In many complex systems a continuous input of energy over time can be suddenly relaxed in the form of avalanches. Conventional avalanche models disregard the possibility of internal dynamical effects in the interavalanche periods, and thus miss basic features observed in some real systems. We address this issue by studying a model with viscoelastic relaxation, showing how coherent oscillations of the stress field can emerge spontaneously. Remarkably, these oscillations generate avalanche patterns that are similar to those observed in seismic phenomena.

Negative radiation forces act opposite to the direction of propagation, or net momentum, of a beam but have previously been challenging to definitively demonstrate. We report an experimental acoustic tractor beam generated by an ultrasonic array operating on macroscopic targets ($>1\mathrm{cm}$) to demonstrate the negative radiation forces and to map out regimes over which they dominate, which we compare to simulations. The result and the geometrically simple configuration show that the effect is due to nonconservative forces, produced by redirection of a momentum flux from the angled sides of a target and not by conservative forces from a potential energy gradient. Use of a simple acoustic setup provides an easily understood illustration of the negative radiation pressure concept for tractor beams and demonstrates continuous attraction towards the source, against a net momentum flux in the system.

We introduce a linked-cluster based computational approach that allows one to study quantum quenches in lattice systems in the thermodynamic limit. This approach is used to study quenches in one-dimensional lattices. We provide evidence that, in the thermodynamic limit, thermalization occurs in the nonintegrable regime but fails at integrability. A phase transitionlike behavior separates the two regimes.

We find an attractor for an axially symmetric magnetic field evolving under the Hall effect and subdominant Ohmic dissipation, resolving the question of the long-term fate of the magnetic field in neutron star crusts. The electron fluid is in isorotation, analogous to Ferraro’s law, with its angular velocity being approximately proportional to the poloidal magnetic flux, $\mathrm{\Omega }\propto \mathrm{\Psi }$. This equilibrium is the long-term configuration of a magnetic field evolving because of the Hall effect and Ohmic dissipation. For an initial dipole-dominated field, the attractor consists mainly of a dipole and an octupole component accompanied by an energetically negligible quadrupole toroidal field. The field dissipates in a self-similar way: Although higher multipoles should decay faster, the toroidal field mediates transfer of energy into them from the lower ones, leading to an advection diffusion equilibrium and keeping the ratio of the poloidal multipoles almost constant. This has implications for the structure of the intermediate-age neutron stars, suggesting that their poloidal field should consist of a dipole and an octupole component accompanied by a very weak toroidal quadrupole. For initial conditions that have a higher multipole $\ell$ structure, the attractor consists mainly of $\ell$ and $\ell +2$ poloidal components.

We show that the $B$-mode polarization signal detected at low multipoles by BICEP2 cannot be entirely due to topological defects. This would be incompatible with the high-multipole $B$-mode polarization data and also with existing temperature anisotropy data. Adding cosmic strings to a model with tensors, we find that $B$ modes on their own provide a comparable limit on the defects to that already coming from Planck satellite temperature data. We note that strings at this limit give a modest improvement to the best fit of the $B$-mode data, at a somewhat lower tensor-to-scalar ratio of $r\simeq 0.15$.

Scaling networks of cosmic defects, such as strings and textures, actively generate scalar, vector, and tensor metric perturbations throughout the history of the Universe. In particular, vector modes sourced by defects are an efficient source of the cosmic microwave background $B$-mode polarization. We use the recently released BICEP2 and POLARBEAR $B$-mode polarization spectra to constrain properties of a wide range of different types of cosmic strings networks. We find that in order for strings to provide a satisfactory fit on their own, the effective interstring distance needs to be extremely large—spectra that fit the data best are more representative of global strings and textures. When a local string contribution is considered together with the inflationary $B$-mode spectrum, the fit is improved. We discuss implications of these results for theories that predict cosmic defects.

The critical nuclear charge $Z_c$ required to bind a nucleus plus two electrons in a heliumlike atom has recently been an area of active study, resulting in a disagreement with earlier calculations and with the value obtained from the radius of convergence $1/Z^{*}$ of a $1/Z$ expansion of the energy. In order to resolve the disagreement, have performed high-precision variational calculations in Hylleraas coordinates. With the double basis set method, we have been able to obtain good convergence for $Z$ very close to $Z_c$, which together with the Hellmann-Feynman theorem yields the value $Z_c=0.91102822407725573(4)$, corresponding to $1/Z_c=1.09766083373855980(5)$. This value is in agreement with the value obtained by Baker et al. [Phys. Rev. A 41, 1247 (1990)]. A significant feature of the results is that the outer electron remains localized near the nucleus, even at $Z=Z_c$, and the bound state evidently changes smoothly into a shape resonance for $Z<Z_c$. A qualitative polarization potential is proposed to account for the resonance, and the radial distribution function for the electron density is calculated.

We report a high-accuracy measurement of the differential static scalar polarizability $\mathrm{\Delta }{\alpha }_0$ of the $5s{}^2{S}_{1/2}-4d{}^2{D}_{5/2}$ transition of the ${}^{88}{\mathrm{Sr}}^{+}$ ion. The high accuracy is obtained by comparing the micromotion-induced positive scalar Stark shift to the negative time-dilation shift. Measurement of the trap drive frequency where these shifts cancel is used to determine $\mathrm{\Delta }{\alpha }_0$ without the need to determine the electric field. $\mathrm{\Delta }{\alpha }_0$ is a critical parameter for the operation of frequency standards as it determines the blackbody radiation frequency shift coefficient, the largest source of uncertainty in the ${}^{88}{\mathrm{Sr}}^{+}$ ion clock. The measured value of $\mathrm{\Delta }{\alpha }_0$ is $-4.7938(71)\times 10^{-40}\mathrm{J}{\mathrm{m}}^2/{\mathrm{V}}^2$. Taking into account the dynamic correction, the blackbody shift at 300 K is 0.247 99(37) Hz. The contribution of the blackbody shift coefficient to the uncertainty of the ion standard has been reduced by a factor of 24, from $2\times 10^{-17}$ to $8.3\times 10^{-19}$. The revised total uncertainty of our reference standard is $1.2\times 10^{-17}$, limited by the blackbody field evaluation. An additional benefit of the low uncertainty of $\mathrm{\Delta }{\alpha }_0$ is the ability to suppress, by a factor of about 200, the net micromotion frequency shifts.

We show experimentally that the sliding friction on sand is greatly reduced by the addition of some—but not too much—water. The formation of capillary water bridges increases the shear modulus of the sand, which facilitates the sliding. Too much water, on the other hand, makes the capillary bridges coalesce, resulting in a decrease of the modulus; in this case, we observe that the friction coefficient increases again. Our results, therefore, show that the friction coefficient is directly related to the shear modulus; this has important repercussions for the transport of granular materials. In addition, the polydispersity of the sand is shown to also have a large effect on the friction coefficient.

The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.

The hyperfine splittings of ground state ${}^{11}{\mathrm{Be}}^{+}$ have been measured precisely by laser-microwave double resonance spectroscopy for trapped and laser cooled beryllium ions. The ions were produced at relativistic energies and subsequently slowed down and trapped at mK temperatures. The magnetic hyperfine structure constant of ${}^{11}{\mathrm{Be}}^{+}$ was determined to be $A_{11}=-2677.302988(72)\mathrm{MHz}$ from the measurements of the $m_F-{m_F}^{\prime }=0–0$ field independent transition. This measurement provides essential data for the study of the distribution of the halo neutron in the single neutron halo nucleus ${}^{11}\mathrm{Be}$ through the Bohr-Weisskopf effect.

Hydrodynamics of the spin-1 Bose-Einstein condensate is investigated without restriction of its allowed manifold to either the ferromagnetic or the antiferromagnetic phase. Mixing of the two manifolds is found to be a generic feature in the temporal evolution of the condensate, regardless of the sign and strength of the spin-dependent interaction. Hydrodynamic theory is developed based on a new representation of the spin-1 condensate wave function as a linear combination of the well-known wave functions specific to antiferromagnetic and ferromagnetic manifolds only. Dynamical constraints unique to each submanifold are derived for the first time, demonstrating that efforts to write down hydrodynamic theory in one specific manifold are generally invalid. Certain exceptions, such as a uniform spiral state in the antiferromagnetic manifold, are shown to sustain dynamical evolution within the same manifold over time.

A measurement-based quantum computer could consist of a local-gapped Hamiltonian system, whose thermal states—at sufficiently low temperature—are universal resources for the computation. Initialization of the computer would correspond to cooling the system. We perform an experimental quantum simulation of such a cooling process with entangled photons. We prepare three-qubit thermal cluster states exploiting the equivalence between local dephasing and thermalization for these states. This allows us to tune the system’s temperature by changing the dephasing strength. We monitor the entanglement as the system cools down and observe the transitions from separability to bound entanglement, and then to free entanglement. We also analyze the performance of the system for measurement-based single-qubit state preparation. These studies constitute a basic characterization of experimental cluster-state computation under imperfect conditions.

This Letter presents a study of the flavor-changing neutral current radiative $B^{\pm }\to K^{\pm }{\pi }^{\mp }{\pi }^{\pm }\gamma$ decays performed using data collected in proton-proton collisions with the LHCb detector at 7 and 8 TeV center-of-mass energies. In this sample, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, nearly 14 000 signal events are reconstructed and selected, containing all possible intermediate resonances with a $K^{\pm }{\pi }^{\mp }{\pi }^{\pm }$ final state in the $[1.1,1.9]\mathrm{GeV}/c^2$ mass range. The distribution of the angle of the photon direction with respect to the plane defined by the final-state hadrons in their rest frame is studied in intervals of $K^{\pm }{\pi }^{\mp }{\pi }^{\pm }$ mass and the asymmetry between the number of signal events found on each side of the plane is obtained. The first direct observation of the photon polarization in the $b\to s\gamma$ transition is reported with a significance of $5.2\sigma$.

We introduce off-axis chromatic scanning confocal electron microscopy, a technique for fast mapping of inelastically scattered electrons in a scanning transmission electron microscope without a spectrometer. The off-axis confocal mode enables the inelastically scattered electrons to be chromatically dispersed both parallel and perpendicular to the optic axis. This enables electrons with different energy losses to be separated and detected in the image plane, enabling efficient energy filtering in a confocal mode with an integrating detector. We describe the experimental configuration and demonstrate the method with nanoscale core-loss chemical mapping of silver ($M_{4,5}$) in an aluminium-silver alloy and atomic scale imaging of the low intensity core-loss La ($M_{4,5}@840\mathrm{eV}$) signal in ${\mathrm{LaB}}_6$. Scan rates up to 2 orders of magnitude faster than conventional methods were used, enabling a corresponding reduction in radiation dose and increase in the field of view. If coupled with the enhanced depth and lateral resolution of the incoherent confocal configuration, this offers an approach for nanoscale three-dimensional chemical mapping.