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Wednesday 30 September

Quantum Physics

Technical analysis of the Resource-efficient Quantum Walkers Quantum Random Access Memory

Giuseppe De Riso, Giuseppe Catalano, Seth Lloyd et al. (+2 more)

Quantum Random Access Memory (qRAM) is a critical component for achieving quantum advantage in algorithms ranging from database search to quantum machine learning. In a recently introduced model [arXiv:2508.02855], we proposed a resource-efficient qRAM architecture based on discrete-time quantum walkers. This article serves as a comprehensive technical follow-up, providing the full mathematical derivations, detailed protocol specifications, and in-depth resource analysis. Moreover, we extend the original proposal with novel techniques for the purpose of making the qRAM implementation more realistic. Our model resolves the primary drawbacks of leading qRAM proposals: it avoids the exponential number of active nodes $\mathcal{O}(2^n)$ required by the "Bucket Brigade" architecture by employing a number of quantum walkers that scales linearly with the address and message sizes, $n$ and $m$, respectively. Simultaneously, it overcomes the spatial bottlenecks of previous quantum-walker schemes by eliminating the need for multiple parallel trees. We propose two algorithmic paradigms: the long- and the short-range approaches, which differ by the length of interaction of the main routing gates employed in the qRAM. We formalize the routing, message-copy, and walker retrieval phases for both variants and we show how, while the long-range scheme employs controlled gates having a multitude of target systems, the short-range approach decomposes these interactions into sequences of 2- and 3-body local gates, which improves the architecture's feasibility for near-term experimental implementation. Finally, our comprehensive resource analysis confirms that the short-range approach achieves the optimal $\mathcal{O}(n+m)$ circuit depth. Within these two paradigms, we explore the potential of different types of quantum walkers, namely bosons, dual-rail qubits and four-level qudits.
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All authors Giuseppe De Riso, Giuseppe Catalano, Seth Lloyd, Vittorio Giovannetti, Dario De Santis

Nuclear Theory

Comparison of chisquare variants for Poisson-distributed data and ratios

Mate Csanad, Yan Huang, Daniel Kincses et al. (+2 more)

Fits to binned, Poisson-distributed data are common in high-energy and heavy-ion physics, and they are particularly delicate in correlation function measurements, where the fitted observable is the ratio of two histograms. We compare several goodness-of-fit estimators on large toy data sets: the Neyman and Pearson $\chi^2$, their Yates continuity-corrected versions, a Neyman $\chi^2$ with the variance shifted by $1/2$, the Poisson log-likelihood, and the correlation function likelihood in which both the signal and the reference histogram are treated as Poisson distributed. For a single histogram with mean occupancy $\lambda$, the Neyman $\chi^2$ underestimates the bin content by approximately one count, while the Pearson $\chi^2$ overestimates it by approximately half a count; shifting the variance by $1/2$ changes the Neyman result only at order $1/\lambda$, and only the log-likelihood recovers the mean without bias. For the ratio of two histograms, Neyman-type estimators are biased by approximately $-3/\lambda$ in relative terms (3% at $\lambda=100$), whereas the Pearson and likelihood-based fits are unbiased in the symmetric configuration studied here. The Yates correction leaves the fitted values essentially unchanged but systematically deflates the $\chi^2$, resulting in unrealistically high confidence levels. Simple analytic expressions are derived that reproduce all observed biases. Since these biases do not decrease with the number of bins, whereas the statistical uncertainties do, they can dominate the uncertainty of high-statistics, finely binned measurements. We therefore recommend likelihood-based fits, in particular the correlation function likelihood, for femtoscopic and similar ratio analyses.
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All authors Mate Csanad, Yan Huang, Daniel Kincses, Marton I. Nagy, Barnabas Porfy

Mathematical Physics · Cross-listed from math.DG

On Lorentzian Lie groups of Nonzero Constant Curvature

Mohamed Boucetta

We investigate Lie groups endowed with left-invariant Lorentzian metrics of nonzero constant sectional curvature. We first revisit Heintze's theory and obtain a characterization of the Lie algebras of Riemannian Lie groups of negative constant curvature. We also extend classical results of Nomizu and Barnet to the pseudo-Riemannian setting by constructing a large family of Lie groups carrying incomplete left-invariant metrics of constant sectional curvature. We then describe Lorentzian Lie algebras of nonzero constant curvature according to the causal nature of their center and derived ideal. This description is complete except when the derived ideal is Lorentzian, for which we obtain a complete classification in dimension four. We show that every left-invariant Lorentzian metric of nonzero constant curvature on \(\mathrm{SL}(2,\mathbb R)\) is bi-invariant and complete. Moreover, a semisimple Lie group admits a complete left-invariant Lorentzian metric of nonzero constant curvature if and only if it is locally isomorphic to \(\mathrm{SL}(2,\mathbb R)\). We also characterize \(\mathrm{SL}(2,\mathbb R)\) through the existence of a noncentral spacelike left-invariant Killing vector field. Finally, we classify Lorentzian Lie algebras of nonzero constant curvature in dimensions at most four.
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Earth and Planetary Astrophysics

Project CERES: Nuclear Thermal Transportation and In-Situ Propellant Production A Foundational Architecture for Solar System Logistics

S.P. Worden and F.G. Kennedy

Project CERES proposes a reusable deep-space transportation architecture centered on the dwarf planet Ceres, combining nuclear thermal propulsion (NTP) with in-situ production of water, hydrogen, oxygen and ammonia. Its first flight, a Pathfinder that surveys Ceres and returns samples, also bears on the question of life: Dawn found the ingredients of prebiotic chemistry there, and returned material would show just how far that chemistry went. We survey every direct Earth-Ceres opportunity from 2032 through 2042 with ephemeris-based Lambert analysis and compare chemical, gravity-assisted, nuclear-electric and nuclear thermal vehicles under one parametric mass model. Impulsive requirements vary from 10.2 to 14.1 km/s across the decade. A 900 s NTP stage delivering 35 t closes near 220 t in low Earth orbit and does so at every surveyed window; chemical delivery is marginal and sensitive to opportunity and staging, and nuclear-electric propulsion requires multi-megawatt power to match one-to-two-year trip times. Refueling at Ceres reduces the launch mass of an Earth-Ceres-Earth sortie by 61%, and the specific impulse of the propellant the node produces decides whether Ceres water becomes cargo or tanker mass. We conclude that Ceres uniquely combines abundant volatiles, exceptionally low gravity and a strategic Main Belt position, and that nuclear thermal propulsion is the only propulsion class that closes mass, time and the return leg together. Together they convert deep-space exploration from independent expeditions into a network operating on a timetable.
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Materials Science

Training and memory in a randomly driven fractal gel

Chloe W. Lindeman, Joshua D. Clugston, Justin C. Goodrich et al. (+3 more)

A variety of disordered materials, including jammed particulate systems and crumpled paper, can be trained to exhibit memory of a cyclic strain amplitude. Only recently, however, have questions emerged around the whether random driving can impart similar training. Here, we employ x-ray photon correlation spectroscopy to study a fractal nanoparticle gel trained via either deterministic cyclic shear or random shear bounded by strain amplitudes $-\gamma_t$ and $+\gamma_t$. Both types of training lead to microstructural reversibility, with a slower and more irregular training for the random protocol. In both cases, the shear induces redistribution of internal stress in the gel with corresponding irreversible strain displacements whose magnitudes decrease during training. Finally, we show that memory of the random driving can be read out and quantified using a standard protocol.
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All authors Chloe W. Lindeman, Joshua D. Clugston, Justin C. Goodrich, Mark Sutton, James L. Harden, Robert L. Leheny

Quantum Gases

Room-temperature polariton supersolids

Yuanhao Gong, Jingwen Ma, Shuang Zhang et al. (+2 more)

Exploring exotic quantum phases of matter at room temperature represents a frontier challenge in modern physics. The supersolid phase, uniquely merging crystalline order with frictionless superfluid flow, stands among the most intriguing macroscopic quantum phenomena. However, all previous demonstrations of supersolidity, whether in ultracold atomic gases or III-V semiconductor-based polariton systems, have been strictly confined to cryogenic temperatures. Here, we report the observation of room-temperature supersolids in photonic-crystal polariton condensates. By integrating a room-temperature-stable perovskite semiconductor with a dispersion-engineered photonic-crystal waveguide, we create a polariton condensate with multi-mode dispersion landscapes and pronounced parametric nonlinearities. Above a critical condensation density, the interacting condensates spontaneously break continuous translational symmetry, creating a non-rigid supersolid phase that simultaneously exhibits emergent crystalline order and global quantum coherence. This work establishes a room-temperature platform for investigating quantum hydrodynamics and developing coherent quantum simulation devices.
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All authors Yuanhao Gong, Jingwen Ma, Shuang Zhang, Xiaobo Yin, Xiang Zhang

High Energy Physics (Phenomenology)

PINNing the pion: conformal deep learning for $F_\pi(s)$ and the $(g-2)_\mu$ hadronic contribution

Mayank Goel, Subhadip Mitra and Monalisa Patra

Extracting the pion electromagnetic form factor $F_{\pi}(s)$ through phenomenological curve-fitting models introduces model dependence, unphysical artefacts, and kinematic inconsistencies. We introduce a Physics-Informed Neural Network (PINN) embedded in a conformal $z$-plane that constructs $F_{\pi}(s)$ directly from first principles across spacelike and timelike domains: charge normalisation and Schwarz reflection are enforced by construction, while Cauchy-Riemann analyticity, dispersion relations, Watson's theorem, and perturbative QCD asymptotics enter through the loss functional. Thus, the fundamental S-matrix principles dictate the form factor's behaviour while data act as constraints. Mapping the cut complex plane onto the unit disk bounds the Hessian norm and prevents Neural Tangent Kernel spectral starvation, two known failure modes of deep-learning optimisation. Besides $e^+e^-$ scattering data, we also incorporate $\tau$-decay data through a switch that isolates the pure isovector form factor natively, bypassing model-dependent isospin-breaking pre-corrections. The network organically yields an interior zero-free form factor, while the framework tests experimental tensions around the $\rho(770)$ peak against analyticity and dispersion constraints. We obtain model-independent estimates of the pion charge radius, $\langle r_{\pi}^2 \rangle = 0.435 \pm 0.008_{\text{stat}} \pm 0.007_{\text{cali}}$ fm$^2$, the second-sheet pole parameters, $m_{\rho}^{\text{pole}} = 761.72\pm 1.04$ MeV and $\Gamma_{\rho}^{\text{pole}} = 135.99 \pm 1.20$ MeV, and the two-pion contribution to the muon anomalous magnetic moment, $a_{\mu}^{\pi\pi} = (506.48 \pm 2.02_{\text{stat}} \pm 1.70_{\text{cali}}) \times 10^{-10}$.
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Accelerator Physics

Compact ultrafast intense LWFA-driven crystal-based source of $\gamma$-radiation, positrons and neutrons

Alexei Sytov, Hyeon Myung Jeong, Leejin Bae et al. (+5 more)

Compact sources of intense $\gamma$-rays, positrons and neutrons can bring radiation and particle-beam capabilities to small university-scale laboratories that would otherwise rely on large accelerator facilities. Here we propose and simulate a compact, laser-driven source that combines laser plasma wakefield acceleration (LWFA) with oriented crystalline targets to enhance the conversion of relativistic electron beams into high-energy photons and secondary particles. Electrons aligned with major crystallographic directions undergo coherent interactions, including channeling radiation and coherent bremsstrahlung, producing substantially enhanced photon emission compared with amorphous targets. Moreover, the strong angular-spectral correlation of the emitted radiation enables the generation of collimated $\gamma$-ray beams with reduced spectral bandwidth. Consequently, crystal-enhanced emission can increase particle-production efficiency, namely that of positrons through $\gamma$ conversion into electron-positron pairs and of neutrons through photonuclear reactions. Using Geant4 simulations, we investigate electron energies of 300 MeV, 1 GeV and 3 GeV interacting with an oriented tungsten crystal. Crystal orientation increases the $\gamma$-ray, positron and neutron yields by up to a factor of $\sim2$, reaching production rates of $\sim10^{24}\gamma/s$, $\sim10^{23}e^+/s$ and $\sim10^{21}neutrons/s$ for a 200 pC electron bunch with a duration of a few fs. The collimated $\gamma$-ray brightness reaches $\sim10^{24}\gamma/s/mm^2/mrad^2/0.1\%BW$, with an enhancement of 6-8 compared with random crystal alignment. We further demonstrate tunable quasi-monochromatic radiation through coherent bremsstrahlung in a thin diamond crystal, reaching a brilliance of $\sim3.5\cdot10^{20}\gamma/s/mm^2/mrad^2/0.1\%BW$. Furthermore, we discuss the potential applications of the technique proposed.
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All authors Alexei Sytov, Hyeon Myung Jeong, Leejin Bae, Iryna Chaikovska, Hyoeun Choi, Gianfranco Paternò, Laura Bandiera, Inhyuk Nam

Statistical Mechanics

Many-body topology in parity-preserving tensor networks

Maksimilian Usoltcev, Nguyen Hanh Dung, Carolin Wille et al. (+2 more)

Planar parity-preserving tensor networks (ppTNs) admit a fermionic formulation in which the Gaussian, efficiently contractible limit is deformed by local interactions. We investigate how many-body methods can be used to analyze such contractions in a minimal two-parameter ppTN. Its contraction is simultaneously an interacting fermionic partition function, a loop gas, a deformed toric-code norm, and a quartic Ising model, allowing the same phase diagram to be approached with complementary tools. At its center lies a `topological island', characterized beyond the Gaussian limit by a boundary-twist $\mathbb Z_2$ indicator that reduces to Chern-number parity and admits a loop-winding interpretation under duality. Fermionic perturbation theory predicts the interacting phase boundaries, tensor-network numerics establish their critical behavior, and the loop and spin descriptions reveal a self-dual line with a $c=1$ four-state-Potts multicritical fixed point.
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All authors Maksimilian Usoltcev, Nguyen Hanh Dung, Carolin Wille, Matteo Rizzi, Alexander Altland

Strongly Correlated Electrons

Entropy-Driven Altermagnetism from Thermal Magnons

Tanaya Halder, Ashis K. Nandy and Anamitra Mukherjee

We identify a route to altermagnetism driven by entropy upon heating and demonstrate it in a minimal square-lattice antiferromagnet. An exchange modulation is entropically favored because the resulting momentum-dependent magnon splitting increases the magnon entropy, thereby lowering the free energy and favoring altermagnetism. We derive an instability criterion for this exchange-modulated state, predict its temperature-driven reentrant behavior, and show that its overlap with Néel order defines a finite-temperature altermagnetic phase. Variational spin-wave theory, spin–lattice Monte Carlo, and cluster diagonalization establish the entropy-driven instability and its coexistence with magnetic order. The resulting state supports a transverse spin conductivity. The mechanism provides a route to thermally induced altermagnetism in compensated magnets coupled to sufficiently soft physical modes.
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Quantum Physics

An exponential query advantage from access to Stinespring dilation unitaries in quantum channel learning

Manaki Arihara, Satoshi Yoshida, Philip Taranto et al. (+1 more)

We present a learning task for quantum channels that exhibits an exponential separation in query complexity (in terms of the number of qubits) between two access models: i) black-box channel access and ii) access to a Stinespring dilation unitary of the channel along with its inverse. We consider the task of learning how close the unknown channel is to the set of unitary channels. This is characterized by the optimal average gate fidelity (OAGF), defined as the average gate fidelity maximized over all unitary channels. For constant Kraus rank and a dilation environment of dimension $\operatorname{polylog}(d)$, we provide an algorithm that estimates the OAGF to fixed additive accuracy using $\operatorname{polylog}(d)$ queries to the Stinespring dilation unitary and its inverse. In contradistinction, under the black-box access model, this estimation requires at least $\Omega(\sqrt{d})$ queries, even for channels of constant Kraus rank and constant additive accuracy. As a corollary, we rule out a universal simulator that uses only $\operatorname{poly}(\log d,q)$ queries to the unknown channel to approximate the output of arbitrary $q$-query algorithms with access to a randomly-sampled Stinespring dilation unitary and its inverse, even to sufficiently small constant error. These results demonstrate that access to the system-environment joint evolution and its inverse can provide an exponential advantage in estimating an intrinsic property of a quantum channel.
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All authors Manaki Arihara, Satoshi Yoshida, Philip Taranto, Mio Murao

Instrumentation and Methods for Astrophysics

Toward the Italian Astronomical Community's Contribution to HWO: Science, Technology, and Instrument Concepts

A. Fontana, F. Annibali, N. Antonietti et al. (+101 more)

This paper summarizes the interest of the Italian astronomical community in the possible participation in the development of the Habitable Worlds Observatory (HWO). It is largely based on the outcome of a dedicated meeting held in Rome in July 2025, and it includes further revisions and new suggestions that have arisen in the following months. It includes a succinct description of the main scientific interests and of the key technologies that the Italian community can contribute to the development of the mission. It also includes a short section dedicated to the instrument concepts that we deem important to develop for potential adoption on HWO, and that the Italian community is interested in developing. Finally, it includes a concise presentation of the main instruments for ground-based telescopes (VLT and ELT) and space missions that will be operational before HWO and in which the Italian community has leading positions. They will be invaluable to settle the scientific landscape for the mission and to prepare the Italian scientists to make optimal use of HWO. This paper is primarily aimed at paving the way to discussions and collaborations with worldwide scientists and agencies interested in the development of the mission.
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All authors A. Fontana, F. Annibali, N. Antonietti, S. Bianchi, L. Biasiotti, K. Biazzo, E. Bisesi, A. S. Bonomo, V. Bozza, R. Briguglio, M. Brogi, D. Busonero, A. Cabras, C. Cecchi Pestellini, G. Cescutti, A. Ciaravella, G. Cresci, A. J. Corso, S. Covino, F. D'Ammando, A. De Rosa, P. D'Incecco, V. D'Odorico, V. D'Orazi, S. Desidera, P. Di Marcantonio, M. P. Di Mauro, N. Elias-Rosa, S. Fineschi, E. Fiorellino, C. Fiorini, D. Fulvio, A. Garufi, G. L. Granato, N. Greco, G. Guilluy, S. Ivanovski, L. Izzo, A. Jimenez-Escobar, N. La Rocca, M. Landoni, C. Lazzoni, E. Liistro, F. Z. Majidi, L. Mancini, A. Mangione, F. Mannucci, M. Marcellino, A. Marconi, M. Maris, D. Massari, A. Melis, D. Mesa, M. Messa, G. Micela, D. Milaković, S. Monai, M. Moresco, E. Munari, G. Murante, L. Napolitano, L. Naponiello, V. Nascimbeni, I. Pagano, M. E. Palumbo, G. Pareschi, P. Pari, F. Pedichini, M. G. Pelizzo, L. Pentericci, C. Pezzotti, F. Piazzese, M. Pilia, L. Pino, G. Piotto, S. Piranomonte, D. Polychroni, R. Ragazzoni, E. Rigliaco, F. Romanato, G. Ruffato, G. Sacco, F. Salvestrini, P. Saracco, G. Scandariato, L. Schreiber, P. Severgnini, L. Silva, P. M. Simonetti, A. Sozzetti, R. Spinelli, L. Testi, A. Tozzi, A. Traficante, M. Tsantaki, R. G. Urso, M. Uslenghi, E. Vanzella, D. Vassallo, C. Vignali, G. Vladilo, A. Vogliardi, A. Zanella, T. Zingales

Fluid Dynamics

A Reynolds-like Criterion for the Onset of Universal Active Turbulence

Kunal Kumar, Sandip Sahoo, Kirti Kashyap et al. (+4 more)

Beyond a critical Toner–Tu drive, bacterial turbulence exhibits universal statistics, including a parameter-independent energy spectrum $E(k)\sim k^{-3/2}$. We show that the same universal state can be reached not only by increasing activity, but also by strengthening nonlinear advection, weakening nonlinear saturation, or tuning the Swift–Hohenberg instability. Motivated by the competition between these processes, we identify the independent dimensionless groups governing the dynamics and construct a Reynolds-like control parameter from them. A single critical value, calibrated from the known activity-controlled transition, predicts the onset of universality along the other parameter routes and collapses them onto a common threshold. These results identify a dynamical organizing parameter for the onset of universal active turbulence and provide a Reynolds-like measure for the Toner–Tu–Swift–Hohenberg universality class.
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All authors Kunal Kumar, Sandip Sahoo, Kirti Kashyap, Amal Manoharan, Siddhartha Mukherjee, Anupam Gupta, Samriddhi Sankar Ray

High Energy Physics (Experiment)

Non-parametric and continuous extraction of amplitudes in realistic electroweak penguin decays

Tingzhen Xiao, Anja Beck and Eluned Smith

A novel approach to extract decay amplitudes in $B\to V(\to M_1M_2)\ell^+\ell^-$ processes, where $V$ represents a meson with either $J = 0$ (S-wave) or $J = 1$ (P-wave), was recently proposed. In this method, the dependence of the amplitudes on the dihadron and dimuon invariant masses is extracted model-independently through a fit to the helicity angles and a subsequent calculation of sPlot weights. In this paper, we show that this method still performs well when applied to data with experimental realism, namely in the presence of detection efficiencies and background candidates, both of which can have non-factorising dependencies between the fitted helicity angles and extracted two-particle masses. We show that a non-factorising efficiency can be accommodated with a small change to the conventional calculation of the sPlot weights, and a non-factorising background by expanding the description of the background distribution into individually factorising terms. We illustrate the method using simulated $B\to K^+\pi^-\mu^+\mu^-$ data, containing both S- and P-wave contributions to the $K^+\pi^-$ system, mixed with non-factorising background candidates and generated with a realistic function for the detection efficiency. We also show that this method can be applied over a much larger dimuon invariant-mass range than studied previously. This work suggests that the method is suitable for application to real experimental data, where it could provide model-independent decay-amplitude shapes for direct comparison with theoretical predictions. A measurement using this technique can improve sensitivity to potential new-physics effects in $b\to s\ell\ell$ transitions, as well as the understanding of hadronic form factors, particularly in the S-wave system.
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Quantum Physics

Quantum state learning beyond approximate unitary designs

Gyungmin Cho, Changhun Oh and Dohun Kim

Approximate unitary designs reproduce the statistics of Haar-random unitaries to a given order and accuracy. Recent constructions realize such designs with logarithmic-depth circuits, enabling shallow measurement protocols for various quantum state-learning tasks while preserving performance. These results raise the question of whether approximate designs can replace exact designs more generally. We show that even exponentially small design error need not preserve the learning guarantees of exact designs. This motivates deriving learning guarantees directly from the measurement circuit structure. For observable estimation with classical shadows, we prove that logarithmic-depth two-layer Clifford circuits yield an unbiased estimator matching the global Clifford variance scaling for every state and Hermitian observable. Beyond observable estimation, this bound allows shallow measurements to retain global Clifford guarantees for other tasks, including setting-efficient tomography, mixed-state metrology, and stabilizer structure learning. We complement these statistical guarantees with a compact, exact tensor-network representation of the inverse shadow channel. When measurement bases are reused, approximate unitary designs of arbitrarily high order need not uniformly match the Haar variance in the many-shot limit. For the all-to-all random two-local circuits considered, doing so requires nearly linear depth. Together, our results reveal the capabilities and limitations of shallow quantum state learning, highlighting the distinction between approximating Haar randomness and reproducing its learning guarantees.
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Quantum Physics

Predicting properties of Scrooge ensembles with high accuracy and low sample complexity

Yue Wu, Yuzhi Tong, Helen Propson et al. (+2 more)

The Scrooge ensemble associated with a density matrix $\rho$ is the maximally random ensemble whose average density matrix is $\rho$. It has attracted growing interest for its applications to quantum many-body dynamics and quantum information. The intrinsic statistical properties of the Scrooge ensembles are encoded in their higher-order moments, yet they are nontrivial to analyze and prepare for they feature a non-polynomial Haar integrand, blocking direct use of Haar integration machinery. Existing physical mechanisms to sample multiple copies of states from the Scrooge ensembles, meanwhile, typically require costly postselection, rendering them inefficient for large system size. In this work, we develop efficient and accurate methods for analyzing and implementing Scrooge moments. We first introduce a polynomial approximation to the Scrooge $k$-th moments whose error is tunable down to a threshold that improves from inverse polynomial to exponential in $1/||\rho||_\infty$ over previous results. Building on our approximation, we then give the first efficient quantum algorithm that estimates observable expectation values using $\widetilde{\mathcal{O}}(k^3/\epsilon^5)$ copies of $\rho$ for any target error $\epsilon$ above this threshold. We further provide a construction of a block encoding of the Scrooge $k$-th moment when the preparation circuit of $\rho$ is known. Of independent interest could be a key subroutine in our algorithms, which efficiently implements projection onto the $k$-fold symmetric subspace for $\rho^{\otimes k}$ using $\mathcal{O}(k^3/\epsilon^2)$ samples of $\rho$, avoiding the naive $k!$ postselection cost of such a projection. These results provide new tools for both the theoretical analysis and practical prediction of properties of Scrooge ensembles, with potential applications to quantum many-body dynamics and quantum information processing.
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All authors Yue Wu, Yuzhi Tong, Helen Propson, Yihui Quek, Liang Mao

Statistical Mechanics

Minimizing capture time with many small traps in heterogeneous media

Denis S. Grebenkov and Theodore Kolokolnikov

We study the problem of optimally placing a large number of small absorbing traps to minimize the mean first-passage time (MFPT) of particles diffusing in a heterogeneous medium with space-dependent diffusivity $D(x)$ and prescribed initial particle distribution $\omega(x)$. In two and three dimensions we identify two distinct regimes, depending on how strongly the trap ensemble depletes the particles. In the weak trapping regime (fewer traps) the optimal trap density $\rho$ is the arithmetic average $\rho = \frac12(\mu+\omega)$ of the normalized inverse diffusivity $\mu \propto 1/D$ and the initial particle distribution $\omega$. In the strong trapping regime (more traps), it is proportional to their geometric average, $\rho \propto \sqrt{\mu \omega}$. We revisit the classical Green's function approach and show that it applies to weak trapping only; in 2D this requires the trap size to be exponentially small in the number of traps, so that most applications of interest fall in the strong trapping regime instead. We therefore develop a homogenization approach that captures both regimes. A special choice of initial particle distribution is $\omega \propto 1/D$, which corresponds to the equilibrium particle distribution in the absence of traps according to the Itô interpretation. In this case we show that the optimal trap distribution is the stationary particle distribution itself, regardless of the trapping strength, and this makes the MFPT constant throughout the domain; the same trap distribution also minimizes the worst-case capture time. Finally, in one dimension we find instead $\rho\propto\left(\mu \omega\right) ^{1/3}$, and a similar analysis covers space-dependent drift and thin domains of variable cross-section. Direct numerical optimization confirms the analytical results.
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General Relativity and Quantum Cosmology

Projective Symmetry and Its Breaking in Quadratic Metric-Affine Gravity

Carmen Ferrara, María José Guzmán and Laur Järv

We investigate generalized projective symmetry and its breaking in four-dimensional parity-even metric-affine gravity, considering an action linear in curvature and at most quadratic in torsion and nonmetricity. We derive the action of the generalized projective transformation on the irreducible components of the affine geometry and determine the coupling relations defining the axial, metric-trace, and fully projectively invariant theories. Complete invariance reduces the $12$ gravitational coefficients to a five-parameter family and generates four vectorial Noether identities for the connection equations. Projective invariance makes the connection operator singular, thus we develop a symmetry-adapted method for solving its vacuum field equations. We then analyze couplings to Dirac, electromagnetic, and complex Klein-Gordon fields. Locally exact axial-projective transformations correspond to chiral rotations for massless fermions at the classical level, while a combination of covectors acts as an Abelian connection linking a locally exact vector-projective representative to scalar $U(1)$ symmetry. Standard Maxwell theory is independent of the affine connection, whereas a torsion-dependent Maxwell-like extension can preserve both electromagnetic and projective invariance through compensating Stückelberg fields. Exact gravitational projective invariance makes the corresponding connection directions nondynamical and forces matter currents sourcing them to vanish. Controlled explicit breaking lifts these zero modes and converts them into auxiliary fields. Thus, projective symmetry provides a unified principle for identifying affine gauge modes, constraining matter couplings, and relating symmetry breaking in the gravitational sector to effective matter interactions.
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General Relativity and Quantum Cosmology

On the topology of the space of vacuum initial data sets

Romain Gicquaud and Jonathan Glöckle

We show that the space of vacuum initial data sets on a closed manifold often has many non-trivial homotopy groups. The starting point is a result of the second named author, which constructs non-trivial elements in the homotopy groups of the space of initial data sets satisfying the strict dominant energy condition, together with a later result in joint work with Bernd Ammann showing that these elements often persist when the strictness assumption is dropped. In this work, we use a parametrized version of the conformal method to show that these elements may also be represented by maps into the space of vacuum initial data sets. This requires two results that may be of independent interest: metrics admitting conformal Killing vectors can be removed from the space of metrics without changing its weak homotopy type, and over the remaining metrics the York decomposition can be carried out in families, the TT-tensors forming a trivial Hilbert bundle. To our knowledge, this is the first result on the global topology of this space.
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Mesoscale and Nanoscale Physics

Atomic-Scale Imaging of Lattice Relaxation and Topological Flat Bands in Helical Trilayer Graphene

Shudan Jiang, Zonglin Li, Yu Gu et al. (+15 more)

Helical trilayer graphene (HTG) has emerged as a highly tunable moire quantum material that hosts strong electronic correlations and nontrivial band topology. However, the atomic-scale lattice structure and local electronic properties have remained largely unexplored. Here we present a comprehensive real-space study of HTG using a combination of scanning near-field optical microscopy and low-temperature scanning tunneling microscopy. We directly image supermoire lattice relaxation, revealing large triangular domains separated by sharp domain walls, as well as stripe domains connected by smoothly varying boundaries. Atomic-scale spectroscopy uncovers flat bands with a honeycomb electronic texture and one-dimensional boundary states confined to domain walls. By systematically varying the twist angle, we identify a magic angle of approximately 1.9{\deg}, substantially larger than the 1.6 degree predicted by theory. Our results establish a direct microscopic link between lattice relaxation and flat bands in HTG.
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All authors Shudan Jiang, Zonglin Li, Yu Gu, Liang Liu, Dandan Guan, Yaoyi Li, Hao Zheng, Canhua Liu, Kenji Watanabe, Takashi Taniguchi, Shengwei Jiang, Xiaoxue Liu, Zhiwen Shi, Guorui Chen, Jinfeng Jia, Tingxin Li, Can Li, Shiyong Wang

Nuclear Theory

N = 32 Shell Evolution in Calcium Isotopes: Evidence from Macroscopic to Microscopic Models

A. Anupam, C. Dash, Yashraj et al. (+2 more)

The well-established N=32 subshell closure in calcium serves as an important benchmark for nuclear structure evolution toward neutron-rich nuclei. Recent charge-radius measurements of potassium isotopes [Nat. Phys. 17, 439-443 (2021)] reveal no corresponding signature of enhanced stability at the same neutron number. Motivated by this discrepancy, we investigate the structural evolution of even-even Ca isotopes from N=22 to 36 using macroscopic, relativistic mean-field (RMF), and large-scale shell-model approaches, with the KB3G, GXPF1A, GXPF1B, and FPD6 interactions in the KSHELL framework. The macroscopic analysis, evaluated against seven theoretical frameworks and NNDC data, reveals a coherent change in neutron structure at N=32, most clearly in the shell-gap energy $\Delta E$ and the three-point binding-energy filter, corroborated by the charge-radius filter. The RMF spectrum with NL3* shows a clear $\nu2p_{3/2}$-$\nu2p_{1/2}$ separation consistent with this signature. KB3G and FPD6 reproduce the N=32 enhancement of $E(2_1^+)$ and reduction of $R_{4/2}$, whereas GXPF1A and GXPF1B give a weaker N=32 signature than N=34 signature, the reverse of the experimental pattern, showing that the two subshells depend differently on the monopole interaction. The deformation parameter and B(E2) values have been obtained with above four interactions and are in good agreement with the experimental results.
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All authors A. Anupam, C. Dash, Yashraj, R. P. Singh, B. B. Sahu

Materials Science

Cluster-based Structural Similarity for Dataset Visualization and Data Selection for Machine Learning Interatomic Potentials

Yuto Iwasaki and Miguel A. Caro

Machine learning interatomic potentials (MLIPs) are essential components for accelerating simulation-driven materials design. Data-efficient MLIP training relies on data-selection strategies that maximize structural diversity while limiting computationally expensive first-principles calculations. A key challenge in such strategies is evaluating structural similarity, which involves a trade-off between retaining information on individual atomic environments and reducing computational cost. Here, we propose similarity evaluation methods that achieve both representational fidelity and computational efficiency. Our method represents each structure using a small set of characteristic atomic environments identified by k-medoids clustering and computes pairwise similarity through optimal matching between these representatives or their distributions. Molecular benchmarks demonstrate that our method is approximately 50 times faster than the baseline method while also more clearly distinguishing structures with different chemical compositions. Similarity-based data-selection benchmarks demonstrate that our methods improve the data efficiency and stability of force prediction in MLIPs.
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Quantum Physics

Logical information localisation in stabiliser codes via single-qubit measurements

Jelena Mackeprang, Hemant Sharma and Jonas Helsen

Stabiliser path finding (SPF) has previously been introduced as a method to localise logical information in a stabiliser code undergoing loss onto a single pre-specified target qubit, using only one round of single-qubit measurements. When working with limited resources and flying qubits, this fast read-out of logical information is a helpful tool for fault-tolerant communication. In this work, we provide a broad analytical and computational study of localisation via SPF. We introduce $g$-SPF, where the task is to localise the logical information onto a set of at most $g$ unspecified target qubits. Through analytical arguments based on percolation theory and the disjointness of stabiliser codes, we prove that for i.i.d. qubit loss with probability $p<1/2$ and sufficiently large planar surface codes, localisation via $g$-SPF for constant $g$ succeeds with a probability converging to one, which establishes a localisation threshold. Furthermore, we propose and implement two algorithms to solve SPF. The first is exact and formulates SPF as an integer linear program, whereas the second is heuristic and formulates SPF as a decoding problem. We validate both algorithms by numerically reproducing the localisation threshold for the surface code and demonstrate that the heuristic algorithm is considerably faster. Our work significantly reduces the time required to solve SPF compared to current state-of-the-art algorithms, allowing us to study localisation in substantially larger stabiliser codes than previously considered in the literature. Together, these theoretical and computational results open the door to various applications, such as fault-tolerant teleportation and efficient logical fusion.
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Astrophysics of Galaxies

A New Sample of Closely Separated Dual and Binary AGN Candidates Revealed with the Radio Fundamental Catalog

Emma Schwartzman, Ryan Pfeifle, Tracy Clarke et al. (+3 more)

The Big Multi-AGN Catalog (BigMAC) provides the first literature-complete compilation of confirmed and candidate dual, binary, and recoiling active galactic nuclei (AGN). The BigMAC provides a basis for the first population-wide radio study of multiple AGN (multi-AGN). The Radio Fundamental Catalog (RFC) is composed of four decades of Very Long Baseline Interferometry (VLBI) observations capable of resolving parsec-scale structure. All BigMAC candidate and confirmed multi-AGN were cross-matched to the RFC, highlighting a total of 241 BigMAC sources with available RFC imaging. We present the first detailed analysis of the ten systems exhibiting multiple compact components, most likely to be radio multi-AGN. Using multi-frequency radio and supporting multiwavelength information, we assess whether each system is more consistent with dual/binary AGN or intrinsic jet structure. The dual/binary nature of six systems is further supported by our radio analysis, while the four remaining systems are more plausibly explained by compact jet activity. Several candidate multi-AGN systems exhibit projected separations of only a few to tens of parsecs. In many cases, the parsec-scale radio separations are orders of magnitude smaller than the separations inferred from the observations that originally motivated their selection, emphasizing the importance of VLBI follow-up. Because all ten sources are members of the International Celestial Reference Frame (ICRF) and display significant intrinsic radio structure, they also represent potential sources of astrometric error. This work establishes the RFC as an archival resource for characterizing the closest SMBH pairs while improving future ICRF realizations.
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All authors Emma Schwartzman, Ryan Pfeifle, Tracy Clarke, Nathan Secrest, Henrique Schmitt, Barry Rothberg

Quantum Physics

The quantum query complexity of the semigroup product problem

Troy Lee and Miklos Santha

We study the quantum query complexity of computing a semigroup product $x_1\cdots x_n$, when one query reveals one input element and the multiplication table is given. For a finite aperiodic semigroup of size $N-1$, the argument of Aaronson, Grier, and Schaeffer gives an upper bound of $\sqrt n\,(N\log(nN+2))^{O(N)}$ queries. To obtain query bounds that reflect algebraic structure, we study the product breadth $\beta$: the smallest bound such that every input word has a subsequence of at most $\beta$ letters with the same product. For nontrivial finite commutative aperiodic monoids, the bounded-error quantum query complexity is $\Theta(\min\{n,\sqrt{n\beta}\})$, and is thus characterized by product breadth. We further show that if such a monoid $M$ has aperiodicity index $k$ (the least positive integer satisfying $x^k=x^{k+1}$ for every $x\in M$), then $\beta=O(k\log(|M|+1)\log\log(|M|+2))$. For monoids with a stable partial order in which the identity is the minimum element, we prove that the bounded-error quantum query complexity is at most $\sqrt{n+1}((\beta+2)\log(n+2))^{O(\log(\beta+2))}$. For arbitrary finite aperiodic semigroups of order $N-1$, we improve the bound of Aaronson, Grier, and Schaeffer, obtaining a bounded-error quantum query complexity of at most \[ \min\left\{n,\sqrt n\, \log^{O((N\log(N+2))^{1/3})}(n+2)\right\}. \] The dependence on semigroup size is nearly tight: the bounded-depth Dyck lower bound of Ambainis et al. yields aperiodic monoids requiring $\sqrt n\,2^{\Omega(N^{1/3})}$ quantum queries in the relevant parameter range.
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Strongly Correlated Electrons

Strong Dimerization and Field-Induced Reconstruction of the Low-Energy Spectrum in $\mathrm{Cu}_3(\mathrm{OH})_4(\mathrm{HCO}_2)_2$

Sk Saniur Rahaman and S. R. Hassan

We investigate the field-dependent low-energy thermodynamics of the distorted triangular quantum antiferromagnet $\mathrm{Cu}_3(\mathrm{OH})_4(\mathrm{HCO}_2)_2$ using a sector-resolved Superblock Diagonalization Method (SBDM) supplemented by a transfer-matrix treatment of weakly coupled layers. The strong exchange hierarchy, dominated by the Cu1–Cu1 intradimer coupling $J_2=150\,\mathrm{K}$, separates a high-energy dimer sector from a much softer magnetic manifold formed predominantly by the Cu2 moments. In the 24-site cluster, sixteen Cu1 spins form the strongly bound sector while eight Cu2 spins remain magnetically active; polarization of these eight spins gives $S^z=4$ compared with $S^z_{\rm sat}=12$, providing a direct microscopic origin for the one-third magnetization scale. The finite-temperature thermodynamics reveals a non-monotonic field evolution of the low-energy scale: the dominant $C/T$ feature softens with increasing field, reaches a minimum near the field region around $2\,\mathrm{T}$, and subsequently hardens as the low-temperature magnetization approaches $M_{\rm sat}/3$. Temperature and field sweeps thus expose a common field-induced spectral reconstruction, while the strongly reduced entropy reflects the restricted number of thermally active degrees of freedom below the dimer excitation scale. Our results identify strong-dimer-induced reduction of the active magnetic Hilbert space, followed by field-driven reorganization of the residual spin sector, as the common microscopic origin of the one-third magnetic response and the non-monotonic low-temperature thermodynamics.
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Quantum Physics

Single-shot state preparation threshold: rigorous theorem and statistical mechanical mapping

Yuanchen Zhao, Yi Yuan, Zhengyi Han et al. (+1 more)

Preparing logical codewords with a single round of noisy syndrome measurements can reduce the time overhead of fault-tolerant quantum computation. It remains unclear which structural properties of the code suffice to guarantee a threshold for single-shot state preparation. Here we prove that linear confinement suffices for a nonzero single-shot state-preparation threshold for CSS quantum low-density parity-check codes, in contrast to the common belief that soundness is necessary. When the distance is at least logarithmic, we prove that below nonzero local stochastic readout and data error thresholds, the logical error probability after a formal ideal final recovery is exponentially small and approaches zero when the code size approaches infinity. Under the stronger assumption of linear soundness, we show that the residual data error of the preparation protocol is local stochastic, directly allowing composition with other fault-tolerant gadgets. To investigate single-shot state-preparation threshold beyond the above assumptions, we further develop a statistical mechanical mapping that expresses the final logical error rate in terms of a classical partition function. We perform Monte Carlo simulations of this model for the three-dimensional toric code. The numerical results suggest that sublinear soundness can also support a single-shot state-preparation threshold.
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All authors Yuanchen Zhao, Yi Yuan, Zhengyi Han, Dong E. Liu

Quantum Physics

Quantum Algorithms for Computational Fluid Dynamics

Mario Guillaume Cecile, Nis-Luca van Hülst, Tomohiro Hashizume et al. (+19 more)

We present a comprehensive review of quantum approaches for solving partial differential equations (PDEs) arising in computational fluid dynamics (CFD). We examine fully quantum approaches, including quantum linear system algorithms (QLSAs), ranging from the Harrow–Hassidim–Lloyd (HHL) algorithm to quantum singular value transformation (QSVT), Hamiltonian simulation, and quantum lattice Boltzmann methods (QLBMs), while emphasizing hybrid quantum–classical approaches, including quantum physics-informed neural networks (QPINNs) and amplitude-encoded variational PDE solvers. We focus on hardware-agnostic algorithms compatible with present noisy processors and emerging fault-tolerant architectures. For each framework, we analyze the mathematical formulation, algorithmic structure, and principal limitations. We also examine tensor-network (TN) representations, since CFD fields, differential operators, and geometrical information can often be encoded efficiently in low-rank form. The TN formalism bridges CFD discretizations and quantum states, operators, and circuits, enabling compact representations to be translated into tensor-programmable variational quantum algorithms (TP-VQAs). We further review benchmark problems, including Poisson, reaction, diffusion, and nonlinear model equations, and assess how well quantum algorithms capture key features of fluid dynamics. Our analysis highlights that potential quantum advantage is highly problem dependent and governed by condition number, representational complexity, state preparation, and measurement constraints. We outline capabilities, limitations, and challenges toward scalable quantum algorithms for CFD.
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All authors Mario Guillaume Cecile, Nis-Luca van Hülst, Tomohiro Hashizume, Pia Siegl, Abhishek Setty, José Diogo da Costa Jesus, Paul Over, Sergio Bengoechea, Muhammad Umer, Spyros Tserkis, Eleftherios Mastorakis, Tristan Kraft, Francisco Cárdenas-López, Leonardo Scandurra, Thomas Rung, Felix Motzoi, Belda Yesil, Barbara Kraus, Martin Kiffner, Dimitris G. Angelakis, Eugene de Villiers, Dieter Jaksch

Quantum Physics

Haah's 3D cubic code thermalizes rapidly

Sebastian Stengele, Libor Caha, Ángela Capel et al. (+1 more)

Haah's 3D cubic code was introduced as a candidate for a self-correcting quantum memory in three dimensions, using the absence of string logical operators to obstruct thermal errors. It has been a long-standing open question whether it is a self-correcting quantum memory. Even though the memory time was explored numerically, no analytical upper bound was known. Here, we settle this question rigorously and show that the cubic code thermalizes rapidly at every positive temperature. Our proof relies on the framework of [Commun. Math. Phys. 407, 195 (2026)] to turn a decay-of-correlations condition into a modified logarithmic Sobolev inequality for the Davies generator. To the best of our knowledge, this is the first analytic proof of rapid mixing for all positive temperatures for any fracton code.
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All authors Sebastian Stengele, Libor Caha, Ángela Capel, Simone Warzel

Optics

Confocal imaging from biphoton correlations

Euan Millar, Emma Pearce, Daniele Faccio et al. (+1 more)

Confocal microscopy provides optical sectioning for three-dimensional imaging but conventionally relies on point-by-point scanning and physical pinholes, limiting imaging speed. Here, we demonstrate that confocal sectioning can instead arise directly from quantum measurement, using spatial correlations in entangled biphoton states. Spatially resolved coincidence measurements in a widefield imaging system suppress contributions from out-of-focus planes, producing optical sectioning in parallel across the whole field of view without physical pinholes or mechanical scanning. In addition, this quantum approach obtains an axial response that, as a result of position-momentum entanglement, is $\sqrt{2}$-times narrower than that of classical confocal imaging. Our work establishes biphoton correlations as a mechanism for improved confocal imaging and enables parallel, pinhole-free optical sectioning without mechanical scanning.
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All authors Euan Millar, Emma Pearce, Daniele Faccio, Miles J. Padgett