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Physics Physics

physics Watch Physics

Monday 5 October

Atmospheric and Oceanic Physics · Cross-listed from cs.LG

Skillful Data-Driven Subseasonal Soil Moisture Forecasting: Prospects and Limits for Flash Drought Prediction

Noelia Otero, Atahan Özer, Miguel-Ángel Fernández-Torres et al. (+1 more)

Despite substantial progress in short-to-medium-range weather forecasting, predicting high-impact events such as flash droughts remains a key challenge for both early warning operations and physically-based subseasonal-to-seasonal (S2S) prediction systems. Here we demonstrate that, for S2S soil-moisture forecasting over Europe, forecast skill depends as much on how the prediction problem is formulated as on the forecasting model itself. Using a Vision Transformer-based architecture with dual-pathway temporal and spatial attention, we show that residual learning is essential to outperform persistence. This advantage is realized only when forecasting root-zone soil moisture in physical units rather than standardized anomalies, revealing that the target representation itself constrains predictability. A probabilistic extension via quantile-head fine-tuning further provides well-calibrated predictive distributions. Benchmarked against deep-learning and operational ECMWF S2S baselines over 2021-2022, our model achieves the highest deterministic and probabilistic skill at all lead times and reliably detects anomalously dry root-zone states (below the 20th percentile). Yet flash drought onset, defined by multi-pentad intensification criteria, remains a fundamental challenge shared across all current S2S systems. These findings advance data-driven S2S soil-moisture forecasting while highlighting the remaining challenge of predicting rapid drought development.
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All authors Noelia Otero, Atahan Özer, Miguel-Ángel Fernández-Torres, Jackie Ma

Computational Physics · Cross-listed from cond-mat.soft

Spontaneous Motion Generates Reversible Nonreciprocity in an Achiral Active Elastic Ring

Wen-de Tian, Kang Chen and Tianhui Zhang

Nonreciprocal mechanical response is usually associated with built-in directional couplings, structural chirality, or external driving. Whether an achiral active body can instead generate and reverse such directionality through its own motion remains less clear. Here we show that spontaneous rotation makes an active elastic ring with reciprocal passive interactions mechanically nonreciprocal. Off-center elastic forces reorient propulsion, coupling deformation back to motion. Reversing the rotation reverses the antisymmetric response, while a compensating control preserves finite directionality but strongly suppresses the near-Hopf resonance. Approaching Hopf, the selected response grows inversely with distance to onset and becomes a self-sustained traveling deformation above threshold. Thus nonreciprocity is selected by the dynamical state, while active feedback controls its critical amplification.
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Applied Physics · Cross-listed from eess.SP

Goal-Oriented Wave Computing for Direction-of-Arrival Estimation

Mert Kalfa

Programmable metasurfaces—arrays of elements whose electromagnetic response can be tuned electronically—can process signals as a wave propagates through or reflects off them, a paradigm known as wave-domain analog computing. The design strategy is typically indirect: the surface is fitted to a prescribed linear operator, such as a discrete Fourier transform, and the result is read from output power measurements. Using direction-of-arrival (DoA) estimation as a canonical benchmark, we study what this indirection costs across analog computing topologies, namely reflective reconfigurable intelligent surfaces (RIS), beyond-diagonal RIS (BD-RIS), and stacked intelligent metasurfaces (SIM). We first derive Cramér–Rao bounds for the complete physical estimation chain, including one that accounts for output detectors measuring only intensity. We then formulate a goal-oriented alternative where the surfaces are trained directly through the deployed readout, with no operator target. We observe that goal-oriented training lets sparsely connected or shallower surfaces reach the task performance of their more complex operator-fitted counterparts—despite nearly identical Fisher information—and that the advantage persists under coarsely quantized (1–3 bit) control. From 16 to 64 elements, the depth saving persists, while the connectivity a beyond-diagonal RIS needs grows with the aperture. Under tuner and per-layer loss, the goal-oriented designs keep their advantage.
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Optics · Cross-listed from quant-ph

Energy-constrained two-way capacities of pure-loss and quantum-limited amplifier channels

Stefano Pirandola

We determine the two-way quantum, entanglement-distribution, private, and secret-key capacities of the pure-loss bosonic channel under an unconditional mean transmitted-photon-number constraint. For transmissivity $\eta$ and mean photon number $N$, all four capacities coincide and are given by $g(N)-g((1-\eta)N)$, where $g$ is the entropy of a thermal mode. This resolves the longstanding energy-constrained capacity problem by establishing the optimality of the reverse-coherent-information rate introduced in 2009. Our central tool is sector teleportation simulation, which we develop specifically to transfer the input-energy constraint to the shared entanglement resource, thereby preserving the constraint throughout converse arguments for general adaptive protocols. For every $N>0$, we further show that the corresponding strong-converse thresholds coincide with the unconstrained value $-\log_2(1-\eta)$. For parallel protocols with a hard total-photon-number cutoff, the same energy-constrained capacity formula instead satisfies an exponential strong converse. As an extension, we determine the energy-constrained two-way capacities of the quantum-limited amplifier, showing that classical assistance does not improve its unassisted quantum and private rates. Together, these results establish a unified finite-energy framework for adaptive bosonic communication and reveal how energy constraints fundamentally reshape both attainable rates and converse bounds.
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Data Analysis and Statistics · Cross-listed from hep-ex

How to scale your HEP ML models: A recipe for robust architecture comparisons at scale

Matthias Vigl, Nikita Pond, Jackson Barr et al. (+5 more)

Much of the recent progress in machine learning domains such as language models has come from scaling laws that predict performance as a function of training effort. In high-energy physics (HEP) similar behavior has now been observed. To aid further study, we present a systematic procedure to derive robust scaling laws and compare design choices on the relevant budget axes for HEP tasks. We first validate the full scaling trajectory on toy problems and then apply the procedure to multi-task transformers on the ~11 billion-jet ATLAS JetSet2 dataset, in both the compute- and data-constrained regimes. For the latter, we predict, to the best of our knowledge for the first time, the jointly optimal model size, training horizon, learning rate and batch size under early stopping. At compute-optimal scaling, we recover a near-equal $\sqrt{C}$ dependence of model and dataset size, and find that auxiliary objectives lower the primary jet-classification loss at equal compute budget. Expanding the inputs toward lower-level data systematically lowers the loss while leaving the scaling exponent nearly unchanged. The onset of the power-law regime is itself set by scale: below a threshold in dataset size the loss carries little information about high-compute scaling, underscoring the value of large, high-quality full-simulation datasets as a foundation for scaling studies and the development of foundation models in HEP.
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All authors Matthias Vigl, Nikita Pond, Jackson Barr, Alexander Froch, Dan Guest, Nicole Hartman, Michael Kagan, Lukas Heinrich

Optics

Novel functionalities enabled by asymmetric reconfiguration in VO2-dielectric metasurfaces

P. Hostalet-Vicent, L.M. Máñez-Espina, K. Schouteden et al. (+5 more)

Bianisotropic metasurfaces provide electromagnetic functionalities that cannot be achieved through electric and magnetic responses alone, enabling asymmetric scattering, directional absorption, and advanced wavefront control. At optical frequencies, however, most demonstrations rely on static nanostructures whose magnetoelectric response is fixed after fabrication. Here, we investigate the opportunities enabled by dynamic reconfiguration of electromagnetic bianisotropy using a thermally tunable silicon–VO$_2$ metasurface operating in the near-infrared. By combining a collective-polarizability framework with numerical simulations and experimental characterization, we show that the insulator-to-metal transition of VO$_2$ modifies not only the electric and magnetic responses of the metasurface, but also its magnetoelectric coupling, providing access to distinct bianisotropic states within a single platform. Temperature-dependent experimental characterization confirms the thermally driven evolution of the absorption asymmetry. Based on this framework, we identify different operational regimes and demonstrate how bianisotropy reconfiguration enables transmissive and reflective directional functionalities, including transmission power-limiting, power-dependent asymmetric reflection, and power-dependent absorptivity/emissivity. These results establish dynamic control of magnetoelectric coupling as a powerful degree of freedom for multifunctional and adaptive photonic metasurfaces for optical and thermal applications.
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All authors P. Hostalet-Vicent, L.M. Máñez-Espina, K. Schouteden, J. P. Locquet, M. Seo, T. Mengual-Chulia, P. Sanchis, A. Díaz-Rubio

Atomic Physics · Cross-listed from quant-ph

Out-of-control Hamiltonian Learning

Weiyuan Gong, Muzhou Ma, Sitan Chen et al. (+2 more)

Learning the Hamiltonian of a many-body system from its dynamics is a central task in quantum science, yet the algorithms with the strongest provable guarantees assume some level of quantum control–fast, arbitrary single-qubit gates interleaved with time evolution, and measurements in arbitrary bases–that is beyond the capabilities of near-term analog quantum simulators. Motivated by analog atom- and ion-based platforms, we study Hamiltonian learning under minimal access models. Uniform state preparation and measurements: We first consider the setting where in every experiment, one can rotate each qubit to the same state, perform short-time evolution, and measure every qubit in the same basis. Surprisingly, we show that for generic 2-local Hamiltonians on any interaction graph, all of the parameters can be reconstructed from such experiments. Computational basis state preparation and measurements: We then consider a similarly constrained setting, but where state preparation and measurement are restricted to the computational basis. For nearest-neighbor Hamiltonians with only Pauli $X/Z$ interactions, a class which captures contemporary Rydberg atom platforms, we show that over 1D and 2D rectangular lattices, all of the parameters can be reconstructed from such experiments up to unavoidable gauges. Our protocols introduce new techniques for solving structured polynomial systems over an extensive number of parameters. Taken together, our results suggest that one can learn a great deal from the dynamics of quantum many-body systems even under the most stringent experimental constraints.
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All authors Weiyuan Gong, Muzhou Ma, Sitan Chen, Jordan Cotler, Hsin-Yuan Huang

Computational Physics

Mechanism-resolved phase-field fracture of composite shells with a certified admissible constitutive operator

Altaf Ahmad Lone, R. T. Durai Prabhakaran, Timon Rabczuk et al. (+1 more)

Phase-field models of fracture in fibre-reinforced shells are usually formulated against a single closed-form stored energy, so that the stress, consistent tangent and plane-stress condensation are derived and verified for that expression alone. Here the constitutive law enters instead as a replaceable operator. The Green-Lagrange strain of a geometrically exact Reissner-Mindlin shell is pulled back through the Cholesky factor of the reference metric and resolved on four mutually orthogonal invariants with an exact closure identity, making the separation into fibre, inter-fibre and interaction channels a change of basis rather than a modelling assumption. On a curved midsurface, the metric, Cholesky factor and fibre direction vary through the thickness through the shifter I - zeta b, and the invariants inherit that dependence. The tension-compression split, channel degradation, plane-stress condensation, consistent tangent and finite-element assembly are all expressed in the channel potentials and their derivatives, allowing either closed-form or learned operators. The fibre-transverse interaction energy is stored once and degraded by the product of the fields whose mechanisms it couples. Six conditions define admissible operators; the two linear invariants are proved convex in the deformation gradient, the geometric tangent is shown independent of the material tangent, and the plane-stress condensation is regular wherever the residual stiffness is positive. A three-tier protocol certifies the algebra, solver state and admission of a state as training data. Numerical studies on notched IM7/8552 shells verify the discretisation in thin and curved regimes and show that fibre orientation governs the damage envelope and load capacity, while curvature drives a through-thickness fracture asymmetry that a single midsurface field cannot represent.
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All authors Altaf Ahmad Lone, R. T. Durai Prabhakaran, Timon Rabczuk, Xiaoying Zhuang

Applied Physics · Cross-listed from cond-mat.mtrl-sci

Structural, electronic, and magnetic properties of HoCo$_2$ thin films grown by pulsed laser deposition

Ajay Kumar, Christopher Burgio, Anis Biswas et al. (+1 more)

We report the growth and physical properties of HoCo$_2$ thin films on Si substrate by pulsed laser deposition. Well-defined Kiessig fringes in the x-ray reflectivity data yield a film thickness of $35 \pm 1$~nm and demonstrate smooth film surface and interface profiles. Atomic force microscopy confirms the high morphological quality of the films, revealing subnanometer surface roughness of $\sim$170~pm. Two independently prepared films exhibit a reproducible ferrimagnetic transition at $T_{\mathrm{C}}=77\pm3$~K, demonstrating retention of the characteristic magnetic ordering of bulk HoCo$_2$. However, the absence of resolvable thermal hysteresis indicates a predominantly continuous, second-order like transition in the films, in contrast to the first-order transition of bulk HoCo$_2$. At 5~K, the films exhibit clear magnetic hysteresis with a coercive field of $\sim$600~Oe, substantially larger than the negligible coercivity of bulk HoCo$_2$. X-ray photoelectron spectroscopy reveals comparable Ho $4d$ and Co $2p$ core-level line shapes and broadly similar valence-band spectra for the bulk and film, indicating preservation of the essential local electronic environment and occupied electronic structure.
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All authors Ajay Kumar, Christopher Burgio, Anis Biswas, Yaroslav Mudryk

Fluid Dynamics

Accurate contact discontinuity resolving Boltzmann schemes based on peculiar velocity

Balwinder Singh and S. V. Raghurama Rao

Starting from the robust foundation of the peculiar velocity based upwind (PVU) Boltzmann scheme, which fails to recognize contact discontinuities, this paper introduces new variants for the numerical solutions of equations of gas dynamics. A modified partial differential equation (MPDE) analysis of the original scheme reveals that the numerical diffusion coefficient corresponding to the peculiar velocity part does not vanish as the Mach number goes to zero. The numerical discretization of the peculiar velocity term is redesigned so that the numerical diffusion depends only on the bulk fluid velocity across the stationary contact discontinuities, enabling its exact capture in 1D and exact capture of slip surfaces in 2D. The resulting variants achieve low numerical diffusion while preserving the robustness of the original scheme, as demonstrated through several 1D and 2D benchmark compressible flow test cases.
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Computational Physics · Cross-listed from cs.DC

GPU-Initiated Discrete Simulated Bifurcation: Low-Latency Requests and Streaming Dense Couplings

Yaocheng Chen

GPU-based optimization faces two communication bottlenecks: coordinating frequent requests and delivering dense models that exceed device memory. We present a discrete simulated bifurcation (dSB) architecture that addresses both through NVIDIA DOCA GPUNetIO. For resident models, a persistent service receives field updates, executes each solve within one GPU thread block, and returns the result. Exact integer coupling sums, GPU work queues, and batched transmission keep the receive–solve–reply path on the device without a dedicated CPU data-path core. In comparisons with socket-based servers using the same solver, the largest latency gains occur under concurrent load. As the offered load increases from 400 to 800 thousand requests per second, median round-trip latency rises by only 6%. At the highest tested load, median and 99th-percentile latencies are 189 and 218~$\mu$s, compared with 288 and 609~$\mu$s for the tuned persistent CPU proxy across repeated runs. For models larger than device memory, a streaming solver retains dynamical state on the GPU and reuses incoming coupling tiles across replicas. It evaluates ten-million-variable dense binary matrices at approximately 307~Gb/s, consuming a 12.5-TB logical matrix through a 64-MiB packet buffer. Ground-state recovery on planted instances and agreement with reference executions verify the computation. Together, the two modes scale dSB to concurrent requests and dense models beyond GPU memory.
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Optics

Heterodyne-Enhanced Sensing of Cavity Mode Spacing

Aaron D. Spector and Laura Roberts

Characterizing the frequencies of resonant modes is critical to a wide variety of experimental applications. In this work we demonstrate a heterodyne-enhanced sensing scheme capable of precisely measuring the frequency spacing between longitudinal modes of a 19 m optical cavity. The technique utilizes two resonant lasers to sense the differential phase accumulated between them as they propagate through the cavity by measuring their interference beatnote before their injection to the cavity and in transmission of it. With these measurements we were able to mitigate the effects of noise in our control loops in post-processing by up to a factor of 30 in the frequency range from 1 mHz to 100 Hz while suppressing injected calibration signals by over two orders of magnitude.
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Optics

Experimental Demonstration of Autonomous Class Aware Optimization via Digital Twin Aided Optical Network Agent

Marc Lawson, Ambashri Purkayastha, Camille Delezoide et al. (+3 more)

We demonstrate on a network testbed a Digital Twin-aided multi-agent LLM for autonomous class-aware optical network management. The system detects soft failures and autonomously adapts modulation format per service class requirements, ensuring continuous, marginless channel quality with minimal human intervention. {\copyright}2026 The
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All authors Marc Lawson, Ambashri Purkayastha, Camille Delezoide, Andrea Sgambelluri, Nicola Sambo, Patricia Layec

Chemical Physics

Optimizing the Optimizer: Language Models Discover Faster Molecular Relaxation Algorithms

Artem Tsypin, Vladimir Deshchenya, Kuzma Khrabrov et al. (+4 more)

Geometry optimization is a major cost in many quantum-chemical workflows: each optimization step requires one force evaluation, and at the density-functional level that evaluation dominates the wall time. Research in this area has produced a broad range of optimization methods, and we ask whether a language model can improve on the best of them through autoresearch. An agent rewrites the optimizer itself to minimize force-call counts, restrained by two admission gates that reject premature stopping and improvements that do not generalize to unseen molecules. Starting from Sella, the fastest open-source optimizer available, the search produces AutoSella, a family of two optimizers. Both of them deliver consistent force-call reductions relative to Sella across held-out molecular benchmarks and potentials not used during the search. Most notably, at the r2SCAN-3c DFT level, the best variant requires only 40.2-77.2% of Sella's force calls while achieving the same energy reduction, even though agent used no DFT gradients.
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All authors Artem Tsypin, Vladimir Deshchenya, Kuzma Khrabrov, Denis Potapov, Maxim Radchenko, Artur Kadurin, Michael G. Medvedev

Optics

Digital confocal imaging in holographic optical coherence tomography at high space-bandwidth product

N. P. Klooster, S. A. Thakur, F. Zeisberger et al. (+4 more)

Full-field Fourier-domain optical coherence tomography (FF-FD-OCT) enables rapid volumetric imaging but lacks the confocal gating inherent to point-scanning OCT. By acquiring volumes under multiple oblique illuminations, a digital confocal volume can be synthesized in post-processing, a technique originally developed within the framework of reflection matrix imaging (RMI). Existing implementations, however, are often time-domain, off-axis, or use a Michelson interferometer, which reduces sensitivity, speed and the usable space-bandwidth product (SBP). Here, we present an oblique-illumination FF-FD-OCT system in a Mach-Zehnder configuration. This setup enables optimal use of SBP, providing up to a fourfold increase in effective acquisition throughput. In addition, we pose the scattering and aberrations as an overdetermined system, allowing a closed-form solution of an alternating least squares problem. The resulting iterative optimization is equivalent to the state-of-the-art closed-loop accumulation of single scattering (CLASS) algorithm. We demonstrate the setup and reconstruction method in lens tissue, a PSF phantom, a USAF target, and a model eye. The resulting digital confocal volumes provide locally optimized focusing and detection quality, and show improved resolution and signal-to-noise ratio (SNR) compared to conventional FF-FD-OCT. The technique combines the advantages of FF-FD-OCT and confocal OCT while mitigating the limitations of both approaches.
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All authors N. P. Klooster, S. A. Thakur, F. Zeisberger, J. R. Hagen, S. Eggert, U. Najar, D. Hillmann

Classical Physics · Cross-listed from gr-qc

Cherenkov Bursts are Memory Steps

Jann Zosso, Guangzi Xu and Vitor Cardoso

We show that the familiar Cherenkov burst from a finite superluminal charge track can be understood as a pair of interfering electromagnetic memory transitions. Endpoint accelerations of the track produce opposite memory steps in the transverse vector potential, such that the corresponding finite-frequency radiation is universally fixed, below the endpoint-acceleration-scale, by the memory amplitude and the time for which the temporary memory offset persists. As the Cherenkov direction is approached, the memory amplitudes grow while their separation in observer time shrinks, until the two vector-potential steps overlap and their divergent contributions cancel into a finite Cherenkov burst. This perspective opens the door to using the Askaryan effect as a setting in which individual macroscopic memory transitions may be resolved, providing a new experimental arena for memory-linked infrared physics.
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Atmospheric and Oceanic Physics · Cross-listed from cs.LG

Xaurora: Generative Weather Forecasting with Denoising Stochastic Interpolants from a Foundation Model Prior

Eliot Walt, Miltiadis Kofinas, Nikolaj Mücke et al. (+2 more)

Deep learning has revolutionised weather forecasting in recent years, especially through atmospheric foundation models, which offer competitive skill for a fraction of the computational costs of classic physics-based models. However, most existing foundation models are deterministic, limiting the generation of large ensembles for accurate uncertainty quantification, extreme weather risk assessment, and long-range weather forecasting. Furthermore, these models incur a large, often prohibitive, computational overhead to train from scratch. To address these shortcomings, we turn a pretrained deterministic prior model, namely the Aurora foundation model, into a generative ensemble-prediction model. To that end, we introduce a novel generative method, Denoising Stochastic Interpolants, combined with a replay buffer for Stochastic Differential Equation (SDE) rollout, enabling probabilistic training of SDE trajectories. Our stochastic foundation model, Xaurora, is finetuned from the small Aurora version, yet it approaches the state-of-the-art on global ensemble metrics and is competitive with the large version of Aurora. Our method is parameter and sample efficient, and generates skilful 15-day forecasts in 13 minutes. Our results demonstrate that deterministic foundation models can be efficiently extended into even stronger stochastic models.
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All authors Eliot Walt, Miltiadis Kofinas, Nikolaj Mücke, Efstratios Gavves, Dim Coumou

Instrumentation and Detectors

Polyurethane Scintillators for High-Energy Physics Detectors: Optimization, Scale-up, and Radiation Response

Rafael Angel Narcio Laveaga, Ildefonso León Monzón, Juan Carlos Cabanillas Noris et al. (+8 more)

Plastic scintillators remain attractive for large-area particle detectors because of their fast response, mechanical flexibility, and relatively simple fabrication. Motivated by future applications including the ALICE~3 Muon Identifier (MID) and the upgrade of the ALICE Fast Interaction Trigger (FIT), this work investigates polyurethane (PU)-based plastic scintillators doped with 2,5-diphenyloxazole (PPO) and 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP). Systematic information on the optimization and radiation response of this material system remains limited. A series of compositions was therefore fabricated and characterized using minimum-ionizing particles (MIP), optical spectroscopy, and time-correlated single-photon counting. The measurements show that the scintillation response is governed primarily by the PPO concentration, while POPOP mainly modifies the spectral response, and fast effective decay times of approximately 1.6–1.75~ns are maintained across the doped samples. The best-performing formulation was subsequently scaled to 20-$cm$ and 1-m scintillator bars and successfully operated with a single embedded wavelength-shifting fiber and SiPM readout, demonstrating its suitability for meter-scale light collection. Proton-irradiation measurements at the CERN IRRAD facility further revealed systematic degradation of the optical and scintillation responses with increasing exposure, together with substantial recovery after approximately two months. These results demonstrate that optimized PU–PPO–POPOP scintillators can combine fast timing, high light output, scalable detector geometry, and recoverable radiation response, supporting their further development for future large-area particle-detector systems.
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All authors Rafael Angel Narcio Laveaga, Ildefonso León Monzón, Juan Carlos Cabanillas Noris, Thanh Luan Nguyen, Jan Král, Vítezslav Jarýd, Gerardo Herrera Corral, Vojtech Zabloudil, Etiennette Auffray, Carlos Duarte Galván, Solangel Rojas Torres

Biological Physics · Cross-listed from cond-mat.soft

Self-assembly Monte Carlo reveals localized entanglement in giant polymer melts

Enrico Fornasa, Francesco Slongo and Cristian Micheletti

Topological entanglements are central to understanding and predicting the properties of polymer melts. Yet, they make equilibrium sampling computationally challenging, as decorrelation times grow rapidly with chain length. Here, we introduce a Monte Carlo scheme that bypasses typical computational bottlenecks by working in a self-assembly ensemble rather than at fixed composition. Strictly local moves efficiently propagate backbone reconnections across scales while conserving the number of linear chains, achieving near-linear scaling of decorrelation time with system size, $\tau_{\rm eq}\sim V^{1.0}$. With this method, formulated for a fully-packed lattice, we equilibrate periodic systems totalling up to $\simeq 1.1\times 10^9$ monomers, accessing a universal melt regime insensitive to lattice details. We analyze intra- and inter-chain entanglements for chains of up to $N\simeq 5 \times 10^5$ monomers, revealing that they manifest as localized knots and links rather than as global tangles. Finally, we show that the magnitude of the Gauss linking integral between neighbouring chains grows only as $N^{1/4}$.
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Optics

Continuous-wave 148.4-nm generation in top-seeded solution-grown strontium tetraborate

Yanzhang Wu, Peng Yang, Lingfeng Yan et al. (+6 more)

We report an implementation of continuous-wave (CW) vacuum-ultraviolet (VUV) generation at 148.4 nm by single-pass second-harmonic generation of 296.8 nm light in strontium tetraborate (SBO). At an incident fundamental power of approximately 580~mW, we estimate a VUV output power of approximately 1.7 nW at the crystal exit. The crystal is grown by the top-seeded solution growth method and operated in a helium-filled chamber. The quadratic power dependence observed under vacuum, together with its strong suppression in air, supports the identification of the generated second harmonic. We further characterize the dependence of the VUV signal on beam position, incidence angle, polarization, and crystal temperature, providing practical guidance for SBO-based CW VUV sources for nuclear spectroscopy and clock applications.
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All authors Yanzhang Wu, Peng Yang, Lingfeng Yan, Qi Xiao, Jiahong Li, Juxian Li, Beichen Huang, Zhiwei Jiao, Shiqian Ding

Fluid Dynamics

Direct Numerical Simulation of Transonic Flows Induced Pitching of NACA Airfoil

Chandan Kumar Bhardwaj and Swagata Bhaumik

A Direct Numerical Simulation (DNS) investigates transonic flow over a NACA0012 airfoil coupled with single-DOF structural dynamics. Analysis is performed for angle of attack ({\alpha}) 8{\deg} with varying reduced velocity (U*) from 2 to 8, reduced mass (m*) 5.0, Reynolds number (Re_\infty) of 3 \times 10^6, and Mach number (M_\infty) 0.8. The speed of sound in air is calculated as a = sqrt({\gamma}RT_\infty). It is noted that the magnitude of the aerodynamic loads and moments increased with higher reduced velocity. The Fast Fourier Transform (FFT) of the moment coefficient (C_M) in the fixed-airfoil condition is carried out to extract the non-dimensional dominant frequency, referred to as the Strouhal number (St), along with the coupled oscillation frequency of the airfoil (Stc). The FFT analysis for the stationary case at {\alpha} = 8{\deg} yields a value of St = 1.3161. The presence of high-frequency components indicates the onset of instability associated with shock-boundary layer interaction. The computed values of the coupled oscillation frequency (Stc ) are 0.0762 and 0.1716 for the cases with angle of attack (AoA) 8{\deg} and reduced velocities U* = 2.0 and 8.0, respectively. The corresponding natural aeroelastic frequencies (Stn) are taken as 1/U*, yielding values of 0.5 and 0.125. It is observed that for U* = 2, the coupled frequency (Stc) aligns with the subharmonic of shock-buffet frequency, indicating flow-dominated dynamics. In contrast, for U* = 8, Stc coincides with the natural pitching frequency, indicating a dominant influence of the structural mode. Pressure fluctuation contours are examined to analyze the characteristics of acoustic radiation. Vorticity and schlieren visualizations are employed to identify the nature of vortex structures, the strength of the shock, and its spatial location, respectively.
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Accelerator Physics

Construction of Approximate Invariants Across Nonlinear Resonances

Yongjun Li, Derong Xu, Yue Hao et al. (+1 more)

Our previous work [Y.~Li, D.~Xu, and Y.~Hao, Phys. Rev. Accel. Beams 28, 074001 (2025)] introduced a square-matrix method for constructing approximate invariants recursively, order by order, from the one-turn map of non-integrable Hamiltonian systems. At fourth and higher even orders, the recursive construction is intrinsically non-unique because rotationally invariant polynomials generate a null space in the corresponding homological equations. In the resonant regime, this ambiguity can lead to an incorrect description of the resonance-island topology and is therefore particularly relevant to accelerator applications that rely on stable resonance islands. In this work, we select the null-space contributions by explicitly incorporating the null-space basis vectors and determining their coefficients through an averaging procedure over a prescribed region of interest. The method is demonstrated using the Kobayashi map near a third-order resonance, extending the square-matrix construction to resonant Hamiltonian dynamics.
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All authors Yongjun Li, Derong Xu, Yue Hao, Sergei Nagaitsev

Medical Physics

A software tool for parametric collimator creation for linac-based preclinical irradiation studies

Joseph B. Schulz, Dixin Chen, Lloyd Emmanuel Kamole Ghomsi et al. (+5 more)

Designing custom Cerrobend collimators for linac-based preclinical electron irradiation is labor intensive and susceptible to design inconsistencies. We developed Collimator Creator (CC), an open source parametric workflow that converts aperture dimensions and jig offsets into printable STL models for a collimator mold, collimator shell, Cerrobend holder, and beam degrader. Two designs were fabricated and characterized: a 4.0 x 1.5 cm$^2$ bilateral lung collimator centered on the central axis and a 1.5 x 1.5 cm$^2$ unilateral lung collimator with a 7.5 mm jig offset, each with a 15 mm thick Cerrobend collimator and a 10 mm PLA degrader. Kilovoltage planar images were used to assess aperture geometry and casting uniformity. Radiochromic film was used to measure full width at half maximum (FWHM) and 80%-20% penumbra under conventional and ultra high dose rate (UHDR) electron delivery on a Varian TrueBeam, and off axis leakage was measured with an ionization chamber at conventional dose rate. The cast Cerrobend apertures deviated by less than 0.2 mm from their specified designs. FWHM values were within 3% of the nominal field dimensions, and penumbra ranged from 0.73 to 1.05 mm under conventional and UHDR delivery. Crossline leakage 2 cm beyond the collimator edge was below 2% for both collimators at conventional dose rate. Parametric collimator generation standardized digital design inputs for the two evaluated rectangular linac-based electron fields. Further work will assess specimen-level dosimetry, additional aperture geometries, and Monte Carlo integration.
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All authors Joseph B. Schulz, Dixin Chen, Lloyd Emmanuel Kamole Ghomsi, M. Ramish Ashraf, Lawrie Skinner, Edward Elliot Graves, Billy W. Loo Jr, Stavros Melemenidis

Accelerator Physics

Effect of transition layer on wakefield in plasma waveguide

Peter Markov, Kostyantyn Galaydych and Gennadiy Sotnikov

Using 2.5D particle-in-cell simulations, we investigate the parameters of a plasma wakefield accelerator (PWFA) for positrons and electrons in a layered plasma waveguide (LPW) in the presence of a transition layer (TL) between two layers of uniform plasma with different densities. The layered plasma is modeled as a combination of a near-wall tubular plasma of higher density and an on-axis plasma column of substantially lower density, separated by a TL in which the plasma density varies smoothly from the lower to the higher value according to a specified law. It is shown that using the PWFA parameters determined for a layered plasma waveguide without the transition layer changes the acceleration conditions when the TL is present, and that this change depends on the TL thickness. It is demonstrated that when the TL thickness exceeds the skin-layer depth in the tubular plasma, the positron acceleration rate decreases substantially. The methods are proposed for correcting the parameters of the plasma waveguide under consideration that compensate for the decrease in the energy gain of the accelerated bunches and ensure their stable transport.
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Chemical Physics

Spinorial Weyl Operator Reduction of Dirac (SWORD): Exact Relativistic Quantum Chemistry with Half the Spinor

Jacopo Masotti, Quentin Pitteloud and Luca Frediani

We present an exact self-consistent two-component reformulation of the Dirac equation in the Weyl representation. By exploiting the diagonal structure of the kinetic operator and the purely scalar mass coupling in the Weyl basis, the four-component Dirac problem is reduced to an exact two-component form without introducing inverse-potential operators that arise in conventional energy-decoupling approaches. The resulting second-order differential equations are cast into an integral formulation and solved iteratively via convolution with the Helmholtz Green's function. We implement this formalism within an adaptive multiwavelet basis framework, providing rigorous, user-defined error control. Proof-of-concept calculations for simple atomic systems demonstrate the numerical stability, accuracy, and efficiency of the proposed algorithm.
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Optics

Symmetry Control in Coupled Quantum Cascade Ring Lasers

DOMINIK SCHOCK, ANNA INVERNICI, MICHAEL JAIDL et al. (+5 more)

Optical frequency combs with fast tunability are essential for integrated spectroscopy and sensing. However, achieving such functionality in the terahertz regime presents a significant challenge. This work introduces a novel approach that exploits supermode symmetry control in terahertz lasers. By electrically manipulating the gain ratio between coupled concentric ring lasers, we selectively excite symmetric and antisymmetric supermodes, allowing frequency tuning and spectral modulation across different frequency regimes and operation modes. Moreover, simultaneous lasing of both supermodes results in a reduced total integrated dispersion, widening the comb locking range. Supported by matrix formalism and numerical simulations, this work demonstrates the potential of symmetry control for enhanced electro-optic modulation, opening new avenues for on-chip spectroscopy and sensing applications.
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All authors DOMINIK SCHOCK, ANNA INVERNICI, MICHAEL JAIDL, MIRIAM GIPARAKIS, SASCHA PREU, AARON M. ANDREWS, JURAJ DARMO, KARL UNTERRAINER

Instrumentation and Detectors · Cross-listed from hep-ex

FPGA-based muon shower identification and graph neural network tracking algorithms for HL-LHC triggers

Daniel Estrada, Elena Aller, Andrea Cardini et al. (+3 more)

This work presents two hardware-accelerated strategies for the upgrade of the CMS Level-1 Trigger (L1T) to target efficiency losses due to highly energetic radiating muons and unconventional physics signatures, such as Long-Lived Particles (LLPs). Operating directly on low-level detector hits, the first strategy implements a dedicated muon shower identification algorithm in the CMS barrel region that monitors hit multiplicities in the Drift Tube chambers to generate Shower Primitives. Executing with sub-bunch-crossing latency and ultra-low resource consumption, this algorithm successfully tags showered muons and provides essential context for downstream track finders. The second approach targets the barrel-endcap transition (overlap) region by evaluating Graph Neural Networks to reconstruct displaced muon tracks. Following a divided methodology that first isolates hardware feasibility from evolving physics refinements, an initial proxy model based on a GraphSAGE architecture is evaluated to validate its viability on FPGAs within strict latency limits. This implementation utilizes an INT8-PO2 quantization technique coupled with a data-driven bit-width optimization. By replacing DSP-heavy fixed-point multiplications with fast compile-time arithmetic bit-shifts and allocating the minimum required bits per signal, this hardware-software co-design reduces DSP utilization to $20\%$ and achieves a deterministic inference latency of just $19$ clock cycles. Together, these developments pave the way for triggering on non-standard muon signatures, such as those from LLP decays, at L1T.
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All authors Daniel Estrada, Elena Aller, Andrea Cardini, Santiago Folgueras, Pelayo Leguina, Javier Prado

Physics Education

Damping by design in a bilateral electromechanical transducer

Angelo Maria Sabatini

In a bilateral electromechanical transducer, energy exchange across the electromechanical interface allows the electrical termination to modify the mechanical response through electromagnetic feedback. The paper develops this idea from a simple magnetic pendulum, describing the electromechanical coupling through the flux linkage as a function of rotation angle. Torque and back electromotive force then follow from the same coupling function, making the bilateral nature of the energy conversion explicit. A radial-field moving-coil system provides a particularly simple realization in which the coupling becomes independent of angular position and the resulting electromagnetic damping is controlled by the electrical termination. A two-port representation then identifies this damping as an impedance-reflection effect and provides a quantitative criterion for separating the electrical and mechanical time scales. A numerical example compares ideal current drive with critical electrical termination and confirms the reduced second-order description against the full third-order dynamics. The resulting treatment provides a systematic and physically transparent route from familiar undergraduate electromagnetism and oscillator dynamics to electromechanical feedback, bilateral transduction, and the effect of loading in coupled physical systems.
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Optics

Extraordinary Optical Spin Hall Effect in 2D Exciton Emission in Open Cavities

Kunal Banerjee, Ben Johns, Nitin Yadav et al. (+2 more)

Open cavities are emerging as a promising platform for studying light-matter interactions. A lossy open cavity couples to an atomic monolayer, resulting in parabolic dispersive emission at room temperature. Here, light-matter interaction is weak, and the dispersion arises from a phenomenon known as dark strong coupling. The bright 2D exciton rapidly decays through the cavity mode, leaving behind spin-dark excitons of the 2D lattice, thereby undergoing symmetry breaking. Further, mapping the polarisation dependence in the k-space reveals spin-orbit interaction in the coupled system. A synthetic spin-orbit Hamiltonian is constructed to extract the effective contributions of Rashba- and Dresselhaus-type interactions in the open cavity. The system shows a clear evolution of the spin-orbit effect at room temperature. Here, emission is polarisation-sensitive, yielding high degrees of linear and circular polarisation. This is one of the simplest configurations for studying photonic spin-orbit coupling, in which an atomic monolayer provides Rashba and Dresselhaus coupling constants of 24 eV Å, and ~10^5 eV Å^2, respectively. Therefore, an open cavity platform is promising for polarisation-sensitive measurements and device applications.
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All authors Kunal Banerjee, Ben Johns, Nitin Yadav, Anil Shaji, Jino George

Chemical Physics · Cross-listed from cond-mat.mtrl-sci

First-Principles Investigation of Multimodal Toxic Gas Sensing in Carbon-Tuned hBN-Graphene Alloys: Chemiresistive, Work-Function, and Optical Responses

Tanjuma Shikder Jhumu and Ahmed Zubair

Compact, reliable, and cost-effective gas sensors have become a highly demanding subject for safety management in the medical sector, chemical manufacturing, food quality monitoring, agriculture, and industrial safety. Hazardous gas emissions need to be controlled and monitored with fast-responsive and highly sensitive sensing devices. A first-principles study employing density functional theory (DFT) was used to investigate the adsorption behavior of Cl2, CO, CO2, NO, NO2, and HCN gas molecules with our proposed alloys, which consisted of hexagonal boron nitride (hBN) and graphene (Gr). The alloy consisting of 22% carbon (BNGr-2) was found to be most competent for sensing Cl2, CO, CO2, and HCN with sufficient adsorption energy, charge transfer, and bandgap alteration. However, NO and NO2 gas molecules showed more engagement with 33% carbon-proportioned alloy (BNGr-3) in terms of adequate gas sensing properties. NOx gases exhibited the most chemiresistive sensitivity towards the adsorbents. Other gases also showed significant chemiresistive sensitivity and distinct selectivity ratios, which would facilitate these alloys as chemiresistive sensors. Besides, noticeable work function variation (~20%) of these systems manifested potential as work function based sensors. Cl2 and NO2 showed strong physical adsorption, while the rest of the gases were weakly to moderately physisorbed, resulting in very short recovery times (10-1 ~ 10-6 seconds). Additionally, the distinctive absorption spectra observed for the gas analyte systems highlighted the potential of the proposed alloys as optical gas sensors. Temperature variation revealed that all gas molecules can be freed from the adsorbent BNGr-2 at 425 K. These findings imply hBN-Gr alloys as promising gas sensors for pollution auditing.
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