In Mott insulators with the $t_{2g}^4$ electronic configuration such as of ${\mathrm{Re}}^{3+}$, ${\mathrm{Ru}}^{4+}$, ${\mathrm{Os}}^{4+}$, and ${\mathrm{Ir}}^{5+}$ ions, spin-orbit coupling dictates a Van Vleck–type nonmagnetic ground state with an angular momentum $J=0$, and the magnetic response is governed by gapped singlet-triplet excitations. We derive the exchange interactions between these excitons and study their collective behavior on different lattices. In perovskites, a conventional Bose condensation of excitons into a magnetic state is found, while an unexpected one-dimensional behavior supporting spin-liquid states emerges in honeycomb lattices, due to the bond directional nature of exciton interactions in the case of 90° $d\mathrm{\text{−}}p\mathrm{\text{−}}d$ bonding geometry.

We demonstrate the transition of a coupled electron shuttle from a stable to a strongly nonlinear response at room temperature. Within this transition we observe the coupled shuttle’s response to change from Coulomb controlled to conventional field emission. This parametric process is fully reversible and occurs within a broad frequency range. In combination, the large current and wide frequency band enable energy harvesting applications. The experimental data and the numerical calculations both indicate that the source of the nonlinearity is given by the electromechanical coupling of electron shuttling.

From microscopic fluid clusters to macroscopic droplets, the structure of fluids is governed by the van der Waals force, a force that acts between polarizable objects. In this Letter, we derive a general theory that describes the nonequilibrium counterpart to the van der Waals force, which emerges in spatially coherently fluctuating electromagnetic fields. We describe the formation of a novel and complex hierarchy of self-organized morphologies in magnetic and dielectric colloid systems. Most striking among these morphologies are dipolar foams—colloidal superstructures that swell against gravity and display a high sensitivity to the applied field. We discuss the dominance of many-body forces and derive the equation of state for a material formed by the coherent van der Waals force. Our theory is applied to recent experiments in paramagnetic colloidal systems and a new experiment is suggested to test the theory.

We show that it is possible to clone quantum states to arbitrary accuracy in the presence of a Deutschian closed timelike curve (D-CTC), with a fidelity converging to one in the limit as the dimension of the CTC system becomes large—thus resolving an open conjecture [Brun et al., Phys. Rev. Lett. 102, 210402 (2009)]. This result follows from a D-CTC-assisted scheme for producing perfect clones of a quantum state prepared in a known eigenbasis, and the fact that one can reconstruct an approximation of a quantum state from empirical estimates of the probabilities of an informationally complete measurement. Our results imply more generally that every continuous, but otherwise arbitrarily nonlinear map from states to states, can be implemented to arbitrary accuracy with D-CTCs. Furthermore, our results show that Deutsch’s model for closed timelike curves is in fact a classical model, in the sense that two arbitrary, distinct density operators are perfectly distinguishable (in the limit of a large closed timelike curve system); hence, in this model quantum mechanics becomes a classical theory in which each density operator is a distinct point in a classical phase space.

Using a Boltzmann equation approach, we analyze how the spin drag of a trapped interacting fermionic mixture is influenced by the nonhomogeneity of the system in a classical regime where the temperature is much larger than the Fermi temperature. We show that for very elongated geometries, the spin damping rate can be related to the spin conductance of an infinitely long cylinder. We characterize analytically the spin conductance both in the hydrodynamic and collisionless limits and discuss the influence of the velocity profile. Our results are in good agreement with recent experiments and provide a quantitative benchmark for further studies of spin drag in ultracold gases.

Entanglement detection and estimation are fundamental problems in quantum information science. Compared with discrete-variable states, for which lots of efficient entanglement detection criteria and lower bounds of entanglement measures have been proposed, the continuous-variable entanglement is much less understood. Here we shall present a family of entanglement witnesses based on continuous-variable local orthogonal observables (CVLOOs) to detect and estimate entanglement of Gaussian and non-Gaussian states, especially for bound entangled states. By choosing an optimal set of CVLOOs, our entanglement witness is equivalent to the realignment criterion and can be used to detect bound entanglement of a class of $2+2$ mode Gaussian states. Via our entanglement witness, lower bounds of two typical entanglement measures for arbitrary two-mode continuous-variable states are provided.

Generic models of propelled particle systems posit that the emergence of polar order is driven by the competition between local alignment and noise. Although this notion has been confirmed employing the Boltzmann equation, the range of applicability of this equation remains elusive. We introduce a broad class of mesoscopic collision rules and analyze the prerequisites for the emergence of polar order in the framework of kinetic theory. Our findings suggest that a Boltzmann approach is appropriate for weakly aligning systems but is incompatible with experiments on cluster forming systems.

We study the time dependent magnetic behavior of uniaxial cobalt films in the vicinity of the dynamic phase transition (DPT) as a function of the period $P$ and bias $H_b$ of an oscillating magnetic field. In addition to the DPT at a critical period $P_c$, we observe the occurrence of transient behavior for $P<P_c$. Our data can be consistently explained by the existence of a phase line at $H_b=0$ for $P<P_c$, which causes a first order phase transition in between two antiparallel dynamic order states, thus indicating far-reaching similarities of the DPT to equilibrium phase transitions in ferromagnets.

We present a measurement of form-factor-independent angular observables in the decay $B^0\to K^{*}(892{)}^0{\mu }^{+}{\mu }^{-}$. The analysis is based on a data sample corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$, collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of 7 TeV. Four observables are measured in six bins of the dimuon invariant mass squared $q^2$ in the range $0.1<q^2<19.0{\mathrm{GeV}}^2/c^4$. Agreement with recent theoretical predictions of the standard model is found for 23 of the 24 measurements. A local discrepancy, corresponding to 3.7 Gaussian standard deviations is observed in one $q^2$ bin for one of the observables. Considering the 24 measurements as independent, the probability to observe such a discrepancy, or larger, in one is 0.5%.

Using a variational method with an explicitly correlated Gaussian basis, we study the $e^{+}$-Li and $e^{+}$-Be complexes in the ground and lowest excited states with higher spin multiplicity. Our calculations provide rigorous theoretical confirmation that a positron can be attached to the excited states: $1s2s2p{}^{4}P^o$ and $1s^{2}2s2p{}^{3}P^o$ for $e^{+}$-Li and $e^{+}$-Be, respectively. The result is particularly notable for the $e^{+}$-Be complex, as the excited ${}^{3}P^o$ state lies below the autoionization threshold. We report accurate binding energies, annihilation rates and structural properties of these positron-atom systems. The existence of the ground and metastable excited states with bound positron opens up a new route to the presently lacking experimental verification of stability of a positron binding to any neutral atom.

We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom. The key element of our experiment is a whispering-gallery-mode bottle microresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers. This system reaches the strong coupling regime of cavity quantum electrodynamics, leading to a vacuum Rabi splitting in the excitation spectrum. We systematically investigate the switching efficiency of our system, i.e., the probability that the fiber-optical switch redirects the light into the desired output. We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without postselection. Moreover, by measuring the second-order correlation functions of the output fields, we show that our switch exhibits a photon-number-dependent routing capability.

We study anisotropic lattice strips in the presence of a magnetic field in the quantum Hall effect regime. At specific magnetic fields, causing resonant umklapp scattering, the system is gapped in the bulk and supports chiral edge states in close analogy to topological insulators. In electron gases with stripes, these gaps result in plateaus for the Hall conductivity exactly at the known fillings $n/m$ (both positive integers and $m$ odd) for the integer and fractional quantum Hall effect. For double strips, we find topological phase transitions with phases that support midgap edge states with flat dispersion. The topological effects predicted here could be tested directly in optical lattices.

Motivated by experiments on ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, we consider metallic pyrochlore systems $A_2{B}_2{\mathrm{O}}_7$, where the $A$ sites are occupied by rare-earth local moments and the $B$ sites host $5d$ transition metal ions with itinerant strongly spin-orbit coupled electrons. Assuming non-Kramers doublets on the $A$ site, we derive the RKKY interaction between them mediated by the $B$-site itinerant electrons and find extended non-Heisenberg interactions. Analyzing a simplified model of the RKKY interaction, we uncover a local moment phase with coexisting spiral Ising-like magnetic dipolar and $XY$-like quadrupolar ordering. This state breaks time-reversal and lattice symmetries, and reconstructs the $B$-site electronic band structure, producing a Weyl metallic phase with an intrinsic anomalous Hall effect and an undetectably small magnetization. We discuss implications of our results for ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

We probe the presence of long-range correlations in phase fluctuations by analyzing the higher-order spectrum of resistance fluctuations in ultrathin NbN superconducting films. The non-Gaussian component of resistance fluctuations is found to be sensitive to film thickness close to the transition, which allows us to distinguish between mean field and Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transitions. The extent of non-Gaussianity was found to be bounded by the BKT and mean field transition temperatures and depends strongly on the roughness and structural inhomogeneity of the superconducting films. Our experiment outlines a novel fluctuation-based kinetic probe in detecting the nature of superconductivity in disordered low-dimensional materials.

Nonlinear conducting states in a strongly correlated organic electronic system $\alpha \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2{\mathrm{I}}_3$ [$\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}=\mathrm{\text{bis}}(\mathrm{\text{ethylenedithio}})$-tetrathiafulvalene] are studied by Raman spectroscopy. Wide-range Raman spectra of nonlinear conducing states provide direct information about conducting properties through the electronic Raman process. A comparison between the behaviors of the electronic modes of BEDT-TTF layers and the vibrational mode of ${\mathrm{I}}_3$ molecules reveals the formation of nonequilibrium states in which only the electronic parts show the change of states. We obtained a spatial map of the conducting regions of the nonlinear conducting states by utilizing the electronic Raman intensity as a measure of the highly conducting states. A spatially inhomogeneous formation of nonlinear conducting states was observed.

In many complex systems, large events are believed to follow power-law, scale-free probability distributions so that the extreme, catastrophic events are unpredictable. Here, we study coupled chaotic oscillators that display extreme events. The mechanism responsible for the rare, largest events makes them distinct, and their distribution deviates from a power law. On the basis of this mechanism identification, we show that it is possible to forecast in real time an impending extreme event. Once forecasted, we also show that extreme events can be suppressed by applying tiny perturbations to the system.

Bit commitment is a fundamental cryptographic primitive in which Bob wishes to commit a secret bit to Alice. Perfectly secure bit commitment between two mistrustful parties is impossible through asynchronous exchange of quantum information. Perfect security is however possible when Alice and Bob split into several agents exchanging classical and quantum information at times and locations suitably chosen to satisfy specific relativistic constraints. Here we report on an implementation of a bit commitment protocol using quantum communication and special relativity. Our protocol is based on [A. Kent, Phys. Rev. Lett. 109, 130501 (2012)] and has the advantage that it is practically feasible with arbitrary large separations between the agents in order to maximize the commitment time. By positioning agents in Geneva and Singapore, we obtain a commitment time of 15 ms. A security analysis considering experimental imperfections and finite statistics is presented.

Different quantities that go by the name of entropy are used in variational principles to infer probability distributions from limited data. Shore and Johnson showed that maximizing the Boltzmann-Gibbs form of the entropy ensures that probability distributions inferred satisfy the multiplication rule of probability for independent events in the absence of data coupling such events. Other types of entropies that violate the Shore and Johnson axioms, including nonadditive entropies such as the Tsallis entropy, violate this basic consistency requirement. Here we use the axiomatic framework of Shore and Johnson to show how such nonadditive entropy functions generate biases in probability distributions that are not warranted by the underlying data.

The result of a search for the decay $B_c^{+}\to B_s^0{\pi }^{+}$ is presented, using the $B_s^0\to D_s^{-}{\pi }^{+}$ and $B_s^0\to J/\psi \varphi$ channels. The analysis is based on a data sample of $pp$ collisions collected with the LHCb detector, corresponding to an integrated luminosity of $1{\mathrm{fb}}^{-1}$ taken at a center-of-mass energy of 7 TeV, and $2{\mathrm{fb}}^{-1}$ taken at 8 TeV. The decay $B_c^{+}\to B_s^0{\pi }^{+}$ is observed with significance in excess of 5 standard deviations independently in both decay channels. The measured product of the ratio of cross sections and branching fraction is $[\sigma (B_c^{+})/\sigma (B_s^0)]\times \mathcal{B}(B_c^{+}\to B_s^0{\pi }^{+})=[2.37\pm 0.31(\mathrm{stat})\pm 0.11(\mathrm{syst}{)}_{-0.13}^{+0.17}({\tau }_{B_c^{+}})]\times {10}^{-3}$, in the pseudorapidity range $2<\eta (B)<5$, where the first uncertainty is statistical, the second is systematic, and the third is due to the uncertainty on the $B_c^{+}$ lifetime. This is the first observation of a $B$ meson decaying to another $B$ meson via the weak interaction.

Waves propagating through a weakly scattering random medium show a pronounced branching of the flow accompanied by the formation of freak waves, i.e., extremely intense waves. Theory predicts that this strong fluctuation regime is accompanied by its own fundamental length scale of transport in random media, parametrically different from the mean free path or the localization length. We show numerically how the scintillation index can be used to assess the scaling behavior of the branching length. We report the experimental observation of this scaling using microwave transport experiments in quasi-two-dimensional resonators with randomly distributed weak scatterers. Remarkably, the scaling range extends much further than expected from random caustics statistics.