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Friday 31 July

Quantum Physics

Phase-Space Methods for Many-Body Quantum Optics

Edgar Guardiola-Navarrete, Silvia Cardenas-Lopez and Ana Asenjo-Garcia

Many-body quantum-optical systems, where a collection of emitters interacts through a common electromagnetic reservoir, exhibit rich out-of-equilibrium behavior and hold promise for applications in quantum technologies. However, exact numerical simulations of their dynamics quickly become unfeasible due to the exponential growth of the Hilbert space with system size. Semiclassical, phase-space approaches – such as the Truncated Wigner approximation (TWA) – provide computationally efficient alternatives by capturing leading-order quantum fluctuations. In this paper, we present a comprehensive overview of how to tackle problems in many-body quantum optics using phase-space methods. We derive the exact partial differential equation governing many-body dissipative evolution in any phase-space representation and discuss the approximations that yield the dissipative TWA proposed by Mink and Fleischhauer [SciPost Phys. 15, 233 (2023)]. We find that $P$ and $Q$ distributions are generally suboptimal for many-body quantum optics. Additionally, we extend the formalism to calculate multi-time correlation functions, thereby broadening the scope of phase-space simulations of open spin systems to include coherence and spectral properties, as well as directional correlations of collectively radiating emitters. These developments provide valuable tools for investigating exotic light sources driven by collective dissipation, driven-dissipative phase transitions, and a wealth of many-body phenomena arising in state-of-the-art experimental platforms.
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Cellular Automata and Lattice Gases · Cross-listed from math.DS

Extinction thresholds in a graph-based model of HIV infection dynamics

Manuel A. Espinosa-García, Ana Paulina Figueroa, Julián A. Fresán-Figueroa et al. (+2 more)

We study a graph-based cellular automaton for HIV infection dynamics in lymph-node networks, originally introduced by Mukwembi. Each vertex represents a cell site that may be healthy, infected, or dead, and the evolution is controlled by a replacement parameter $R$, which determines whether a dead cell is replaced by an infected or a healthy cell according to the number of its infected neighbors. For a graph $G$, we introduce two extinction parameters. The parameter $\mathbf{hiv}(G)$ is the smallest value of $R$ for which extinction occurs for every admissible initial configuration, whereas $\mathbf{HIV}(G)$ is the smallest threshold such that extinction occurs for every replacement parameter greater than or equal to it. We prove the general bounds $2\leq \mathbf{hiv}(G)\leq \mathbf{HIV}(G)\leq \Delta(G)+1$ and characterize the extremal case $\mathbf{HIV}(G)=\Delta(G)+1$. We also show that the gap $\mathbf{HIV}(G)-\mathbf{hiv}(G)$ is unbounded and determine both parameters for some classical families of graphs. Finally, we study the dynamics of the model using the state-transition digraph of the system and the configurations whose trajectories converge to nontrivial periodic orbits. The results show that extinction depends not only on the replacement parameter but also on the structural properties of the underlying graph.
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All authors Manuel A. Espinosa-García, Ana Paulina Figueroa, Julián A. Fresán-Figueroa, Gerardo L. Maldonado, L. Ariadna Sánchez-Solís

Superconductivity

Ferroelectric superconductivity in noncentrosymmetric metals

Jason G. Kattan and T. Pereg-Barnea

It has recently been shown in experiments that certain materials can display both superconductivity and ferroelectricity, contrary to a long-standing conjecture that these two phenomena are incompatible, or at least unrelated. In this work we study superconductivity in ferroelectric metals, using a formalism of ionic polarization fields coupled to itinerant electrons, both of which are treated at the microscopic level. The ferroelectric order manifests as a spontaneous polarization that may be uniform or spatially modulated, and fluctuations of the polarization mediate interactions between the electrons. The polarization fluctuations give rise to attractive interactions that can lead to Cooper pairing in certain lattice configurations, analogous to the nonpolar phonons in conventional BCS theory. Working with a simplified BCS model, we derive conditions under which superconductivity can coexist with and even emerge from ferroelectricity.
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Quantum Physics · Cross-listed from cs.CC

Sharp Hardness for MAX-3-CUT and Quantum MAX-CUT

Steven Heilman

Assuming the Unique Games Conjecture, we show it is NP-hard to approximate MAX-3-CUT within a multiplicative factor of $\alpha_3+\epsilon$ for every $\epsilon>0$, where $\alpha_3\approx.83600811464$ is the approximation ratio of Frieze-Jerrum's polynomial-time algorithm from 1995. That is, we prove sharp hardness of approximation for MAX-3-CUT. This result resolves a conjecture of Khot-Kindler-Mossel-O'Donnell from 2004 by proving the three candidate Plurality is Stablest Conjecture for correlations in $[-1/2,2/5]$ and generalizes the Majority is Stablest Theorem of Mossel-O'Donnell-Oleszkiewicz [Annals of Math, 2010]. With a similar strategy we prove: assuming the Unique Games Conjecture, it is NP-hard to approximate the product-state value of Quantum MAX-CUT within a multiplicative factor of $\alpha_{\rm BOV}+\epsilon$ for every $\epsilon>0$, where $\alpha_{\rm BOV}\approx 0.9563372685$ is the approximation ratio of the Briët-de Oliveira Filho-Vallentin algorithm. This sharp hardness result completes the conjectured hardness of Hwang-Neeman-Parekh-Thompson-Wright from 2021 by proving their $S^{k-1}$-valued Borell inequality for correlations in $[-.5843,.5843]$ for all $k\geq3$.
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High Energy Astrophysical Phenomena

Fast Population Leakage in Astronomical Masers: Maser Amplification and Transient Superradiance

Vahid Anari, Toktam Rashidi and Fereshteh Rajabi

We use a $\Lambda$-type three-level Maxwell–Bloch model to test whether an inverted molecular transition in an astronomical maser source can produce maser amplification or superradiance when its upper level also decays through a second radiative pathway with a much larger spontaneous decay rate. Such shared-upper-level configurations occur in multilevel, radiatively pumped molecules, including Class II methanol masers and several OH maser transitions. The model follows the coupled evolution of level populations, molecular coherences, radiation fields, and phenomenological relaxation and dephasing, separating population leakage from coherence loss. We focus on the mixed configuration in which the observed transition is inverted while the faster pathway is non-inverted and acts as a leakage channel. We find that rapid spontaneous decay through the competing pathway does not, by itself, suppress maser amplification or superradiant emission from the inverted transition. The response is controlled by the shared upper-level population reservoir, the available initial coherence, and the relaxation and dephasing timescales. For small effective coherence in the leakage pathway, its radiative output remains weak, while the inverted transition either amplifies a seed field in the quasi-steady maser regime or develops macroscopic coherence and produces a transient superradiant burst. A larger inversion does not necessarily produce a stronger burst if it is accompanied by a weaker initial coherence seed. As a benchmark, we apply the model to the 6.7 GHz methanol flare in S255IR-NIRS3, whose upper level also decays through the 239.7 GHz transition at a spontaneous rate more than four orders of magnitude larger. The calculated flare remains compatible with a transient-superradiance interpretation when this fast leakage pathway is included explicitly.
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Cosmology and Nongalactic Astrophysics

Spin-1 Ultralight Dark Matter under Cosmological Scrutiny: Mass Constraints from CMB and Distance Probes

Guadalupe Ahumada Acuña, Tomas Ferreira Chase, Diana López Nacir et al. (+1 more)

We present cosmological constraints on spin-1 ultralight dark matter, described by a vector field (VFDM) with mass $m_{\rm A}$, using Planck CMB data and geometrical probes from BAO and SNIa. A key theoretical result is the derivation of the full CMB temperature covariance matrix, including both diagonal and off-diagonal anisotropic contributions induced by the preferred direction of the background vector field. We first constrain the model using the diagonal part of the covariance, together with CMB lensing; the off-diagonal terms, which couple multipoles with $\Delta\ell\in\{2,4\}$, could bias lensing reconstruction, but only at very low multipoles ($L=\{2,4\}$) not included in the Planck likelihood. We consider both a pure VFDM scenario and a mixed VFDM+CDM scenario, characterized by the fraction $f=\Omega_{\rm A}/(\Omega_{\rm A}+\Omega_{\rm cdm})$, obtaining $\log_{10}(m_{\rm A}/\mathrm{eV})>-24.07$ (95% C.L.) in the pure case, and a clear correlation between $f$ and $m_{\rm A}$ in the mixed case, with smaller fractions allowing lighter masses; standard cosmological parameters remain fully consistent with $\Lambda$CDM. For the off-diagonal contributions, we derive the corresponding Bipolar Spherical Harmonic (BipoSH) coefficients and predict their amplitude using our best-fit and bounds. While the anisotropic signal is difficult to detect in the pure VFDM scenario with current Planck data, mixed VFDM+CDM models can produce signals at, or above, Planck sensitivity over a range of multipoles, motivating dedicated searches for this characteristic signature.
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All authors Guadalupe Ahumada Acuña, Tomas Ferreira Chase, Diana López Nacir, Rafael C. Nunes

Instrumentation and Methods for Astrophysics

Design and Performance of 220 and 270 GHz Bandpass Filters for BICEP Array

A. Steiger, The BICEP/Keck Collaboration, P. A. R. Ade et al. (+90 more)

The BICEP Array (BA) is the latest in the BICEP/ Keck series of experiments that aim to measure the polarization of the cosmic microwave background (CMB) with small aperture polarimeters located at the South Pole. To constrain the frequency response of these receivers, each detector is serially coupled to a band-pass filter (BPF). The electric circuits of these BPFs utilize series and shunt capacitors as well as series inductors, but critically do not include shunt inductors which simplifies fabrication. The filters are designed and simulated with Sonnet, and optimized for noise by considering loading from the atmosphere and the CMB. Multiple 220 GHz detector modules have had their frequency response measured at the South Pole. The 270 GHz detector modules have recently begun testing in a lab setting, and their performance in a BA receiver will be measured this winter.
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All authors A. Steiger, The BICEP/Keck Collaboration, P. A. R. Ade, Z. Ahmed, M. Amiri, D. Barkats, R. Basu Thakur, C. A. Bischoff, D. Beck, J. J. Bock, H. Boenish, V. Buza, K. Carter, J. R. Cheshire IV, J. Connors, J. Cornelison, L. Corrigan, M. Crumrine, S. Crystian, A. J. Cukierman, E. Denison, L. Duband, M. Echter, M. Eiben, B. D. Elwood, S. Fatigoni, J. P. Filippini, A. Fortes, M. Gao, C. Giannakopoulos, N. Goeckner-Wald, D. C. Goldfinger, J. A. Grayson, A. Greathouse, P. K. Grimes, G. Hall, G. Halal, M. Halpern, E. Hand, S. A. Harrison, S. Henderson, J. Hubmayr, H. Hui, K. D. Irwin, J. H. Kang, K. S. Karkare, S. Kefeli, J. M. Kovac, C. Kuo, K. Lau, M. Lautzenhiser, A. Lennox, T. Liu, K. G. Megerian, M. Miller, L. Minutolo, L. Moncelsi, Y. Nakato, H. T. Nguyen, R. O'Brient, S. Paine, A. Patel, M. A. Petroff, A. R. Polish, T. Prouve, C. Pryke, C. D. Reintsema, T. Romand, D. Santalucia, A. Schillaci, B. Schmitt, E. Sheffield, B. Singari, K. Sjoberg, A. Soliman, T. St Germaine, B. Steinbach, R. Sudiwala, K. L. Thompson, C. Tsai, C. Tucker, A. D. Turner, C. Vergès, A. G. Vieregg, A. Wandui, A. C. Weber, J. Willmert, W. L. K. Wu, H. Yang, C. Yu, L. Zeng, C. Zhang, S. Zhang

Materials Science

Transient Liquid Phase Bonding of NiTi Using Cu- and Nb-base Interlayers

Zhaoxi Cao, Samuel Price, Alessandra Crippa et al. (+3 more)

Transient liquid phase (TLP) bonding was examined as an approach for joining NiTi to achieve a high joint efficiency while minimizing chemical variance within the joint region. Two bonding interlayer chemistries (Cu-base and Nb-base) were identified by screening thermodynamic criteria for TLP in ternary alloys using the CALPHAD method. These two systems were then experimentally evaluated with respect to their impact on solidification kinetics, microstructure in the joint region, and performance during quasistatic and cyclic tensile loading. For both interlayer chemistries, the composition profile and microstructure in the joint region confirmed an isothermal solidification mechanism. In addition, the joints were found to be fully dense and contain at most 1.2% intermetallic phases. Tensile testing showed excellent load transfer across the joints with approximately 4% recoverable strain and martensite onset stresses reaching 94% and 89% of the unbonded, annealed NiTi values for Cu-base and Nb-base interlayers, respectively. Lastly, a stable superelastic response was observed under cyclic loading for both bond chemistries, with spatial variation in the strain evolution linked to enhanced stiffness and hardness in the joint region arising from the substitutional Cu and Nb solutes, as confirmed via nanoindentation. This study demonstrates that TLP bonding of NiTi can produce high-strength and nearly intermetallic-free joints without sacrificing functional performance, such as the superelastic response.
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All authors Zhaoxi Cao, Samuel Price, Alessandra Crippa, John P. Reidy, Gianna M. Valentino, Ian McCue

Earth and Planetary Astrophysics

Planetary atmospheric escape and disk formation around WDJ0914+1914

C. Villarreal D'Angelo, M. P. Ronco, M. R. Schreiber et al. (+3 more)

The spectrum of the white dwarf WD J091405.30+191412.25 displays the absorption and double-peaked emission lines of the volatiles hydrogen, oxygen, and sulfur. This unique characteristic has been interpreted as evidence of this white dwarf accreting mass from a circumstellar disk that had formed from atmospheric material evaporating off a close-in Neptune-like or super-puff mass planet. Thus far, however, the orbital separation of the planet and its mass-loss rate have only been estimated using simple analytical approximations. We investigate this scenario using 3D radiative-hydrodynamic simulations of irradiated hydrogen atmospheres together with 1D viscous disk evolution models. We compute atmospheric escape from Neptune-like and super-puff planets exposed to extreme ultraviolet (XUV) radiation of the white dwarf at different orbital separations and follow the evolution of the escaping gas after it forms a circumstellar disk. The simulations yield planetary mass-loss rates of $(1.8-4)x10^{12}$ g/s. The injected material forms a gaseous disk that reaches a quasi-steady state in less than $10^5$ through the balance between continuous mass supply and viscous accretion onto the white dwarf. The resulting accretion rates are consistent with observational estimates. In contrast to previous interpretations, our models predict that the disk extends beyond the planetary orbit. We conclude that a gas-rich planet orbiting at 15 solar radii undergoes sustained photoevaporation and naturally produces a circumstellar disk capable of reproducing the observed accretion rates and spectral signatures of WD J0914+1914. These results provide strong support for the evaporating-planet scenario and offer new constraints on the structure and extent of the circumstellar disk.
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All authors C. Villarreal D'Angelo, M. P. Ronco, M. R. Schreiber, O. Toloza, A. Esquivel, B. T. Gänsicke

Biological Physics

Noise-induced stability of insect swarms

Justin Faber, Annabelle Boots and Dolores Bozovic

Flying insect swarms exhibit cohesion without the local velocity alignment observed in flocks of birds or schools of fish. The interaction rules and channels of communication between insects that lead to collective behavior have not yet been determined. We propose a theoretical model based on acoustic communication between swarm members, where each individual is attracted to a weighted mean field of its neighbors. We demonstrate that this simple framework can describe a wide range of swarming dynamics, including a phenomenon where pairs of insects break free from the swarm in a helical orbit around each other. Counterintuitively, our numerical model suggests that stochastic noise enhances the stability of an insect swarm, as it interferes with pair formation. Finally, we demonstrate that a specific species of malarial mosquito produces swarms that reside on the edge of a transition to instability. Our study shows that fairly simple local interaction rules can be sufficient to describe the collective behavior of swarming biological systems.
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Geophysics

A Poromechanics-Based Framework for Fully Coupled Reactive Transport and Geomechanics in Porous Rocks

Fan Fei, Yifan Yang, Aleksander Zibitsker et al. (+4 more)

Mineral precipitation and dissolution in rocks alter pore structure, stress, and hydraulic properties, producing tightly coupled chemo-hydro-mechanical processes that are central to subsurface systems. Yet, existing continuum-scale simulators lack a generalizable, mathematically tractable, and physically grounded chemo-mechanics formulation for modeling these coupled processes. To address this gap, this work presents a poromechanics-based framework for chemo-mechanics coupling and integrates it into coupled reactive transport and geomechanics simulations. Building upon classical poromechanics theory, the framework provides distinct descriptions for the stress states of the host rock and pore minerals, in addition to pore fluid pressure, enabling a rigorous and flexible treatment of their individual behavior and mechanical interactions during precipitation and dissolution. As demonstrated by numerical examples, the poromechanics-based method captures the expected mechanical response by translating pore-scale mineral growth into mineralization pressure. By accounting for mineral compressibility, the method also supports more physically realistic representations of deformation and induced stress. Numerical comparisons further highlight the unique capability of the poromechanics-based approach to model effective stress evolution and fracture development without explicitly representing pore geometry or other microstructural features. Overall, this framework offers a versatile, physics-based predictive approach to modeling coupled chemo-mechanical processes in reactive porous rocks and supports future reservoir-scale analyses of engineered subsurface systems.
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All authors Fan Fei, Yifan Yang, Aleksander Zibitsker, Giuseppe Buscarnera, Randolph Settgast, Kari Finstad, Giovanna Bucci

Optics

Spatiotemporal Metasurface for Ultrafast All-Optical Wavefront Shaping

Mert Akturk, Giulia Crotti, Andrea Schirato et al. (+6 more)

Optical metasurfaces have established themselves as exceptional nanophotonic platforms to control the properties of light at subwavelength scales. By properly designing the geometry and spatial arrangement of their constituent meta-atoms, engineered phase gradients can be imparted to an incoming field, thereby tailoring its wavefront with a virtually unlimited number of control knobs. However, once fabricated, a metasurface has a permanently fixed optical response. Here, we introduce a strategy for dynamic control, by theoretically predicting and experimentally demonstrating all-optical, reversible, and ultrafast wavefront shaping in a periodic semiconductor nanowire metasurface. A strongly astigmatic femtosecond optical pump pulse is used to photoexcite the resonant metasurface non-uniformly, inducing a transient, spatially inhomogeneous permittivity modulation that reshapes the wavefront of a delayed probe pulse. Ultrafast imaging measurements reveal transient defocusing of the transmitted field at selected probe wavelengths, with a switching time below one picosecond. Quantitative numerical modelling elucidates the defocusing mechanism as arising from the interplay between the unperturbed optical response of the resonant metasurface and the pump-induced spatial permittivity gradient. Our work establishes a fully all-optical and ultrafast route to the dynamic analogue of passive wavefront shaping in gradient metasurfaces, paving the way to all-optically reconfigurable metalenses.
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All authors Mert Akturk, Giulia Crotti, Andrea Schirato, Katsuya Tanaka, Isabelle Staude, Thomas Pertsch, Giulio Cerullo, Giuseppe Della Valle, Margherita Maiuri

General Relativity and Quantum Cosmology

Kerr black holes with vector dark matter hair

Fredric Hancock and Helvi Witek

We consider a massive vector-field perturbation on a Kerr black hole background. We fix the field's density at the edge of the black hole sphere of influence to model a bath of wave cold dark matter, from which the black hole can accrete endlessly. We employ the approach of Lunin, Frolov, Krtouš, Kubizňák, and Santos to separate the equations, and solve them with a mix of analytical and numerical techniques. We find that the field forms a density spike around the black hole with profile $\rho \sim r^{-3/2}$, consistent with dark matter spikes studied in the literature, which is not significantly distorted by spacetime rotation, except near the black hole's ergoregion. For any nonzero spin, an additional superradiant regime appears, in which co-rotating modes extract mass and angular momentum from the black hole. We find a mass extraction rate as high as $10\, M_\odot/\text{yr}$ in the superradiant regime for a $10^9\, M_\odot$ black hole. For large field masses, the rate at which the black-hole mass grows behaves similarly to the Schwarzschild case, reaching $\sim 100\, M_\odot/\text{yr}$ for a $10^9\, M_\odot$ black hole. We compare these results to their scalar-field counterparts and discuss how they mix with the superradiant instability.
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General Relativity and Quantum Cosmology

Stationary scalar clouds, quasinormal ringing, and observational signatures of (near-)extremal rotating Kalb-Ramond black holes: from the superradiant threshold to EHT bounds

Gülnihal Tokgöz and İzzet Sakall{\i}

We study a massive scalar field on the rotating Kalb-Ramond black hole of Kumar, Ghosh and Wang, in the (near-)extremal corner of its three-parameter family $(a,\gamma,\lambda)$. We verify by direct substitution that the Klein-Gordon-Fock equation separates on this background, with radial and angular separation constants related by $\Lambda_{rad}=\Lambda_{ang}+a^2\omega^2-2am\omega$. At exact extremality the horizon polynomial and the function $K(r)$ share a zero, rendering $K^2/\Delta$ regular there and reducing the radial problem to Whittaker form on the Kerr line $(\gamma=0)$ and the Kerr-Newman line $(\lambda=1)$. The stationary resonances saturate $\omega=m\Omega_H$ and satisfy the bound-state condition $\mu>m\Omega_H$, with overtones accumulating at the threshold from above; in the genuinely Kalb-Ramond interior we solve the exact radial equation as a two-point boundary value problem. Moving off extremality, the clouds continue onto the co-rotating zero-damped branch $\omega_n=m\Omega_H-i\kappa(n+1/2+\beta)$, governed by the same near-horizon exponent $\beta$; the photon-sphere family is a distinct observable. Greybody factors and superradiant amplification are obtained by direct integration, recovering the classical scalar amplification on Kerr. Strong-field lensing is treated in the Bozza-Tsukamoto limit with rotating photon orbits of both senses, where the prograde logarithmic coefficient departs strongly from its Schwarzschild value. The shadow contour follows from the Carter-Hamilton-Jacobi separation. Crossing the Event Horizon Telescope window of Zubair, Raza and Maqsood with the cloud-existence region yields a narrower bound on the Lorentz-violating parameters $(\gamma,\lambda)$ than either constraint alone.
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High Energy Astrophysical Phenomena

Polarization Angle Swings in Blazars Detected in the Millimeter-wave with the South Pole Telescope

A. Simpson, E. Anderes, A. J. Anderson et al. (+93 more)

We present the first systematic search for electric vector position angle (EVPA) swings in the millimeter-wave (mm-wave) emission of blazars, using five years of observations from the South Pole Telescope SPT-3G camera at 95 and 150 GHz, and investigate their connection to gamma-ray flares. Of the 168 bright sources in the ~1500 square degrees SPT-3G Main Field, eight have sufficient polarization signal-to-noise for reliable EVPA measurement, four of which have continuous Fermi LAT gamma-ray detections. We detect EVPA swings in all four gamma-ray-active blazars and in none of the remaining four, consistent with the established connection between EVPA swings and high-energy emission seen at optical wavelengths. The observed swing amplitudes and rotation rates are smaller than those found in optical studies, consistent with mm-wave EVPA variability being slower than at shorter wavelengths. Random walk simulations of the polarization angle using a multi-cell model fail to reproduce both the number, amplitude, and duration of the observed swings, suggesting that this mechanism is insufficient to explain the observed swing population. Analysis of EVPA variability on one-week timescales is consistent with the anti-correlation between polarization degree and EVPA rotation rate previously observed at optical wavelengths. Of the 14 detected swings, eight are within 30 days of a gamma-ray flare. While many individual swing-flare associations are found to have a very low probability of happening by chance, the full ensembles of 95 and 150 GHz swing time lags with respect to gamma-ray flares are found to be consistent with random coincidence.
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All authors A. Simpson, E. Anderes, A. J. Anderson, B. Ansarinejad, M. Archipley, L. Balkenhol, D. R. Barron, P. S. Barry, K. Benabed, A. N. Bender, B. A. Benson, F. Bianchini, L. E. Bleem, S. Bocquet, F. R. Bouchet, E. Camphuis, M. G. Campitiello, J. E. Carlstrom, J. Carron, C. L. Chang, P. M. Chichura, A. Chokshi, T.-L. Chou, A. Coerver, T. M. Crawford, C. Daley, T. de Haan, K. R. Dibert, M. A. Dobbs, M. Doohan, D. Dutcher, C. Feng, K. R. Ferguson, N. C. Ferree, K. Fichman, A. Foster, S. Galli, A. E. Gambrel, A. K. Gao, F. Ge, F. Guidi, S. Guns, N. W. Halverson, A. D. Hincks, E. Hivon, G. P. Holder, W. L. Holzapfel, J. C. Hood, A. Hryciuk, N. Huang, E. Järvelä, T. Jhaveri, F. Kéruzoré, A. R. Khalife, L. Knox, K. Kornoelje, C.-L. Kuo, K. Levy, Y. Li, A. E. Lowitz, C. Lu, G. P. Lynch, X. Ma, T. J. Maccarone, G. Madejski, A. S. Maniyar, E. S. Martsen, F. Menanteau, M. Millea, J. Montgomery, Y. Nakato, T. Natoli, A. Ouellette, Z. Pan, K. A. Phadke, A. W. Pollak, K. Prabhu, W. Quan, M. Rahimi, A. Rahlin, C. L. Reichardt, M. Rouble, J. E. Ruhl, A. C. Silva Oliveira, J. A. Sobrin, A. A. Stark, C. Tandoi, C. Trendafilova, J. D. Vieira, A. G. Vieregg, A. Vitrier, Y. Wan, N. Whitehorn, W. L. K. Wu, M. R. Young, J. A. Zebrowski

General Relativity and Quantum Cosmology

How greedy is the Universe? An entropy ledger and a causal envelope for early black hole growth

Fernando Izaurieta, Cristian Quinzacara and Omar Valdivia

The entropy of the observable Universe is dominated, by fifteen orders of magnitude, by the horizons of supermassive black holes, and gravitational collapse into black holes has often been proposed as the Universe's preferred channel of entropy production. We examine this suggestion quantitatively, and the analysis largely refutes it. Three results are presented. First, an entropy production history: the rate $dS/dt(z)$ resolved by channel, from the star formation history and a Soltan-normalized accretion history. Black hole horizon growth exceeds all radiative channels by a factor $\sim 10^{16-17}$ after the first seeds; the total rate peaked near $z\approx 1$–$2$ and has since declined; the late peak is structural, because horizon entropy production weights accretion by the accretor's mass and the most massive accretors are assembled last. Second, a causal-hydrodynamic envelope: the fastest entropy-producing trajectory permitted by causality and gas dynamics, $\dot M \sim c_s^3/G$ in atomic-cooling halos, which reaches $10^{7\pm1}\,M_\odot$ by $z\approx 8$–$10$ without radiative throttling, and whose phenomenology (obscured, radiatively inefficient, X-ray weak, red) closely resembles the JWST little red dots. The realized-to-envelope ratio defines an efficiency $F(z)$, which remains between $10^{-7}$ and $10^{-4}$ throughout $4\le z\le 12$, even under the most generous JWST-era bracket. Third, order-of-magnitude no-go results: Boltzmann weighting of collapse channels by entropy gain, dissipative adaptation applied to self-gravitating systems, and any strong maximum-entropy-production principle in cosmology all fail at leading order, by factors of up to $10^{16}$. The Universe produces entropy overwhelmingly through black holes, yet remains far below its own envelope at all times. Whatever selects cosmic structure, it does not maximize entropy production.
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Plasma Physics

On the ideal magnetohydrodynamic stability of the sheared-flow Z pinch

Daniel W. Crews and Jackson C. Turner

Sheared-flow Z-pinch stability has previously been studied within ideal MHD primarily through growth rate calculations, which find that even trans-Alfvénic sheared flows apparently fail to suppress the kink instability. Here, trans-Alfvénic sheared flow is shown to stabilize the MHD kink but also to excite shear-driven instabilities characteristic of high-Reynolds-number supersonic flow. This distinction is evident from the dispersion relations underlying the growth rates, computed as the analytic dispersion function in the complex-frequency plane. Regularization splits this function into adiabatic and resonant parts describing how discrete modes emerge from and interact with the continuous spectrum. The Doppler-shifted flow continuum is then seen to interact with the interchange and kink instabilities in distinct ways. For interchange, the continuum overlaps the instability branch at all wavenumbers, so even sub-Alfvénic sheared flow stabilizes profiles modestly beyond the interchange threshold. The kink, by contrast, is shielded from the continuum by a frequency gap, and trans-Alfvénic flow is required to Doppler-shift the continuum into resonance with it, giving a geometric picture of the stabilization threshold. But shear-driven instabilities arise at this same threshold, including reflection modes and an acoustic kink. It is these shear-driven modes, not the original MHD instabilities, that dominate the ideal-MHD spectrum in trans-Alfvénic conditions. The ideal analysis thus describes the stabilization mechanism while showing that the stability of the sheared-flow Z pinch ultimately rests on non-ideal physics, such as finite orbit width and dissipation.
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Astrophysics of Galaxies

A Review of How Supermassive Black Hole Binaries Can Be Detected and Characterized through Gravitational Lensing of Background Stars

Mu'allim Yakubu

Supermassive black hole binaries are an inevitable outcome of hierarchical galaxy mergers, but their sub-parsec evolutionary phase between the kpc separations resolvable by direct imaging and the millihertz gravitational-wave regime targeted by LISA remains observationally elusive. Gravitational lensing of background stars by SMBH binaries has emerged as a powerful complement to periodic variability searches, periodic Doppler-boost signals, and circumbinary accretion hydrodynamic models. Two distinct lensing channels are reviewed: (i) the gravitational lensing of Milky Way bulge stars by the central Sgr A-star binary, whose secondary image is now within reach of ELT-class instruments; and (ii) the recently introduced Quasi-Periodic Lensing of Starlight (QPLS) mechanism, in which the rotation of caustics produced by a non-active SMBH binary magnifies individual bright stars in its host galaxy on the orbital period of the binary. The QPLS framework predicts 1-50 (190-5000) binaries per cubic parsec with periods below 10 yr and redshift z less than 0.3, opening a multimessenger window onto pulsar timing array sources and LISA-band events. After outlining the theoretical foundation, microlensing formalism, observational status, and the synergy with pulsar timing arrays and LISA, we identify outstanding challenges and project the field into the LSST era.
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Optics

Hybrid BaTiO3/TiO2 Metasurface for Efficient Gigahertz-Speed Free-Space Electro-Optic Modulation

Zhongpeng Sun, Kerolos M. A. Yousef, Michael Domm et al. (+13 more)

Free-space electro-optic modulators are key to emerging photonic systems, yet their performance remains limited by trade-offs between modulation efficiency, bandwidth, and device aperture. Here we report a hybrid BaTiO3 (BTO)/TiO2 metasurface for large-aperture, efficient, gigahertz-speed free-space electro-optic modulation. Combining scalable BTO film growth by radio-frequency magnetron sputtering with mature TiO2 nanofabrication, we pattern the metasurface in TiO2 on an unetched BTO layer. The resulting devices support guided-mode resonances with quality factors exceeding 1300 and an optical confinement factor of ~0.8, while the continuous BTO layer makes efficient use of the applied voltage, together maximizing the overlap between the optical and driving fields within the BTO. A device with a 0.3 mm x 0.3 mm metasurface achieves a transmittance modulation efficiency of ~0.020 per volt and a -3 dB electro-optic bandwidth of ~0.8 GHz, with an effective Pockels coefficient of ~151 pm/V for the BTO. This establishes a scalable route to high-performance free-space electro-optic modulators for LiDAR, free-space optical communication, and reconfigurable optical computing.
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All authors Zhongpeng Sun, Kerolos M. A. Yousef, Michael Domm, Agham Posadas, Xudong Li, Marcus Ossiander, Maryna L. Meretska, Yiwei Ju, Isabel Barth, Jason Tischler, Theodore P. Letsou, Moaz Waqar, Amirhassan Shams-Ansari, Xiaoqing Pan, Alexander A. Demkov, Federico Capasso

General Relativity and Quantum Cosmology

Can machine learning improve the detectability and disentanglement of the gravitational-wave background?

Hugo Einsle, Marie Anne Bizouard, Tania Regimbau et al. (+2 more)

Gravitational waves from compact binary coalescences and from early Universe processes are expected to form a gravitational-wave background. We employ a custom deep learning multi-scale multi-headed autoencoder architecture to isolate gravitational-wave background from detector noise, followed by a Markov chain Monte Carlo inference stage to separate the astrophysical and cosmological components. Analyzing $108$-day mock datasets representative of the first period of the fourth LIGO-Virgo-KAGRA observing run, we show that we can detect with high confidence — $\log_{10}$ noise Bayes factor larger than 3 — a compact binary coalescence gravitational-wave background with an amplitude of $4.3^{+0.5}_{-0.4}\times10^{-9}$ at $f_{\rm ref}=25\,\mathrm{Hz}$, which is a factor $\sim5$ higher than the amplitude expected from compact binary sources. We also show that we can isolate a cosmological – assumed flat spectrum – gravitational-wave background as weak as $ 9.7^{+2.5}_{-2.4} \times 10^{-10}$ from the expected compact binary coalescence gravitational-wave background within simulated Gaussian noise mimicking the LIGO detectors sensitivity achieved in the fourth observing run. In blind-test comparisons with the standard pygwb pipeline, we show that our method achieves more accurate amplitude and spectral-index recovery and enables the separation of astrophysical and cosmological background components.
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All authors Hugo Einsle, Marie Anne Bizouard, Tania Regimbau, Mairi Sakellariadou, Jishnu Suresh

Astrophysics of Galaxies

On an empirical method to build near-equilibrium axisymmetric and triaxial galaxy models

Pierfrancesco Di Cintio, Alessandro Alberto Trani and Valentina Cesare

We present a numerical technique to implement $N$-body realizations of axisymmetric and triax- ial self-gravitating system starting from a spherical self-consistent models for which a phase-space distribution is known. The method builds up on the so-called adiabatic squeezing technique by K. Holley-Bockelmann et al. (2001) and ideally allows to control the effective ellipticities of the particle distribution. We discuss some preliminary tests and sketch a possible improvement of our method
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Instrumentation and Methods for Astrophysics

A Comparative Systems-Engineering Framework for RFI Coexistence in Radio Astronomy and Aviation Safety Systems

Simthembile Dlamini

Radio astronomy and aviation safety systems occupy opposite ends of the signal-power spectrum, yet both depend on the same finite resource and are increasingly squeezed by commercial broadband wireless services. The two fields protect their receivers using separately derived, apparently incommensurable criteria: the ITU-R RA.769 detrimental-interference threshold for radio telescopes, and the radar-altimeter interference thresholds established by RTCA Special Committee 239 for the 5G C-band coexistence problem. We introduce a single coexistence-margin framework, M(f,d) = Pth(f) - Prx(f,d), and show both criteria to be special cases of it. We apply this framework to two case studies, the Karoo Radio Quiet Zone surrounding MeerKAT and SKA-Mid, and the global 5G C-band/altimeter dispute, showing that, despite unrelated regulatory histories, both converged on the same three-lever solution: guard-band separation, bounded exclusion zones, and receiver-side filtering, governed by mandatory coordination rather than static exclusion. We then show that interference-excision, beamforming, and statistical-calibration pipelines developed for radio interferometry are closely analogous to, and in several cases directly reusable for, operations required by ADS-B validation, phased-array radar clutter rejection, and predictive-maintenance anomaly detection. The unifying framework, and the finding that two independently-arising regulatory histories converge on the same design pattern, is this papers central contribution: M(f,d) has not, to our knowledge, been proposed before as a common formalism spanning these two protection regimes, and the toolkit mapping distinguishes genuine reuse from looser analogy. The result is a reusable design pattern and shared skills/infrastructure pipeline, applicable wherever a narrowband receiver must coexist with a growing broadband commercial neighbour.
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Fluid Dynamics

Elasto-hydrodynamics of droplet-pool-interactions

Md Sultan and Purbarun Dhar

In Newtonian fluids, impact of a droplet on a liquid pool births a cavity, crown, capillary waves, and Worthington jet. The corresponding hydrodynamic events for elastic or Boger fluids, however, remain an uncharted domain of comprehension and exploration. We thoroughly investigate, via experiments, theory, and simulations, how elastic energy storage, fluid relaxation, and competitive inertio elasto capillarity govern the spatio temporal evolution of the cavity, the crown, and the ensuing Worthington jet in polymeric elastic fluids. The events are systematically explored over a wide range of impact Weber and Deborah numbers, considering varied Newtonian and elastic fluid droplet pool combinations, and revealing new, and distinct morphological regimes compared to Newtonian counterparts. We illustrate that these new findings are purely driven by fluid elasticity, and not by viscosity or interfacial tension. We derive a theory for cavity radius evolution, using energy conservation within potential-flow framework. We show that 30-40 % of the droplets kinetic impact energy may be stored as elastic energy by the stretching polymer chains during cavity expansion. Appealing to the FENE P model, we derive a theory for the temporal evolution of the radius of the elongated Worthington jet. We show that in elasto capillary regime, competitive elastic and capillary stresses lead to exponential decay of the jet radius. The role of elastic stresses and the local velocity field in governing cavity evolution, morphology, and jet formation are further elucidated through computer simulations. Our findings significantly advance the uncharted paradigm of interplay between inertia, capillarity, and elasticity in droplet-pool interaction elastohydrodynamics.
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Instrumentation and Methods for Astrophysics

A Focal-plane X-ray Polarimeter with Spectral and Timing Capabilities for Future Missions

Hemanth Manikantan, Carlo Lefevre, Lorenzo Petrucci et al. (+16 more)

The NASA-ASI mission Imaging X-ray Polarimeter Explorer (IXPE) firmly established X-ray polarimetry as a core observation pillar of high-energy astrophysics, alongside imaging, timing, and spectroscopy. While IXPE made groundbreaking discoveries in the 2$-$8 keV range on both point and extended X-ray sources, the Gas Pixel Detectors onboard IXPE demand substantial improvements to be ready for the next generation of imaging X-ray polarimeters. Here, we present the development of a detector prototype that can deliver the next generation of sensitive imaging X-ray polarimetry across the energy band of 2–30 keV. We use Ar/DME based gas volume to absorb X-rays by the photoelectric effect and a pixelated CMOS readout ASIC, Timepix3, with excellent timing capabilities to image the resulting photoelectron track in three dimensions. The Timepix3 is integrated with a multiplication stage called InGrid that enables single-primary-electron detection from the gas volume. The photoelectron track reconstructed in three dimensions increases the polarimetric sensitivity towards the lowest operable energies, and the deadtime-free operation of the ASIC facilitates the usage as a focal plane instrument on high-throughput X-ray mirrors. With this prototype, we demonstrate a low-medium-energy X-ray polarimeter with excellent timing and moderate spectral capabilities.
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All authors Hemanth Manikantan, Carlo Lefevre, Lorenzo Petrucci, Paolo Soffitta, Fabio Muleri, Enrico Costa, Alda Rubini, Vladislavs Plesanovs, Markus Gruber, Jochen Kaminski, Klaus Desch, Alessandro Di Marco, Sergio Fabiani, Riccardo Ferrazzoli, Saba Imtiaz, Dawoon E. Kim, Alessandro Lacerenza, John Rankin, Ajay Ratheesh

Materials Science

Resonant Raman spectroscopies beyond density-functional theory

Aleksandr Poliukhin, Corto Babs Aubry, Lorenzo Bastonero et al. (+1 more)

Resonant Raman spectroscopy probes, in a single measurement, how electrons and phonons couple in a material. Density-functional theory (DFT) typically reproduces well phonon frequencies, but resonant Raman intensities hinge on electron-phonon matrix elements and electronic transitions that are far more sensitive to the underlying exchange-correlation approximation. However, electron-phonon coupling has so far been accessible only through linear-response theories developed for a handful of semilocal DFT methods, leaving the sensitivity of resonant Raman intensities to the electronic-structure approximation essentially unexplored. Here, we introduce a general finite-difference framework that can compute resonant Raman tensors for any electronic-structure method capable of delivering forces, eigenvalues, and wavefunctions of pristine and displaced configurations. We apply the formalism to graphene and monolayer MoS$_2$, using hybrid functionals or meta-GGAs, and show that these approaches systematically enhance electron-phonon couplings relative to semilocal DFT, reflecting reduced dielectric overscreening. A decomposition of the Raman tensor shows that accurate intensities require electronic eigenvalues and electron-phonon matrix elements to be treated consistently at the same level of theory. Among the approaches tested, hybrid functionals provide the best overall agreement with experiment. Because the framework needs only quantities every electronic-structure code already produces, it opens the door to systematic, beyond-DFT Raman characterization or benchmarking against experiments, especially for 2D materials.
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All authors Aleksandr Poliukhin, Corto Babs Aubry, Lorenzo Bastonero, Nicola Marzari

Solar and Stellar Astrophysics

An extreme case of X-ray reflection in the symbiotic V648 Car

Jherssica Freitas, Raimundo Lopes de Oliveira and Koji Mukai

V648 Car is a $\delta$-type symbiotic star known for its strong hard thermal X-ray emission, heavy local photoelectric absorption, and stochastic variability. We analyze NuSTAR observations, complemented by nearly contemporaneous Swift/XRT observations, to check for the expected presence and physical implications of Compton reflection in its X-ray spectrum. The spectra were fitted with X-ray models that accounted for thermal plasma components, different prescriptions to quantify the intrinsic absorption, the presence of the fluorescent iron line at 6.4 keV, and a reflection component. Our results reveal a strong Compton reflection contribution, accounting for approximately 37% of the total unabsorbed flux in the 3-50 keV band. The inclusion of reflection significantly improves the spectral fits and reduces the value for the maximum plasma temperature by a factor of 2 (from about 49 keV to 25 keV). If the maximum temperature is attributed to strong shock of a Keplerian flow, the inferred white dwarf mass decreases from about 20% (from $1.2$ to $0.95\,M_{\odot}$), demonstrating that neglecting reflection can lead to significant overestimates of this parameter. Reflection models yield high reflection scaling factors ($R \sim 2$), inconsistent with a simple picture of a primary X-ray source above a reflecting disk. The timing analysis reveals stochastic flickering variability with no evidence for coherent periodic modulations, thus the accreting object is likely a non-magnetic white dwarf. We propose a multiple-reflection model to explain the extraordinary strength of reflection features in V648 Car.
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Quantum Physics

No low-degree tests for quantum states

Omar Alrabiah, Srinivasan Arunachalam, Sabee Grewal et al. (+1 more)

We study the problem of testing low-degree phase states, namely m-qudit quantum states of the form $q^{-m/2} \sum_{x \in \mathbb{F}_q^m} \omega^{f(x)} |x>$, where $f$ is a degree-$d$ polynomial. In contrast to the classical setting, where low-degree polynomials admit highly efficient classical testers, it is not known whether analogous quantum tests exist. We show that no such quantum low-degree test exists: any tester requires $\Omega(\binom{\lfloor m/2\rfloor}{\lfloor (d-1)/2 \rfloor})$ copies to determine whether a given state is a degree-$d$ phase state or is far from every such state. Our results follow from a general framework that relates quantum testing of codeword phase states to classical decoding properties of the dual code, which allows us to leverage known bounds on the tolerance of high-rate Reed–Muller codes to random errors.
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All authors Omar Alrabiah, Srinivasan Arunachalam, Sabee Grewal, John Wright

Astrophysics of Galaxies

Vortex State of Ultralight Dark Matter and the Fornax Timing Problem

V. Gorkavenko, O. Barabash, T. Gorkavenko et al. (+4 more)

We investigate the impact of the vortex state of the ultralight dark matter (ULDM) on the dynamical friction acting on moving globular clusters. Comparing this force with that for the solitonic ground state, it is shown that the internal structure and rotation of the ULDM core strongly affect the orbital decay of globular clusters. In particular, co-directional rotation in a vortex state can lead to significant suppression of dynamic friction at certain distances where globular clusters and ULDM velocities match. Applying these findings to the Fornax dwarf galaxy, it is found that the Fornax timing problem is naturally alleviated.
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All authors V. Gorkavenko, O. Barabash, T. Gorkavenko, K. Korshynska, O. Teslyk, A. Zaporozhchenko, E. Gorbar

Strongly Correlated Electrons

Interplay of spin-orbit coupling, crystal field splitting and correlations: a ghost rotationally invariant slave boson treatment

Xue Sun, Walber Hugo Brito, Andreas Gleis et al. (+4 more)

We investigate the interplay of spin-orbit coupling, crystal field splittings, and electronic correlations in the $t_{2g}$ Hubbard-Kanamori model within the recently formulated ghost rotationally invariant slave-boson method (GRISB). In particular, we study a tight binding model of Sr$_2$RuO$_4$ with parameters extracted from density functional theory and linearized quasiparticle self-consistent GW (LQSGW) calculations; we study the behavior of different physical quantities as the number of ghosts increases to examine the convergence of GRISB to dynamical mean field theory (DMFT) and experimental results; and we leverage the ability of GRISB to investigate the model over a wide range of parameters at low temperature. In particular, we examine both static and dynamical observables driven by the spin orbit coupling (SOC) and study how they vary as a function of the Hubbard $U$ and Hund's coupling $J$. GRISB converges quickly for most of these observables, and the calculations reveal the following: $U$ enhances the spin-orbit coupling while $J$ suppresses it. We also study the shape of the Fermi surface within different methodologies, and examine the Lifshitz transition which takes place as a function of strain in this material.
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All authors Xue Sun, Walber Hugo Brito, Andreas Gleis, Ran Adler, Tsung-Han Lee, Gabriel Kotliar, Corey Peters

Space Physics

An Adaptive Approach for Modeling Collisionless Plasma Shocks and Ion Heating in the Shock Frame

Yuri A. Omelchenko, Igor V. Sokolov, Lulu Zhao et al. (+2 more)

We present a novel computational method for hybrid (kinetic ions, quasi-neutral fluid electrons) simulations of non-relativistic magnetized collisionless plasma shocks, observed near the Sun, in the solar system, and beyond. This Adaptive Frame-Of-Reference Approach (AFORA) enables simulations of shocks in their co-moving frames of reference, where shock evolution remains quasi-steady for quasi-stationary upstream conditions, in contrast to moving shocks. We demonstrate our method in two spatial dimensions for a range of angles between the background magnetic field and the shock normal (obliquities). Shock-frame simulations, conducted with a time-adaptive hybrid code, HYPERS, are shown to self-consistently capture upstream and downstream magnetic fluctuations commonly observed by spacecraft at interplanetary shocks and Earth's bow shock. These fluctuations, which may be crucial for estimating the efficiency of ion acceleration at heliospheric and astrophysical shocks, are notably missing from many hybrid simulations of shocks, unless upstream external turbulence is imposed. The quasi-stationary behavior of numerical shocks in the shock frame is confirmed in a series of animations and by demonstrating converged ion energy spectra in the downstream region. Our approach underscores computationally efficient and physically more accurate modeling of shock-driven turbulence, downstream plasma relaxation, and concomitant ion heating in the shock frame. The new method can be used for exploring ion relaxation at interplanetary (IP) shocks, more efficient studies of cosmic rays (CRs) and plasma turbulence, and more accurate predictions of seed populations of Solar Energetic Particles (SEPs) at heliospheric shocks driven by coronal mass ejections (CMEs).
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All authors Yuri A. Omelchenko, Igor V. Sokolov, Lulu Zhao, Keheng Zhu, Michal Golan