We present the first attempt to access the $x$ dependence of the gluon unpolarized parton-distribution function (PDF), based on lattice simulations using the large-momentum effective theory approach. The lattice calculation is carried out with pion masses of 340 and 678 MeV on a ($2+1$)-flavor domain-wall fermion configuration with lattice spacing $a=0.111\mathrm{fm}$, for the gluon quasi-PDF matrix element with the nucleon momentum up to 0.93 GeV. Taking the normalization from similar matrix elements in the rest frame of the nucleon and pion, our results for these matrix elements are consistent with the Fourier transform of the global fit CT14 and PDF4LHC15 NNLO of the gluon PDF, within statistical uncertainty and the systematic one up to power corrections, perturbative $\mathcal{O}({\alpha }_s)$ matching and the mixing from the quark PDFs.

The isospin character of $p\text{−}n$ pairs at large relative momentum has been observed for the first time in the ${}^{16}\mathrm{O}$ ground state. A strong population of the $J,T=1,0$ state and a very weak population of the $J,T=0,1$ state were observed in the neutron pickup domain of ${}^{16}\mathrm{O}(p,pd)$ at 392 MeV. This strong isospin dependence at large momentum transfer is not reproduced by the distorted-wave impulse approximation calculations with known spectroscopic amplitudes. The results indicate the presence of high-momentum protons and neutrons induced by the tensor interactions in the ground state of ${}^{16}\mathrm{O}$.

We investigate the problem of $N$ identical bosons that are coupled to an impurity particle with infinite mass. For noninteracting bosons, we show that a dynamical impurity-boson interaction, mediated by a closed-channel dimer, can induce an effective boson-boson repulsion which strongly modifies the bound states consisting of the impurity and $N$ bosons. In particular, we demonstrate the existence of two universal “multibody” resonances, where all multibody bound states involving any $N$ emerge and disappear. The first multibody resonance corresponds to infinite impurity-boson scattering length, $a\to +\infty$, while the second corresponds to the critical scattering length $a^{*}>0$ beyond which the trimer ($N=2$ bound state) ceases to exist. Crucially, we show that the existence of $a^{*}$ ensures that the ground-state energy in the multibody bound-state region, $\infty >a>a^{*}$, is bounded from below, with a bound that is independent of $N$. Thus, even though the impurity can support multibody bound states, they become increasingly fragile beyond the dimer state. This has implications for the nature of the Bose polaron currently being studied in cold-atom experiments.

A string of tracers interacting elastically in a turbulent flow is shown to have a dramatically different behavior when compared to the noninteracting case. In particular, such an elastic chain shows strong preferential sampling of the turbulent flow unlike the usual tracer limit: An elastic chain is trapped in the vortical regions. The degree of preferential sampling and its dependence on the elasticity of the chain is quantified via the Okubo-Weiss parameter. The effect of modifying the deformability of the chain via the number of links that form it is also examined.

It is widely accepted that phonon-mediated high-temperature superconductivity is impossible at ambient pressure, because of the very large effective masses of polarons or bipolarons at strong electron-phonon coupling. Here we challenge this belief by showing that strongly bound yet very light bipolarons appear for strong Peierls coupling. These bipolarons also exhibit many other unconventional properties; e.g., at strong coupling there are two low-energy bipolaron bands that are stable against strong Coulomb repulsion. Using numerical simulations and analytical arguments, we show that these properties result from the specific form of the phonon-mediated interaction, which is of “pair hopping” instead of regular density-density type. This unusual effective interaction is bound to have nontrivial consequences for the superconducting state expected to arise at finite carrier concentrations and should favor a large critical temperature.

We introduce the deformable particle (DP) model for cells, foams, emulsions, and other soft particulate materials, which adds to the benefits and eliminates deficiencies of existing models. The DP model combines the ability to model individual soft particles with the shape-energy function of the vertex model, and adds arbitrary particle deformations. We focus on 2D deformable polygons with a shape-energy function that is minimized for area $a_0$ and perimeter $p_0$ and repulsive interparticle forces. We study the onset of jamming versus particle asphericity, $\mathcal{A}=p_0^2/4\pi a_0$, and find that the packing fraction grows with $\mathcal{A}$ until reaching ${\mathcal{A}}^{*}=1.16$ of the underlying Voronoi cells at confluence. We find that DP packings above and below ${\mathcal{A}}^{*}$ are solidlike, which helps explain the solid-to-fluid transition at ${\mathcal{A}}^{*}$ in the vertex model as a transition from tension- to compression-dominated regimes.

We demonstrate that active carpets of bacteria or self-propelled colloids generate coherent flows towards the substrate, and propose that these currents provide efficient pathways to replenish nutrients that feed back into activity. A full theory is developed in terms of gradients in the active matter density and velocity, and applied to bacterial turbulence, topological defects and clustering. Currents with complex spatiotemporal patterns are obtained, which are tunable through confinement. Our findings show that diversity in carpet architecture is essential to maintain biofunctionality.

As a conventional approach, optical dipole trap (ODT) arrays with linear polarization have been widely used to assemble neutral-atom qubits for building a quantum computer. However, due to the inherent scalar differential light shifts (DLS) of qubit states induced by trapping fields, the microwave-driven gates acting on single qubits suffer from errors on the order of ${10}^{-3}$. Here, we construct a DLS compensated ODT array based upon a recently developed magic-intensity trapping technique. In such a magic-intensity optical dipole trap (MI-ODT) array, the detrimental effects of DLS are efficiently mitigated so that the performance of global microwave-driven Clifford gates is significantly improved. Experimentally, we achieve an average error of $(4.7\pm 1.1)\times {10}^{-5}$ per global gate, which is characterized by randomized benchmarking in a $4\times 4$ MI-ODT array. Moreover, we experimentally study the correlation between the coherence time and gate errors in a single MI-ODT with an optimum error per gate of $(3.0\pm 0.7)\times {10}^{-5}$. Our demonstration shows that MI-ODT array is a versatile platform for building scalable quantum computers with neutral atoms.

The far-field spectral and near-field spatial responses of an individual metal-insulator-metal nanoantenna are reported, using thermal fluctuations as an internal source of the electromagnetic field. The far-field spectra, obtained by combining Fourier transform infrared spectroscopy with spatial modulation based on a light falloff effect in a confocal geometry, have revealed two distinct emission peaks attributed to the excitation of the fundamental mode of the nanoantenna at two distinct wavelengths. Superresolved near-field images of the thermally excited mode have been obtained by thermal radiation scanning tunneling microscopy. Experimental results are supported by numerical simulations showing that it is possible to excite the same mode at different wavelengths near a resonance of the insulating dielectric material forming the antenna.

The breaking of ergodicity in isolated quantum systems with a single-particle mobility edge is an intriguing subject that has not yet been fully understood. In particular, whether a nonergodic but metallic phase exists or not in the presence of a one-dimensional quasiperiodic potential is currently under active debate. In this Letter, we develop a neural-network-based approach to investigate the existence of this nonergodic metallic phase in a prototype model using many-body entanglement spectra as the sole diagnostic. We find that such a method identifies with high confidence the existence of a nonergodic metallic phase in the midspectrum at an intermediate quasiperiodic potential strength. Our neural-network-based approach shows how supervised machine learning can be applied not only in locating phase boundaries but also in providing a way to definitively examine the existence or not of a novel phase.

The orientational distribution of free O-H (O-D) groups at the ${\mathrm{H}}_2\mathrm{O}$- (${\mathrm{D}}_2\mathrm{O}$-)air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be $\sim 63°$, substantially larger than previous estimates of 30°–40°. This discrepancy can be traced to erroneously assumed Gaussian or stepwise orientational distributions of free O-H groups. Instead, the MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface.

The effect of quenched (frozen) disorder on the collective motion of active particles is analyzed. We find that active polar systems are far more robust against quenched disorder than equilibrium ferromagnets. Long-ranged order (a nonzero average velocity $⟨\mathbf{v}⟩$) persists in the presence of quenched disorder even in spatial dimensions $d=3$; in $d=2$, quasi-long-ranged order (i.e., spatial velocity correlations that decay as a power law with distance) occurs. In equilibrium systems, only quasi-long-ranged order in $d=3$ and short-ranged order in $d=2$ are possible. Our theoretical predictions for two dimensions are borne out by simulations.

We establish that a Bloch-state ansatz based on periodic uniform matrix product states (PUMPS), originally designed to capture single-quasiparticle excitations in gapped systems, is in fact capable of accurately approximating all low-energy eigenstates of critical quantum spin chains on the circle. When combined with the methods of [Milsted and Vidal, Phys. Rev. B 96, 245105] based on the Koo-Saleur formula, PUMPS Bloch states can then be used to identify each low-energy eigenstate of a chain made of up to hundreds of spins with its corresponding scaling operator in the emergent conformal field theory (CFT). This enables the following two tasks that we demonstrate using the quantum Ising model and a recently proposed generalization thereof due to O’Brien and Fendley [Phys. Rev. Lett. 120, 206403]. (i) From the spectrum of low energies and momenta we extract conformal data (specifying the emergent CFT) with unprecedented numerical accuracy. (ii) By changing the lattice size, we investigate nonperturbatively the renormalization group flow of the low-energy spectrum between two CFTs. In our example, where the flow is from the tricritical Ising CFT to the Ising CFT, we obtain excellent agreement with an analytical result [Klassen and Melzer, Nucl. Phys. B370, 511] conjectured to describe the flow of the first spectral gap directly in the continuum.

New techniques based on weak measurements have recently been introduced to the field of quantum state reconstruction. Some of them allow the direct measurement of each matrix element of an unknown density operator and need only $O(d)$ different operations, compared to $d^2$ linearly independent projectors in the case of standard quantum state tomography, for the reconstruction of an arbitrary mixed state. However, due to the weakness of these couplings, these protocols are approximated and prone to large statistical errors. We propose a method which is similar to the weak measurement protocols but works regardless of the coupling strength: our protocol is not approximated and thus improves the accuracy and precision of the results with respect to weak measurement schemes. We experimentally apply it to the polarization state of single photons and compare the results to those of preexisting methods for different values of the coupling strength. Our results show that our method outperforms previous proposals in terms of accuracy and statistical errors.

We identify a class of one-dimensional spin and fermionic lattice models that display diverging spin and charge diffusion constants, including several paradigmatic models of exactly solvable, strongly correlated many-body dynamics such as the isotropic Heisenberg spin chains, the Fermi-Hubbard model, and the $t\text{−}J$ model at the integrable point. Using the hydrodynamic transport theory, we derive an analytic lower bound on the spin and charge diffusion constants by calculating the curvature of the corresponding Drude weights at half-filling, and demonstrate that for certain lattice models with isotropic interactions some of the Noether charges exhibit superdiffusive transport at finite temperature and half-filling.

We present the first Advanced LIGO and Advanced Virgo search for ultracompact binary systems with component masses between $0.2M_{\odot }–1.0M_{\odot }$ using data taken between September 12, 2015 and January 19, 2016. We find no viable gravitational wave candidates. Our null result constrains the coalescence rate of monochromatic (delta function) distributions of nonspinning ($0.2M_{\odot }$, $0.2M_{\odot }$) ultracompact binaries to be less than $1.0\times {10}^6{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ and the coalescence rate of a similar distribution of ($1.0M_{\odot }$, $1.0M_{\odot }$) ultracompact binaries to be less than $1.9\times {10}^4{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ (at 90% confidence). Neither black holes nor neutron stars are expected to form below $\sim 1M_{\odot }$ through conventional stellar evolution, though it has been proposed that similarly low mass black holes could be formed primordially through density fluctuations in the early Universe and contribute to the dark matter density. The interpretation of our constraints in the primordial black hole dark matter paradigm is highly model dependent; however, under a particular primordial black hole binary formation scenario we constrain monochromatic primordial black hole populations of $0.2M_{\odot }$ to be less than 33% of the total dark matter density and monochromatic populations of $1.0M_{\odot }$ to be less than 5% of the dark matter density. The latter strengthens the presently placed bounds from microlensing surveys of massive compact halo objects (MACHOs) provided by the MACHO and EROS Collaborations.

Molecular ionization in the central molecular zone of our Galaxy is enhanced over the typical galactic value by an order of magnitude or more. This cannot be easily explained for dense Galactic center molecular complexes in the absence of embedded sources of low energy cosmic rays. We provide such a source in the form of ultracompact minihalos of self-annihilating dark matter for a variety of annihilation channels that depend on the particle mass and model. Such sources are motivated for plausible inflationary power spectrum parameters, and while possibly subdominant in terms of the total dark matter mass within the Galactic bulge, might also account for, or at least not be in tension with, the Fermi Galactic center $\gamma$-ray excess.