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Physics Physics

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Sunday 8 February

Instrumentation and Detectors

Mutual Inductance Sensing SQUID: Cryogenic microcalorimeter based on mutual inductance readout of superconducting temperature sensors

Jodok Zeuner, Constantin Schuster and Sebastian Kempf

Superconducting microcalorimeters, such as superconducting transition-edge sensors and magnetic microcalorimeters, have emerged as state-of-the-art detectors for X-ray emission spectroscopy by combining near-unity quantum efficiency with excellent energy resolution. Despite these achievements, their resolving power has not yet reached the level required to rival modern wavelength-dispersive grating or crystal spectrometers. Here, we introduce a next-generation SQUID-based microcalorimeter concept that exploits the strong temperature dependence of the magnetic penetration depth of a superconductor operated close to its critical temperature. The resulting mutual-inductance-based readout enables in situ tunable signal amplification, while inherently avoiding hysteretic effects that commonly limit superconducting sensors. Experiments with prototype devices demonstrate robust and reproducible operation over a wide temperature range. Based on our measurements and modeling, we project that, using an optimized absorber-sensor combination, an energy resolution below 100meV (FWHM) should be achievable for soft X-ray photons with energies below 800eV. This approach therefore represents a promising pathway towards next-generation cryogenic detectors for high-precision X-ray emission spectroscopy.
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Optics

Origin of Experimental Realization of Vector Beams by Superposition Technique: Geometric Phase

A. Srinivasa Rao

Optical vector beam generation based on the superposition technique [Phys. Rev. Lett. 107(5), 053601 (2011)] has attracted significant interest in both fundamental and applied optics due to its simplicity and cost-effectiveness. In this approach, the superposed modes exist in orthogonal polarization states and therefore do not interfere with each other. When optical gadgets are employed for characterization, the corresponding polarization distribution can be observed. In this work, we report that the characteristic (non-uniform) polarization distribution of vector beams is not inherently present in their field distribution. Instead, the observed polarization features arise during characterization as a consequence of the geometric phase difference introduced between the superposed modes by the optical gadgets. This analysis opens a new direction in the understanding of various classes of vector beams and offers potential advances in the field of structured beam optics.
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Chemical Physics · Cross-listed from quant-ph

Doubling the size of quantum selected configuration interaction based on seniority-zero space and its application to QC-QSCI-AFQMC

Yuichiro Yoshida, Takuma Murokoshi, Rika Nakagawa et al. (+6 more)

We propose doubly occupied configuration interaction-quantum selected configuration interaction (DOCI-QSCI), which samples from the seniority-zero space. While the use of this space effectively doubles the qubit budget, equaling the number of spatial orbitals, this sector restriction can compromise quantitative accuracy. To compensate for this, we expand sampled bitstrings via their Cartesian product into a larger space that includes seniority-breaking determinants. The resulting wave function is also proposed using the trial state in phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) to recover dynamical correlations across the full orbital space (DOCI-QSCI-AFQMC). We evaluate the proposed methods on the H6 chain, N2 dissociation, and the addition of singlet O2 to a BODIPY dye. For the H6 chain, DOCI-QSCI-AFQMC reproduces the accuracy of the level of the complete-active-space counterpart with the quantum device ibm kobe. For N2 and BODIPY-O2, with (14e, 28o) and up to (20e, 20o) active spaces, it yields reasonable results, whereas single-reference CCSD(T) fails qualitatively. These results demonstrate that the DOCI-QSCI doubles the orbital space accessible to conventional QSCI and subsequent ph-AFQMC post-processing delivers reasonably high accuracy.
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All authors Yuichiro Yoshida, Takuma Murokoshi, Rika Nakagawa, Chihiro Mori, Yuta Katayama, Naoya Kuroda, Shigeki Furukawa, Hanae Tagami, Wataru Mizukami

Physics and Society

Effects of Stochastic Games on Evolutionary Dynamics in Structured Populations

Yuji Zhang, Minyu Feng, Qin Li et al. (+2 more)

Continuously changing environments have a paramount role in the evolution of cooperative behavior. Previous works have shown that the transitions among different games, as the feedback between behaviors and environments, can promote cooperative behavior significantly. Quantitative analysis, however, is limited to homogeneous populations, while realistic populations in nature are often more complex and highly heterogeneous. We hereby provide an analytical treatment of when the evolution of cooperation can be supported in stochastic games, applying to arbitrary spatial heterogeneity and payoff structure. We highlight that the rule and frequency of game changes can have surprisingly diverse effects on evolutionary outcomes, depending on the governing social dilemmas. While stochastic games favor the evolution of cooperation in donation games, this is not the case for public goods games and snowdrift games. Hence, our framework and model results offer a more subtle insight into the long-standing enigma.
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All authors Yuji Zhang, Minyu Feng, Qin Li, Matjaz Perc, Attila Szolnoki

Chemical Physics · Cross-listed from cond-mat.stat-mech

The Entropies

Roumen Tsekov

Entropy is critically examined as a fundamental concept in contemporary science and informatics. Although the typical Shannon entropy provides a proper framework for describing the canonical ensemble, it fails to represent adequately the microcanonical ensemble. This discrepancy manifests additionally in its inability to support a theoretical derivation of the Second Law of thermodynamics.
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Accelerator Physics · Cross-listed from quant-ph

Geometry-Enabled Radiation from Structured Paraxial Electrons

M.S. Epov, I.E. Shenderovich and S.S. Baturin

We present a microscopic calculation of spontaneous photon emission by twisted (paraxial) electrons propagating through inhomogeneous, axisymmetric magnetic fields. We construct exact electron states that incorporate transverse mode structure and wavefront curvature by combining the Foldy-Wouthuysen transformation with a geometric framework based on Lewis-Ermakov invariants and metaplectic transformations. We show that the evolution of such structured states corresponds to an open path in the space of quadratic forms, giving rise to a geometric contribution to the emission amplitude that cannot be eliminated by gauge choice or adiabatic arguments. The inverse radius of curvature of the electron wavefront controls a finite-window boundary contribution to the emission amplitude in regions where the external magnetic field vanishes locally, this contribution vanishes when the observation window is extended to infinity. Presented mechanism generalizes Landau-level radiation to nonasymptotic, structured electron states and establishes a direct connection between noncyclic geometric evolution and photon emission.
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Applied Physics · Cross-listed from cond-mat.mtrl-sci

Influence of Elastic Oscillations on Nucleation in Metals

A.S. Nuradinov, O.V. Chistyakov, K.A. Sirenko et al. (+2 more)

This work is devoted to establishing the mechanisms of elastic oscillation influence on nucleation processes in metal melts. The method of physical modeling with low-temperature metallic alloys (Wood and Rose) and transparent organic media (salol, camphene, diphenylamine) was used. It was established that vibration and ultrasound significantly reduce the supercooling required to initiate crystallization. The effectiveness of the influence significantly increases for samples with solid substrates. The hypotheses about the influence through changes in melt viscosity and the exclusive role of cavitation were experimentally refuted. The transition from pre-cavitation to cavitation ultrasound regime is not accompanied by qualitative changes in the influence on nucleation. The mechanism of elastic oscillation influence is substantiated, which consists in mechanical impact on adsorbed crystal nuclei on the surfaces of solid substrates. Elastic oscillations increase the nucleation rate by creating growth steps (dislocations) on the surfaces of adsorbed nuclei as a result of mechanical friction of solid substrates and cavitation erosion. The results have fundamental significance for understanding the physical nature of metal crystallization and practical application for developing technologies for controlling structure formation.
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All authors A.S. Nuradinov, O.V. Chistyakov, K.A. Sirenko, I.A. Nuradinov, D.O. Derecha

Computational Physics · Cross-listed from math.NA

Field conserving adaptive mesh refinement (AMR) scheme on massively parallel adaptive octree meshes

Kumar Saurabh, Makrand A. Khanwale, Masado Ishii et al. (+2 more)

Adaptive mesh refinement (AMR) is widely used to efficiently resolve localized features in time-dependent partial differential equations (PDEs) by selectively refining and coarsening the mesh. However, in long-horizon simulations, repeated intergrid interpolations can introduce systematic drift in conserved quantities, especially for variational discretizations with continuous basis functions. While interpolation from parent-to-child during refinement in continuous Galerkin (CG) discretizations is naturally conservative, the standard injection-based child-to-parent coarsening interpolation is generally not. We propose a simple, scalable field-conserving coarsening operator for parallel, octree-based AMR. The method enforces discrete global conservation during coarsening by first computing field conserving coarse-element values at quadrature points and then recovering coarse nodal degrees of freedom via an $L^2$ projection (mass-matrix solve), which simultaneously controls the $L_2$ error. We evaluate the approach on mass-conserving phase-field models, including the Cahn–Hilliard and Cahn–Hilliard–Navier–Stokes systems, and compare against injection in terms of conservation error, solution quality, and computational cost.
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All authors Kumar Saurabh, Makrand A. Khanwale, Masado Ishii, Hari Sundar, Baskar Ganapathysubramanian

Atomic Physics · Cross-listed from cond-mat.quant-gas

Engineering a Bound State in the Continuum via Quantum Interference

Alexander Guthmann, Louisa Marie Kienesberger, Felix Lang et al. (+2 more)

Quantum mechanical interaction potentials typically support either localized bound states below the dissociation threshold or delocalized scattering states above it. While bound states are energetically isolated, scattering states embed a quantum system in a continuum of environmental modes, making dissipation and loss intrisic features of open quantum systems. A striking exception are bound states in the continuum (BICs), which remain localized despite lying within the scattering continuum due to destructive interference. It was predicted that such states can arise from the interference of two Feshbach resonances coupled to a common continuum, yet this mechanism has remained experimentally inaccessible in genuine quantum systems. Here we demonstrate the formation of such an interference-stabilized state in ultracold collisions of ${}^6$Li atoms by coherently coupling two tunable Feshbach resonances using Floquet engineering. At a critical parameter point, both elastic and inelastic coupling to the continuum vanish, yielding a molecular state above the dissociation threshold. Loss spectroscopy, quench dynamics, and rf-photoassociation directly reveal the resulting decoupling from scattering states. Our observations are quantitatively captured by full coupled-channel calculations and a minimal non-Hermitian model, identifying a Friedrich-Wintgen BIC. Our results establish quantum interference as a powerful mechanism for controlling openness in quantum matter and for engineering non-Hermitian Hamiltonians.
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All authors Alexander Guthmann, Louisa Marie Kienesberger, Felix Lang, Eleonora Lippi, Artur Widera

Fluid Dynamics

A Three-Dimensional Two-Temperature Gas-Kinetic Scheme with Generalized Kinetic Boundary Condition for Hypersonic SBLI

Xingjian Gao, Hualin Liu, Fengxiang Zhao et al. (+1 more)

Accurate prediction of aerothermal loads in hypersonic flows is critical yet challenging due to the coupling of Shock-Wave/Boundary-Layer Interactions (SBLI) and thermal non-equilibrium. This work presents the development of a three-dimensional two-temperature Gas-Kinetic Scheme (3D 2T-GKS) on unstructured meshes. The scheme resolves translational-rotational and vibrational energy modes within a unified kinetic framework. A key innovation is the integration of a Generalized Kinetic Boundary Condition (GKBC), which physically decouples the thermal accommodation of vibrational energy from the translational-rotational mode, thereby offering a more accurate model for gas-surface interactions. Additionally, a Discontinuity Feedback Factor (DFF) is employed to capture strong shock waves with reduced numerical dissipation compared to classical limiters. The method is rigorously validated against standard experimental benchmarks, including the sharp double-cone and hollow cylinder-flare configurations. Numerical results demonstrate that the proposed solver, augmented by the GKBC, accurately captures complex wave structures, separation topologies, and surface heat flux distributions. These findings confirm the robustness and fidelity of the 3D 2T-GKS for simulating complex hypersonic non-equilibrium flows.
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All authors Xingjian Gao, Hualin Liu, Fengxiang Zhao, Xing Ji

Optics

Systematic Analysis of Ferroelectric Domain Dynamics in Periodically Poled Lithium Niobate Waveguides Using Two-Photon Microscopy and Digital Imaging Processing

Jiuyi Zhang, Christopher Cullen, Matthew Konkol et al. (+8 more)

We present a characterization and analysis methodology suitable for volume production for characterizing and optimizing x-cut thin-film periodically poled lithium niobate (PPLN) devices using two-photon (2P) microscopy with quantitative image processing. This method enables direct extraction of key structural parameters, such as duty cycle, phase-matching behavior, and domain uniformity, across large device sets in a non-destructive manner. By correlating 2P microscopy-derived structural metrics with systematic variations in poling conditions, we establish a scalable, image-driven approach for evaluating and improving PPLN fabrication. The resulting workflow supports wafer-level process development and accelerates the manufacturing and packaging of lithium niobate photonic integrated circuits (PICs).
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All authors Jiuyi Zhang, Christopher Cullen, Matthew Konkol, Peng Yao, Timothy Creazzo, Janusz Murakowski, Ruidong Xue, Xiaofeng Zhu, Md Omar Faruk Rasel, Yash Kabra, Dennis Prather

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

Proton Quantum Effects on Electronic Excitation in Hydrogen-bonded Organic Solid: A First-Principles Green's Function Theory Study

Sampreeti Bhattacharya, Jianhang Xu, Ruiyi Zhou et al. (+1 more)

Nuclear quantum effects of protons on electronic excitations in hydrogen-bonded organic materials remains underexplored. In theoretical studies, modeling excitons in these extended systems is particularly difficult because they tend to have a large exciton binding energy and sometimes exhibit charge transfer character. We demonstrate how first-principles Green's function theory combined with the nuclear-electronic orbital method enables us to examine the nature of excitons in a prototypical organic solid of eumelanin, for which the extensive hydrogen bonds have been proposed to facilitate the formation of delocalized excitons. We investigate how the quantization of protons impacts electronic excitations. We discuss the extent to which the resulting proton quantum effects can be described as being derived from structure and how they induce molecular-level anisotropy for the excitons in the organic solid.
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All authors Sampreeti Bhattacharya, Jianhang Xu, Ruiyi Zhou, Yosuke Kanai

Computational Physics

Compressed Sensing Methods for Memory Reduction in Monte Carlo Simulations

Ethan Lame, Camille Palmer, Todd Palmer et al. (+1 more)

Monte Carlo simulations of neutronic systems are computationally intensive and demand significant memory resources for high-fidelity modeling. Compressed sensing enables accurate reconstruction of signals from significantly fewer samples than traditional methods. The specific implementation of compressed sensing investigated here involves the use of overlapping cells to collect tallies. Increasing the number of samples improves the reconstruction accuracy, although the marginal gains diminish with more samples. Reconstruction quality is strongly influenced by the sparsity parameter used in basis pursuit denoising. Across the three test cases considered, memory reductions of up to 81.25% (96.25%) are demonstrated for 2D (3D) reconstructions, with select scenarios achieving reconstruction errors within 1 standard deviation of the corresponding high-fidelity reference results.
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All authors Ethan Lame, Camille Palmer, Todd Palmer, Ilham Variansyah

Medical Physics

The Development of a Preclinical Alpha Irradiation Platform with Versatile Control of Dose, Dose Rate, and Spatiotemporal Irradiation Patterns

Harsh Arya, Joshua Rajan, Varghese Chirayath et al. (+7 more)

Objectives. This study develops and validates a vacuum-based alpha irradiation platform to support preclinical radiobiology. We aim to demonstrate precise, independent control over incident energy, fluence rate, and spatiotemporal patterns, which are critical to the mechanisms underlying targeted alpha therapies and low-dose risk assessments. Approach. A vacuum-based system with a radioactive alpha source was designed and fabricated. The platform provides independent modulation of: (i) temporal patterns via a programmable gate valve; (ii) fluence rate across two orders of magnitude by varying source-to-aperture distance (57 to 381 mm); (iii) incident energy (0 to 4.6 MeV) using adjustable absorption layers; and (iv) spatial distributions via a 3D motion stage. Temporal precision was assessed via synchronized audio-electronic recordings. Fluence rates and energies were validated using CR-39 detectors and Monte Carlo (MC) simulations. Spatial precision was verified through programmed continuous and discrete trajectories. Main results. Validation experiments demonstrated high system fidelity. Measured irradiation durations deviated from programmed values by less than 0.3 s. Measured and computed fluence rates agreed within 3%. For energy validation, CR-39 track diameters matched MC model predictions within one standard deviation. Recorded spatial patterns and dimensions aligned well with programmed trajectories. Significance. We successfully validated a versatile vacuum-based platform that overcomes energy-degradation constraints of gas-filled systems. By providing multi-parametric control over alpha-particle delivery, this system enables systematic investigation into how energy, dose rate, and spatiotemporal patterns influence radiobiological responses. This platform is poised to optimize targeted alpha therapies and refine radiation protection frameworks.
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All authors Harsh Arya, Joshua Rajan, Varghese Chirayath, Mainul Arbar, Awat Lotfihagh, Sitmukhambetov Satzhan, Yan Chang, Alex Weiss, Zui Pan, Yujie Chi

Geophysics

Constructive Reuse of Free-Surface Ghost Wavefields in Marine Seismic Data: A Propagation-Based Framework

Hitoshi Mikada

Recent advances in marine seismic acquisition and processing have substantially improved the decomposition of free surface ghost wavefields. Building upon these developments, this study proposes a propagation-based framework in which decomposed ghost wavefields are treated as physically meaningful components that can be constructively reused as additional wavefield components. Starting from a finite inverse expansion of the ghost operator, a recursive formulation is derived and implemented through adaptive predictive decomposition. The formulation is subsequently extended to multidimensional wavefields through wavefield extrapolation and reciprocity, enabling decomposition into components associated with real and mirrored sources and receivers. The decomposed wavefields are then restored to a common acquisition geometry by survey sinking and combined through wavefield superposition, forming a preprocessing module that can be incorporated into conventional marine seismic processing workflows. Synthetic examples demonstrate increased signal strength through ghost reuse, spectral amplitude recovery of ghost-related notches including frequencies near DC, and clear decomposition of ghost-related wavefield components. Field-data examples acquired using a conventional shallow-towed airgun survey further demonstrate improvements in reflector continuity and coherent reflection energy, together with partial restoration of spectral amplitudes degraded by the free surface ghost, supporting the practical applicability of the proposed framework. The proposed framework therefore provides a processing-based complement to conventional ghost decomposition technologies and suggests a unified approach in which acquisition geometry and ghost processing are jointly optimised to maximise the usable wavefield information.
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Optics

Overcoming sensitivity-bandwidth trade-off in mid-infrared spectroscopy by a microresonator-anchored swept laser

Zhaoyu Cai, Zihao Wang, Yulei Ding et al. (+11 more)

Optical frequency combs have revolutionized high-precision spectroscopy, yet an intrinsic trade-off between spectroscopic signal-to-noise ratio (sSNR) and measurement bandwidth ($B$) fundamentally constrains sensitive, broadband measurements. While broadband swept lasers offer a potential solution, generating broadband, ultrafast and linearly sweeping lasers with a narrow linewidth remains a significant challenge, particularly in the fingerprint mid-infrared (mid-IR) band. Here we overcome this limitation by using a microresonator-anchored ultrafast sweeping Fourier domain mode-locked (FDML) laser for mid-IR spectroscopy. We introduce a dual-microresonator-anchor approach: a microcomb provides frequency calibration and a high-Q microresonator resolves the instantaneous FDML lasing lineshape. The strategy enables accurate correction of the FDML laser's sweep nonlinearity and broad linewidth in the near-IR, allowing the FDML laser to function as a high-fidelity mid-IR light via difference frequency generation. The system achieves a record sSNR$\times$$B$ of 1.3$\times$10$^5$ THz$\cdot \sqrt{\rm Hz}$ and methane sensing precision of 9 ppb$\cdot$m$\cdot$$\sqrt{\rm s}$, while retaining GHz resolution to distinguish methane isotope. We further demonstrate broadband, coherent swept laser phase spectroscopy in the mid-IR, tolerating losses up to 78 dB. This work leverages advances in integrated photonics to overcome the fundamental limitations of precision spectroscopy, paving the way for next-generation, broadband, and ultra-sensitive mid-IR spectroscopic sensing systems.
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All authors Zhaoyu Cai, Zihao Wang, Yulei Ding, Yifei Wang, Chengjiu Wang, Changxi Yang, Yanan Guo, Jianchang Yan, Junxi Wang, Xun Liu, Jiangtao Li, Ruocan Zhao, Xianghui Xue, Chengying Bao

Chemical Physics

Charge Transfer with a Spin. II: A Framework for Diabatization which Localizes Charge and Spin

Alok Kumar, Zhen Tao, Zuxin Jin et al. (+2 more)

We investigate a diabatization procedure that localizes charges (in real space) and localizes spins (in spin space) for open-shell systems that exhibit charge transfer in the presence of spin-orbit coupling. The procedure is applied to a two-state crossing between pairs of Kramers-restricted doublet states (which can also be considered effectively a four-state crossing). To generate the relevant diabatic states, we employ a two-step optimization over complex-unitary rotations that sequentially maximizes dipole and spin moments through iterative Jacobi sweeps; the resulting update rules are effectively equivalent to those of approximate joint diagonalization (AJD) applied to charge and spin. The method converges rapidly and yields smooth diabatic potential energy surfaces that preserve dipole and spin properties (e.g., a smoothly varying spin quantization axis) along the reaction coordinate while maintaining time-reversal symmetry.
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All authors Alok Kumar, Zhen Tao, Zuxin Jin, Joseph E. Subotnik, Tian Qiu

Chemical Physics

Charge Transfer with a Spin. I: A Variational Constrained-CASSCF Framework for Investigating Charge Transfer in the Presence of Spin-Orbit Coupling

Alok Kumar, Zhen Tao, Joseph E. Subotnik et al. (+1 more)

Charge transfer in open-shell molecular systems can involve a delicate interplay between charge localization, orbital relaxation, and spin-orbit coupling (SOC), particularly near ground-excited state crossings. Here, we introduce a very inexpensive variational framework for treating these effects in odd-electron systems by extending the electron/hole-transfer Dynamically-weighted State-Averaged Constrained CASSCF (eDSC/hDSC) method to include SOC. Our method incorporates the SOC Hamiltonian directly into the variational orbital optimization through complex-valued spinor orbitals, allowing orbital and spin degrees of freedom to relax self-consistently while preserving the time-reversal symmetry of doublet states. The method achieves smooth potential energy surfaces and rapid self-consistent field (SCF) convergence; moreover, from physically small to artificially large SOC strengths, the calculation can be converged to very tight thresholds across the whole potential energy surface. While the results presented here are for systems with two charge centers, as shown in the appendix, the theory is quite general and can be adapted to either a system of $N$ charge centers or a metal-molecule surface with a continuum of states, so that charge transport (and not just charge transfer) can also be studied. The approach therefore provides a route towards ab initio studies of spin-dependent charge transfer and charge transport in molecular systems with nontrivial spin degrees of freedom.
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All authors Alok Kumar, Zhen Tao, Joseph E. Subotnik, Tian Qiu

Fluid Dynamics

StabOp: A Data-Driven Stabilization Operator for Reduced Order Modeling

Ping-Hsuan Tsai, Anna Ivagnes, Annalisa Quaini et al. (+2 more)

Spatial filters have played a central role in large eddy simulation and, more recently, in reduced order model (ROM) stabilization for convection-dominated flows. Nevertheless, important open questions remain: in under-resolved regimes, which filter is most suitable for a given stabilization or closure model? Moreover, once a filter is selected, how should its parameters, such as the filter radius, be determined? Addressing these questions is essential for the reliable design and performance of filter-based stabilization strategies. To answer these questions, we propose a novel strategy that differs fundamentally from current filter-based approaches: we replace traditional spatial filters with a data-driven stabilization operator (StabOp) that yields accurate results for a given resolution, quantity of interest, and stabilization strategy. Although the new StabOp can be used for both classical discretizations and ROMs, and for different types of filter-based stabilization or closure, for clarity we focus on ROMs and the Leray stabilization. To build the new StabOp, we postulate its model form as a linear, quadratic, or nonlinear mapping, and then solve a PDE-constrained optimization problem to minimize a given loss function. Using the resulting StabOp in the Leray ROM (L-ROM) yields a new stabilized ROM, StabOp-L-ROM. To assess the StabOp-L-ROM, we compare it with the L-ROM and the standard ROM in numerical simulations of four flows: 2D flow past a cylinder at Re=500, lid-driven cavity at Re=10000, 3D flow past a hemisphere at Re=2200, and minimal channel flow at Re=5000. Our numerical results show that the StabOp-L-ROM can be orders of magnitude more accurate than the classical L-ROM tuned with an optimal filter radius in the predictive regime. Furthermore, while the new StabOp smooths the input flow fields, its smoothing mechanism differs from that of classical spatial filters.
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All authors Ping-Hsuan Tsai, Anna Ivagnes, Annalisa Quaini, Traian Iliescu, Gianluigi Rozza

Optics

Terahertz Synthetic FM Triplet for Distortion-Free Stabilization and Lamb-Dip Spectroscopy

Kohei Eguchi, Toki Tanaka, Hiroshi Ito et al. (+1 more)

We demonstrate a distortion-free terahertz frequency stabilization technique using a "synthetic FM triplet" to overcome modulation sideband interference associated with high-density spectral lines in molecular clocks. By applying this method to the rotational transitions of acetonitrile (CH$_3$CN), we successfully generated clean derivative waveforms free from inter-line interference, achieving a fractional frequency instability of $1 \times 10^{-9}$ at an averaging time of $1~\mathrm{s}$. Furthermore, we report the observation of Lamb-dips using this high-fidelity approach. Our results establish acetonitrile as a promising candidate for high-agility molecular clocks and provide a robust solution for precision spectroscopy of molecules with complex hyperfine structures.
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All authors Kohei Eguchi, Toki Tanaka, Hiroshi Ito, Koichiro Tanaka

Saturday 7 February

Computational Physics

Surrogate Modeling for Neutron Transport: A Neural Operator Approach

Md Hossain Sahadath, Qiyun Cheng, Shaowu Pan et al. (+1 more)

This work introduces a neural operator based surrogate modeling framework for neutron transport computation. Two architectures, the Deep Operator Network (DeepONet) and the Fourier Neural Operator (FNO), were trained for fixed source problems to learn the mapping from anisotropic neutron sources, Q(x,{\mu}), to the corresponding angular fluxes, {\psi}(x,{\mu}), in a one-dimensional slab geometry. Three distinct models were trained for each neural operator, corresponding to different scattering ratios (c = 0.1, 0.5, & 1.0), providing insight into their performance across distinct transport regimes (absorption-dominated, moderate, and scattering-dominated). The models were subsequently evaluated on a wide range of previously unseen source configurations, demonstrating that FNO generally achieves higher predictive accuracy, while DeepONet offers greater computational efficiency. Both models offered significant speedups that become increasingly pronounced as the scattering ratio increases, requiring <0.3% of the runtime of a conventional S_N solver. The surrogate models were further incorporated into the S_N k-eigenvalue solver, replacing the computationally intensive transport sweep loop with a single forward pass. Across varying fission cross sections and spatial-angular grids, both neural operator solvers reproduced reference eigenvalues with deviations up to 135 pcm for DeepONet and 112 pcm for FNO, while reducing runtime to <0.1% of that of the S_N solver on relatively fine grids. These results demonstrate the strong potential of neural operator frameworks as accurate, efficient, and generalizable surrogates for neutron transport, paving the way for real-time digital twin applications and repeated evaluations, such as in design optimization.
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All authors Md Hossain Sahadath, Qiyun Cheng, Shaowu Pan, Wei Ji

Chemical Physics

Photoluminescent registration of fullerite C$_{60}$ derivatives during chemical interaction with H$_{2}$ and N$_{2}$ molecules

Victor Zoryansky, Peter Zinoviev and Yuri Semerenko

Single crystals of C$_{60}$ saturated with molecular hydrogen and nitrogen were studied using the spectral-luminescent method of registration in the quantum counting mode at a low temperature of 20 K. Previously, it was found that the temperature limit of the adsorption crossover (the transition from the diffusion mechanism of intercalation - physisorption to chemical interaction - chemisorption) for fullerite C$_{60}$ in an H$_{2}$ atmosphere is about 525 K, and in an N$_{2}$ atmosphere 695 K, respectively. At saturation temperatures above those indicated, the process of chemical interaction of impurity molecules and the fullerite matrix occurs with the formation of new compounds. Therefore, saturation was carried out at temperatures of 570 K for hydrogen and 720 K for nitrogen under a pressure of 30 atm. For the first time, the results of registration of photoluminescent radiation at low temperatures from new substances based on fullerite C60 are presented. A mixture of weakly saturated fulleranes C$_{60}$H$_{X}$ was obtained by saturation of hydrogen from the gas phase at a temperature close to the sorption crossover temperature, and their low-temperature photoluminescence was identified for the first time. The analysis of the "blue" shift of the beginning of the spectrum of such radiation allowed us to more accurately attribute the obtained material to the initial segment of the fullerane series. The presence of radiation of the azafullerene dimer (biazafullerene) (C$_{59}$N)$_{2}$ in the spectra of the reaction products of C$_{60}$ with N$_{2}$ was also detected. It was shown that for polycrystalline samples of C$_{60}$ saturated in a nitrogen atmosphere, the characteristic luminescence of biazafullerene with a maximum at 1.53 eV determines the intensity and shape of the entire short-wave part of the emission spectrum of the complex of synthesized substances.
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Chemical Physics

The Interplay of Pauli Repulsion, Electrostatics, and Field Inhomogeneity for Blueshifting and Redshifting Vibrational Probe Molecules

R. Allen LaCour, Ruoqi Zhao and Teresa Head-Gordon

Many molecules' vibrational frequencies are sensitive to intermolecular electric fields, enabling them to probe the field in complex molecular environments. However, it is often unclear whether the probe is responding to the local electric field or other types of intermolecular interactions, inhibiting interpretation of the frequency and effectiveness as probes. This is especially true of molecules whose vibrational frequencies blueshift instead of the more typical redshift in hydrogen bonding configurations. Here we computationally investigate the causes of redshifting versus blueshifting over a range of vibrational reporters. First, we apply adiabatic energy decomposition analysis to a paradigmatic set of probes, finding that redshifting only occurs when electrostatic interactions are strong enough to overcome the dominant and large blueshifting contribution of Pauli repulsion. Furthermore, we demonstrate that field inhomogeneity can further shift the frequency of many probes substantially to either reinforce or counteract the shift expected from a homogeneous field. We find that redshifting is reinforced by electric field inhomogeneity, otherwise field inhomogeneity further weakens the electrostatic contribution relative to Pauli repulsion, leading to blueshifting. Further calculations indicate that the probe's response to field inhomogeneity can be understood by considering the mass of the atoms involved in the stretching mode and sign of the electric field. In explaining the interplay of different intermolecular interactions and field inhomogeneity for many probes, our results should enable the use and interpretation of spectroscopic probes and their connection to electric fields in more complex systems.
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Physics Education

Notes from the Physics Teaching Lab: Optical Pumping

Kenneth G. Libbrecht

We describe a series of experiments done using a commercially available optical pumping apparatus that is currently being used in physics teaching labs at over one hundred universities. Our focus here is to provide an extensive and detailed examination of the capabilities of this instrument, including numerous examples of measurements and data analysis, presented as a supplement to the manufacturers user manual. Our hope is that instructors using this or similar optical pumping instruments will find the experiments described here useful for designing and implementing the curricula in their own physics teaching labs.
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Plasma Physics

Assessing the impact of Open Research Information Infrastructures using NLP driven full-text Scientometrics: A case study of the LXCat open-access platform

Kalp Pandya, Khushi Shah, Nirmal Shah et al. (+2 more)

Open research information (ORI) play a central role in shaping how scientific knowledge is produced, disseminated, validated, and reused across the research lifecycle. While the visibility of such ORI infrastructures is often assessed through citation-based metrics, in this study, we present a full-text, natural language processing (NLP) driven scientometric framework to systematically quantify the impact of ORI infrastructures beyond citation counts, using the LXCat platform for low temperature plasma (LTP) research as a representative case study. The modeling of LTPs and interpretation of LTP experiments rely heavily on accurate data, much of which is hosted on LXCat, a community-driven, open-access platform central to the LTP research ecosystem. To investigate the scholarly impact of the LXCat platform over the past decade, we analyzed a curated corpus of full-text research articles citing three foundational LXCat publications. We present a comprehensive pipeline that integrates chemical entity recognition, dataset and solver mention extraction, affiliation based geographic mapping and topic modeling to extract fine-grained patterns of data usage that reflect implicit research priorities, data practices, differential reliance on specific databases, evolving modes of data reuse and coupling within scientific workflows, and thematic evolution. Importantly, our proposed methodology is domain-agnostic and transferable to other ORI contexts, and highlights the utility of NLP in quantifying the role of scientific data infrastructures and offers a data-driven reflection on how open-access platforms like LXCat contribute to shaping research directions. This work presents a scalable scientometric framework that has the potential to support evidence based evaluation of ORI platforms and to inform infrastructure design, governance, sustainability, and policy for future development.
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All authors Kalp Pandya, Khushi Shah, Nirmal Shah, Nakshi Shah, Bhaskar Chaudhury

Optics

Terahertz multispectral polarimetric imaging based on intensity measurement

Redwan Ahmad, Charles Simard, Rejeena R Sebastian et al. (+3 more)

A compact terahertz (THz) polarimetric spectrometer and imaging system is demonstrated using polarization-sensitive frequency-selective surfaces (PS-FSS) and rapid, intensity-based detection. Real-time Stokes parameter extraction enables angle-of-linear-polarization (AoLP) measurements and quantitative retrieval of the birefringence of a 1-mm-thick x-cut quartz crystal, used here as a benchmark anisotropic sample. The extracted birefringence is in good agreement with THz time domain spectroscopy-based measurements, validating the accuracy of the proposed THz polarimetric approach. Signal-to-noise ratios (SNR) up to 87 dB across 0.23-0.41 THz ensure reliable discrimination of the measured spectral components above the noise floor. Extending this approach to raster-scanned THz imaging using distinct PS-FSS orientations (0{\deg}, 45{\deg}, 90{\deg}, and 135{\deg}) enables simultaneous mapping of frequency-dependent Stokes parameters. The resulting degree of linear polarization (DoLP) and AoLP maps exhibit well-resolved polarimetric contrast, consistent with numerical simulations and independent visible polarimetric measurements. Operating entirely without field-resolved detection or mechanical delay stages, the system provides a robust and compact platform for THz polarimetric spectroscopy and imaging of anisotropic materials.
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All authors Redwan Ahmad, Charles Simard, Rejeena R Sebastian, Jonathan Lafreniere-Greig, Xavier Ropagnol, Francois Blanchard

Optics

Short mode-locked pulses from planarized Y-coupled THz lasers

Urban Senica, Tabea Bühler, Sara Cibella et al. (+4 more)

Short THz pulses are highly attractive both for fundamental research and practical applications in this underdeveloped region of the electromagnetic spectrum. Typically, THz pulses are generated using nonlinear optical methods, starting from a high-power visible or near-IR mode-locked source, but usually suffer from low conversion efficiencies. Here, we present a direct on-chip THz source of coherent short pulse trains based on active mode-locking of an inverse-designed planarized Y-coupled THz quantum cascade laser. By employing quasi-resonant microwave modulation of the asymmetric laser cavity, mode-locked pulses as short as 2.3 ps are generated, with emission bandwidths spanning 500 and 700 GHz at a central frequency of 2.9 THz.
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All authors Urban Senica, Tabea Bühler, Sara Cibella, Guido Torrioli, Mattias Beck, Jerome Faist, Giacomo Scalari

Medical Physics · Cross-listed from nucl-ex

Photonuclear Cross Sections for the $^{197}$Au($\gamma$,pn)$^{195m}$Pt Reaction Near Threshold

J. Song, J. Nolen, D. Rotsch et al. (+10 more)

Platinum radioisotopes are of growing interest for targeted cancer therapy and diagnostic imaging because their decay delivers highly localized radiation doses in tissue, herewith enabling precise DNA damage through Auger-electron emission. Developing production technologies that provide platinum isotopes with high specific activity is therefore essential, and photonuclear reactions on stable nuclei offer a viable accelerator-based route when supported by reliable cross-section data. We report photonuclear cross-section measurements for the $^{197}$Au($\gamma$,pn)$^{195m}$Pt reaction at incident $\gamma$-ray energies of 27, 29, and 31 MeV using the activation method. The measurements were performed by irradiating a stack of concentric-ring gold targets with a quasi-monoenergetic $\gamma$-ray beam provided by the High Intensity Gamma-ray Source (HI$\gamma$S). The induced $^{195m}$Pt activity was quantified using off-line $\gamma$-ray spectroscopy. These data provide the first experimental constraints on the $^{197}$Au($\gamma$,pn)$^{195m}$Pt cross section in the near-threshold region. The measured excitation function is compared with PHITS and TALYS calculations and indicates that the reaction becomes measurable only near 30 MeV, with substantially higher bremsstrahlung end-point energies required for practically meaningful production.
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All authors J. Song, J. Nolen, D. Rotsch, R. Gampa, R. M. de Kruijff, T. Brossard, C. R. Howell, F. Krishichayan, S. W. Finch, Y. K. Wu, S. Mikhailov, M. W. Ahmed, R. V. F. Janssens

Atomic Physics · Cross-listed from quant-ph

Kinematic Modulation in Driven Spin Resonance

Sunghyun Kim

The transition probability of a spin driven by a rotating magnetic field is reformulated. This work shows that, once projection onto the measurement basis is properly accounted for, the laboratory measured probability is governed by both intrinsic spin dynamics and the time dependence of the measurement basis. For the rotating-field eigenbasis, this yields an additional kinematic modulation, leading to measurable deviations under strong driving. A unified probability expression is derived that subsumes the classic 1937 and 1954 formulations as limiting cases, while correcting the conventional treatment of magnetic resonance transitions.
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Plasma Physics

2DESR: a two-dimensional Fourier-space gyrokinetic eigenvalue code for the ion-temperature-gradient modes in tokamaks

Haochuan Wang, Jie Wang, Yuefeng Qiu et al. (+6 more)

A two-dimensional (2D) gyrokinetic eigenvalue solver, 2DESR, has been developed to solve the 2D gyrokinetic eigenvalue problem in the poloidal Fourier space for the ion-temperature-gradient (ITG) modes in tokamaks. With full kinetic effects of ions retained, the 2D gyrokinetic eigenvalue equations in the poloidal Fourier space have been derived and numerically solved in the 2DESR code. In the linear ITG Cyclone test with adiabatic electrons, the 2DESR code benchmarks well against the gyrokinetic initial-value codes GENE and NLT. It is found that two branches of ITG modes coexist in the system.
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All authors Haochuan Wang, Jie Wang, Yuefeng Qiu, Shaojie Wang, Zihao Wang, Tiannan Wu, Yuesong Li, Yicheng Cai, Shiqi Xiao