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Physics Physics

physics Watch Physics

Friday 3 July

Instrumentation and Detectors

A New High-Intensity Source for Ultracold Neutrons

K. Abe, S. Ahmed, B. Algohi et al. (+78 more)

The TRIUMF UltraCold Advanced Neutron (TUCAN) collaboration has completed a new superthermal source for ultracold neutrons (UCNs) at TRIUMF. Neutrons produced in a spallation target are moderated in heavy water and liquid deuterium, and are converted to ultracold neutrons by inelastic scattering inside superfluid $^4$He at around 1.1 K. During commissioning of the completed moderation and conversion system, including the liquid-deuterium moderator, up to $1.47(2)\times 10^7$ UCNs were detected outside the source in the experimental area following a 60 s accumulation period. At the same location, continuous operation produced detector rates of up to $6.75(3)\times 10^7$ UCN/s, and a density of 165.6(1) UCN/cm$^3$ was achieved in a 25.4 L external vessel. These measurements demonstrate high-intensity operation of a spallation-driven He-II UCN source, including extraction and transport to an external experimental area.
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All authors K. Abe, S. Ahmed, B. Algohi, D. Anthony, L. Barron-Palos, Y. Bylinsky, M. Bosse, M.P. Bradley, A. Brossard, J. Chak, R. Chiba, C. Davis, R. de Vries, K. Dong, K. Drury, B. Franke, D. Fujimoto, R. Fujitani, M. Gericke, P. Giampa, C. Gibson, R. Golub, K. Hatanaka, T. Hepworth, T. Higuchi, A. Jaison, B. Jamieson, K. Jorgensen-Fullam, M. Katotoka, S. Kawasaki, M. Kitaguchi, W. Klassen, E. Klemets, E. Korkmaz, E. Korobkina, F. Kuchler, C. Lamb, M. Lavvaf, A. Lejuez, T. Lightbody, T. Lindner, S. Longo, K.W. Madison, J. Malcolm, J. Mammei, R. Mammei, C. Marshall, J.W. Martin, R. Matsumiya, M. McCrea, E. Miller, M. Miller, K. Mishima, T. Mohammadi, T. Momose, M. Nalbandian, T. Okamura, S. Pankratz, R. Patni, R. Picker, V. Purcell, K. Qiao, Y.-N. Rao, W. Rathnakela, T. Reimer, D. Salazar, W. Schreyer, T. Shima, H.M. Shimizu, S. Sidhu, S. Stargardter, R. Stutters, P. Switzer, Tushar, M. Uzair, S. Vanbergen, W.T.H. van Oers, N. Yazdandoost, Q. Ye, A. Zahra, L. Zhang

Optics

MEMS Fiber-Tip Photoacoustic Spectrometer for In Situ Microscale Trace Gas Sensing

Xingjie Liu, Mengqi Wu and Ping Lu

To meet the stringent requirements for miniaturized and highly sensitive trace gas sensing in space-constrained scenarios, including power equipment monitoring, minimally invasive biomedical diagnostics, and in situ lithium-battery analysis, we report a MEMS-integrated fiber-tip photoacoustic spectrometer (MFPAS). The device incorporates a Fabry-Perot (F-P) photoacoustic sensor formed by directly butt-coupling a single-mode fiber (SMF) to a 3 mm x 3 mm MEMS chip with a 100-nm-thick low-pressure chemical vapor deposition (LPCVD) Si$_3$N$_4$ diaphragm. The resulting approximately 200-$\mu$m deep silicon microcavity functions simultaneously as a photoacoustic gas cell and an acoustic confinement cavity. A micro-aperture fabricated at the diaphragm periphery by focused ion beam (FIB) milling serves as both a gas diffusion channel and an acoustic high-pass filter, suppressing ambient low-frequency pressure fluctuations and stabilizing the F-P quadrature point without active servo control. In gas-phase measurements, the sensor achieves a noise-equivalent concentration (NEC) of 58.5 ppb@1s, with a rapid response time of 6 s. Benefiting from its ultra-small cavity volume of approximately 1.5 nL, the device is further adapted through structural packaging for in situ dissolved gas analysis in transformer oil, where it achieves an NEC of 230 ppb@1s and a T90 response time of 320 s in the oil phase. By combining nanoliter-scale detection volume, ppb-level sensitivity, rapid response, and wafer-scale batch fabrication compatibility, the proposed MFPAS bridges MEMS diaphragm micromachining and FIB-enabled gas exchange engineering. This design overcomes the intrinsic gas-exchange limitation of conventional sealed-diaphragm optical microphones and offers significant potential for power equipment monitoring and in situ health diagnostics.
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Fluid Dynamics

Stability of gas flow past viscoelastic compliant solid

Mandeep Deka

Stability of a high-speed gas flow past a compliant solid is impacted by two distinct features: high solid-to-fluid density ratio ($\rho_r$), and flow compressibility when flow speeds are comparable to acoustic speed. This study investigates the linear stability of a shear-driven compressible gas flow past a compliant substrate modelled as a continuum Neo-Hookean solid. Numerical solutions of the eigenvalue problem reveal that at high density ratios, the dominant instabilities are the elastic shear-waves of the solid. Our study shows that flow compressibility exerts a non-monotonic effect on the growth rate of the elastic modes; the growth rate increases with increase in Mach number up to $\mbox{Ma} \approx 2$ before subsequently decreasing. Furthermore, for compressible flows, strong thermal-coupling renders the base state highly sensitive to both the solid-to-fluid thermal conductivity ratio and the substrate's bottom-surface temperature. Numerical results demonstrate that increasing the conductivity ratio destabilizes the system, whereas increasing the bottom wall temperature is stabilizing. The stability equations, analyzed in the asymptotic limit of $\rho_r \mbox{Re} \gg 1$, reveals that the fluid-solid system de-couples at the leading order with the elastic modes emerging as a solution of the linear elasticity equations under free-shear condition at the interface. We derive a closed-form expression for the leading-order growth rate of the instability, which shows an excellent agreement with the numerical solution. This expression explicitly quantifies the influence of fluid stresses at the interface, which can in-turn be expressed as integrals of the flow solution isolating the distinct physical mechanisms driving the instability.
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Physics and Society

Community structure of the pseudofractal web

Alexei Vazquez

The Ramsey community number $r_\kappa$ is the smallest network size at which a graph is better described by a partition into communities than by no partition, under a prescribed detection rule. On a scale-free graph this question is confounded: a block model can split the network merely to absorb its degree distribution. I compute $r_\kappa$ analytically for the deterministic pseudofractal scale-free web of Dorogovtsev, Goltsev, and Mendes, separating genuine community structure from degree heterogeneity with two closed-form detection rules. Under a plain Bernoulli stochastic block model, the web's natural recursive bipartition is unpreferred while small and breaks at $r_\kappa=1095$ nodes, with a log-evidence growing as $(\ln 3-\tfrac{2}{3}\ln 2)n$. Under a degree-corrected model tested against the configuration-model null, the same partition survives, breaking far earlier at $r_\kappa=42$, with a log-evidence growing as $(2\ln 3-\tfrac{4}{3}\ln 2)n$ – exactly twice the plain slope, and independent of the prior. Degree correction reverses the ordering of the candidate cuts, demoting the hub-leaf split and elevating the recursive one. Because the web is self-similar, the best description is not two communities but a nested hierarchy: the degree-corrected evidence keeps rising as the partition is refined, and is maximised at of order $\sqrt{n}$ communities of $\sim\sqrt{n}$ nodes. A purely local recursive rule thus builds true hierarchical community structure, over and above the scale-free degree sequence it also produces, in an exactly solvable setting.
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Optics

Heterogeneously Integrated Balanced Photodetector on an Ultra-Low Loss Silicon Nitride Delay Line Interferometer

Rahul Chawlani, Fatemehsadat Tabatabaei, Mark W. Harrington et al. (+7 more)

Thin core silicon nitride photonics enables ultra-low loss, CMOS foundry compatible integration that supports wavelengths from the visible to shortwave infrared. Applications that can benefit from the resulting lower cost, improved robustness, and portability include quantum sensing and computing, ultra-low noise microwave generation, optical clocks, optical gyros, coherent fiber communications, and fiber sensing. An important next step is integration of functional circuits and systems on chip with heterogeneous integration of active components such as high-performance photodetection. Yet to date integrated high-performance photodetectors on the thin film silicon nitride platform has remained elusive. In this work, we demonstrate heterogeneous integration of an InGaAs on InP substrate Modified Uni-Traveling Carrier balanced photodetector with a 15-meter-long unbalanced thin core silicon nitride Mach-Zehnder Interferometer with a bandwidth of 0.92 GHz and a responsivity of 0.305 A/W at 1550 nm with a propagation loss as low as 2.5 dB/m at 1600 nm. Using this circuit we demonstrate two functions, a meter-scale differential interferometer laser stabilization circuit achieving a nearly 23 dB noise suppression at 1 kHz offset and an optical frequency discriminator frequency noise measurement with high sensitivity across 6 orders of magnitude from 10 Hz to 10 MHz. These results demonstrate that the high performance of thin core silicon nitride devices can be combined with integrated high-performance photodetection to realize on-chip stabilized lasers and circuits and pave the path towards full systems on chip.
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All authors Rahul Chawlani, Fatemehsadat Tabatabaei, Mark W. Harrington, Kaikai Liu, Steven M. Zhu, Jiawei Wang, Meiting Song, Xiangwen Guo, Andreas Beling, Daniel J. Blumenthal

Accelerator Physics · Cross-listed from quant-ph

Fringe field induced spin-OAM mixing of twisted electrons

S. V. Gatalina and S. S. Baturin

We study spin effects in twisted-electron propagation through the entrance or exit region of an axially symmetric magnetic coil. Starting from the Foldy-Wouthuysen reduction of the Dirac equation, we derive a paraxial spinor equation in which the longitudinally varying solenoidal field produces, in addition to the usual diagonal Zeeman term, a transverse Pauli coupling proportional to the fringe-field gradient. The scalar transverse dynamics is treated exactly by the Ermakov mapping, which absorbs the longitudinally dependent focusing into a metaplectic scaling transformation and reduces the orbital evolution to that of a stationary two-dimensional oscillator. On this background, the transverse Pauli term is treated perturbatively and yields an explicit first-order correction for arbitrary realistic solenoidal profiles. Axial symmetry implies conservation of the total projection of angular momentum, so each spin flip is accompanied by a compensating one-quantum change of orbital angular momentum. In addition, the linear coordinate structure of the perturbation restricts the first-order dynamics to at most two neighboring radial sidebands for each incoming oscillator component. We derive the corresponding transition amplitudes and show how their phases are governed jointly by the Ermakov accumulation and the diagonal spin-orbital rotation. The resulting framework provides a direct way to quantify spin-orbit mixing of twisted electrons in realistic magnetic lenses and solenoidal beam-line elements, and it identifies a route toward controlled spin-OAM conversion in engineered sequences of magnetic-field edges.
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Fluid Dynamics

An inertial slender-body theory

Anmol Joshi, Anubhab Roy, Arjun Sharma et al. (+1 more)

We present a fully inertial slender-body theory (SBT) that incorporates the effect of fluid inertia on the scale of the length (the "outer" region) as well as the characteristic diameter (the "inner" region) of a steadily translating slender particle. This is achieved by matching the solution of the quasi-two-dimensional full Navier-Stokes equations in the inner region to an outer solution that consists of a superposition of a solution of the linearized Navier-Stokes equations driven by a line of forces and a potential flow solution driven by a line distribution of sources and source dipoles. The drag and lift forces result from the distribution of Oseen force singularities. These Oseenlets also predominantly govern the torque at small Reynolds numbers and large aspect ratios. However, the potential flow singularities play a crucial role in yielding a torque that grows with increasing Reynolds number at large Reynolds numbers and finite aspect ratios. By comparing the forces and torque on the steadily translating particle with those obtained from a finite difference Navier-Stokes solution, we demonstrate the accuracy of the resulting inertial SBT for $\mathrm{Re}_D$ up to 10, where $\mathrm{Re}_D$ is the Reynolds number based on the smallest dimension, i.e., the characteristic cross-sectional diameter of the slender particle.
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All authors Anmol Joshi, Anubhab Roy, Arjun Sharma, Donald L. Koch

Geophysics · Cross-listed from cs.LG

MABLE: Masked Autoencoding with Bi-Lipschitz Decoding for Embeddings and Graph Metric Learning

Yaniv Shulman, Shaghayegh Akbarpour and Jack B. Muir

We propose MABLE (Masked Autoencoding with Bi-Lipschitz Decoding for Embeddings and Graph Metric Learning), a self-supervised framework for learning node and graph embeddings from large, heterogeneous graphs, demonstrated here on geospatial mineral-exploration data. MABLE combines masked reconstruction with fixed cosine-similarity losses that align matched augmented views while keeping unpaired embeddings well spread. A bi-Lipschitz feature decoder ties a low-dimensional reconstruction component of each node embedding to feature similarity, while matched-node consistency shapes the remaining context used by graph pooling. Lipschitz-controlled pooling helps stabilize graph-level representations under perturbations of retained node embeddings, while augmentation alignment trains robustness to masking, node dropping, and sampling variation. Across local copper and regional Arabian Shield studies, MABLE embeddings provide complementary downstream signal and produce coherent embedding-derived layers for hypothesis generation without learned discriminators or hard-negative selection.
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Fluid Dynamics

The Excess Dissipation of Energy in a Turbulent Boundary-Layer and its Departure from Log-Normality

Benjamin Musci, Sebastien Aumaitre, Enzo Francisco et al. (+2 more)

We investigate turbulent dissipation in a von Karman flow using PIV and Diffusing Wave Spectroscopy measurements to directly compare bulk and wall dynamics. While bulk dissipation conforms to the dissipative anomaly, wall dissipation exhibits a clear excess that grows with Re, consistent with velocity-gradient dominated scaling. Decomposition into dissipation intensity bands reveals that this excess is mainly driven by progressive redistribution toward high-intensity events, larger than 10 x mean, as Re increases. From these measurements, we infer the skin-friction coefficient, finding a decreasing trend with Re fairly consistent with classical power-law behavior despite increasing dissipation. Statistically, the wall shows strong departures from log-normality at low Re that diminishes with increasing Re, reflecting an increase in the effective dimensionality of the near-wall gradient field with Re. In contrast, the bulk dissipation remains near log-normal across all Re with slowly growing log-dissipation variance, consistent with K62 refined similarity. These results suggest distinct origins of log-normal behavior which are multiplicative cascade dynamics in the bulk versus the combined effect of persistent shear and a superposition of an increasing number of independent gradient contributions at the wall.
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All authors Benjamin Musci, Sebastien Aumaitre, Enzo Francisco, Adam Cheminet, Berengere Dubrulle

Chemical Physics

Chemical applicability of $\pi$-Sombor index

Monika S and H. M. Nagesh

The Sombor index ($SO$) and the modified Sombor index (${}^m SO$) are two closely related vertex-degree-based graph invariants. Both were introduced in the early 2020s and have found diverse applications in chemical, physicochemical, and network-theoretical studies. In 2023, the $\pi$-Sombor index was introduced as the product $SO \cdot {}^m SO$, with an initial focus on its fundamental mathematical properties. However, its potential chemical applicability has remained unexplored. In the present work, we address this gap by employing regression models to analyze the $\pi$-Sombor index in 22 benzenoid hydrocarbons. This study investigates the chemical significance of the $\pi$-Sombor index through curvilinear regression analysis against the physicochemical properties of these hydrocarbons.
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Optics

Optomechanical transducers for gigahertz frequency ultrasound imaging

Lisa Hackett, Chang Ge, Alex Miera et al. (+2 more)

Ultrasound is a noninvasive, real-time, and therefore widely used imaging modality; yet its application in cellular and sub-cellular biology is significantly limited by rapidly increasing acoustic losses in aqueous solutions with decreasing wavelength. Here we introduce a nano-optomechanical cavity transducer platform to generate and detect ultrasound in aqueous solutions with a sub-micron acoustic wavelength. We analyze the full signal pathway through a combination of finite element method modeling and the coupled differential equations that describe the dispersive optomechanical interaction. Our findings project a signal-to-noise ratio in the thousands at ~5 GHz, limited by diffraction losses and thermal-acoustic noise. This work establishes a viable path towards optomechanical ultrasound systems capable of label-free imaging at cellular and sub-cellular length scales while also providing a broader framework for optomechanical crystal device operation in aqueous environments relevant to biochemical sensing, medical diagnostics, underwater acoustic sensing, and nanoscale imaging.
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All authors Lisa Hackett, Chang Ge, Alex Miera, Brandon Smith, Matt Eichenfield

Medical Physics

Harmonic-Aware Transformer for Real-Time Catheter Localization in Interventional Procedures of Magnetic Particle Imaging

Abuobaida M. Khair, Wenjing Jiang, Xiaoli Yang et al. (+2 more)

Magnetic particle imaging (MPI) enables real-time, radiation-free tracking of magnetic nanoparticle-coated instruments, making it highly suitable for interventional procedures. This study proposes a harmonic-aware transformer framework that directly predicts catheter tip positions from raw MPI voltage signals, eliminating the need for image reconstruction and reducing computational latency. The framework incorporates frequency-domain preprocessing to isolate the 2nd to 8th drive-field harmonics, enhancing the signal-to-noise ratio while preserving motion-relevant features. A transformer architecture with six encoder layers and eight attention heads is employed to learn spatio-temporal dependencies across the three receive axes (x, y, z) for accurate three-dimensional position estimation. The model is trained on simulated MPI signals and evaluated on real in vitro datasets under standard, bending, and heartbeat-like motion conditions. The proposed method achieves sub-millimeter localization accuracy, with a minimum L2 error of 0.103 +/- 0.092 mm and mean absolute errors (MAEs) of 0.039 +/- 0.046 mm, 0.054 +/- 0.049 mm, and 0.060 +/- 0.044 mm along the (x, y, z) axes, respectively, for the bending dataset. Across all datasets, the MAE ranges from 0.165 mm to 0.655 mm, demonstrating consistent performance. The optimized inference achieves a latency of 0.55 ms per frame and a throughput of approximately 1800 frames per second, confirming real-time capability. Compared with conventional MPI-guided approaches relying on image reconstruction, the proposed framework provides improved accuracy, reduced latency, and enhanced robustness under complex motion conditions. These results highlight the potential of harmonic-aware transformer models as efficient and scalable solutions for real-time catheter localization in interventional MPI.
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All authors Abuobaida M. Khair, Wenjing Jiang, Xiaoli Yang, Moritz Wildgruber, Xiaopeng Ma

Atomic Physics · Cross-listed from quant-ph

Coherent Control of Three-Level System Using Shaped Free Electrons

Dixuan Wu, Jing Li, Yuhan Jiang et al. (+1 more)

Three-level systems exhibit quantum interference effects absent in two-level systems, making them important for quantum optics. Here, we study the coherent interaction of a Lambda-type three-level system with free electrons shaped by optical near fields. By treating the electron train as a quantum drive, we show that the interplay between electron modulation and the three-level system's transition pathways induces tunable interference patterns. This interaction effectively realizes electron-mediated coherent population trapping (CPT). We identify a regime that enables complete population transfer between the two lower states and the preparation of a high-coherence superposition, manifested as dark states. In particular, these driven-dissipative steady states are independent of the initial state. Our work proposes shaped free electrons as a platform for steady-state coherent control of three-level systems, enabling atomic-scale state engineering.
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All authors Dixuan Wu, Jing Li, Yuhan Jiang, Yunquan Liu

Optics

Demonstration and Design of Uni-Directional and Ultra-Low Threshold Hybrid Quantum Dot III-V/Si Micro-Ring Laser

Xucheng Yang, Yingtao Hu, Antoine Descos et al. (+8 more)

Micro-ring lasers (MRLs) are attractive light sources for energy-efficient optical interconnects, but their intrinsic directional bistability leads to unpredictable clockwise/counter-clockwise emission. We demonstrate stable unidirectional emission in hybrid quantum-dot (QD) III-V/Si MRLs using passive reflective feedback integrated on the bus waveguide, leaving the ring cavity unperturbed. Three reflector architectures - Y-splitter loop mirrors, adiabatic Y-splitter loop mirrors, and distributed Bragg reflectors (DBRs) - are benchmarked against a reflector-free bidirectional baseline through combined experiment and coupled-mode-theory rate-equation modeling. All designs preserve ultra-low thresholds of 0.79-1.12 mA (112-158 A/cm^2, roughly an order of magnitude below prior quantum-well unidirectional ring lasers) while enhancing single-facet output power and wall-plug efficiency, with directional isolation up to 27.65 dB for the DBR. The reflectors impose no penalty on the 4-5 GHz modulation bandwidth or its thermal robustness, establishing passive external feedback as a practical route to unidirectional QD MRLs for DWDM-scale optical interconnects.
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All authors Xucheng Yang, Yingtao Hu, Antoine Descos, Yuan Yuan, Bassem Tossoun, Geza Kurczveil, Yatiraj Ramanujam, Jonathan Wierer, Raymond G. Beausoleil, Di Liang, Stanley Cheung

Optics

Second-harmonic generation from an optically levitated KTP nanocrystal in vacuum

Yuanbin Jin, Chenli Gao, Jiayu Feng et al. (+3 more)

The optically levitated system in vacuum has emerged as a powerful platform for studies of fundamental physics and precision measurements. Although various nanoparticles have been successfully levitated in vacuum, they typically lack the capability to support optical nonlinear processes. Here, we experimentally demonstrate the stable levitation of a potassium titanyl phosphate (KTP) nonlinear nanocrystal in vacuum and investigate its second-harmonic generation (SHG) properties. This levitated system intrinsically provides a pristine dark-background environment with a high signal-to-noise ratio. The trapping laser simultaneously serves as a fundamental light for efficient SHG. Moreover, the polarization of the collected SHG signal is correlated with that of the fundamental laser, providing clear evidence of the optical torque enabling controllable alignment of the nanocrystal with the driving field. Our work establishes a new route, to our knowledge, toward exploring nonlinear optical processes in vacuum levitation systems and designing novel nanodevices with high manipulation agility in a fully contact-free environment.
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All authors Yuanbin Jin, Chenli Gao, Jiayu Feng, Yuxuan Hao, Xudong Yu, Jing Zhang

Optics

Fluctuation – dissipation physics of stimulated light scattering from laser-driven density gratings

Aleksei Zheltikov

The fluctuation - dissipation theorem (FDT) is shown to provide a powerful resource for the analysis of stimulated light scattering from laser-driven density gratings, including stimulated Brillouin scattering (SBS) and its kinetic-regime extension. In the physical setting of stimulated light scattering by density gratings, the FDT establishes that the dynamics of disturbances induced in a medium by a laser-driven electrostrictive force unfolds via the same physical pathways as the dynamics of internal, spontaneous fluctuations in this medium at equilibrium. When integrated into a suitable kinetic framework, the FDT leads to a significant simplification of the analysis of stimulated light scattering, allowing the stimulated gain/loss spectrum to be found directly from the spectrum of spontaneous density fluctuations without the need to solve kinetic equations with an external-field term. Such FDT-based framework enables an accurate description of stimulated light scattering within a vast parameter space, recovering all the signature properties of sound-wave-mediated Stokes amplification in the hydrodynamic regime, providing a continuous crossover from the hydrodynamic to kinetic regime of stimulated scattering, and explaining distinctly different properties of kinetic stimulated scattering from density gratings, consistent with experiments on stimulated scattering in moderate-pressure gases. As important physical benchmarks, in the limits of vanishingly low and very high collision frequencies, this solution for the SBS gain recovers the FDT-transformed solutions of, respectively, the Vlasov and Navier - Stokes equations.
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Optics

Electron-beam Writing of Spectrally Uniform Green Single-photon Emitters in Hexagonal Boron Nitride

Qingsong Tao, Fuyi Zhou, Zhijie Li et al. (+16 more)

Scalable quantum photonic technologies require single-photon emitters whose positions and emission energies can be engineered simultaneously. Hexagonal boron nitride (hBN) is an attractive room-temperature host, but deterministic creation of spectrally reproducible emitters remains challenging. Here, we use a standard scanning electron microscope as a direct-writing tool to activate bright green single-photon emitters in hBN at predefined sites, without ion implantation or post-fabrication thermal annealing. The written emitters exhibit reproducible zero-phonon-line emission centered near 536 nm, room-temperature antibunching with g(2)(0) as low as 0.08, high brightness, strong linear polarization, and stable emission. Thickness-dependent activation, stacking experiments, cathodoluminescence spectroscopy, and first-principles calculations support a carbon-related defect complex as the most plausible origin of the emission. As a proof of nanophotonic compatibility, we further activate emitters in a nanoparticle-on-mirror plasmonic nanocavity and observe photoluminescence enhancement accompanied by shortened emission lifetimes. These results establish electron-beam direct writing as a practical route to site-selective, spectrally uniform green quantum emitters in hBN, offering a promising basis for integrated room-temperature quantum photonic architectures.
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All authors Qingsong Tao, Fuyi Zhou, Zhijie Li, Yihao Yan, Shuangyue Li, Yuelan Gao, Zijing Wu, Yizhou Liu, Tao Liang, Shuai Yuan, Dakun Wu, Hongzhi Zhou, Qi Zhang, Zhenyi Ni, Chunlei Yu, Pan Wang, Fei Yu, Lili Hu, Ning Zhou

Physics and Society · Cross-listed from eess.SY

Overload-Based Cascades in Multiplex Flow Networks with Partial Functionality

Orkun İrsoy and Osman Yağan

Cascading failures driven by load or flow redistribution arise in networked systems such as power grids, supply chains, and cloud computing centers. Most flow-network models assume that a node either functions or fails as a whole. In many real systems, however, a node supports several distinct flows that share node-level resources, and failure in one of them does not necessarily imply failure in the others. We study this setting through multiplex flow networks with partial functionality, where a node can remain operational in some functionalities while failing in others. A heavy load on one functionality reduces the capacity available to the others, as quantified by cross-layer influence factors. When a node fails in one layer, its load is redistributed among surviving nodes in that layer, while the node may continue to operate in the others. Using mean-field analysis, we derive recursive equations for the final system sizes, namely the fraction of surviving nodes in each layer after the cascade stops. We validate the analysis through simulations for several load-capacity distributions. We then examine key features of the cascade dynamics, including non-monotone robustness curves, different cascade-outcome regimes, and their relation with cross-layer influence. We map the outcomes to distinct steady-state regimes, including single-layer survival phases absent in joint-functionality models, and show that partial functionality can increase robustness relative to the joint-functionality case. Finally, we study robustness maximization under a fixed total capacity budget by comparing several capacity allocation strategies. We propose a strategy that combines cross-layer influence with local neighborhood information on load and degree, and show that it gives the strongest robustness performance across the configurations considered.
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Medical Physics

Geometry-Consistent Discretization of Local Correlation Equations for the Radon Transform in Clinical CT

Xuanqing Mou, Jiayu Duan and Yang Mou

In this work, we extend the local correlation equation (LCE) framework of the 2D Radon transform to clinical computed tomography (CT) geometry and propose a set of geometry-consistent discretization schemes. Building upon the dual-center-of-rotation formulation, the 1st and 2nd LCEs are reformulated under circular fan-beam geometry and discretized using finite difference methods aligned with clinical acquisition configurations. The numerical validity of the proposed discretized circular LCEs (cLCEs) is evaluated using projection data generated from clinical CT datasets. Quantitative analysis based on residual error, mean absolute error (MAE), and mean percentage error (MPE) demonstrates strong consistency between both sides of the equations, with the 2nd cLCE exhibiting the highest accuracy. Furthermore, the proposed cLCE framework is applied to sparse-view sinogram interpolation. By refining interpolated projections using a gradient-descent-based scheme without additional regularization, improved reconstruction quality is achieved. Experimental results show that the cLCE-refined projections significantly enhance image quality compared to sparse-view reconstruction and can outperform clinical complete view (CCV) reconstruction under certain conditions. These results demonstrate that the proposed cLCE discretization provides an effective tool for exploiting projection redundancy in clinical CT and has strong potential for sparse-view reconstruction applications.
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Thursday 2 July

Computational Physics · Cross-listed from cs.LG

Scale-Aware Learning of Chaotic Dynamics on Unstructured Meshes via Binned Spectral Losses

Kanad Sen and Romit Maulik

Surrogate modeling for high-dimensional nonlinear dynamical systems that exhibit chaos requires mechanisms that preserve not only pointwise accuracy but also the scale-dependent structure of physical fields. Bandwise spectral power losses, such as the binned spectral loss function, provide such supervision on structured grids, where Fourier modes define a standard frequency decomposition. On irregular meshes, however, no canonical Fourier basis exists, and spectral representations must be constructed from graph operators induced by mesh connectivity and geometry. In this study, we extend the binned spectral power loss for application to unstructured-mesh surrogate modeling of nonlinear dynamical systems. This is obtained by replacing Fourier bands with graph-Laplacian frequency bands, and we provide scalable Chebyshev and multilevel approximations for improving long-horizon rollout fidelity. In its full-spectrum form, our approach uses graph Laplacian eigenspaces to provide a graph analogue of Fourier band-power matching, but incurs the high cost of spectral decomposition. As a scalable approximation, we replace exact band projectors with sparse Chebyshev polynomial graph filters, avoiding explicit eigendecomposition. When utilizing multilevel graph architectures, we introduce Graph Laplacian Energy Alignment for Meshes (GLEAM), which applies retained-subspace scale-aware supervision across graph hierarchies so that coarse and fine representations are regularized during autoregressive rollout. Our results show that the proposed spectral losses improve long-horizon rollout fidelity and preserve statistical invariants for the forecasting of turbulent flows on unstructured meshes, compared to deterministic baselines.
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Plasma Physics

Integrated 3D fully kinetic simulation of field-reversed-configuration formation with embedded coils

Bowen Zhu and Jian Wu

We present an integrated, three-dimensional, fully kinetic particle-in-cell simulation of field-reversed-configuration (FRC) formation at the device scale. To our knowledge, this is the first fully kinetic model of whole-device FRC formation. The model embeds the drive coils directly inside the computational domain as physical conductors, advancing them self-consistently with the plasma on a single explicit grid and coupling them in closed loop to an external circuit. We apply this unified framework to the Yingguang-1 $\theta$-pinch. Unlike the magnetohydrodynamic and hybrid models used previously, our framework advances the electrons as kinetic particles rather than a fluid, capturing fast magnetic reconnection and electron heating from first principles. The simulation reproduces the complete formation sequence, from reversed-bias lock-in through reconnection to the emergence of a closed-flux FRC, reaching a peak ion density ${\sim}2.2\times10^{22}\,\mathrm{m^{-3}}$ consistent with experiment. The compressed core is electron-dominated, with $T_e\approx1.7\,$keV exceeding $T_i\approx1.2\,$keV, and is pinched to a separatrix radius $r_s\approx1\,$cm, several times below the equilibrium-inferred value, indicating that the plasma never relaxes to a pressure-balanced equilibrium within the microsecond pulse. The model further reproduces a non-axisymmetric, four-fold ($m=4$) deformation of the compressed column, matching the square cross-section recorded by the experiment's end-on framing camera, a feature beyond the reach of the two-dimensional models previously applied to this device. Running on modest GPU hardware, this work brings integrated, first-principles kinetic modeling of fusion-relevant FRCs within reach.
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General Physics

Induced Couplings and Causal Bounds from Nondegenerate Dirac Lagrangians

Marcelo Netz-Marzola, David Vasak, Vladimir Denk et al. (+2 more)

The standard Dirac Lagrangian is linear in the field derivatives and therefore has a vanishing Hessian. We identify the minimal null deformations of this Lagrangian that make the covariant Legendre map locally invertible. Imposing global phase invariance, reality, proper Poincaré covariance, and absence of external background tensors leaves the two-parameter spinorial bivector $\mathcal E^{\mu\nu}=\ell\sigma^{\mu\nu} +\ell_5{}^\star\sigma^{\mu\nu}$, where the star denotes the Hodge dual and $\ell^2+\ell_5^2\neq0$. This extends the analysis of \cite{struckmeier2024pauli} by a parity-odd term. After minimal $U(1)$ gauging, this free null term is no longer variationally trivial and induces magnetic- and electric-dipole Pauli operators, together with an identically conserved dipole current. These dipole terms make the species-dependent regularization lengths directly constrainable by precision moment measurements. Metric-affine gauging in a Lorentz-spinor prescription then produces spin-curvature, torsion, and nonmetricity corrections to the Dirac operator. Applying the Velo-Zwanziger criterion, we exclude all nonzero pure axial, pure trace, and mixed axial-trace torsion backgrounds, as well as nonzero pure Weyl and second-trace nonmetricity backgrounds. The combined trace-vector nonmetricity sector is not excluded only when the effective trace vector vanishes. Tensor torsion and general tracefree nonmetricity remain unclassified without further algebraic assumptions, while the Levi-Civita limit preserves the metric light cone and leaves only lower-order curvature-dependent mass terms. Thus, gauging a Legendre-regular Dirac representative turns a free variational ambiguity into observable couplings, while non-Riemannian sectors are sharply constrained by causality bounds.
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All authors Marcelo Netz-Marzola, David Vasak, Vladimir Denk, Jürgen Struckmeier, Horst Stoecker

Chemical Physics

Q-Score: A Quantum-Native Scoring Function for Molecular Docking

Kangyu Zheng, Yidong Zhou, Ruihao Li et al. (+3 more)

Molecular docking predicts how a small molecule binds to a protein and is a key bottleneck in drug discovery. Classical scoring functions sum empirical pairwise contacts, blind to quantum-mechanical effects like orbital charge transfer that govern binding specificity. We introduce Q-Score, encoding GNN-predicted orbital donor-acceptor energies into a weighted graph and scoring binding by solving a maximum-weight vertex clique problem via Digitized-Counterdiabatic QAOA. Each interaction anchor maps to one qubit and compatibility constraints become edges. Across 11 protein targets, DC-QAOA recovers the exact optimum on 8 at 10 qubits. On 1000 AI-generated molecules, Q-Score is orthogonal to classical scoring with Spearman rho of 0.05, driven by orbital quality with rho of 0.90, and free of molecular-weight bias, enriching for strong orbital interactions at twice the random rate. DC-QAOA achieves a mean approximation ratio of 0.94 with 52 percent exact. Execution of 1000 circuits on IBM Eagle confirms 6-qubit solvability on NISQ hardware.
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All authors Kangyu Zheng, Yidong Zhou, Ruihao Li, Zixin Ding, Zhiding Liang, Shaohua Li

Applied Physics · Cross-listed from cs.RO

Saturation-Aware Robust Trajectory Optimization for Reusable Launch Vehicles via Differentiable Physics

Liwei Chen and Tong Qin

The high-angle-of-attack flip maneuver of reusable launch vehicles presents significant challenges for robust trajectory optimization due to the combined effects of highly nonlinear dynamics, aerodynamic uncertainties, and actuator saturation. This paper presents a differentiable physics framework for saturation-aware robust trajectory optimization. At its core, a Differentiable Particle Tube Control (DPTC) scheme is developed to optimize uncertainty evolution through an ensemble-based distribution shaping strategy. State uncertainty is represented by a Lagrangian particle ensemble, while hard actuator projection operators are embedded directly into the computational graph, enabling the joint optimization of the nominal feedforward trajectory and a time-varying feedback policy via end-to-end backpropagation. The proposed framework is evaluated against an automatic differentiation-based Successive Convexification (AD-SCvx) baseline combined with a conventional covariance steering feedback strategy. Six-degree-of-freedom Monte Carlo simulations demonstrate that, although the baseline achieves nominal fuel-optimal solutions, its unconstrained feedback formulation becomes susceptible to actuator saturation under aerodynamic disturbances, leading to degraded closed-loop robustness. In contrast, the proposed DPTC framework proactively performs a constraint-aware performance trade-off by relaxing spatial tracking to preserve critical control authority. These results demonstrate that integrating differentiable physics with ensemble-based optimization provides an effective and practical framework for robust guidance in highly constrained aerospace flight systems.
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General Physics

On the Classical Limit of Quantum Mechanics

Marcello Baldo

One of the main unsolved question in Quantum Mechanics (QM) is its compatibility with classical mechanics. The laws of QM, which describe the microscopic world, must merge into the classical ones for large enough physical systems, but it is still unknown at which point, if any, the transition occurs and if the transition is smooth or sudden as the size increases. Furthermore, a strict extension of QM to the macroscopic world leads to well known 'paradoxes', which is not straightforward to solve. This question is tightly connected with the measurement problem, since any measurement apparatus must give a response which can be described at classical level. Many experiments have been performed to answer to this question. They mainly try to find to which extent the number of particles can be increased in order to observe clear evidence of violation of standard QM. In this paper we argue that the macroscopicity parameter is not the number of particles but the number of independent degrees of freedom, as proposed in a recent model for the completion of QM. This introduces a sort of change of paradigm. To support this claim a representative set of well known experiments are analyzed from this point of view. This brings to a new interpretation of the experiments. A general scheme for the quantum-classical transition is discussed in some details.
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General Physics

Why Do Light Nuclei Survive at the Large Hadron Collider?

Sushanta Tripathy and Raghunath Sahoo

Light nuclei and antinuclei, such as deuterons, are produced abundantly at the Large Hadron Collider (LHC) in hadronic and nuclear collisions. Even though their binding energies are only a few MeV, they survive in the extremely high temperatures of the order of a few hundred MeV. This contradiction, often referred to as “Snowballs in Hell”, has become a sharp test of how quantum chromodynamics (QCD) turns quarks and gluons into composite matter. Strikingly, two very different frameworks can reproduce the same inclusive yields, i.e., late-stage nucleon coalescence, where nuclei form from nearby nucleons as the system dilutes, and statistical thermal models, where nuclei emerge as part of an equilibrated hadronization chemistry at a temperature close to 155 MeV. Here, we review how recent LHC measurements and model developments are shifting the question–from whether light nuclei are produced, to when and how they form, with broader implications for QCD matter and cosmic-ray antinuclei searches.
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General Physics

Late-time cosmological constraints on three holographic dark energy models with DESI DR2 BAO and Type Ia supernovae

Jiayuan Huang, Tonghua Liu and Chenggang Shao

We constrain three holographic-inspired dark energy models, namely holographic dark energy (HDE), agegraphic dark energy (ADE), and Ricci dark energy (RDE), using late-time observations from cosmic chronometers, Type Ia supernovae (SNe Ia), DESI DR2 baryon acoustic oscillations (BAO), and {redshift-space distortion (RSD) growth measurements}. Five data combinations are considered: $H(z)+$Pantheon+, $H(z)+$DESI DR2+Pantheon+, $H(z)+$DESI DR2+DES-Dovekie, $H(z)+$DESI DR2+DESY5, and {$H(z)+$DESI DR2+DES-Dovekie+RSD}. We perform Bayesian Markov chain Monte Carlo parameter estimation and compare the models with AIC and BIC. In the BAO-included combinations, HDE gives $H_0\simeq67.3$–$68.0~\mathrm{~km~s^{-1}~Mpc^{-1}}$, $\Omega_{m0}\simeq0.270$–$0.272$, and $c\simeq1$, indicating an expansion history close to the de Sitter boundary rather than a robust phantom regime. ADE yields a stable agegraphic parameter $n\simeq2.78$–$2.81$, while RDE gives $\gamma\simeq0.53$–$0.55$ and persistently favors a low matter density, $\Omega_{m0}\simeq0.215$–$0.219$. {Treating $r_d$ as a free parameter reveals a strong negative correlation between $H_0$ and $r_d$, and the RSD-included combination provides a growth-level consistency check through $f\sigma_8(z)$ without constituting a full perturbative stability analysis.} None of the three models significantly alleviates the Hubble tension. Overall, HDE shows the most balanced phenomenological behavior among the three models, although current late-time data do not decisively prefer it over $\Lambda\text{CDM}$.
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Atmospheric and Oceanic Physics

Enhancing a high resolution data-driven weather prediction model with surface descriptors

Åsmund Bakketun, Håvard Homleid Haugen, Jostein Blyverket et al. (+2 more)

We study the importance of surface characteristics when forecasting near-surface variables with a data-driven weather prediction model. To target the challenge of predicting small-scale weather conditions at high resolution, we introduce a range of surface descriptors in the training of a state-of-the-art data-driven model. The input data includes surface descriptors inherited from the numerical weather prediction model used to produce the training dataset and topographic neighbourhood indices. We found that errors of 2-metre temperature and 10-metre wind speed forecasts were reduced by 1.9% and 3.0% respectively compared to a baseline model over the model domain. Over certain surfaces, the improvements were significantly larger. For example, we found a 12% reduction of temperature mean absolute errors over urban areas when the urban fraction was included in the model input. Furthermore, we investigated how the model responded to removal of glaciers, resulting in an increase of temperature. This indicates that 1) the model produce a physically reasonable response and 2) input datasets can be updated without the need to retrain the model. The latter suggests a great benefit for operational systems as training is expensive compared to running these models. This study highlights the importance of including surface conditions in the prediction of near-surface variables.
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All authors Åsmund Bakketun, Håvard Homleid Haugen, Jostein Blyverket, Thomas Nils Nipen, Malte Müller

Physics and Society

From graphons to real-world networks: kinetic opinion dynamics under selective media influence

Bertram Düring, Martina Fraia and Alessandro Licciardi

We propose a kinetic model of opinion dynamics under selective media influence on both graphon-based and real-world networks. The media action, inspired by Hallin's theory of spheres, is incorporated through a model predictive control strategy designed to steer agents' opinions toward a desired target opinion. For the resulting Boltzmann-type description, we analyse the evolution of the moments and, in the quasi-invariant interaction limit, derive a Fokker–Planck-type equation together with a characterisation of its stationary states. We also prove exponential convergence to equilibrium in the Fourier metric. Numerical experiments are performed on networks generated by a Gaussian graphon and on real-world, single-issue Twitter networks, allowing us to investigate the role of control and interaction parameters, as well as the impact of the subset of agents subject to media influence. Using real-world social networks data to initialise opinions and infer the interaction structure, we then compare the dynamics obtained on the original networks with those produced by Gaussian graphons fitted to their adjacency matrices, thereby assessing the descriptive power of the graphon approach for real-world opinion dynamics.
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Medical Physics

Body Habitus Dominates Solver Choice as a Source of Uncertainty in MRI Safety Assessment of Active Implantable Medical Devices

Safa Hameed, Fuchang Jiang, Bhumi Bhusal et al. (+3 more)

MRI is increasingly critical for patients with active implantable medical devices (AIMDs), yet access depends on safety labeling derived from computational heating predictions under ISO/TS 10974 Tier 3. Published assessments have relied predominantly on a single electromagnetic solver class and one or two standard-BMI reference anatomies, leaving the relative contributions of solver choice, tissue property uncertainty, and patient anatomy to predictive variability uncharacterized within a common workflow. We performed a cross-platform evaluation of finite-difference time-domain (FDTD, Sim4Life) and finite element method (FEM, ANSYS HFSS) implementations of the full Tier 3 workflow for a deep brain stimulation system at 1.5 T, extending the analysis across more than 250 clinically realistic trajectories spanning standard male and female references (Duke, HBM, Ella), an elderly male (Glenn), and elevated-BMI models of both sexes (Fats, Ella BMI 30). FDTD and FEM agreed closely in standard anatomies, with Maximum Allowable B1+ limits converging near 2.6-3.0 uT. The elderly male model produced a comparable limit to Duke, indicating BMI rather than age drives heating variability. Elevated BMI reduced safe B1+ by 19-31% in both sexes, while sex at matched BMI had no significant effect. Geometric morphing approximated the native obese limit, whereas dielectric property sweeps failed to reproduce elevated-BMI heating distributions. Body habitus is the dominant source of predictive uncertainty in Tier 3 assessment, exceeding solver choice, dielectric assumptions, and sex. Anatomical diversity, including elevated-BMI female phenotypes, should be treated as a primary variable.
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All authors Safa Hameed, Fuchang Jiang, Bhumi Bhusal, Sana Ullah, Pia Sanpitak, Laleh Golestanirad