Plasma Physics · Cross-listed from astro-ph.HE
1D PIC Simulations of Resonant Scattering-Driven Pair Cascades in Magnetar Magnetospheres
Jens F. Mahlmann, Alexander A. Philippov, Adrien Soudais et al. (+2 more)
Hard X-rays in persistent magnetar emission originate from radiative processes in pair-loaded magnetospheric
plasma, yet self-consistent kinetic simulations of such radiation-rich systems remain limited. We perform 1D particle-in-
cell simulations of pair creation mediated by resonant inverse Compton scattering (RICS) along isolated field lines in twisted magnetospheres, capturing particle acceleration and radiative drag from first principles. The drag force, which strongly depends on the particle Lorentz factor and location, plays a key role in the magnetospheric
circuit. Strong RICS drag impedes the flow of electrons and positrons, particularly along extended field lines. Nevertheless, the
plasma sustains the
circuit by self-organizing into a single accelerating gap near the
star in one hemisphere. The accelerated
plasma flow from the gap becomes loaded with copious electron-positron pairs in extended RICS zones, and also carries ions extracted from the
star and accelerated in the gap. When radiative drag stops the pair-loaded flow at the
magnetic equator, the ion component transfers momentum through streaming instabilities, delivering a small lepton population into the opposite hemisphere and sustaining the
circuit. The gap confined to a single (anode) hemisphere implies asymmetric hard X-ray production, possibly detectable in phase-resolved magnetar spectra.
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All authors Jens F. Mahlmann, Alexander A. Philippov, Adrien Soudais, Andrei M. Beloborodov, Lorenzo Sironi
Fluid Dynamics
Self-similar swirl between contracting porous walls: the GD1998 exact Navier-Stokes solution revisited in the similarity variables of the OpenAI 2026 forced blow-up construction
Ramani Duraiswami
The finite-time blow-up construction for the forced Navier-Stokes equations released by OpenAI in Sep. 2026 (OpenAI 2026) has as its object an axisymmetric swirl core in anisotropic similarity variables. Its geometry is that of an exact steady swirl between rotating porous cylinders found by Gumerov and Duraiswami in 1998 (GD1998). We find that the GD1998 boundary value problem does not recast into the similarity variables and that its generalization is a two-dimensional profile problem between porous walls held at fixed similarity radii, second order in the radial variable X and first order in the time-like axial variable $\eta$. We solve it by tensor Chebyshev collocation with a pinned pressure gauge, complex-step Newton and arclength continuation, verify it three ways, and sweep the radial Reynolds number, the wall swirl and a
symmetry-breaking datum. Below inflow 9 the solution is one smooth branch; above it the symmetric branch folds at a critical swirl $F^*=0.41V_0^2$. The Dirichlet axis problem has no resolution-stable solution; the core must be a Cauchy problem from the axis. Its moment identities cannot be met by a core symmetric about the dividing plane: the core is an axial through-flow, as its authors chose it; a non-symmetric core meets them to 0.2%. The cone condition is Rayleigh's criterion with axial shear and requires radii of order $10^{20}$. Dynamic rescaling shows the profile to be an attractor at weak inflow and toward the fold, a saddle-node; at moderate inflow and strong swirl the spectrum does not converge in the axial resolution and is left open. A real fluid cavitates (water) or shocks (air) first; the GD1998 chamber is proposed as the experiment. Nothing found suggests the mechanism is reachable in a flow one computes or builds; the forced
theorem, and this study, leave the unforced equations of practice as they were. Code, tests and log accompany the paper.
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Computational Physics · Cross-listed from cs.LG
Lecture notes on Physics Informed Neural Networks, Neural Operators, and their applications
Alessandro Bombini
This is the set of lecture notes for the PhD course \href{https://www.unibz.it/en/faculties/engineering/phd-computer-science/study-course-offering/2025/36967}{Physics Informed Neural Network, held at the University of Bozen/Bolzano} in the academic year 2025/2026. The goal of the course was to introduce the concept of Physics Informed Deep Neural Networks (PINN) and Neural Operators (NOs), discuss their implementation from scratch in PyTorch and using advanced ad-hoc developed open-source libraries such as NVIDia PhysicsNeMo to address real-world problems in various fields (engineering, physics, petroleum reservoir). We discuss recent topics such as Mixture-of-Models,
Fourier Neural Operators, Physics-Informed Kolmogorov-Arnold Networks (PIKANs) and
Fourier Neural Operators.
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Fluid Dynamics
Low order Carleman simulation of steady state flows at mild Reynolds numbers
Luca Cappelli, Massimo Berneschi and Sauro Succi
It is shown that the second-order Carleman truncation (C2) captures the steady state of the decaying forced logistic with second-order accuracy in the forcing parameter, provided such forcing remains below a critical value of 1/2. We then investigate the extension of these results to spatially extended systems by means of Carleman
Lattice Boltzmann simulations of a two-dimensional periodically forced flow. It is found that C2 correctly reproduces the low-Reynolds-number (between 1 and 10) quasi-linear dynamics, well above the critical value of the forced logistic equation. However, C2 fails to capture more complex steady states characterised by persistent vortical structures at Reynolds numbers around 20. Interestingly, the divergence observed in the latter case shows qualitative similarities with the runaway behaviour of the C2 linearization of the growing logistics, suggesting that suitable closures and/or change of variables may improve the convergence of the C2 procedure.
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Physics and Society
Modelling sexual partnership dynamics and population heterogeneities in agent-based dynamic network models
Priyanka Nair-Turkich, Patricia T. Campbell and Nicholas Geard
Population-level heterogeneities, combined with temporal fluctuations in sexual partnerships, shape the structure of sexual contact networks and can substantially influence the spread of sexually transmitted infections (STIs). Traditional static network models, which assume fixed attributes of partnerships, such as count and duration, may not adequately capture the effects of partnerships on STI transmission. In contrast,
agent-based dynamic network models offer a flexible framework for incorporating individual and population-level heterogeneities. We developed an
agent-based dynamic network model in which partnership formation and dissolution probabilities, stratified by age, sex, and sexual orientation (including bisexual individuals), govern the formation of monogamous and concurrent partnerships and their dissolution via a duration-dependent hazard. Partnership statistics from the National Survey of Sexual Attitudes and Lifestyles (NATSAL-3) were used as model calibration targets, and Latin Hypercube Sampling (LHS) was used to generate candidate parameter combinations. Parameter estimation was performed by selecting the combination that produced the lowest Mean Squared Error (MSE) between the model outputs and the calibration targets. Our study addresses three questions: (1) how well can the observed characteristics of sexual partnerships in NATSAL-3 be reproduced using an
agent-based model; (2) how does concurrency shape the structure of dynamic sexual contact networks; and (3) how do concurrent partnerships affect the dynamics of STI transmission. In this study, we find that interactions between individual characteristics such as age, sex, and sexual orientation, and partnership attributes such as count, duration, and concurrency play a critical role in shaping the population-level sexual contact network and, in turn, the dynamics of STI transmission.
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Optics · Cross-listed from cond-mat.mtrl-sci
Enhanced thermal stability of SiGeSn by suppressing surface-mediated degradation
Anis Attiaoui, Sebastian Koelling, Lu Luo et al. (+2 more)
$\text{SiGeSn}$
alloys are promising silicon-compatible
semiconductors for monolithic infrared photonics. However, their metastable nature limits the thermal budgets available for post-growth device processing, and the mechanisms governing their thermal degradation remain unresolved. Here, we investigate the thermal stability of $\text{Si}_{0.08}\text{Ge}_{0.83}\text{Sn}_{0.04}$
alloys using in situ spectroscopic ellipsometry (SE) during isothermal annealing at 550 {\deg}C. We show that adding an ultrathin oxide cap kinetically suppresses Sn exchange with the free surface while leaving bulk
diffusion pathways largely unaffected. Uncapped films undergo phase separation after 50 min, accompanied by void formation, a 60% thickness reduction, and a 400 meV blueshift of the $E_{2}$ critical point (CP) transition, consistent with substitutional Sn depletion from the probed volume through surface segregation. In contrast, oxide-capped films exhibit a small compositional change (<1 at.% Sn) and optical shift (<20 meV) over the same period, with suppressed void formation, strain relaxation, and
alloy decomposition. This surface-kinetic control additionally yields a 25-fold reduction in contact resistivity relative to annealed uncapped
alloys. These results identify surface Sn transport as the dominant degradation pathway in SiGeSn and demonstrate that an ultrathin oxide cap extends the thermal stability of metastable group-IV
alloys, providing a practical route toward their integration into advanced silicon photonic and electronic platforms.
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All authors Anis Attiaoui, Sebastian Koelling, Lu Luo, Simone Assali, Oussama Moutanabbir
Optics
Hysteresis and trap emission in dc-biased integrated lithium niobate electro-optic modulators
Matthew Yeh, CJ Xin, Donald Witt et al. (+3 more)
The electro-optic effect is crucially important for low power and efficient tuning of integrated photonic
circuits. However, in electro-optic materials such as lithium niobate, dc biasing for an extended duration of time results in the emergence of numerous nonidealities, including hysteresis – a persistent degradation of the magnitude and linearity of the dc electro-optic response. We show that electro-optic hysteresis can be reversed under both zero-bias and reverse-bias conditions, given sufficient time or reverse voltage and consistent with a defect model of the underlying physics. Specifically, we find that drift phenomena at short time scales can be explained by charge trapping dynamics near the contact junction, and thereby devise an active reset protocol that restores the magnitude of the response and partially restores the drift time scales.
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All authors Matthew Yeh, CJ Xin, Donald Witt, David R. Barton, Evelyn L. Hu, Marko Lončar
Optics
Stitch-Free, Diamond-Scribed Silicon Nitride Photonic Integrated Circuits for the Visible Band
Kishor Kumar Mandal, Lekshmi Eswaramoorthy, Parul Sharma et al. (+5 more)
Silicon nitride photonic integrated
circuits for the visible band are conventionally built with a buried oxide overcladding and singulated with wafer-scale tooling, constraints that preclude evanescent access to the guided mode for externally integrated emitters. We report a PECVD grown $Si_3N_4$ platform designed around an air-clad waveguide whose evanescent field remains accessible along the full device length. Two process elements make this geometry practical at chip scale. Fixed-beam moving-stage electron-beam lithography writes 500 nm single-mode waveguides as one continuous exposure across the 5 mm chip, removing write-field stitching which, given the $\sigma^{2}/d^{4}$ scaling of sidewall scattering in this high-confinement geometry at 635 nm, would otherwise dominate the loss budget. Chip singulation is performed by pen-type diamond scribing along lithographically patterned markers registered to in-plane direction, cleaving the Si(100) substrate to yield end-facets within $2^\circ$ of normal at $80 \%$ yield. Structural characterization by scanning electron microscopy confirms stitch-free waveguide geometry and undamaged, near-vertical scribed facets; light is coupled end-fire into fabricated devices and guided to a microring with evanescent bus-to-ring coupling confirmed by scattering imaging, and a sidewall-roughness-dependent scattering-loss model indicates that loss remains low in the roughness regime consistent with the observed facet and sidewall quality. Building on the intrinsic emitter-resonator coupling demonstrated in, this platform extends monolithic $Si_3N_4$ photonics toward scalable visible-to-near-infrared
quantum and classical
circuits.
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All authors Kishor Kumar Mandal, Lekshmi Eswaramoorthy, Parul Sharma, Anuj Kumar Singh, Brijesh Kumar, Tanmay Gupta, Venu Gopal Achanta, Anshuman Kumar
Atomic Physics
Angle-of-Arrival Determination of Radio-Frequency Fields Using Stark-Shifted Rydberg-EIT Spectra
Rajavardhan Talashila, Zoya Popovic, Nikunjkumar Prajapati et al. (+2 more)
We demonstrate a method for determining the magnitude of the angle of arrival (AoA) of a linearly polarized
radio-frequency (RF) field using the angle-dependent spectral amplitudes of AC Stark-shifted electromagnetically induced transparency (EIT) resonances. An off-resonant RF field produces distinct Stark shifts of the $|m_J|$ sublevels, while their relative excitation strengths depend strongly on the relative orientation of the RF and optical polarizations. Under the controlled polarization geometry considered here, this dependence enables determination of the AoA magnitude without requiring RF phase measurements or spatially separated sensing locations. We experimentally demonstrate the method at RF frequencies of 1.27 GHz, 2 GHz, 3 GHz, and 4 GHz using a subwavelength cesium vapor
cell. Within the $20^\circ-40^\circ$ region of highest angular sensitivity, the estimated repeatability-based $1\sigma$ AoA uncertainty ranges from approximately $0.6^\circ$ to $2^\circ$ across the investigated RF frequencies. A model combining Floquet-Shirley calculations of the RF-dressed Rydberg states with a coherent hyperfine-to-fine-structure optical excitation model qualitatively reproduces the experimental observations. These results establish a path toward compact, phase-independent AoA sensing using spatially localized Rydberg-atom spectroscopy.
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All authors Rajavardhan Talashila, Zoya Popovic, Nikunjkumar Prajapati, Noah Schlossberger, Christopher L. Holloway
Atomic Physics
Improved calculation of the hyperfine structure of muonic helium
F. A. Martynenko, K. A. Seredina and A. P. Martynenko
Using perturbation theory for the fine-structure constant $\alpha$ and the electron-to-
muon mass ratio, we calculated new contributions to the hyperfine structure of the ground state of the
muon-electron helium atom. Compared to our previous results, we calculated new corrections in second-order perturbation theory. This calculation reduces the discrepancy between the theoretical value and the new experimental hyperfine structure measurement.
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Applied Physics
Fast, optimal readout of a tethered optomechanical pressure sensor
Stephen R. Chen, Yiliang Bao, John R. Lawall et al. (+2 more)
We demonstrate a fast readout method for tethered optomechanical pressure sensors. The fast readout method allows mechanical ring-down measurement rates that exceed the total mechanical damping rate $\Gamma_t$. We model the uncertainty of our ring-down measurements with the Cramér-Rao bound, including the effect of thermomechanical noise. Both our data and the Cramér-Rao bound indicate that measurement rates on the order of the mechanical damping rate produce the lowest uncertainty. At low pressures, using the optimal measurement rate allows 10$\times$ faster measurements or 3$\times$ lower uncertainty compared to allowing our sensor to ring down to the thermomechanical noise floor. Our results allow tethered optomechanical pressure sensors to operate at rates comparable to commercially available pressure gauges, removing an obstacle to adoption of optomechanical pressure sensors in industrial settings.
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All authors Stephen R. Chen, Yiliang Bao, John R. Lawall, Jason J. Gorman, Daniel S. Barker
Optics
Bridging high-Q and Kerr-nonlinear photonics using modal phase matching
Jordan R. Stone, Saleha Fatema, Christopher V. Poulton et al. (+3 more)
Integrated Kerr microresonators provide on-chip optical nonlinearity for wavelength conversion, optical frequency combs, and
quantum light sources. Traditionally, their dispersion engineering has tied nonlinear functionality to resonator geometry, forcing trade-offs with other device objectives. In particular, Kerr microresonators usually feature narrow resonator waveguides, but wide waveguides support higher Q through reduced sidewall scattering. We propose that modal phase matching - invoking multiple spatial mode families to satisfy dispersion requirements - facilitates Kerr nonlinear optics beyond traditional geometries. Working with a commercial foundry, we design and fabricate high-$ (>10^7) microresonators on a 160-nm-thick silicon nitride platform and demonstrate Kerr optical parametric oscillation. We achieve 20% conversion efficiency and gap-free wavelength tuning over >1 nm for parametric oscillation at the cesium D1 transition. Modal phase matching further supports pumping in both 1060-nm and 795-nm bands, without custom device layers, for wavelength generation between 600 nm to 1400 nm. Our work expands the Kerr design space, effectively decoupling Q and dispersion to create new opportunities with high-Q nonlinear devices.
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All authors Jordan R. Stone, Saleha Fatema, Christopher V. Poulton, Michael G. Wood, Gordon A. Keeler, Kartik Srinivasan
Optics · Cross-listed from cs.IT
Analytical Channel Modeling and Stability Aware Optimization of Optical Inter Satellite Links
Hossein Safi, Ziheng Wang, Stijn Mast et al. (+2 more)
Optical inter-satellite links (OISLs) are key enablers for high-capacity space networks and next-generation satellite constellations. However, their extreme directionality makes link reliability highly sensitive to platform-induced pointing jitter, which causes random misalignment between the transmitter and receiver beams. In this paper, we develop a tractable closed-form statistical channel model for point-to-point OISLs subject to independent pointing errors at both terminals. Accurate Gaussian main-lobe approximations are applied to the transmitter far-field pattern and receiver coupling efficiency. This transforms the diffraction-based channel response into closed-form expressions for the channel-gain distribution, outage probability, and ergodic capacity. The analytical results are validated through Monte Carlo simulations and used to study the impact of terminal stability, beam divergence, and link margin on OISL performance. The results show that outage probability is governed by the weaker terminal in terms of pointing stability, while improving only the stronger terminal provides minimal additional benefit. In contrast, the ergodic-capacity penalty depends on the combined stability of both terminals, revealing a fundamental distinction between reliability and throughput metrics. The proposed framework provides practical design guidelines for selecting beam parameters and specifying pointing and tracking requirements under varying levels of platform instability.
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All authors Hossein Safi, Ziheng Wang, Stijn Mast, Harald Haas, Iman Tavakkolnia
Plasma Physics
On energy conservation laws in the drift-reduced Braginskii model
Sergio García Herreros, Brenno de Lucca, Davide Mancini et al. (+5 more)
A revision of the drift-reduced Braginskii model is presented, focusing on its practical implementation in numerical codes, and systematically analyzing the effect of several commonly-used approximations on energy conservation. As a practical example, the effect of the approximations is quantified in the GBS code. By dropping the most important approximations and related energy sinks, the simulation shows increased transport levels and a target heat flux with closer agreement to experimental observations.
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All authors Sergio García Herreros, Brenno de Lucca, Davide Mancini, Zeno Tecchiolli, Micol Bassanini, Paolo Ricci, Louis Stenger, Christian Theiler
Plasma Physics
Electrons quench ion tails during evaporative cooling in hot ion mode plasmas
Eric Palmerduca, Hong Qin and Nathaniel J. Fisch
It is shown how evaporative cooling in hot
plasma is essentially different from evaporative cooling in other media, such as neutral gases or Bose-Einstein condensates. The fundamental difference in
plasmas arises both from the large mass ratio between electrons and ions in fully ionized
plasma and the unusually sensitive dependence of
plasma collisionality on speed. Thus, a hot ion mode
plasma ($T_e < T_i$) is shown to support a distinctive evaporative cooling regime, where a strong reduction in the ion evaporation rate appears. This new regime may have application to approaches to economical nuclear fusion, where the ion tail plays an outsized role in the fusion reaction rate.
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Chemical Physics
A note on the accuracy of spin-densities from Kohn-Sham Density Functional Theory
Rodrigo A. Mendes, Zachary W. Windom, Ajith Perera et al. (+2 more)
Quantifying the
magnetic properties of open-shell molecules is a common task in chemistry and is increasingly performed in silico using Kohn-Sham density functional theory (KS-DFT). Previous work demonstrates that the predictive accuracy of a few functionals for one such property - hyperfine coupling constants (HFCCs) - is possible, implying that such approximations must yield accurate
spin-densities at the
nucleus. However, the ability of such functionals to globally predict accurate
spin-densities of comparable quality to rigorous ab initio coupled cluster theory, for example, is dubious, despite this being a fundamental quantity for KS-DFT. This work intends to explore the matter by evaluating moments of the
spin-density, $\langle r^n \rangle = \int \rho(r)r^n d\tau$, $n=-2,\cdots,2,3$, for second-, third-, and fourth-row atoms to compare various KS-DFT functionals against CCSD. Our results broadly indicate that the tested functionals experience significant deviations with respect to CCSD
spin-densities in regions up to 1 Bohr away from the
nuclei, with most errors occurring in the immediate vicinity of the
nucleus. We find evidence of extreme errors by some functionals for individual $\alpha$/$\beta$
spin-densities, although several ultimately end up benefiting from significant error cancellation once the corresponding global
spin-density is formed. Nevertheless, a comparison between CAM-B3LYP and the
Quantum Theory Project (QTP)-family of DFT functionals based on Correlated Orbital Theory conditions across all error metrics demonstrates that QTP00 more accurately reproduces the global
spin-density as well as HFCCs, generally offering results in better agreement with CCSD. In line with previous work, we also corroborate the success of PBE0 and the TPSS-family of functionals for HFCCs, further finding that both approximations generally yield
spin-densities that are amongst the best.
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All authors Rodrigo A. Mendes, Zachary W. Windom, Ajith Perera, Roberto L. A. Haiduke, Rodney J. Bartlett
Instrumentation and Detectors · Cross-listed from astro-ph.IM
Characterization of IBEX LmAPD detectors at CEA for future low-noise NIR astronomy instruments
Jean Le Graët, Vincent Affatato, Marion Baumann et al. (+9 more)
Future near-infrared (NIR) astronomy missions dedicated to
photon-starved science cases, in particular an all-sky astrometric survey extending the legacy of Gaia into the NIR, as identified within the ESA Voyage~2050 programme, require large-format detectors combining sub-electron effective read noise with very low dark current. Linear-mode avalanche photodiodes (LmAPDs) based on HgCdTe meet this need by multiplying the photo-generated charge by an avalanche gain before the readout stage, thereby reducing the relative contribution of the read noise. We report the first electro-optical characterization of an IBEX detector, a $2048\times2048$, \SI{15}{\micro\meter}-pitch HgCdTe LmAPD array developed by Leonardo with the European Space Agency and operated at \SI{80}{\kelvin} on a dedicated bench at CEA-IRFU. We first discuss the central difficulty of characterizing an APD array, the degeneracy between avalanche gain,
quantum efficiency (QE), and conversion gain in the measured response, and we then separate the measurements into two categories. Among the quantities that are directly measurable without assumptions, we report a signal-to-noise ratio for a CDS measurement that exceeds that of a Euclid-type H2RG above \SI{8}{\volt} pixel bias under identical low-flux conditions, a photo-response non-uniformity stable at the $\sim$10% level, and a
quantum efficiency-to-excess-noise ratio (QEFR) of \num{0.40} at \SI{10.5}{\volt}. Among the quantities derived under an explicit set of assumptions, we obtain a conversion gain corresponding to a sense-node capacitance of \SI{27}{\femto\farad}, a
quantum efficiency of $46\pm12\%$ at \SI{2.5}{\volt}, and an excess noise factor $F=1.15\pm0.14$ at \SI{10.5}{\volt}. These results establish IBEX as a promising European large-format detector for future ultra-low-flux NIR instruments.
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All authors Jean Le Graët, Vincent Affatato, Marion Baumann, Olivier Boulade, Cyrille Delisle, Eloy Hernandez, Christian Ketchazo, Frederic Lemmel, Vincent Moreau, Salima Mouzali, Thibault Pichon, François Visticot
Optics
Bound-State Engineered Quantum Batteries Against Decoherence
J. L. Li, C. Cui, W. Y. Hu et al. (+3 more)
Quantum batteries promise revolutionary advantages for energy storage but are fundamentally crippled by environmental decoherence, which induces self-discharge and rapid "aging." Here, we crack this critical bottleneck by exploiting the decoherence-suppression mechanism: the formation of system-environment bound states. We consider a charger-battery system embedded in a three-dimensional anisotropic photonic
crystal bath and derive exact non-Markovian dynamics. Strikingly, we reveal a tunable phase diagram where modulating the atomic eigenfrequency and coupling strength switches the system between zero, one, and two bound states. In the presence of two bound states, the battery energy evolves into a persistent periodic oscillation, enabling lossless energy storage and on-demand extraction indefinitely, effectively realizing an aging-free
quantum battery. Conversely, the absence of bound states leads to complete energy decay. We also demonstrate that even during self-discharge, a single bound state can stabilize extractable energy. This work establishes bound-state formation as a powerful and feasible strategy for combating decoherence, offering a clear blueprint for designing durable solid-state
quantum batteries in non-Markovian photonic platforms.
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All authors J. L. Li, C. Cui, W. Y. Hu, Cheng Shang, Yan-Hui Zhou, H. Z. Shen
Fluid Dynamics · Cross-listed from astro-ph.HE
Large ${\rm Pm}$ small-scale kinematic dynamo in protoneutron stars
Shipra Verma, Kannabiran Seshasayanan, Raphaël Raynaud et al. (+1 more)
Magnetars are young, isolated
neutron stars that possess an exceptionally strong
magnetic field, with surface dipolar strengths on the order of $10^{15}$ G. One of the plausible scenarios for generating such a strong field is an exponential amplification by a turbulent convective dynamo during the protoneutron
star phase. However, the short expected duration of the convection ($\sim 10$ s) imposes a stringent constraint on the dynamo growth rate. We perform an extensive set of 82 three-dimensional convective dynamo simulations in the anelastic approximation and investigate the kinematic phase to quantify the dynamo growth rate $\gamma$. We find that $\gamma$ increases with both the
magnetic Prandtl number ${\rm Pm}$ and the Rayleigh number ${\rm Ra}$, with the most unstable mode becoming highly non-axisymmetric and multipolar. We further observe a gradual transition from large-scale to small-scale dynamo as the
magnetic Reynolds number ${\rm Rm}$ increases, resulting in a
magnetic field that is predominantly concentrated at small scales. The trend remains unchanged when the outer
magnetic boundary condition is varied. Since resolving the increasingly small scales becomes numerically impractical, we employ the theoretical small-scale Kazantsev dynamo model to explore the large ${\rm Pm}$ regime characteristic of protoneutron
stars. The model qualitatively captures the growth rate behaviour observed in simulations and, upon extrapolation to the large ${\rm Pm}$ limit, indicates that $\gamma$ is only weakly dependent on the resistivity in a PNS. Under conditions relevant to the PNS, this model predicts a
magnetic energy growth rate of the order of $\sim 1$ ms$^{-1}$.
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All authors Shipra Verma, Kannabiran Seshasayanan, Raphaël Raynaud, Jérôme Guilet
Physics and Society
Multiscale Reconstruction of Multiplex Networks with Higher-Order Interactions
Guangyuan Mei, Yao Cai, Suoyi Tan et al. (+4 more)
Inferring network structure from dynamical observations is a fundamental inverse problem in complex systems. Existing approaches have largely focused on single-layer or pairwise interaction networks, leaving the reconstructability of multiplex systems with higher-order interactions poorly understood. Here, we show that structural identifiability in such systems is jointly governed by cross-layer spreading couplings and network heterogeneity. Building on coupled awareness-behavior dynamics, we develop a multiscale inference framework that disentangles interacting spreading processes and enables the simultaneous reconstruction of microscopic simplicial interactions and macroscopic metapopulation organization. Numerical experiments on synthetic and empirical networks demonstrate high reconstruction performance, particularly in non-absorbing dynamical regimes sustained by weak inhibitory and strong facilitatory couplings. Results reveal the mechanisms governing structural identifiability in higher-order multiplex networks and provide a general route for inverse inference in multiscale complex systems.
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All authors Guangyuan Mei, Yao Cai, Suoyi Tan, Chuang Liu, Xiu-Xiu Zhan, Xin Lu, Yi-Cheng Zhang
Optics · Cross-listed from quant-ph
Adaptive Relational Learning on Multi-instance Quantum Data with Photonic Processors
Marcin Jastrzebski, Shang Yu, Raj B. Patel et al. (+1 more)
Loading multiple
quantum states in parallel into a
quantum machine learning (QML) model can unlock learning tasks where key information resides in the relations between states rather than in individual states. We introduce an adaptive relational learning framework for such multi-instance
quantum data that accesses pairwise and higher-order relations. Our model combines global measurements via SWAP or CYCLE tests for evaluating an $n$-state Bargmann invariant with shallow trainable transformations applied locally to each input state. We demonstrate the approach for continuous-variable (CV) photonic systems, which naturally provide access to
quantum data and necessary computing operations. We solve tasks involving hidden relationship detection,
geometric phase classification, and sensing in the presence of an unknown shared nuisance interaction. We
benchmark the adaptive model against a non-adaptive “measure-first” approach based on continuous-variable classical shadows, and show that the cost of shadow estimation grows rapidly with $n$, while our model avoids this dependence. Already for $n=2$, we achieve perfect test accuracy $A=1.0$ with $500$ inference shots, improving average test accuracy over the shadow-based method by $\Delta A=0.15$ while using $100$ times fewer shots per data point. Our work opens routes to sensing and
quantum-data applications where adaptive photonic QML can access relational features that are costly to recover with non-adaptive, measure-first models.
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All authors Marcin Jastrzebski, Shang Yu, Raj B. Patel, Oleksandr Kyriienko
Plasma Physics
Spatially-resolved methane decomposition in a short glow discharge: insights into suprathermal hydrogen and radical chemistry
Shurik Yatom
This work presents the spatially resolved mapping of methane decomposition chemistry in a short direct-current glow discharge, revealing segregated zones for dissociation and polymerization in an Ar-CH4 mixture at 400 mTorr.
Laser-induced fluorescence (LIF), two-
photon absorption LIF (TALIF), and optical emission spectroscopy (OES), are used to map the absolute number densities of atomic hydrogen (H), methylidyne (CH), and dicarbon (C2) radicals. The results reveal a segregated chemical environment driven by the non-local electron kinetics. The primary dissociation of methane is confined to the cathode sheath (< 4 mm), where the H atom density peaks at a value (). Analysis of the Halpha} line profile identifies two distinct suprathermal H atom populations with peak kinetic energies of ~110 eV and ~17.5 eV, attributed to ion-surface reflection and electron-impact dissociation, respectively. The densities of secondary radicals CH and C2 peak further from the cathode at the sheath-negative glow boundary (y=4-6 mm), identifying this region as the primary zone for polymerization, with the peak C2 density higher than that of CH by a factor of ~370. The discharge operates under strong non-
equilibrium conditions, confirmed by the disparity between the low bulk gas temperature (~570 K) and the high vibrational temperature (3300-4700 K) of the emitting CH(A) radicals, the latter serving as a signature of a specific high-energy dissociative excitation pathway. This work demonstrates how the structured energy landscape of a short glow discharge spatially separates methane dissociation from subsequent polymerization. Kinetic estimates suggest that suprathermal H atoms may play a role in bulk
plasma chemistry via abstraction reactions, with predicted CH4 activation rates exceeding those of the thermal H population.
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Chemical Physics · Cross-listed from q-bio.MN
Local energetic coupling enhances the expressivity of chemical computation
Marco Tuccio, Jason W. Rocks and Joshua E. Goldford
Living systems compute with chemistry by mapping environmental signals onto specific internal chemical states. Despite recent advances in molecular programming, it remains unclear which physicochemical features control the computational expressivity of chemical systems. Here we inverse-design thermodynamically consistent chemical reaction networks whose steady-state response to an environmental input computes a target nonlinear function. Using implicit differentiation we train the free-energy landscape directly: standard chemical potentials, transition-state energies and thermodynamic drives. Increasingly large networks generated by elementary ligation and cleavage steps fit increasingly complex nonmonotonic
polynomial functions, with expressivity scaling logarithmically with network size, predicted primarily by the number of reactions. Training individual energetic parameter classes reveals that internal thermodynamic drives, capable of breaking detailed balance, dominate trainability, with comparable performances achieved only by pairs of parameter classes. These results identify nonequilibrium drive as the most effective single resource for steady-state computational expressivity in chemical reaction networks.
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Fluid Dynamics
A derivative-free framework for capturing macroscopic behavior of incompressible turbulent flows
Jihun Han and Yoonsang Lee
The derivative-free loss method (DFLM) is a mesh-free neural network approach for solving partial differential equations by simulating
stochastic walkers rather than computing derivatives directly. Although DFLM has previously been applied to the Navier–Stokes equations, extending it to turbulent flows reveals two limitations of its simplest implementation. The walkers do not account for the swirling, directionally stretching motion of the flow, and randomly sampling the walkers used to evaluate the target introduces noise into the learned solution. We address both limitations with an analytic approximation of the walkers' local motion that captures directional stretching and eliminates this sampling noise, while remaining computationally efficient. We study the resulting method as a non-intrusive multiscale solver capable of learning macroscopic flow behavior without resolving every scale on a fine grid or explicitly coupling coarse- and fine-scale models. For two turbulent two-dimensional flows, the proposed method reproduces the macroscopic energy spectrum of fully resolved reference simulations more accurately than the standard method, particularly where directional stretching is strongest, confirming DFLM's efficacy as a multiscale solver for turbulent fluid systems.
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Computational Physics · Cross-listed from math.NA
The sharp CFL condition of the piecewise constant sparse grid discontinuous Galerkin method for high-dimensional transport equations
Juntao Huang
We establish the sharp CFL condition for the piecewise constant sparse grid discontinuous Galerkin (DG) method with forward Euler time stepping, applied to transport equations with constant coefficients on periodic domains in arbitrary dimensions. For the transport velocity $\boldsymbol c=(c_1,\ldots,c_d)$ and a uniform mesh of size $h$, we prove that the scheme is $L^2$ stable if and only if $\Delta t \leq {h}/{\max_{1\leq \ell\leq d}|c_\ell|}$, whereas the corresponding full grid upwind scheme is well-known to require $\Delta t\leq h/\sum_{\ell=1}^d |c_\ell|$. The sparse grid discretization therefore enlarges the admissible time step by a factor of ${(\sum_{\ell=1}^d |c_\ell|)}/{(\max_{1\leq \ell\leq d}|c_\ell|)}$, which lies between $1$ and $d$ and reaches $d$ for isotropic transport. The proof of sufficiency relies on projection leakage identities for the multilevel Haar decomposition, which allow the mixed directional terms in the energy estimate to be absorbed by the energy discarded by the sparse grid projection. The proof of sharpness follows from alternating modes in one dimension at the finest level. As a by-product, we obtain explicit formulas for the $L^2$ operator norm and the spectral radius of the amplification operator. For spaces over general downward closed index sets, we derive an explicit sufficient CFL condition and a
geometric criterion for its sharpness. Numerical experiments in two and four dimensions confirm the theoretical results.
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Atomic Physics · Cross-listed from quant-ph
Optimal Linear-Rate Conversion of Unknown Mixed Qubit States via SWAP Tests
Sujay Kazi and Iman Marvian
By consuming multiple copies of an unknown
qubit state, one can modify its purity while preserving the direction of its Bloch vector. We determine the maximum linear rate at which
qubit states of different purities can be interconverted, allowing a nonzero error, quantified, for instance, by the trace distance, provided that it vanishes in the limit of infinitely many copies. Interestingly, the optimal conversion rate is determined by the two
eigenvalues of the complex right-logarithmic-derivative (RLD) Fisher information
matrix associated with $\mathrm{SU}(2)$ rotations of the
qubit state. When the output
qubits have higher purity, corresponding to concentration, the optimal rate is given by the ratio of the maximum
eigenvalues of the input and output RLD
matrices. In contrast, when the output
qubits have lower purity, corresponding to dilution, the optimal rate is given by the ratio of their minimum
eigenvalues. Remarkably, both concentration and dilution can be implemented using SWAP tests as the only nontrivial two-
qubit measurement primitive, together with ancillary
qubits initially prepared in maximally mixed states, without requiring any additional two-
qubit gates. Our work thus provides a novel operational interpretation of the full complex RLD Fisher information
matrix. Crucially, its antisymmetric, purely imaginary part encodes
geometric information beyond the statistical distance between density operators and plays an essential role in determining the optimal state-conversion rates.
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Fluid Dynamics
Comparative Evaluation of Carotid Artery Hemodynamics: Patient-Specific CFD Simulations vs. 4D flow MRI
Ali Mokhtari, Merel Meulendijks, Ariel Bergmann et al. (+4 more)
Wall shear stress (WSS) is implicated in carotid atherosclerosis, and 4D flow MRI offers a non-invasive route to its estimation, but the agreement of MRI-derived plaque-surface WSS with computational fluid dynamics (CFD) has not been quantified in a large cohort. We compared peak-systolic velocity and WSS from 4D flow MRI and patient-specific CFD in 240 carotid arteries of 120 patients, with CFD geometries and boundary conditions derived from MRI. Velocity and circumferential WSS were compared at three standardized cross-sections, and plaque-surface WSS in 63 of 149 stenosed arteries with a reliably segmented plaque. Agreement was assessed with Bland-Altman analysis, Spearman correlation with stenosis degree, and exploratory multivariable regression of the plaque-surface discrepancy on hemodynamic and
geometric variables. MRI reproduced the broad flow features but underestimated peak velocity, increasingly toward the distal internal carotid artery. WSS disagreement was location dependent: mean circumferential WSS showed little average bias in the common carotid artery, whereas MRI missed the high WSS at the bifurcation apex and underestimated plaque-surface mean and maximum WSS by $40.9 \pm 28.8\%$ and $44.5 \pm 26.8\%$; the difference in Pa increased with stenosis degree ($\rho = 0.47$ and $0.50$). In exploratory regression, the CFD-derived pressure drop across the plaque was the only significant correlate of the mean WSS discrepancy, whereas stenosis degree and
geometric variables were associated with the maximum WSS discrepancy. Under the present protocol, MRI-derived plaque-surface WSS showed magnitude- and location-dependent disagreement with CFD, greater in more severely stenosed arteries and, in exploratory analyses, associated with factors beyond stenosis degree.
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All authors Ali Mokhtari, Merel Meulendijks, Ariel Bergmann, Christoph Strecker, Jonathan Andrae, Andreas Harloff, Dominik Obrist
Chemical Physics · Cross-listed from astro-ph.IM
PyExoCross 2.0: A Python Framework for LTE and non-LTE Spectra, Cross Sections, and Spectroscopic Post-processing of Atomic and Molecular Line Lists
Jingxin Zhang, Sergei N. Yurchenko and Jonathan Tennyson
PyExoCross is a Python-based spectroscopic post-processing framework for converting large atomic and molecular line lists into scientifically useful quantities, including partition function, specific heats, cooling functions, lifetimes, oscillator strengths, line intensities, stick spectra, and absorption and emission cross sections. It is intended for applications in astrophysics, planetary atmospheres, laboratory spectroscopy, and other high-temperature environments where large modern databases require efficient and reproducible computational treatment. The new release provides both a configuration-file-driven command-line interface (CLI) and a Python application programming interface (API) within a unified computational framework. The original input file workflow is retained, while the Python API enables direct use in scripts, notebooks, and automated pipelines. The version PyExoCross 2.0 also adds explicit support for non-local thermodynamic
equilibrium (non-LTE) calculations, including two-temperature models and user-defined density and population treatments for both absorption and emission spectra. In addition, database compatibility has been expanded beyond ExoMol, HITRAN, and HITEMP line lists to include high-resolution molecular database ExoMolHR and atomic database ExoAtom. GPU acceleration is also introduced for computationally intensive intensity and cross-section calculations and is available for all currently supported database formats. These developments improve workflow flexibility, reproducibility, and integration with modern data-analysis environments, while extending the physical modelling capabilities of the code. PyExoCross therefore provides a more general and extensible platform for large-scale spectroscopic simulations based on modern atomic and molecular databases.
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Fluid Dynamics · Cross-listed from cs.RO
Swim-and-Breach at Palm Scale: A Rudder-Steered Two-Propeller Underwater Robot Platform with Differential-Thrust Pitch Control
Daehyun Choi, Ian Bergerson, Hengjia Zhu et al. (+2 more)
We present a palm-scale (65 mm, 34 g) swim-and-breach
robot platform. Two vertically stacked propellers provide both propulsion and differential-thrust pitch control under a proportional-integral-derivative (PID) loop, and a tail rudder adds yaw control. The hull, evaluated by flow simulation, reduces the drag five-fold relative to an equivalent cuboid, and the propellers are optimized using B-series modeling validated by dynamometer measurements. The current
robot swims at 13.9 body lengths per second and turns at 209 deg per second, corresponding to the upper limits reported for underwater
robots. In free swimming, the pitch loop turns the body to any commanded nose-up pitch angle, and, with the rudder stabilizing the exit, the current
robot leaps 1.6 body lengths high and 3.7 long in a seamless cruise-leap-cruise sequence. The platform can be used to build small-scale
robots that cross barriers and dry gaps between pools for inspection in streams, flooded structures, and industrial systems.
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All authors Daehyun Choi, Ian Bergerson, Hengjia Zhu, Tianjun Lan, Saad Bhamla
Instrumentation and Detectors · Cross-listed from cond-mat.mtrl-sci
A transfer-rod-mounted rotatable sample holder enabling a compact modular vacuum system for ion-beam milling, oblique magnetron sputtering, and co-sputtering
Tiago de Oliveira Schneider, Folke Kaiser, Andreas Semrad et al. (+2 more)
We present a novel multifunctional sample-holder concept for compact research-scale
vacuum systems. The sample holder is mounted directly on the transfer rod and remains mechanically connected to it throughout loading, transfer, deposition, and ion-beam milling. In contrast to conventional layouts, no handover to a separate substrate stage is required. By combining axial transfer motion, rotation around the transfer-rod axis, and independent substrate rotation, the holder itself defines the relevant process geometries and enables standard deposition, oblique deposition at freely selectable incidence angles, substrate rotation, and co-sputtering from adjacent sources. A shielding plate integrated into the sample holder acts as both a substrate shutter and a source shield. This reduces the need for separate shutter mechanisms and allows a compact radial arrangement of four 2-inch magnetron sputter sources without source-tilting mechanisms. The concept was implemented in a modular
vacuum system assembled from standard DN100CF components, combining a load lock, magnetron sputtering, and ion-beam milling. The system reaches a base pressure of $5 \cdot 10^{-8}\,\mathrm{mbar}$ without bakeout. The functionality of the sample-holder concept is validated by oblique deposition of Ta/CoFeB bilayers, which exhibit pronounced uniaxial in-plane
magnetic anisotropy, and by co-sputtering of an Au–Ag composition series, whose resistivity follows the expected Nordheim-type composition dependence.
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All authors Tiago de Oliveira Schneider, Folke Kaiser, Andreas Semrad, Peter Kießlich, Markus Meinert