The communication complexity of a quantum channel is the minimal amount of classical communication required for classically simulating a process of state preparation, transmission through the channel and subsequent measurement. It establishes a limit on the power of quantum communication in terms of classical resources. We show that classical simulations employing a finite amount of communication can be derived from a special class of hidden variable theories where quantum states represent statistical knowledge about the classical state and not an element of reality. This special class has attracted strong interest very recently. The communication cost of each derived simulation is given by the mutual information between the quantum state and the classical state of the parent hidden variable theory. Finally, we find that the communication complexity for single qubits is smaller than 1.28 bits. The previous known upper bound was 1.85 bits.

We study the sudden expansion of spin-imbalanced ultracold lattice fermions with attractive interactions in one dimension after turning off the longitudinal confining potential. We show that the momentum distribution functions of majority and minority fermions quickly approach stationary values due to a quantum distillation mechanism that results in a spatial separation of pairs and majority fermions. As a consequence, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) correlations are lost during the expansion. Furthermore, we argue that the shape of the stationary momentum distribution functions can be understood by relating them to the integrals of motion in this integrable quantum system. We discuss our results in the context of proposals to observe FFLO correlations, related to recent experiments by Liao et al., Nature (London) 467, 567 (2010).

Two mechanisms of two-color ($\omega +2\omega$) laser-induced field-free molecular orientation, based on the hyperpolarizability and ionization depletion, are explored and compared. The CO molecule is used as a computational example. While the hyperpolarizability mechanism generates small amounts of orientation at intensities below the ionization threshold, ionization depletion quickly becomes the dominant mechanism as soon as ionizing intensities are reached. Only the ionization mechanism leads to substantial orientation (e.g., on the order of $⟨\cos \theta ⟩\gtrsim 0.1$). For intensities typical of laser-induced molecular alignment and orientation experiments, the two mechanisms lead to robust, characteristic timings of the field-free orientation wave-packet revivals relative to the alignment revivals and the revival time. The revival timings can be used to detect the active orientation mechanism experimentally.

We produce oriented rotational wave packets in CO and measure their characteristics via high harmonic generation. The wave packet is created using an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm pulse probes the wave packet, generating even-order high harmonics that arise from the broken symmetry induced by the orientation dynamics. The even-order harmonic radiation that we measure appears on a zero background, enabling us to accurately follow the temporal evolution of the wave packet. Our measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation.

In this Letter, we present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. The dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region as a function of spin-orbit coupling parameters as well as the oscillation amplitude. The anharmonic properties beyond the effective-mass approximation are revealed, such as the amplitude-dependent frequency and finite oscillation frequency at a place with a divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of the spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility—a static physical quantity—can be measured via the dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates the magnetic nonzero-momentum condensate from the nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a benchmark for recently developed theoretical approaches.

We use a simple mesoscopic Landau-Safran theory of network fluids to show that a reentrant phase diagram, in the “empty liquid” regime, leads to nonmonotonic surface tension and reentrant wetting, as previously reported for binary mixtures. One of the wetting transitions is of the usual kind, but the low temperature transition may allow the display of the full range of fluctuation regimes predicted by renormalization group theory.

We calculate the bulk photovoltaic response of the ferroelectrics ${\mathrm{BaTiO}}_3$ and ${\mathrm{PbTiO}}_3$ from first principles by applying the “shift current” theory to the electronic structure from density functional theory. The first principles results for ${\mathrm{BaTiO}}_3$ reproduce experimental photocurrent direction and magnitude as a function of light frequency, as well as the dependence of current on light polarization, demonstrating that shift current is the dominant mechanism of the bulk photovoltaic effect in ${\mathrm{BaTiO}}_3$. Additionally, we analyze the relationship between response and material properties in detail. Photocurrent does not depend simply or strongly on the magnitude of material polarization, as has been previously assumed; instead, electronic states with delocalized, covalent bonding that is highly asymmetric along the current direction are required for strong shift current enhancements. The complexity of the response dependence on both external and material parameters suggests applications not only in solar energy conversion, but in photocatalysis and sensor and switch type devices as well.

With high quality topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films, we report thickness-independent transport properties over wide thickness ranges. Conductance remained nominally constant as the sample thickness changed from 256 to $\sim 8\mathrm{QL}$ (where QL refers to quintuple layer, $1\mathrm{QL}\approx 1\mathrm{nm}$). Two surface channels of very different behaviors were identified. The sheet carrier density of one channel remained constant at $\sim 3.0\times {10}^{13}{\mathrm{cm}}^{-2}$ down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at $\sim 8\times {10}^{12}{\mathrm{cm}}^{-2}$ only down to $\sim 8\mathrm{QL}$. The weak antilocalization parameters also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively.

Scanning tunneling spectroscopy is used to study the real-space local density of states of a two-dimensional electron system in a magnetic field, in particular within higher Landau levels. By Fourier transforming the local density of states, we find a set of $n$ radial minima at fixed momenta for the $n$th Landau levels. The momenta of the minima depend only on the inverse magnetic length. By comparison with analytical theory and numerical simulations, we attribute the minima to the nodes of the quantum cyclotron orbits, which decouple in a Fourier representation from the random guiding center motion due to disorder. Adequate Fourier filtering reveals the nodal structure in real space in some areas of the sample with relatively smooth potential disorder.

We propose a systematic methodology to construct a probabilistic epigenetic landscape of cell-fate attainment associated with $N$-node Boolean genetic regulatory networks. The general derivation proposed here is exemplified with an Arabidopsis thaliana network underlying floral organ determination grounded on qualitative experimental data.

The advent of solid state nanodevices allows for interrogating the physicochemical properties of a polyelectrolyte chain by electrophoretically driving it through a nanopore. Salient dynamical aspects of the translocation process have been recently characterized by theoretical and computational studies of model polymer chains free from self-entanglement. However, sufficiently long equilibrated chains are necessarily knotted. The impact of such topological “defects” on the translocation process is largely unexplored, and is addressed in this Letter. By using Brownian dynamics simulations on a coarse-grained polyelectrolyte model we show that knots, despite being trapped at the pore entrance, do not per se cause the translocation process to jam. Rather, knots introduce an effective friction that increases with the applied force, and practically halts the translocation above a threshold force. The predicted dynamical crossover, which is experimentally verifiable, ought to be relevant in applicative contexts, such as DNA nanopore sequencing.

We demonstrate the emergence of criticality due to power-law cross correlations in an ensemble of noninteracting particles propagating in an infinite Lorentz channel. The origin of these interparticle long-range correlations is the intermittent dynamics associated with the ballistic corridors in the single particle phase space. This behavior persists dynamically, even in the presence of external driving, provided that the billiard’s horizon becomes infinite at certain times. For the driven system, we show that Fermi acceleration permits the synchronization of the particle motion with the periodic appearance of the ballistic corridors. The particle ensemble then acquires characteristics of self-organization as the weight of the phase space regions leading to critical behavior increases with time.

The density of states is calculated for the SU(2), SU(3), and a compact U(1) lattice gauge theories using a modified version of the Wang-Landau algorithm. We find that the density of states of the SU(2) gauge theory can be reliably calculated over a range of 120 000 orders of magnitude for lattice sizes as big as ${20}^4$. We demonstrate the potential of the algorithm by reproducing the SU(2) average action, its specific heat, and the critical couplings of the weak first order transition in U(1).

In thermal equilibrium the ground state of the plasma of Standard Model particles is determined by temperature and exactly conserved combinations of baryon and lepton numbers. We show that at nonzero values of the global charges a translation invariant and homogeneous state of the plasma becomes unstable and the system transits into a new equilibrium state, containing a large-scale magnetic field. The origin of this effect is the parity-breaking character of weak interactions and chiral anomaly. This situation could occur in the early Universe and may play an important role in its subsequent evolution.

We report a measurement of the difference ($\Delta A_{CP}$) between time-integrated $CP$-violating asymmetries in $D^0\to K^{+}{K}^{-}$ and $D^0\to {\pi }^{+}{\pi }^{-}$ decays reconstructed in the full data set of proton-antiproton collisions collected by the Collider Detector at Fermilab, corresponding to $9.7{\mathrm{fb}}^{-1}$ of integrated luminosity. The strong decay $D^{*+}\to D^0{\pi }^{+}$ is used to identify the charm meson at production as $D^0$ or ${\overline{D}}^0$. We measure $\Delta A_{CP}=[-0.62\pm 0.21(\mathrm{stat})\pm 0.10(\mathrm{syst})]\%$, which differs from zero by 2.7 Gaussian standard deviations. This result supports similar evidence for $CP$ violation in charm-quark decays obtained in proton-proton collisions.

A measurement of the electron charge asymmetry in inclusive $pp\to W+X\to e\nu +X$ production at $\sqrt{s}=7\mathrm{TeV}$ is presented based on data recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of $840{\mathrm{pb}}^{-1}$. The electron charge asymmetry reflects the unequal production of $W^{+}$ and $W^{-}$ bosons in $pp$ collisions. The electron charge asymmetry is measured in bins of the absolute value of electron pseudorapidity in the range of $|\eta |<2.4$. The asymmetry rises from about 0.1 to 0.2 as a function of the pseudorapidity and is measured with a relative precision better than 7%. This measurement provides new stringent constraints for parton distribution functions.

By irradiating a flat Al target with femtosecond laser pulses at moderate intensities of $\sim {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$, we obtained stable collimated quasimonoenergetic electrons in the specular direction but deviated somewhat toward the target normal. An associated local minimum located on the other side of the specular direction seems to indicate that the peak actually results from the deflection of the collimated electrons from their initial ejection direction. We have proposed a two-step model in which some laser-accelerated electrons are able to leave the plasma in a narrow phase-locked window of the moving wave interference pattern, and are then steered toward the target normal by the ponderomotive force of the interference field. The periodic repetition of the electron emission leads to a pulse train of collimated quasimonoenergetic electrons with subcycle duration.

Wave profile measurement and interpretation play a central role in shock physics research. We propose an approach to revealing the direct link between the wave profile and nucleation and growth process in shock-induced phase transition. Through phase field simulations, we show for the first time that it is possible to explain experimentally measured wave profiles directly from the viewpoint of nucleation and growth, and constrain from such profiles real nucleation and growth scenarios that are otherwise impossible to study.

The heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ displays an additional transition within its superconducting (SC) state, whose nature is characterized by high-precision studies of the isothermal field dependence of the entropy, derived from combined specific heat and magnetocaloric effect measurements at temperatures $T\ge 100\mathrm{mK}$ and fields $H\le 12\mathrm{T}$ aligned along different directions. For any of these conditions, we do not observe an additional entropy contribution upon tuning at constant temperature by magnetic field from the homogeneous SC into the presumed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) SC state. By contrast, for $H\parallel [100]$ a reduction of entropy was found that quantitatively agrees with the expectation for spin-density-wave order without FFLO superconductivity. Our data exclude the formation of a FFLO state in $\mathrm{CeCoI}{\mathrm{n}}_5$ for out-of-plane field directions, where no spin-density-wave order exists.

In graphene, as in most metals, electron-electron interactions renormalize the properties of electrons but leave them behaving like noninteracting quasiparticles. Many measurements probe the renormalized properties of electrons right at the Fermi energy. Uniquely for graphene, the accessibility of the electrons at the surface offers the opportunity to use scanned probe techniques to examine the effect of interactions at energies away from the Fermi energy, over a broad range of densities, and on a local scale. Using scanning tunneling spectroscopy, we show that electron interactions leave the graphene energy dispersion linear as a function of excitation energy for energies within $\pm 200\mathrm{meV}$ of the Fermi energy. However, the measured dispersion velocity depends on density and increases strongly as the density approaches zero near the charge neutrality point, revealing a squeezing of the Dirac cone due to interactions.