We present a first-principles lattice QCD investigation of the $R$ ratio between the $e^{+}{e}^{-}$ cross section into hadrons and into muons. By using the method of Ref. [1], that allows one to extract smeared spectral densities from Euclidean correlators, we compute the $R$ ratio convoluted with Gaussian smearing kernels of widths of about 600 MeV and central energies from 220 MeV up to 2.5 GeV. Our theoretical results are compared with the corresponding quantities obtained by smearing the KNT19 compilation [2] of $R$-ratio experimental measurements with the same kernels and, by centering the Gaussians in the region around the $\rho$-resonance peak, a tension of about 3 standard deviations is observed. From the phenomenological perspective, we have not included yet in our calculation QED and strong isospin-breaking corrections, and this might affect the observed tension. From the methodological perspective, our calculation demonstrates that it is possible to study the $R$ ratio in Gaussian energy bins on the lattice at the level of accuracy required in order to perform precision tests of the standard model.

The elliptic ($v_2$) and triangular ($v_3$) azimuthal anisotropy coefficients in central ${}^3\mathrm{He}+\mathrm{Au}$, $d+\mathrm{Au}$, and $p+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ are measured as a function of transverse momentum ($p_T$) at midrapidity ($|\eta |<0.9$), via the azimuthal angular correlation between two particles both at $|\eta |<0.9$. While the $v_2(p_T)$ values depend on the colliding systems, the $v_3(p_T)$ values are system independent within the uncertainties, suggesting an influence on eccentricity from subnucleonic fluctuations in these small-sized systems. These results also provide stringent constraints for the hydrodynamic modeling of these systems.

Owing to the chirality of Weyl nodes characterized by the first Chern number, a Weyl system supports one-way chiral zero modes under a magnetic field, which underlies the celebrated chiral anomaly. As a generalization of Weyl nodes from three-dimensional to five-dimensional physical systems, Yang monopoles are topological singularities carrying nonzero second-order Chern numbers $c_2=\pm 1$. Here, we couple a Yang monopole with an external gauge field using an inhomogeneous Yang monopole metamaterial and experimentally demonstrate the existence of a gapless chiral zero mode, where the judiciously designed metallic helical structures and the corresponding effective antisymmetric bianisotropic terms provide the means for controlling gauge fields in a synthetic five-dimensional space. This zeroth mode is found to originate from the coupling between the second Chern singularity and a generalized 4-form gauge field—the wedge product of the magnetic field with itself. This generalization reveals intrinsic connections between physical systems of different dimensions, while a higher-dimensional system exhibits much richer supersymmetric structures in Landau level degeneracy due to the internal degrees of freedom. Our study offers the possibility of controlling electromagnetic waves by leveraging the concept of higher-order and higher-dimensional topological phenomena.

Optically induced mechanical torque driving rotation of small objects requires the presence of absorption or breaking cylindrical symmetry of a scatterer. A spherical nonabsorbing particle cannot rotate due to the conservation of the angular momentum of light upon scattering. Here, we suggest a novel physical mechanism for the angular momentum transfer to nonabsorbing particles via nonlinear light scattering. The breaking of symmetry occurs at the microscopic level manifested in nonlinear negative optical torque due to the excitation of resonant states at the harmonic frequency with higher projection of angular momentum. The proposed physical mechanism can be verified with resonant dielectric nanostructures, and we suggest some specific realizations.

The Wigner crystal, an ordered array of electrons, is one of the very first proposed many-body phases stabilized by the electron-electron interaction. We examine this quantum phase with simultaneous capacitance and conductance measurements, and observe a large capacitive response while the conductance vanishes. We study one sample with four devices whose length scale is comparable with the crystal’s correlation length, and deduce the crystal’s elastic modulus, permittivity, pinning strength, etc. Such a systematic quantitative investigation of all properties on a single sample has a great promise to advance the study of Wigner crystals.

Contagion processes on networks, including disease spreading, information diffusion, or social behaviors propagation, can be modeled as simple contagion, i.e., as a contagion process involving one connection at a time, or as complex contagion, in which multiple interactions are needed for a contagion event. Empirical data on spreading processes, however, even when available, do not easily allow us to uncover which of these underlying contagion mechanisms is at work. We propose a strategy to discriminate between these mechanisms upon the observation of a single instance of a spreading process. The strategy is based on the observation of the order in which network nodes are infected, and on its correlations with their local topology: these correlations differ between processes of simple contagion, processes involving threshold mechanisms, and processes driven by group interactions (i.e., by “higher-order” mechanisms). Our results improve our understanding of contagion processes and provide a method using only limited information to distinguish between several possible contagion mechanisms.

We propose relativistic tests of quantum gravity using the gravitational self-interaction of photons in a cavity. We demonstrate that this interaction results in a number of quantum gravitational signatures in the quantum state of the light that cannot be reproduced by any classical theory of gravity. We rigorously assess these effects using quantum parameter estimation theory and discuss simple measurement schemes that optimally extract their signatures. Crucially, the proposed tests are free of QED photon-photon scattering, are sensitive to the spin of the mediating gravitons, and can probe the locality of the gravitational interaction. These protocols provide a new avenue for studying the quantum nature of gravity in a relativistic setting.

Contextuality is a distinctive feature of quantum theory and a fundamental resource for quantum computation. However, existing examples of contextuality in high-dimensional systems lack the necessary robustness required in experiments. Here, we address this problem by identifying a family of noncontextuality inequalities whose maximum quantum violation grows with the dimension of the system. At first glance, this contextuality is the single-system version of multipartite Bell nonlocality taken to an extreme form. What is interesting is that the single-system version achieves the same degree of contextuality but uses a Hilbert space of lower dimension. That is, contextuality “concentrates” as the degree of contextuality per dimension increases. We show the practicality of this result by presenting an experimental test of contextuality in a seven-dimensional system. By simulating sequences of quantum ideal measurements with destructive measurements and repreparation in an all-optical setup, we report a violation of 68.7 standard deviations of the simplest case of the noncontextuality inequalities identified. Our results advance the investigation of high-dimensional contextuality, its connection to the Clifford algebra, and its role in quantum computation.

The task of learning a probability distribution from samples is ubiquitous across the natural sciences. The output distributions of local quantum circuits are of central importance in both quantum advantage proposals and a variety of quantum machine learning algorithms. In this work, we extensively characterize the learnability of output distributions of local quantum circuits. Firstly, we contrast learnability with simulatability by showing that Clifford circuit output distributions are efficiently learnable, while the injection of a single $T$ gate renders the density modeling task hard for any depth $d=n^{\mathrm{\Omega }(1)}$. We further show that the task of generative modeling universal quantum circuits at any depth $d=n^{\mathrm{\Omega }(1)}$ is hard for any learning algorithm, classical or quantum, and that for statistical query algorithms, even depth $d=\omega [\log (n)]$ Clifford circuits are hard to learn. Our results show that one cannot use the output distributions of local quantum circuits to provide a separation between the power of quantum and classical generative modeling algorithms, and therefore provide evidence against quantum advantages for practically relevant probabilistic modeling tasks.

We revisit the theory and constraints on axionlike particles (ALPs) interacting with leptons. We clarify some subtleties in the constraints on ALP parameter space and find several new opportunities for ALP detection. We identify a qualitative difference between weak-violating and weak-preserving ALPs, which dramatically change the current constraints due to possible “energy enhancements” in various processes. This new understanding leads to additional opportunities for ALP detection through charged meson decays (e.g., ${\pi }^{+}\to e^{+}\nu a$, $K^{+}\to e^{+}\nu a$) and $W$ boson decays. The new bounds impact both weak-preserving and weak-violating ALPs and have implications for the QCD axion and addressing experimental anomalies using ALPs.

Designing the morphotropic phase boundary (MPB) has been the most sought-after approach to achieve high piezoelectric performance of piezoelectric materials. However, MPB has not yet been found in the polarized organic piezoelectric materials. Here, we discover MPB with biphasic competition of $\beta$ and $3/1$-helical phases in the polarized piezoelectric polymer alloys (PVTC-PVT) and demonstrate a mechanism to induce MPB using the compositionally tailored intermolecular interaction. Consequently, PVTC-PVT exhibits a giant quasistatic piezoelectric coefficient of $>32\mathrm{pC}/\mathrm{N}$ while maintaining a low Young’s modulus of 182 MPa, with a record-high figure of merit of piezoelectricity modulus of about $176\mathrm{pC}/(\mathrm{N}·\mathrm{GPa})$ among all piezoelectric materials.

High-performance piezoelectrics have been extensively reported with a typical perovskite structure, in which a huge breakthrough in piezoelectric constants is found to be more and more difficult. Hence, the development of materials beyond perovskite is a potential means of achieving lead-free and high piezoelectricity in next-generation piezoelectrics. Here, we demonstrate the possibility of developing high piezoelectricity in the nonperovskite carbon-boron clathrate with the composition of ${\mathrm{ScB}}_3{\mathrm{C}}_3$ using first-principles calculations. The robust and highly symmetric B-C cage with mobilizable Sc atom constructs a flat potential valley to connect the ferroelectric orthorhombic and rhombohedral structures, which allows an easy, continuous, and strong polarization rotation. By manipulating the cell parameter $b$, the potential energy surface can be further flattened to produce an extra-high shear piezoelectric constant $d_{15}$ of $9424\mathrm{pC}/\mathrm{N}$. Our calculations also confirm the effectiveness of the partial chemical replacement of Sc by Y to form a morphotropic phase boundary in the clathrate. The significance of large polarization and high symmetric polyhedron structure is demonstrated for realizing strong polarization rotation, offering the universal physical principles to aid the search for new high-performance piezoelectrics. This work takes ${\mathrm{ScB}}_3{\mathrm{C}}_3$ as an example to exhibit the great potential for realizing high piezoelectricity in clathrate structure, which opens the door to developing next-generation lead-free piezoelectric applications.

We analyze the onset of diffusive hydrodynamics in the one-dimensional hard-rod gas subject to stochastic backscattering. While this perturbation breaks integrability and leads to a crossover from ballistic to diffusive transport, it preserves infinitely many conserved quantities corresponding to even moments of the velocity distribution of the gas. In the limit of small noise, we derive the exact expressions for the diffusion and structure factor matrices, and show that they generically have off diagonal components. We find that the particle density structure factor is non-Gaussian and singular near the origin, with a return probability showing logarithmic deviations from diffusion.

Heisenberg’s uncertainty principle implies fundamental constraints on what properties of a quantum system we can simultaneously learn. However, it typically assumes that we probe these properties via measurements at a single point in time. In contrast, inferring causal dependencies in complex processes often requires interactive experimentation—multiple rounds of interventions where we adaptively probe the process with different inputs to observe how they affect outputs. Here, we demonstrate universal uncertainty principles for general interactive measurements involving arbitrary rounds of interventions. As a case study, we show that they imply an uncertainty trade-off between measurements compatible with different causal dependencies.

Finding the transient and steady state properties of open quantum systems is a central problem in various fields of quantum technologies. Here, we present a quantum-assisted algorithm to determine the steady states of open system dynamics. By reformulating the problem of finding the fixed point of Lindblad dynamics as a feasibility semidefinite program, we bypass several well-known issues with variational quantum approaches to solving for steady states. We demonstrate that our hybrid approach allows us to estimate the steady states of higher dimensional open quantum systems and discuss how our method can find multiple steady states for systems with symmetries.

The fractional Fourier transform (FrFT), a fundamental operation in physics that corresponds to a rotation of phase space by any angle, is also an indispensable tool employed in digital signal processing for noise reduction. Processing of optical signals in their time-frequency degree of freedom bypasses the digitization step and presents an opportunity to enhance many protocols in quantum and classical communication, sensing, and computing. In this Letter, we present the experimental realization of the fractional Fourier transform in the time-frequency domain using an atomic quantum-optical memory system with processing capabilities. Our scheme performs the operation by imposing programmable interleaved spectral and temporal phases. We have verified the FrFT by analyses of chroncyclic Wigner functions measured via a shot-noise limited homodyne detector. Our results hold prospects for achieving temporal-mode sorting, processing, and superresolved parameter estimation.

The QCD axion cosmology depends crucially on whether the QCD axion is present during inflation or not. We point out that contrary to the standard criterion, the Peccei-Quinn (PQ) symmetry could remain unbroken during inflation, even when the axion decay constant, $f_a$, is (much) above the inflationary Hubble scale, $H_I$. This is achieved through the heavy lifting of the PQ scalar field due to its leading nonrenormalizable interaction with the inflaton, encoded in a high-dimensional operator which respects the approximate shift symmetry of the inflaton. The mechanism opens up a new window for the post-inflationary QCD axion and significantly enlarges the parameter space, in which the QCD axion dark matter with $f_a>H_I$ could be compatible with high-scale inflation and free from constraints on axion isocurvature perturbations. There also exist nonderivative couplings, which still keep the inflaton shift symmetry breaking under control, to achieve the heavy lifting of the PQ field during inflation. Additionally, by introducing an early matter domination era, more parameter space of high $f_a$ could yield the observed DM abundance.

Contemporary gravitational-wave detectors are fundamentally limited by thermal noise—due to dissipation in the mechanical elements of the test mass—and quantum noise—from the vacuum fluctuations of the optical field used to probe the test-mass position. Two other fundamental noises can in principle also limit sensitivity: test-mass quantization noise due to the zero-point fluctuation of its mechanical modes and thermal excitation of the optical field. We use the quantum fluctuation-dissipation theorem to unify all four noises. This unified picture shows precisely when test-mass quantization noise and optical thermal noise can be ignored.