications for storing T-matrices and associated metadata. The specifications should allow maximum freedom to accommodate as many use cases as possible without introducing any ambiguity in the stored data. The common format will assist in setting up a public database of T-matrices.

physics.optics↗

arXiv2026

Passive quantum state transfer in a dispersion-engineered waveguide

High-fidelity state transfer is fundamentally limited by time-reversal symmetry: one qubit emits a photon with a certain temporal pulse shape, whereas a second qubit requires the time-reversed pulse shape to efficiently absorb this photon. This limit is often overcome by introducing active elements. Here, we propose an alternative solution: by tailoring the dispersion relation of a waveguide, the photon pulse emitted by one qubit is passively reshaped into its time-reversed counterpart, thus enabling perfect absorption. We analytically derive the optimal dispersion relations in the limit of small and large qubit-qubit separations, and numerically extend our results to arbitrary separations via multiparameter optimization. We further propose a spatially inhomogeneous waveguide that renders the state transfer robust to variations in qubit separations. In all cases, we obtain near-unity transfer fidelity (>= 98%) that is robust against imperfections in parameter values and propagation loss. Our dispersion-engineered waveguide provides a compact and passive route toward on-chip quantum networks, highlighting dispersion as a powerful resource in waveguide quantum electrodynamics.

quant-ph↗

arXiv2026

Brewster-anomaly delocalization for free-electron radiation

Localization effects are central to disordered electronics and photonics. In electronics, Anderson localization governs electron confinement in randomly perturbed lattices. Similarly, its photonic counterpart inhibits light transport via disorder -- but with a unique exception: Brewster-anomaly delocalization, where the Brewster effect prevents multiple-scattering interference and counteracts the localization. Despite extensive research in electronics and photonics separately, the intricate role of localization effects in free-electron--light interactions -- vital for lasers, accelerators, microscopy and spectroscopy, and quantum information -- remains largely unexplored. At the same time, localization effects are widely regarded as a key factor limiting the efficient coupling between free electrons and light in random media. Here we overcome this key limitation via the unconventional interplay between Brewster-anomaly delocalization and free-electron radiation. In this way, free-electron radiation can be localization-free, intense and directional even in strongly disordered, unengineered multilayers. Essentially, this delocalization-mediated free-electron radiation is remarkably invariant not only to the random-medium configuration, but also to the light frequency and the electron velocity. Our findings unlock new opportunities for particle detectors and achromatic light sources operating in easy-to-fabricate complex media at previously inaccessible frequencies.

physics.optics↗

arXiv2026

Universal Statistics of Energy and Information Flow in Random Electromagnetic Fields

We establish a universal statistical description for the local flow of energy and information in random electromagnetic fields. The longitudinal Poynting flux, written as a Hermitian quadratic form of the transverse electric and magnetic field components, follows a probability distribution that is completely determined by four eigenvalues of an electromagnetic covariance matrix. These flux eigenvalues quantify forward transport, optical backflow, and polarization mixing, and reduce to the known paraxial and isotropic limits in the appropriate regimes -- including strongly nonparaxial fields, where no universal description was known so far. Full-vector simulations of continuous and discrete disordered media confirm this universality. The same framework applies to the recently introduced Fisher-information flux, with the fields replaced by their sensitivity to a parameter, thereby unifying the statistics of local energy and information transport in random light and revealing the reversal of information flow across a parameter-dependent object.

physics.optics↗

arXiv2026

Fundamental Limits of Non-Hermitian Sensing from Quantum Fisher Information

Exceptional points (EPs) exhibit strongly enhanced spectral responses and are therefore promising candidates for sensing applications. Whether these non-Hermitian degeneracies provide a genuine advantage in the quantum regime has been the subject of ongoing debate. Here, we address this issue within a scattering-matrix formalism for sensing with coherent light, which allows the quantum Fisher information (QFI) to be evaluated directly from experimentally accessible scattering data without introducing additional noise channels beyond those inherent to the scattering process. We analyze both nondegenerate and degenerate scattering-matrix poles, including EPs of arbitrary order, and show that the QFI per incoming photon flux is governed by three key factors: the decay rate of the resonant mode, the strength of the spectral response associated with non-normality, and the adjustment between the scattering states and the information source. For spatially localized perturbations, this implies that the Fisher information is fully determined by the local density of states at the perturbation site. Within this framework, we demonstrate that EPs can enhance the QFI compared to isolated modes or diabolic points with identical decay rates, and that the QFI can be further increased by moving away from the EP toward parameter regimes where non-Hermitian linewidth splitting reduces the decay rate of one mode. We further show that sufficiently small additional internal losses do not alter this overall picture, thereby providing a unified and experimentally relevant perspective on the design of quantum-limited non-Hermitian sensors.

quant-ph↗

arXiv2026

Massively Degenerate Coherent Perfect Absorption in Gradient-Index Fibers

Coherent perfect absorbers (CPAs) have recently attracted considerable attention due to their ability to enhance light--matter interaction. By exploiting interference, CPAs enable even weakly absorbing materials to achieve complete absorption under appropriate excitation conditions. Generalizing this concept to the simultaneous absorption of arbitrary multimode input states remains challenging, however, since conventional implementations typically operate only for a single or a very small number of input channels. Here, we propose a compact realization of a multimode coherent perfect absorber based on a gradient-index (GRIN) fiber. Using the self-imaging property of the fiber, the bulky free-space architecture of previous approaches is replaced by a monolithic waveguiding platform that supports near-degenerate rephasing of many spatial modes. We show that standard GRIN profiles optimized for minimal intermodal dispersion enable highly efficient absorption of complex multimode fields, with field-of-view reflectivities well below \(1\%\) for realistic parameters. This approach provides a practical and scalable route toward efficient multimode absorption in fiber-based and integrated photonic systems, with potential applications in light harvesting, optical control, and imaging.

physics.optics↗

arXiv2026

Multi-Objective Tweezers in Scattering Media

Radiation forces and torques enable the manipulation of objects with acoustic and electromagnetic waves. Yet, harnessing them in complex scattering media remains a formidable challenge, especially when multiple objects must be controlled under competing objectives. Here, we demonstrate that sound or light can be shaped to tailor momentum transfer to multiple objects simultaneously in a complex scattering medium. For a single object, our theory yields the maximal achievable force or torque; for multiple objects, it produces Pareto-optimal actuation and exact bounds on the simultaneous realization of incompatible objectives. This opens new applications for wave tweezers, enabling selective and precise manipulation of objects within complex media, ranging from the handling of cells, organoids, or microrobots, to targeted drug delivery in biological media.

physics.app-ph↗

arXiv2026

Controlling the Flow of Information in Optical Metrology

Optical metrology has progressed beyond the Abbe-Rayleigh limit, unlocking (sub)atomic precision by leveraging nonlinear phenomena, statistical accumulation, and AI estimators trained on measurand variations. Here, we show that Fisher information, which defines the fundamental precision limit, can be viewed as a physical entity that propagates through space, and we derive a wave equation for sensitivity fields describing its flow, which can resonate, diffract, and interfere. We reveal how material composition, geometry, and environmental design dictate where information is generated and how it travels, analogous to antennas and metasurfaces sculpting electromagnetic energy. Plasmonic and dielectric resonances enhance information flow, while gratings and near-field structures reshape radiation patterns. This perspective reframes metrology as a discipline in which resolution can be engineered by tailoring information sources and flow for applications in atomic-scale diagnostics and beyond, including optimisation of Light Detection and Ranging (LiDAR), remote sensing, and radar technologies.

physics.optics↗

arXiv2025

Dynamic and Geometric Shifts in Wave Scattering

Since Berry's pioneering 1984 work, the separation of geometric and dynamic contributions in the {\it phase} of an evolving wave has become fundamental in physics, underpinning diverse phenomena in quantum mechanics, optics, and condensed matter. Here we extend this geometric-dynamic decomposition from the wave-evolution phase to a distinct class of wave scattering problems, where observables (such as frequency, momentum, or position) experience shifts in their expectation values between the input and output wave states. We describe this class of problems using a unitary scattering matrix and the associated generalized Wigner-Smith operator (GWSO), which involves gradients of the scattering matrix with respect to conjugate variables (time, position, or momentum, respectively). We show that both the GWSO and the resulting expectation-values shifts admit gauge-invariant decompositions into dynamic and geometric parts, related respectively to gradients of the eigenvalues and eigenvectors of the scattering matrix. We illustrate this general theory through a series of examples, including frequency shifts in polarized-light transmission through a time-varying waveplate (linked to the Pancharatnam-Berry phase), momentum shifts at spatially varying metasurfaces, optical forces, beam shifts upon reflection at a dielectric interface, and Wigner time delays in 1D scattering. This unifying framework illuminates the interplay between geometry and dynamics in wave scattering and can be applied to a broad range of physical systems.

physics.optics↗

arXiv2025

Detection and characterization of targets in complex media using fingerprint matrices

When waves propagate through a complex medium, they undergo several scattering events. This phenomenon is detrimental to imaging, as it causes full blurring of the image. Here we describe a method for detecting, localizing and characterizing any scattering target embedded in a complex medium. We introduce a fingerprint operator that contains the specific signature of the target with respect to its environment. When applied to the recorded reflection matrix, it provides a likelihood index of the target state. This state can be the position of the target for localization purposes, its shape for characterization or any other parameter that influences its response. We demonstrate the versatility of our method by performing proof-of-concept ultrasound experiments on elastic spheres buried inside a strongly scattering granular suspension and on lesion markers, which are commonly used to monitor breast tumours, embedded in a foam mimicking soft tissue. Furthermore, we show how the fingerprint operator can be leveraged to characterize the complex medium itself by mapping the fibre architecture within muscle tissue. Our method is broadly applicable to different types of waves beyond ultrasound for which multi-element technology allows a reflection matrix to be measured.

physics.app-ph↗

arXiv2025

Fisher information flow in artificial neural networks

The estimation of continuous parameters from measured data plays a central role in many fields of physics. A key tool in understanding and improving such estimation processes is the concept of Fisher information, which quantifies how information about unknown parameters propagates through a physical system and determines the ultimate limits of precision. With Artificial Neural Networks (ANNs) gradually becoming an integral part of many measurement systems, it is essential to understand how they process and transmit parameter-relevant information internally. Here, we present a method to monitor the flow of Fisher information through an ANN performing a parameter estimation task, tracking it from the input to the output layer. We show that optimal estimation performance corresponds to the maximal transmission of Fisher information, and that training beyond this point results in information loss due to overfitting. This provides a model-free stopping criterion for network training-eliminating the need for a separate validation dataset. To demonstrate the practical relevance of our approach, we apply it to a network trained on data from an imaging experiment, highlighting its effectiveness in a realistic physical setting.

cs.LG↗

arXiv2025

Precision and cost of feedback cooling

We investigate the consequences of information exchange between a system and a measurement-feedback apparatus that cools the system below the environmental temperature. A quantitative relationship between entropy pumping and information acquired about the system is derived, showing that, independent of the concrete realization of the feedback, the latter exceeds the former by a positive amount of excess information flow. This excess information flow satisfies a trade-off relation with the precision of the feedback force, which places strong constraints on both the information-theoretic cost of feedback cooling and the required magnitude of the feedback force. From these constraints, a fundamental lower bound on the energetic cost of optical feedback cooling is derived. Finally, the results are demonstrated for feedback cooling by coherent light scattering. We show that measurement precision is the major factor determining the attainable temperature. Precise measurements can also be leveraged to reduce the required feedback force, leading to significantly more energy-efficient cooling close to the fundamental bound for realistic parameter values.

cond-mat.stat-mech↗