We study the time taken by a language learner to correctly identify the meaning of all words in a lexicon under conditions where many plausible meanings can be inferred whenever a word is uttered. We show that the most basic form of cross-situational learning—whereby information from multiple episodes is combined to eliminate incorrect meanings—can perform badly when words are learned independently and meanings are drawn from a nonuniform distribution. If learners further assume that no two words share a common meaning, we find a phase transition between a maximally efficient learning regime, where the learning time is reduced to the shortest it can possibly be, and a partially efficient regime where incorrect candidate meanings for words persist at late times. We obtain exact results for the word-learning process through an equivalence to a statistical mechanical problem of enumerating loops in the space of word-meaning mappings.

The generation of ultrarelativistic positron beams with short duration (${\tau }_{e^{+}}\simeq 30\mathrm{fs}$), small divergence (${\theta }_{e^{+}}\simeq 3\mathrm{mrad}$), and high density ($n_{e^{+}}\simeq {10}^{14}–{10}^{15}{\mathrm{cm}}^{-3}$) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and $\gamma$ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

A sheared microscopic graphite mesa retracts spontaneously to minimize interfacial energy. Using an optical knife-edge technique, we report first measurements of the speeds of such self-retracting motion (SRM) from the $\mathrm{mm}/\mathrm{s}$ range at room temperature to $25\mathrm{m}/\mathrm{s}$ at $235°\mathrm{C}$. This remarkably high speed is comparable with the upper theoretical limit found for sliding interfaces exhibiting structural superlubricity. We observe a strong temperature dependence of SRM speed which is consistent with a thermally activated mechanism of translational motion that involves successive pinning and depinning events at interfacial defects. The activation energy for depinning is estimated to be 0.1–1 eV.

The nature of the glass transition is one of the most important unsolved problems in condensed matter physics. The difference between glasses and liquids is believed to be caused by very large free energy barriers for particle rearrangements; however, so far it has not been possible to confirm this experimentally. We provide the first quantitative determination of the free energy for an aging hard sphere colloidal glass. The determination of the free energy allows for a number of new insights in the glass transition, notably the quantification of the strong spatial and temporal heterogeneity in the free energy. A study of the local minima of the free energy reveals that the observed variations are directly related to the rearrangements of the particles. Our main finding is that the probability of particle rearrangements shows a power law dependence on the free energy changes associated with the rearrangements similar to the Gutenberg-Richter law in seismology.

The competition between two ecologically similar species that use the same resources and differ from each other only in the type of spatial motion they undergo is studied. The latter is assumed to be described either by Brownian motion or Lévy flights. Competition is taken into account by assuming that individuals reproduce in a density-dependent fashion. It is observed that no influence of the type of motion occurs when the two species are in a well-mixed unstructured state. However, as soon as the species develop spatial clustering, the one forming more concentrated clusters gets a competitive advantage and eliminates the other. A similar competitive advantage would occur between walkers of the same type but with different diffusivities if this leads also to different clustering. The coexistence of both species is also possible under certain conditions.

We demonstrate the unambiguous entangling operation of a photonic quantum-logic gate driven by an ultrabright solid-state single-photon source. Indistinguishable single photons emitted by a single semiconductor quantum dot in a micropillar optical cavity are used as target and control qubits. For a source brightness of 0.56 photons per pulse, the measured truth table has an overlap with the ideal case of $68.4\pm 0.5\%$, increasing to $73.0\pm 1.6\%$ for a source brightness of 0.17 photons per pulse. The gate is entangling: At a source brightness of 0.48, the Bell-state fidelity is above the entangling threshold of 50% and reaches $71.0\pm 3.6\%$ for a source brightness of 0.15.

We study the process $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ at a center-of-mass energy of 4.260 GeV using a $525{\mathrm{pb}}^{-1}$ data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be $(62.9\pm 1.9\pm 3.7)\mathrm{pb}$, consistent with the production of the $Y(4260)$. We observe a structure at around $3.9\mathrm{GeV}/c^2$ in the ${\pi }^{\pm }J/\psi$ mass spectrum, which we refer to as the $Z_c(3900)$. If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the ${\pi }^{\pm }J/\psi$ invariant mass spectrum, neglecting interference, results in a mass of $(3899.0\pm 3.6\pm 4.9)\mathrm{MeV}/c^2$ and a width of $(46\pm 10\pm 20)\mathrm{MeV}$. Its production ratio is measured to be $R=(\sigma \mathbf{(}{e}^{+}{e}^{-}\to {\pi }^{\pm }{Z}_c(3900{)}^{\mp }\to {\pi }^{+}{\pi }^{-}J/\psi \mathbf{)}/\sigma (e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi ))=(21.5\pm 3.3\pm 7.5)\%$. In all measurements the first errors are statistical and the second are systematic.

The cross section for $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ between 3.8 and 5.5 GeV is measured with a $967{\mathrm{fb}}^{-1}$ data sample collected by the Belle detector at or near the ${\rm Y}(nS)$ ($n=1,2,\dots ,5$) resonances. The $Y(4260)$ state is observed, and its resonance parameters are determined. In addition, an excess of ${\pi }^{+}{\pi }^{-}J/\psi$ production around 4 GeV is observed. This feature can be described by a Breit-Wigner parametrization with properties that are consistent with the $Y(4008)$ state that was previously reported by Belle. In a study of $Y(4260)\to {\pi }^{+}{\pi }^{-}J/\psi$ decays, a structure is observed in the $M({\pi }^{\pm }J/\psi )$ mass spectrum with $5.2\sigma$ significance, with mass $M=(3894.5\pm 6.6\pm 4.5)\mathrm{MeV}/c^2$ and width $\Gamma =(63\pm 24\pm 26)\mathrm{MeV}/c^2$, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmoniumlike state.

We investigate the relationship between communication and search efficiency in a biological context by proposing a model of Brownian searchers with long-range pairwise interactions. After a general study of the properties of the model, we show an application to the particular case of acoustic communication among Mongolian gazelles, for which data are available, searching for good habitat areas. Using Monte Carlo simulations and density equations, our results point out that the search is optimal (i.e., the mean first hitting time among searchers is minimum) at intermediate scales of communication, showing that both an excess and a lack of information may worsen it.

Quantum mechanics predicts that measurements of incompatible observables carry a minimum uncertainty which is independent of technical deficiencies of the measurement apparatus or incomplete knowledge of the state of the system. Nothing yet seems to prevent a single physical quantity, such as one spin component, from being measured with arbitrary precision. Here, we show that an intrinsic quantum uncertainty on a single observable is ineludible in a number of physical situations. When revealed on local observables of a bipartite system, such uncertainty defines an entire class of bona fide measures of nonclassical correlations. For the case of $2\times d$ systems, we find that a unique measure is defined, which we evaluate in closed form. We then discuss the role that these correlations, which are of the “discord” type, can play in the context of quantum metrology. We show in particular that the amount of discord present in a bipartite mixed probe state guarantees a minimum precision, as quantified by the quantum Fisher information, in the optimal phase estimation protocol.

There is an approximately 9% discrepancy, corresponding to $2.4\sigma$, between two independent constraints on the expansion rate of the Universe: one indirectly arising from the cosmic microwave background and baryon acoustic oscillations and one more directly obtained from local measurements of the relation between redshifts and distances to sources. We argue that by taking into account the local gravitational potential at the position of the observer this tension—strengthened by the recent Planck results—is partially relieved and the concordance of the Standard Model of cosmology increased. We estimate that measurements of the local Hubble constant are subject to a cosmic variance of about 2.4% (limiting the local sample to redshifts $z>0.010$) or 1.3% (limiting it to $z>0.023$), a more significant correction than that taken into account already. Nonetheless, we show that one would need a very rare fluctuation to fully explain the offset in the Hubble rates. If this tension is further strengthened, a cosmology beyond the Standard Model may prove necessary.

We demonstrate 97%, 89%, and 76% transmission of sound amplitude in air through walls perforated with subwavelength holes of areal coverage fractions 0.10, 0.03, and 0.01, respectively, producing 94-, 950-, and 5700-fold intensity enhancements therein. This remarkable level of extraordinary acoustic transmission is achieved with thin tensioned circular membranes, making the mass of the air in the holes effectively vanish. Imaging the pressure field confirms incident-angle independent transmission, thus realizing a bona fide invisible wall. Applications include high-resolution acoustic sensing.

We demonstrate that the pattern forming partial differential equation derived from the auxin distribution model proposed by Meyerowitz, Traas, and others gives rise to all spiral phyllotaxis properties observed on plants. We show how the advancing pushed pattern front chooses spiral families enumerated by Fibonacci sequences with all attendant self-similar properties, a new amplitude invariant curve, and connect the results with the optimal packing based algorithms previously used to explain phyllotaxis. Our results allow us to make experimentally testable predictions.

Jet reheating via nuclear collisions has recently been proposed as the main mechanism for gamma-ray burst (GRB) emission. In addition to producing the observed gamma rays, collisional heating must generate 10–100 GeV neutrinos, implying a close relation between the neutrino and gamma-ray luminosities. We exploit this theoretical relation to make predictions for possible GRB detections by $\mathrm{\text{IceCube}}+\mathrm{\text{DeepCore}}$. To estimate the expected neutrino signal, we use the largest sample of bursts observed by the Burst and Transient Source Experiment in 1991–2000. GRB neutrinos could have been detected if $\mathrm{\text{IceCube}}+\mathrm{\text{DeepCore}}$ operated at that time. Detection of 10–100 GeV neutrinos would have significant implications, shedding light on the composition of GRB jets and their Lorentz factors. This could be an important target in designing future upgrades of the $\mathrm{\text{IceCube}}+\mathrm{\text{DeepCore}}$ observatory.

We use computer simulations to investigate the static properties of a simple glass-forming fluid in which the positions of a finite fraction of the particles have been frozen. By probing the equilibrium statistics of the overlap between independent configurations of the liquid, we find strong evidence that this random pinning induces a glass transition. At low temperatures, our numerical findings are consistent with the existence of a random first-order phase transition rounded by finite size effects.

It is suggested that Goldstone bosons may be masquerading as fractional cosmic neutrinos, contributing about 0.39 to what is reported as the effective number of neutrino types in the era before recombination. The broken symmetry associated with these Goldstone bosons is further speculated to be the conservation of the particles of dark matter.

We investigate the current noise correlations at a quantum point contact in a quantum spin Hall structure, focusing on the effect of a weak magnetic field in the presence of disorder. For the case of two equally biased terminals we discover a robust peak: the noise correlations vanish at $B=0$ and are negative for $B\ne 0$. We find that the character of this peak is intimately related to the interplay between time reversal symmetry and the helical nature of the edge states and call it the ${\mathbb{Z}}_2$ peak.

We demonstrate an organic molecule with an energy gap between its singlet and triplet excited states of almost zero ($\Delta E_{\mathrm{ST}}\sim 0\mathrm{eV}$). Such separation was realized through proper combination of an electron-donating indolocarbazole group and a diphenyltriazine electron-accepting moiety. Calculated and measured $\Delta E_{\mathrm{ST}}$ were 0.003 and 0.02 eV, respectively. A total photoluminescence efficiency of $59\%\pm 2\%$ with $45\%\pm 2\%$ from a delayed component and $14\%\pm 2\%$ from a prompt component was obtained for a doped film. Organic light emitting diodes containing this molecule as an emitting dopant exhibited an unexpectedly high external electroluminescence efficiency of ${\eta }_{\mathrm{EQE}}=14\%\pm 1\%$.

We propose an experimental scheme to verify the quantum nonequilibrium fluctuation relations using current technology. Specifically, we show that the characteristic function of the work distribution for a nonequilibrium quench of a general quantum system can be extracted by Ramsey interferometry of a single probe qubit. Our scheme paves the way for the full characterization of nonequilibrium processes in a variety of quantum systems, ranging from single particles to many-body atomic systems and spin chains. We demonstrate our idea using a time-dependent quench of the motional state of a trapped ion, where the internal pseudospin provides a convenient probe qubit.

We propose an interferometric setting for the ancilla-assisted measurement of the characteristic function of the work distribution following a time-dependent process experienced by a quantum system. We identify how the configuration of the effective interferometer is linked to the symmetries enjoyed by the Hamiltonian ruling the process and provide the explicit form of the operations to implement in order to accomplish our task. We finally discuss two physical settings, based on hybrid optomechanical-electromechanical devices, where the theoretical proposals discussed in our work could find an experimental demonstration.