We present a classical algorithm for approximating the expectation values of observables in linear optical circuits with arbitrary product input states, achieving additive-error accuracy. This result indicates that current applications of photonic systems aimed at demonstrating practical quantum advantage through expectation value estimation, such as photonic variational algorithms, may face challenges in attaining the computational advantage. It also implies the output probabilities of boson sampling with arbitrary product input states can be efficiently approximated by our method, so that boson sampling becomes efficiently simulable when its output probability distribution is polynomially sparse. We also develop an efficient classical algorithm for estimating transition amplitudes of arbitrary product states in linear optical circuits. This provides additive-error approximation algorithms for matrix functions associated with linear optical circuits, such as the (loop-)Hafnian, which are of independent interest. As an application, it solves the generalized molecular vibronic spectra problem [C. Oh et al., Nat. Phys. 20, 225 (2024)], previously suggested as a candidate for practical quantum advantage. Finally, we extend our framework to near-Clifford circuits, enabling classical approximation of their expectation values.

Magnetic fields between a supernova (SN) and Earth convert axions into gamma rays. The absence of such a signal in coincidence with SN 1987A neutrinos, using the coherent Milky Way field, provides well-studied constraints on $g_{ap}\times g_{a\gamma }$ (axion-proton times axion-photon couplings) and on $g_{a\gamma }$ alone. We show that the small-scale power of the turbulent magnetic field component boosts axion-photon conversion and, crucially, extends sensitivity to larger masses. The turbulent field components of the Milky Way and of the Large Magellanic Cloud (hosting SN 1987A) yield improvements of up to 2 orders of magnitude in $g_{ap}\times g_{a\gamma }$. Turbulence should likely impact the sensitivity of other searches based on other axion-photon conversion sites, such as starburst galaxies.

We present greyring, a new model for the postmerger signal in black-hole binary coalescences based on the graybody factor of the remnant. The model accurately reproduces the full frequency-domain ringdown signal for a large set of comparable-mass, aligned-spin numerical relativity waveforms, including the $(\ell ,m)=(2,2)$, (3,3), and (4,4) modes. It achieves mismatches of order $\mathcal{O}({10}^{-6})$ for the dominant (2,2) mode, demonstrating that the graybody-factor parametrization provides a very accurate phenomenological description of the postmerger signal. Building on this model, we introduce a novel consistency test of strong gravity based on the graybody factor: the remnant mass and spin inferred from greyring can be compared with those obtained through standard black-hole spectroscopy. This agnostic test relies exclusively on the postmerger signal and does not require the inclusion of overtones or the choice of very early ringdown starting times, combining the advantages of inspiral-merger-ringdown consistency tests and traditional black-hole spectroscopy. We apply the test to GW250114 and find that the remnant mass and spin inferred from greyring are consistent with those measured from the full signal. Remarkably, the inferred parameters can be measured with a precision comparable to, or slightly better than, that achieved with standard black-hole spectroscopy. Our graybody-factor waveform model allows for new precision tests of strong gravity using the ringdown signal.

High-energy QCD evolution beyond leading order suffers from instabilities driven by large collinear logarithms. We present a framework, consistent with the standard high-energy operator product expansion, that restores perturbative stability order by order. The method involves a change of basis in the space of high-energy operators, which modifies both the evolution kernel and the coefficient functions while leaving physical observables invariant. Within this factorization scheme, we derive a next-to-leading-order renormalization-group equation whose numerical solution exhibits stable evolution up to large rapidities, paving the way for a systematic framework for precision studies of gluon saturation at current and future colliders.

Thermometry is essential for studying many-body physics with ultracold atoms. Accurately measuring low temperatures in these systems, however, remains a significant challenge due to the absence of a universal thermometer. Most widely applicable methods, such as fitting of in situ density profiles, are limited by the requirement of global thermal equilibrium and inapplicability to homogeneous systems. In this Letter, we introduce a novel in situ thermometry for quantum gases, leveraging single-atom-resolved measurements via quantum gas microscopy, and demonstrate it on an ideal Fermi gas. By analyzing number fluctuations in probe volumes containing as little as one atom on average, we extract both global and local temperatures over a broad dynamic range. Our method does not rely on the fluctuation-dissipation theorem and is based instead on the exact relationship between number fluctuations and density-density correlations. In the low-temperature regime, it allows us to observe significant deviations from fluctuation-dissipation predictions, uncovering subextensive fluctuations. Our method is applicable to systems with arbitrary trapping potentials, requiring neither precise trap calibration nor global thermal equilibrium. This thermometer for quantum gases overcomes key limitations of existing techniques, paving the way for more accurate and versatile temperature measurements in ultracold quantum systems.

Reconstructive spectrometers are an emerging class of devices that combine complex light scattering with inference. Thus far, the physical determinants of their performance remain underexplored. Within the regime of chaotic or diffusive scattering, the noise-induced error for spectral reconstruction is governed by Fisher information. We use random matrix theory to derive a closed-form relation linking the variance bound to physical parameters: the spectral correlation length, mean transmittance, and the number of frequency and measurement channels. This analysis reveals fundamental trade-offs between the physical parameters and establishes the conditions for “super-resolution” below the limit set by the spectral correlation length. Our theory is validated numerically using a random matrix model as well as full-wave simulations. These results establish a physically grounded framework for designing compact, performant, and robust reconstructive spectrometers.

Manipulating the spin of ultrarelativistic charged particle beams is a cornerstone of high-energy physics, yet its practical implementation has been limited by intrinsic energy dependence and a lack of universality across particle species. Here, we introduce the concept of a “unipolar surface wave spin rotator” that overcomes these barriers. In this scheme, a laser-driven, unipolar surface wave generates an extreme traveling electromagnetic impulse which imparts a unidirectional spin kick to a counterpropagating charged particle beam without requiring phase matching. We theoretically show that spin rotation from transverse to longitudinal polarization achieves $\sim 100\%$ efficiency within 25 mm for both leptons and ions, and across an energy range from GeV to TeV while preserving beam quality. This approach establishes a new paradigm for universal spin manipulator in future particle accelerators.

We report ultrasound velocity measurements on an ultraclean ${\mathrm{UTe}}_2$ single crystal with $T_c>2\mathrm{K}$ for $H\parallel b$, performed up to 18 T and down to 0.33 K. The measurements provide the missing bulk thermodynamic evidence for an additional high-field phase boundary near ${\mu }_{0}H\sim 14–15\mathrm{T}$. A distinct, nonhysteretic anomaly in the longitudinal $C_{33}$ mode, together with a weaker response in $C_{44}$, no resolvable anomaly in $C_{55}$, and a coincident kink in transverse magnetostriction, reveals a symmetry-selective coupling to lattice strain. This mode selectivity places constraints on the symmetry of the field-induced superconducting component. The phase line remains nearly field constant near 14 T and meets three other phase boundaries at a tetracritical point near 13.5 T and 1.25 K. The results complete the local tetracritical topology of the $H\text{−}T$ phase diagram and support field-induced multicomponent superconductivity in ${\mathrm{UTe}}_2$.

We study the effect of density-assisted hopping on different dimerized lattice geometries, such as bilayers and ladder structures. We show analytically that the density-assisted hopping induces an attractive interaction in the lower (bonding) band of the dimer structure and a repulsion in the upper (antibonding) band. Overcoming the on-site repulsion, this can lead to the appearance of superconductivity with a pairing structure more complex than $s$-wave pairing. Combining numerical and analytical methods such as the matrix product states ansatz, bosonization, and perturbative calculations, we map out the phase diagram of the two-leg ladder system and identify its superconducting phase. We characterize the transition from the non-density-assisted repulsive regime to the spin-gapped superconducting regime as a Berezinskii-Kosterlitz-Thouless transition.

Patterning of precipitates along dislocation lines arising from nonequilibrium segregation during ion irradiation is investigated in model binary alloys. Lattice kinetic Monte Carlo simulations reveal that the competition between solute advection by point defects to the dislocation and thermal diffusion along the dislocation can stabilize self-organized nanostructures with distinct morphologies, including tubes and quasiperiodic necklaces. The stabilization of nanonecklaces is rationalized by heavy-tail power-law distributions for solute redistribution along the dislocation due to advection.

We construct a Hubbard model with a nearly flat band whose quantum geometry can be tuned independent of the energy dispersion and the Coulomb interaction. We show that, when the nearly flat band is half filled, the exact ground state of the model exhibits ferromagnetism and that this ferromagnetism is stabilized by the quantum metric through the spin stiffness. Furthermore, we demonstrate that tuning the quantum geometry alone drives a magnetic phase transition. Our nonperturbative results without resorting to mean-field approximations reveal the quantum-geometric origin of ferromagnetism and the underlying many-body physics in dispersive-band systems.

The Fermi-Hubbard model describes a large variety of condensed matter systems with spinful fermions and strong interactions. On the other hand, the Kitaev chain model deals with noninteracting spinless fermions and produces robust ground-state degeneracies that give rise to Majorana bound states. In this Letter, we connect these two domains by experimentally studying small arrays of quantum dots (QDs) coupled via superconductors. Without a magnetic field, this system constitutes a Fermi-Hubbard model with intersite superconducting correlations. Using two coupled QDs, we find sweet spots in parameter space where we observe zero-bias conductance peaks that are robust against on-site perturbations of the QDs, similar to the behavior expected of a two-site Kitaev chain. This observation is consistent with the presence of Majorana Kramers pairs or ${\mathbb{Z}}_3$ parafermions in this minimal system. Extending to three sites, we find that the spinful system scales very differently compared to the spinless Kitaev chain. When the same sweet-spot conditions are satisfied, the ground state degeneracy of the full three-site system is lifted. The degeneracy can be restored by tuning the superconducting phase difference between the hybrid segments, but the robustness against on-site detunings is lost. Our findings are a first step in studying ground-state degeneracies in strongly interacting Fermi-Hubbard systems with superconducting correlations.

Non-Hermitian Hamiltonians enrich quantum physics by extending conventional phase diagrams, enabling novel topological phenomena, and realizing exceptional points with applications in quantum sensing. Here, we present a programmable liquid-crystal-based photonic platform for implementing nonunitary quantum walks with topological band structures. The novelty of this platform lies in its direct access to the reciprocal space, which enables us to perform a momentum-resolved characterization of the underlying Hamiltonian. We report a complete process tomography of a single step of several walks, from which we retrieve complex-valued energy bands, resolve exceptional points, and detect the associated parity-time symmetry breaking through eigenvector coalescence. Our Letter offers a powerful complementary route to retrieving topology beyond dynamical measurements.

The non-Hermitian skin effect (NHSE), characterized by extensive boundary accumulation of eigenstates under open boundary conditions, has emerged as a central phenomenon in non-Hermitian physics. Established routes to the NHSE are usually based on Abelian nonreciprocal mechanisms, including an equivalent approach involving gain and loss combined with synthetic gauge fields. Here, we demonstrate that non-Abelian couplings can generate the NHSE through a distinct reciprocal mechanism, giving rise to a time-reversal-symmetry-protected $Z_2$ skin effect with pseudospin-resolved boundary localization and dynamical pseudospin separation. We further show that local breaking of time-reversal symmetry at a boundary induces a pseudospin-inversion reflection process, converting the bidirectional $Z_2$ skin response into unidirectional boundary localization. Experimentally, we implement a representative four-level model using a programmable topolectrical circuit and directly observe both the predicted NHSE and the boundary-induced pseudospin-inversion reflection. Our results identify non-Abelian coupling as a mechanism for symmetry-controlled skin effects in reciprocal systems, opening new avenues for realizing nonreciprocity-free topological materials and devices.

Topologically protected (anti)skyrmions are promising information carriers for next-generation spintronics. Their fractional counterparts, however, have so far been observed only in bound pairs or lattice states, leaving a major challenge for the stabilization and electrical manipulation of an isolated fractional topological spin texture. Here, we report the generation of an isolated meron with half topological charge ($Q=1/2$) and its electric-current-driven linear motion at room temperature in the chiral magnet ${\mathrm{Mn}}_{1.4}\mathrm{PtSn}$. Both Lorentz transmission electron microscopy and micromagnetic simulations provide evidence for the emergence of a unique fractional topological state due to the edge-induced topological transformation, distinct from bulk excitations. Importantly, the sample edge simultaneously acts as a one-dimensional track that enables robust current-driven linear motion by compensating the transverse Magnus force, thereby completely suppressing the intrinsic skyrmion Hall effect. These results establish an experimental pathway for generating and manipulating isolated fractional topological spin textures in real space, thereby offering a platform to explore fractional topological physics and spintronic functionalities.

Sliding ferroelectrics possess vertical polarization via stackings of monolayer van der Waals (vdW) materials, exhibiting energy efficiency and ultrafast switching dynamics. Here, we uncover an intriguing polarization-switching behavior in bilayer BN, wherein the interlayer sliding is locked toward a specific direction under uniaxial strain, unable to reverse motion. The in-plane driving force for guiding directional-locked sliding is determined by the emergent off-diagonal Born effective charge (BEC), along with its sign reversal when strain changes from compressive to tensile. The sign reversal of BEC is governed by an improper bilinear coupling between strain and in-plane polarization. Strikingly, the polarization switching barrier is drastically reduced and nearly vanishes under compressive strain. Microscopically, reduced charge density gradient is utilized to capture intricate vdW interactions, providing an intuitive understanding for the sign reversal of BEC and reduced switching barrier. Finally, we demonstrated promising applications in long-range structural precision control, dynamical multiferroicity and terahertz-light emission based on switching performances in sliding ferroelectrics.