Table of contents

Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum (minimum-B) trap formed by superconducting octupole and mirror magnet coils. The trapped antiatoms were detected by rapidly turning off these magnets, thereby eliminating the magnetic minimum and releasing any antiatoms contained in the trap. Once released, these antiatoms quickly hit the trap wall, whereupon the positrons and antiprotons in the antiatoms annihilate. The antiproton annihilations produce easily detected signals; we used these signals to prove that we trapped antihydrogen. However, our technique could be confounded by mirror-trapped antiprotons, which would produce seemingly identical annihilation signals upon hitting the trap wall. In this paper, we discuss possible sources of mirror-trapped antiprotons and show that antihydrogen and antiprotons can be readily distinguished, often with the aid of applied electric fields, by analyzing the annihilation locations and times. We further discuss the general properties of antiproton and antihydrogen trajectories in this magnetic geometry, and reconstruct the antihydrogen energy distribution from the measured annihilation time history.

We obtain the effective action of four-dimensional quantum gravity, induced by N massless matter fields, by integrating the renormalization group (RG) flow of the relative effective average action. By considering the leading approximation in the large N limit, where one neglects the gravitational contributions with respect to the matter contributions, we show how different aspects of quantum gravity, such as asymptotic safety, quantum corrections to the Newtonian potential and the conformal anomaly-induced effective action, are all represented by different terms of the effective action when this is expanded in powers of the curvature.

We investigate the thermodynamics of artificial square spin ice systems assuming only dipolar interactions among the islands that compose the array. Emphasis is given to the effects of temperature on elementary excitations (magnetic monopoles and their strings). By using Monte Carlo techniques we calculate the specific heat, the density of poles and their average separation as functions of temperature. The specific heat and average separation between monopoles with opposite charges exhibit a sharp peak and a local maximum, respectively, at the same temperature, T_p ≈ 7.2D/k_B (here, D is the strength of the dipolar interaction and k_B the Boltzmann constant). When the lattice size is increased, the amplitude of these features also increases but very slowly. Really, the specific heat and the maximum of the average separation d_max between oppositely charged monopoles increase logarithmically with system size, indicating that completely isolated charges could be found only at the thermodynamic limit. In general, the results obtained here suggest that, for temperatures T ⩾ T_p, these systems may exhibit a phase with separated monopoles, although the quantity d_max should not be larger than a few lattice spacings for viable artificial materials.

We present a proposal for a versatile cold-atom-based quantum simulator of relativistic fermionic theories and topological insulators in arbitrary dimensions. The setup consists of a spin-independent optical lattice that traps a collection of hyperfine states of the same alkaline atom, to which the different degrees of freedom of the field theory to be simulated are then mapped. We show that the combination of bi-chromatic optical lattices with Raman transitions can allow the engineering of a spin-dependent tunneling of the atoms between neighboring lattice sites. These assisted-hopping processes can be employed for the quantum simulation of various interesting models, ranging from non-interacting relativistic fermionic theories to topological insulators. We present a toolbox for the realization of different types of relativistic lattice fermions, which can then be exploited to synthesize a majority of phases in the periodic table of topological insulators.

Using first-principles calculations, we consider the bond between thiolate and small Au clusters, with particular emphasis on the resulting magnetic moment. The moment of pure gold clusters is 1 μ_B for clusters with an odd number of Au atoms and zero for those with an even number. The addition of the thiolate, having an odd number of electrons itself, shifts the phase of the odd–even oscillations so that particles with an even number of Au atoms now have unit moment. Surprisingly, gold thiolate exhibits a dramatic and non-intuitive distribution of charge and spin moment. Our results show that the S–Au bond is such that sulfur does not get charge and an electron is transferred to the Au cluster. This extra electron is mainly sp in character and resides in an electronic shell below the Au surface. The calculations suggest that any thiolate-induced magnetism occurs in the gold nanoparticle and not the thiolate, and can be controlled by modifying the thiolate coverage.

It has recently been shown that small quantum subsystems generically equilibrate, in the sense that they spend most of the time close to a fixed equilibrium state. This relies on just two assumptions: that the state is spread over many different energies, and that the Hamiltonian has non-degenerate energy gaps. Given the same assumptions, it has also been shown that closed systems equilibrate with respect to realistic measurements. We extend these results in two important ways. Firstly, we prove equilibration over a finite (rather than infinite) time-interval, allowing us to bound the equilibration time. Secondly, we weaken the non-degenerate energy gaps condition, showing that equilibration occurs provided that no energy gap is hugely degenerate.

The technique of time-resolved pump–probe x-ray photoelectron spectroscopy using the free-electron laser in Hamburg (FLASH) is described in detail. Particular foci lie on the macrobunch resolving detection scheme, the role of vacuum space-charge effects and the synchronization of pump and probe lasers. In an exemplary case study, the complete Ta 4f core-level dynamics in the layered charge-density-wave (CDW) compound 1T-TaS₂ in response to impulsive optical excitation is measured on the sub-picosecond to nanosecond timescale. The observed multi-component dynamics is related to the intrinsic melting and reformation of the CDW as well as to extrinsic pump-laser-induced vacuum space-charge effects.

We study the principle of information-causality (IC) in the presence of extremal no-signaling correlations on a tripartite scenario. We prove that all, except one, of the non-local correlations lead to violation of IC. The remaining non-quantum correlation is shown to satisfy any bipartite physical principle.

We present a field theoretical analysis of the 2 + 1 dimensional BF model with boundary in the Abelian and the non-Abelian case based on Symanzik's separability condition. Our aim is to characterize the low-energy properties of time reversal invariant topological insulators. In both cases, on the edges, we obtain Kač–Moody algebras with opposite chiralities reflecting the time reversal invariance of the theory. While the Abelian case presents an apparent arbitrariness in the value of the central charge, the physics on the boundary of the non-Abelian theory is completely determined by time reversal and gauge symmetry. The discussion of the non-Abelian BF model shows that time reversal symmetry on the boundary implies the existence of counter-propagating chiral currents.

Localized surface plasmon resonance (LSPR) and photoactivation (PA) effects are combined for the tuning of fluorescent colors of colloidal CdSe quantum dots (QDs). It is found that LSPR with QD emitters intensely enhances surface state emission, accompanied by a remarkable red-shift of fluorescent colors, while PA treatment with colloidal QDs leads to a distinct enhancement of band-edge emission, accompanied by a peak blue-shift. Furthermore, the LSPR effect on QD emitters can be continuously tuned by the PA process. The combination of the post-synthetic approaches allows feasible realization of multi-color patterns from one batch of QDs and the approaches can also be compatible with other micro-fabrication technologies of QD embossed fluorescent patterns, which undoubtedly provides a way of precisely tuning the colors of light-emitting materials and devices that use colloidal QDs.

When a beam of light is reflected by a smooth surface its behavior deviates from geometrical optics predictions. Such deviations are quantified by the so-called spatial and angular Goos–Hänchen (GH) and Imbert–Fedorov (IF) shifts of the reflected beam. These shifts depend upon the shape of the incident beam, its polarization and on the material composition of the reflecting surface. In this paper we suggest a novel approach that allows one to unambiguously isolate the beam-shape dependent aspects of GH and IF shifts. We show that this separation is possible as a result of some universal features of shifted distribution functions which are presented and discussed.

Results are presented for positron binding to a selection of molecules containing the hydroxyl functional group. These molecules, which span in the range of carbon atoms from 1 (methanol) to 4 (1-butanol), have moderate permanent dipole moments ranging from about 1.4 to 2.4 D. The dependence of the binding energy on the magnitude of the molecular dipole polarizability and static dipole moment is studied. An effect that appears to be due to the localization of the bound positron is discussed.

Extreme ultraviolet (XUV) pulses with a duration of tens of femtoseconds initiate 4s⁻¹ or 4p⁻¹ photoionization of krypton, which populates highly excited satellite states through the electron correlation. The excited ions are then tunnel ionized to Kr²⁺4s⁻¹4p⁻¹ or 4p⁻² by a strong-field near-infrared (NIR) pulse of a similar duration. The XUV pulses are produced by high harmonic generation in a gas jet and we employ a state-of-the-art time-preserving monochromator to isolate individual XUV harmonic orders. An enhancement of the Kr²⁺ yield as a function of harmonic photon energy and XUV-pump NIR-probe delay is observed and compared with a two-step model, which allows the population of the satellite states to be inferred. Furthermore, relative 4s and 4p satellite excitation cross-sections are predicted at the photon energies studied. This proof-of-principle experiment demonstrates that isolated harmonics can be employed to pump specific electronic states, which will be highly complementary to synchrotron, attosecond and x-ray free-electron laser studies of complex systems.

Several widely used methods for the calculation of band structures and photo emission spectra, such as the GW approximation, rely on many-body perturbation theory. They can be obtained by iterating a set of functional differential equations (DEs) relating the one-particle Green's function (GF) to its functional derivative with respect to an external perturbing potential. In this work, we apply a linear response expansion in order to obtain insights into various approximations for GF calculations. The expansion leads to an effective screening while keeping the effects of the interaction to all orders. In order to study various aspects of the resulting equations, we discretize them and retain only one point in space, spin and time for all variables. Within this one-point model we obtain an explicit solution for the GF, which allows us to explore the structure of the general family of solutions and to determine the specific solution that corresponds to the physical one. Moreover, we analyze the performances of established approaches like GW over the whole range of interaction strength, and we explore alternative approximations. Finally, we link certain approximations for the exact solution to the corresponding manipulations of the DE which produces them. This link is crucial in view of a generalization of our findings to the real (multidimensional functional) case where only the DE is known.

The temporal communication patterns of human individuals are known to be inhomogeneous or bursty, which is reflected as heavy tail behavior in the inter-event time distribution. As the cause of such a bursty behavior two main mechanisms have been suggested: (i) inhomogeneities due to the circadian and weekly activity patterns and (ii) inhomogeneities rooted in human task execution behavior. In this paper, we investigate the role of these mechanisms by developing and then applying systematic de-seasoning methods to remove the circadian and weekly patterns from the time series of mobile phone communication events of individuals. We find that the heavy tails in the inter-event time distributions remain robust with respect to this procedure, which clearly indicates that the human task execution-based mechanism is a possible cause of the remaining burstiness in temporal mobile phone communication patterns.

We report the experimental verification of metamaterial cloaking for a 3D object in free space. We apply the plasmonic cloaking technique, based on scattering cancellation, to suppress microwave scattering from a finite-length dielectric cylinder. We verify that scattering suppression is obtained all around the object in the near- and far-field and for different incidence angles, validating our measurements with analytical results and full-wave simulations. Our near-field and far-field measurements confirm that realistic and robust plasmonic metamaterial cloaks may be realized for elongated 3D objects with moderate transverse cross-section at microwave frequencies.

Nanotubes have recently been experimentally demonstrated to be perfect phonon waveguides. To explore the underlying physics, we present atomic scale calculations of thermal transport in carbon nanotubes under radial strain using the nonequilibrium Green's function method. It is found that the thermal conductance exhibits a robust linear response behavior to radial strain over the whole elastic range. A detailed analysis of phonon transmission reveals that an elastic radial strain can be viewed as a perturbation of the transport of most of the low-frequency phonons. This is attributed to the unique bonding configuration of nanotubes, which can be well preserved even under severe deformation. Such a structural response to deformation, which is rare in other systems, explains the robust thermal transport in nanotubes against severe radial deformation.

We numerically study the dynamics and frequency response of the recently proposed Ising machine based on the polarization degrees of freedom of an injection-locked laser network (Utsunomiya et al 2011 Opt. Express19 18091). We simulate various anti-ferromagnetic Ising problems, including the ones with symmetric Ising and Zeeman coefficients, which enable us to study the problem size up to M = 1000. Transient time, to reach a steady-state polarization configuration after a given Ising problem is mapped onto the system, is inversely proportional to the locking bandwidth and does not scale exponentially with the problem size. In the Fourier analysis with first-order linearization approximation, we find that the cut-off frequency of a system's response is almost identical to the locking bandwidth, which supports the time-domain analysis. It is also shown that the Zeeman term, which is created by the horizontally polarized injection signal from the master laser, serves as an initial driving force on the system and contributes to the transient time in addition to the inverse locking bandwidth.

We formulate a model for the steady state response of a nonlinear quantum oscillator structure, such as those used in a variety of superconducting qubit experiments, when excited by a steady, but not necessarily small, ac tone. We show that this model can be derived directly from a circuit description of some recent qubit experiments in which the state of the qubit is read out directly, without a superconducting quantum interference device (SQUID) magnetometer. The excitation profile has a rich structure depending on the detuning of the tone from the small-signal resonant frequency, on the degree of damping and on the excitation amplitude. We explore two regions in detail. Firstly, at high damping there is a trough in the excitation response as a function of detuning, near where the classical Duffing bifurcation occurs. This trough has been understood as a classical interference between two metastable responses with opposite phase. We use Wigner function studies to show that while this picture is roughly correct, there are also more quantum mechanical aspects to this feature. Secondly, at low damping we study the emergence of sharp, discrete spectral features from a continuum response. We show that these the structures, associated with discrete transitions between different excited-state eigenstates of the oscillator, provide an interesting example of a quantum Fano resonance. The trough in the Fano response evolves continuously from the 'classical' trough at high damping.

We study the gas-surface dynamics of O₂ at Ag(111) with the particular objective to unravel whether electronic non-adiabatic effects are contributing to the experimentally established inertness of the surface with respect to oxygen uptake. We employ a first-principles divide and conquer approach based on an extensive density-functional theory mapping of the adiabatic potential energy surface (PES) along the six O₂ molecular degrees of freedom. Neural networks are subsequently used to interpolate these grid data to a continuous representation. The low computational cost with which forces are available from this PES representation allows then for a sufficiently large number of molecular dynamics trajectories to quantitatively determine the very low initial dissociative sticking coefficient at this surface. Already these adiabatic calculations yield dissociation probabilities close to the scattered experimental data. Our analysis shows that this low reactivity is governed by large energy barriers in excess of 1.1 eV very close to the surface. Unfortunately, these adiabatic PES characteristics render the dissociative sticking a rather insensitive quantity with respect to a potential spin or charge non-adiabaticity in the O₂–Ag(111) interaction. We correspondingly attribute the remaining deviations between the computed and measured dissociation probabilities primarily to unresolved experimental issues with respect to surface imperfections.

We propose two algorithms to factor numbers using Gauss sums and entanglement: (i) in a Shor-like algorithm we encode the standard Gauss sum in one of two entangled states and (ii) in an interference algorithm we create a superposition of Gauss sums in the probability amplitudes of two entangled states. These schemes are rather efficient provided that there exists a fast algorithm that can detect a period of a function hidden in its zeros.

We analyse the single-mode transmission of microwaves in a guide with internal random structure. The waveguide contains scatterers characterized by random heights and positions, corresponding to compositional and structural disorder. We measure the effects of cross-correlations between two kinds of disorder, showing how they enhance or attenuate the experimentally found transmission gaps generated by long-range self-correlations. The results agree with the theoretical predictions obtained for the aperiodic Kronig–Penney model and prove that self- and cross-correlations also have relevant effects in finite disordered samples of small size.

We investigate theoretically the magnetoplasmon spectrum of two-dimensional helical metals. The effects of the Zeeman energy and Coulomb interaction on the collective excitation modes are examined through the charge-density excitation (CDE) and spin-density excitation (SDE) intensities. At low excitation frequencies, the CDE is always accompanied by an SDE and is almost unaffected by the change in the Zeeman energy or Coulomb interaction intensity. However, at some higher excitation frequencies, the CDE and SDE can be decoupled and a strong SDE with a weak CDE can be realized by tuning either the Zeeman energy or the Coulomb interaction intensity.

It is well known in quantum field theory that the off-shell effective action depends on the gauge choice and field parametrization used in calculating it. Nevertheless, the typical scheme in which the scenario of asymptotically safe gravity is investigated is an off-shell version of the functional renormalization group equation. Working with the Einstein–Hilbert truncation as a test bed, we develop a new scheme for the analysis of asymptotically safe gravity in which the on-shell part of the effective action is singled out and we show that the beta function for the essential coupling has no explicit gauge dependence. In order to reach our goal, we introduce several technical novelties, including a different decomposition of the metric fluctuations, a new implementation of the ghost sector and a new cut-off scheme. We find a nontrivial fixed point, with a value of the cosmological constant that is independent of the gauge-fixing parameters.

We provide a unified parameter-free explanation of the observed oxygen-isotope effects on the critical temperature, the magnetic-field penetration depth and on the normal-state pseudogap for underdoped cuprate superconductors within the framework of the multi-(bi)polaron theory with strong Coulomb and Fröhlich interactions. We also quantitatively explain the measured critical temperature and the magnitude of the magnetic-field penetration depth. This paper thus represents an important support for the bipolaron theory of high-temperature superconductivity, compatible with many other independent observations.

An absolute frequency measurement has been made of the ²S_1/2–²F_7/2 electric octupole transition in a single ion of ¹⁷¹Yb⁺. The implementation of a diode-based probe laser stabilized to this highly forbidden transition has resulted in an improvement of more than one order of magnitude upon the lowest published uncertainty. After correcting for systematic shifts, the frequency was determined to be 642 121 496 772 646.22 (67) Hz. This corresponds to a fractional uncertainty of 1.0 × 10⁻¹⁵.

We study different types of stationary dynamos observed in the Von Kármán sodium (VKS) experiment when varying the electromagnetic boundary conditions on (and in) the impellers. The flow is driven with two impellers made of soft iron (Monchaux et al 2007 Phys Rev. Lett.98 044502) or using one soft-iron impeller and one stainless steel impeller. The magnetic field is mapped using 40 three-dimensional probes distributed within the flow and its surroundings. Symmetry and coupling properties are then retrieved from direct probe measurements and/or from the field structure as reconstructed using the inversion procedure described by Boisson and Dubrulle (2011 New J. Phys.13 023037). Several salient results are obtained: (i) dynamo action is not achieved unless at least one iron impeller is rotating, at a frequency larger than 15 Hz; (ii) the resulting dynamo is a dipolar, mostly axisymmetric structure; and (iii) the self-sustained magnetic field properties depend on the sodium flow structure between the two impellers. We propose to interpret the stationary dynamos generation as the (constructive or destructive) superposition of two one-impeller fluid dynamos generated close to the soft-iron impellers, nonlinearly coupled through the turbulent flow, as suggested by Verhille et al (2010 New J. Phys.12 033006). The normal form equation describing this coupling is similar to the one obtained in a theoretical model (Pétrélis et al 2009 Phys. Rev. Lett.102 144503).

It has been shown that inter-spin interaction strengths in a spin-1/2 chain can be evaluated by accessing one of the edge spins only. We demonstrate this experimentally for the simplest case, a three-spin chain, with the nuclear magnetic resonance technique. The three spins in the chain interact through nearest-neighbor Ising interactions under site-dependent transverse fields. The employed molecule is an alanine containing three ¹³C nuclei, each of which has spin-1/2.

The results of experimental pump–probe spectroscopy of a quantum dash optical amplifier biased at transparency are presented. Using strong pump pulses we observe competition between free carrier absorption and two-photon-induced stimulated emission that can have drastic effects on the transmission dynamics. Thus, both an enhancement as well as a suppression of the transmission can be observed even when the amplifier is biased at transparency. A simple theoretical model taking into account two-photon absorption and free carrier absorption is presented that shows good agreement with the measurements.

We study transitions between phases of matter with topological order. By studying these transitions in exactly solvable lattice models we show how universality classes may be identified and critical properties described. As a familiar example to elucidate our results concretely, we describe in detail a transition between a fully gapped achiral 2D p-wave superconductor (p + ip for pseudo-spin up/p − ip for pseudo-spin down) to an s-wave superconductor. We show in particular that this transition is of the 2D transverse field Ising universality class.

The emission of positronium negative ions from Na- and K-coated W(100) surfaces has been studied. The emission efficiencies (the fractions of incident slow positrons yielding the ions) for both samples were found to be as high as 1.5%. Although the efficiencies decreased with time after coating, the effects were more durable than that for Cs coating. In particular, the efficiency after Na coating was still higher than 0.5% after three days. The successful development of a durable Ps⁻ source opens the door to a new era of experimental investigations on the Ps⁻ ion.

We report on the realization of a compact atomic Mach–Zehnder-type Sagnac interferometer of 13.7 cm length, which covers an area of 19 mm² previously reported only for large thermal beam interferometers. According to Sagnac's formula, which holds for both light and atoms, the sensitivity for rotation rates increases linearly with the area enclosed by the interferometer. The use of cold atoms instead of thermal atoms enables miniaturization of Sagnac interferometers without sacrificing large areas. In comparison with thermal beams, slow atoms offer better matching of the initial beam velocity and the velocity with which the matter waves separate. In our case, the area is spanned by a cold atomic beam of 2.79 m s⁻¹, which is split, deflected and combined by driving a Raman transition between the two hyperfine ground states of ⁸⁷Rb in three spatially separated light zones. The use of cold atoms requires a precise angular alignment and high wave front quality of the three independent light zones over the cloud envelope. We present a procedure for mutually aligning the beam splitters at the microradian level by making use of the atom interferometer itself in different configurations. With this method, we currently achieve a sensitivity of $6.1\times 10^{-7}\,\mathrm {rad}\,\mathrm {s}^{-1}\,{\rm Hz}^{-1/2}$.

We use statistically validated networks, a recently introduced method of validating links in a bipartite system, to identify clusters of investors trading in a financial market. Specifically, we investigate a special database allowing us to track the trading activity of individual investors of Nokia stock. We find that many statistically detected clusters of investors show a very high degree of synchronization in time when they decide to trade and in the trading action taken. We investigate the composition of these clusters and find that several of them show an over-expression of specific categories of investors.

We show how to use excitable regimes mediated by localized structures (LSs) to perform AND, OR and NOT logical operations providing full logical functionality. Our scheme is general and can be implemented in any physical system displaying LSs. In particular, LSs in nonlinear photonic devices can be used for all-optical computing applications where several reconfigurable logic gates can be implemented in the transverse plane of a single device, allowing for parallel computing.

Using an optimal control hydrodynamic modeling approach and irradiation adaptive time-design, we indicate excitation channels maximizing heat load in laser ablated aluminum at low energy costs. The primary relaxation paths leading to an emerging plasma are particularly affected. With impulsive pulses on ps pedestals, thermodynamic trajectories are preferentially guided in ionized domains where variations in ionization degree occur. This impinges on the gas-transformation mechanisms and triggers a positive bremsstrahlung absorption feedback. The highest temperatures are thus obtained in the expanding ionized matter after a final impulsive excitation, as the electronic energy relaxes recombinatively. The drive relies on transitions to weakly coupled front plasmas at the critical optical density, favoring energy confinement with low mechanical work. Alternatively, robust collisional heating occurs in denser regions above the critical point. This impacts the nature, the excitation degree and the energy content of the ablated matter. Adaptive modeling can therefore provide optimal strategies with information on physical variables not readily accessible and, as experimentally confirmed, databases for pulse shapes with interest in remote spectroscopy, laser-induced matter transfer, laser material processing and development of secondary sources.

We consider a Bose–Einstein condensate driven by periodic δ-kicks. In contrast to first-order descriptions, which predict rapid, unbounded growth of the noncondensate in resonant parameter regimes, the consistent treatment of condensate depletion in our fully time-dependent, second-order description acts to damp this growth, leading to oscillations in the (non)condensate population and the coherence of the system.

We report on the observation of macroscopically coherent states of exciton–polaritons in a ZnO-based bulk planar microcavity up to 250 K. Excitation power-dependent photoluminescence investigations show a clear threshold behaviour and corresponding spectral narrowing of the emission for negative detunings, revealing clear signatures of a Bose–Einstein condensate. For positive detunings, no condensation occurred but the emission from an electron–hole plasma was detected. Above threshold interscattering phenomena of condensate polaritons between roughly equidistant energy levels have been observed. As a special feature we found ballistic propagation of the condensate in the pump-induced potential landscape, making these ZnO-based microcavities promising candidates for applications based on polariton transport. This effect is caused by strong repulsive interactions in our system, leading to an immense blueshift of the condensate emission and hence to pronounced dynamic effects.

We investigate the coupling between a quantized electromagnetic field in a cavity resonator and a Coulomb interacting electronic system in a nanostructure in an external magnetic field. The effects caused by the geometry of the electronic system and the polarization of the electromagnetic field are explicitly taken into account. Our numerical results demonstrate that the two-level system approximation and the Jaynes–Cummings model remain valid in the weak electron–photon coupling regime, while the quadratic vector potential in the diamagnetic part of the charge current leads to significant correction to the energy spectrum in the strong coupling regime. Furthermore, we find that coupling to a strong cavity photon mode polarizes the charge distribution of the system, requiring a large basis of single-electron eigenstates to be included in the model.

We calculate the angular emission characteristics of phonons from parabolic confined quantum dots containing a single or two interacting electrons. The emission spectra are shown to be generally characterized by a narrow polar angle giving a phonon propagation direction explicitly characterized by the energy difference of the transition. In addition, the phonon emission spectra contain a given number of azimuthally oriented lobes which reflect the quantum structure of the initial excited state. This implies that measurements of angular resolved phonon emission spectra can give detailed information on the electronic charge distribution and energy spectra of excited quantum states. When such a structure is known, a large-scale ordering of identical quantum dots with respect to the emission angles may realize phonon amplification by stimulated emission.

Complex-oxide superlattices (SLs) with atomic-scale periodicity have dynamical properties that are distinct from thin films of uniform composition. The origins of these properties are closely related to the dynamics of polarization domains and to field-driven changes in the symmetries resulting from interfacial coupling between different components. These dynamics are apparent at timescales from a few nanoseconds to several milliseconds in experiments probing the piezoelectricity of a ferroelectric/dielectric BaTiO₃(BTO)/CaTiO₃ (CTO) SL using time-resolved x-ray microdiffraction. At the 100 ns timescale, the piezoelectric distortion is approximately ten times smaller than in the millisecond regime. This reduced piezoelectricity at short timescales is not observed in previously studied PbTiO₃/SrTiO₃ SLs or compositionally uniform ferroelectrics such as tetragonal compositions of Pb(Zr,Ti)O₃. The unusual behavior of the BTO/CTO SL can be linked to the switching of a nanodomain state into a uniform polarization state or to a field-induced crystallographic symmetry transition. A comparison of the results with the characteristic timescales of these two dynamical phenomena in other complex oxides with different compositions suggests that the phase transition is a more likely possibility.

Puzzling magnetic data on the Zn_1−xMn_xO system such as a small magnetization values or a large negative values of the Curie–Weiss temperature have been obtained in many experimental investigations. Here we report element-specific structural and magnetic investigations on a high-quality Zn_0.95Mn_0.05O nanocrystalline sample. Combining low-temperature x-ray absorption spectroscopy and theoretical simulations, we show that the formation of substitutional spin-antiparallel pairs induces a large local distortion involving a contraction of the Mn–Mn distance and a reduced Mn–O–Mn bond angle. The first-principles calculation considering hole-doping reveals that such a distortion can result in a localized hole around a dopant atom, generating a ferrimagnetic ordering with a magnetization of 0.45 μ_B/Mn. This result may give a new insight for a better understanding of the reported magnetic data.

We benchmark many-body perturbation theory by studying neutral, as well as non-neutral, excitations of finite lattice systems. The neutral excitation spectra are obtained by time-propagating the Kadanoff–Baym equations in the Hartree–Fock and the second Born approximations. Our method is equivalent to solving the Bethe–Salpeter equation with a high-level kernel while respecting self-consistency, which guarantees the fulfillment of a frequency sum rule. As a result, we find that a time-local method, such as Hartree–Fock, can give incomplete spectra, while already the second Born approximation, which is the simplest time-non-local approximation, reproduces well most of the additional excitations, which are characterized as double excitations.

We propose an inverse method to accelerate without final excitation the adiabatic transport of a Bose–Einstein condensate. The method is based on a partial extension of the Lewis–Riesenfeld invariants and provides transport protocols that satisfy exactly the no-excitation conditions without approximations. This inverse method is complemented by optimizing the trap trajectory with respect to different physical criteria and by studying the effect of perturbations such as anharmonicities and noise.

Recently, a new form of quantum memory was proposed. The storage medium is an ensemble of electron spins, coupled to a stripline cavity and an ancillary readout system. Theoretical studies suggest that the system should be capable of storing numerous qubits within the ensemble, and an experimental proof-of-concept has already been performed. Here, we show that this minimal architecture is not limited to storage but is in fact capable of full quantum processing by employing measurement-based entanglement. The technique appears to be remarkably robust against the anticipated dominant error types. The key enabling component, namely a readout technology that non-destructively determines 'are there n photons in the cavity?', has already been realized experimentally.

We thoroughly investigate vibrational quantum dynamics of dimers attached to He droplets, motivated by recent measurements with K₂ (Claas et al 2006 J. Phys. B: At. Mol. Opt. Phys.39 S1151). For those femtosecond pump–probe experiments, the crucial observed features are not reproduced by gas-phase calculations, but agreement is found using a description based on dissipative quantum dynamics, as briefly shown in the work by Schlesinger et al (2010 Chem. Phys. Lett.490 245–8). Here we present a detailed study of the influence of possible effects induced by the droplet. The helium droplet causes electronic decoherence, shifts of potential surfaces and relaxation of wave packets in attached dimers. Moreover, a realistic description of (stochastic) desorption of dimers off the droplet needs to be taken into account. Step by step, we include and study the importance of these effects in our full quantum calculation of the effective dimer dynamics. This approach allows us to reproduce and explain all major experimental findings. We find that desorption is fast and occurs within 2–10 ps after electronic excitation. A further finding is that slow vibrational motion in the ground state can be considered frictionless.

We theoretically study the bichromatic driving of a solid-state cavity quantum electrodynamics (QED) system as a means of probing cavity dressed state transitions and observing the coherent interaction between the system and the light field. We show that this method can enable the observation of the higher order cavity dressed states, supersplitting and ac-Stark shift in a solid-state system comprised of a quantum dot (QD) strongly coupled to a photonic crystal cavity for the on- and far off-resonant cases. For the off-resonant case, phonons mediate off-resonant coupling between the QD and the photonic resonator, a phenomenon unique to solid-state cavity QED.

Quantum discord (QD) measures the fraction of the pairwise mutual information that is locally inaccessible in a multipartite system. Fundamental aspects related to two important measures in quantum information theory, namely the entanglement of formation (EOF) and the conditional entropy, can be understood in terms of the distribution of this form of local inaccessible information (LII). As such, the EOF for an arbitrarily mixed bipartite system AB can be related to the gain or loss of LII due to the extra knowledge that a purifying ancillary system E has on the pair AB. Similarly, a clear meaning of the negativity of the conditional entropy for AB is given. We employ these relations to elucidate important and yet not well-understood quantum features, such as the bipartite entanglement sudden death and the distinction between EOF and QD for quantifying quantum correlation. For that we introduce the concept of LII flow that quantifies the LII shared in a multipartite system when sequential local measurements are carried out.

A new method of momentum measurement of charged particles through multiple Coulomb scattering (MCS) in the OPERA lead-emulsion target is presented. It is based on precise measurements of track angular deviations carried out thanks to the very high resolution of nuclear emulsions. The algorithm has been tested with Monte Carlo pions. The results are found to describe within the expected uncertainties the data obtained from test beams. We also present a comparison of muon momenta evaluated through MCS in the OPERA lead-emulsion target with those determined by the electronic detectors for neutrino-charged current interaction events. The two independent measurements agree within the experimental uncertainties, and the results validate the algorithm developed for the emulsion detector of OPERA.

By means of polarized small-angle neutron scattering, we have resolved the long-standing challenge of determining the magnetization distribution in magnetic nanoparticles in absolute units. The reduced magnetization, localized in non-interacting nanoparticles, indicates strongly particle shape- dependent surface spin canting with a 0.3(1) and 0.5(1) nm thick surface shell of reduced magnetization found for ∼9 nm nanospheres and ∼8.5 nm nanocubes, respectively. Further, the reduced macroscopic magnetization in nanoparticles results not only from surface spin canting, but also from drastically reduced magnetization inside the uniformly magnetized core as compared to the bulk material. Our microscopic results explain the low macroscopic magnetization commonly found in nanoparticles.

We present a new technique that improves the scaling of the error in the adiabatic approximation with respect to the evolution duration, thereby permitting faster transfer at a fixed error tolerance. Our method is conceptually different from previously proposed techniques: it exploits a commonly overlooked phase interference effect that occurs predictably at specific evolution times, suppressing transitions away from the adiabatically transferred eigenstate. Our method can be used in concert with existing adiabatic optimization techniques, such as local adiabatic evolutions or boundary cancelation methods. We perform a full error analysis of our phase interference method along with existing boundary cancelation techniques and show a tradeoff between error-scaling and experimental precision. We illustrate these findings using two examples, showing improved error-scaling for an adiabatic search algorithm and a tunable two-qubit quantum logic gate.

Recently, it was shown that the non-local correlations needed for measurement-based quantum computation (MBQC) can be revealed in the ground state of the Affleck–Kennedy–Lieb–Tasaki (AKLT) model involving nearest-neighbour spin-3/2 interactions on a honeycomb lattice. This state is not singular but resides in the disordered phase of the ground states of a large family of Hamiltonians characterized by short-range-correlated valence bond solid states. By applying local filtering and adaptive single-particle measurements, we show that most states in the disordered phase can be reduced to a graph of correlated qubits that is a scalable resource for MBQC. At the transition between the disordered and Néel ordered phases, we find a transition from universal to non-universal states as witnessed by the scaling of percolation in the reduced graph state.

We demonstrate an x-ray imaging method that combines Fourier transform holography with tomography ('tomoholography') for three-dimensional (3D) microscopic imaging. A 3D image of a diatom shell with a spatial resolution of 140 nm is presented. The experiment is realized by using a small gold sphere as the reference wave source for holographic imaging. This setup allows us to rotate the sample and to collect a number of 2D projections for tomography.

The time-dependent surface flux (t-SURFF) method is introduced for computing strong-field infrared (IR) photo-ionization spectra of atoms by numerically solving the time-dependent Schrödinger equation on minimal simulation volumes. The volumes only need to accommodate the electron quiver motion and the relevant range of the atomic binding potential. Spectra are computed from the electron flux through a surface, beyond which the outgoing flux is absorbed by infinite range exterior complex scaling (irECS). Highly accurate IR photo-electron spectra are calculated in single active electron approximation and compared to the literature results. Detailed numerical evidence for performance and accuracy is given. Extensions to multi-electron systems and double ionization are discussed.

We theoretically investigate the amplification of extraordinary optical transmission (EOT) phenomena in periodic arrays of subwavelength apertures incorporating gain media. In particular, we consider a realistic structure consisting of an opaque silver film perforated by a periodic array of slits and clad on each side by an optically pumped dielectric thin film containing rhodamine dye molecules. By solving the semiclassical electronic rate equations coupled to rigorous finite-element simulations of the electromagnetic fields, we show how the resonant electric-field enhancement associated with EOT properties enables complete ohmic loss compensation at moderate pump intensity levels. Furthermore, our calculations show that, as a consequence of the strong spatial hole-burning effects displayed by the considered structures, three separate regimes of operation arise: the system can behave as an absorber, an optical amplifier or a laser, depending on the value of the pump intensity. A discussion on the feasibility of reaching the lasing regime in the considered class of structures is also presented.

The insulating state of magnetite (Fe₃O₄) can be disrupted by a sufficiently large dc electric field. Pulsed measurements are used to examine the kinetics of this transition. Histograms of the switching voltage show a transition width that broadens as the temperature is decreased, consistent with trends seen in other systems involving 'unpinning' in the presence of disorder. The switching distributions are also modified by an external magnetic field on a scale comparable to that required to reorient the magnetization.

We show that the sensitivity and robustness of a label-free optical imaging technique based on stimulated Raman scattering (SRS) can be enhanced by using resonant optical transitions in a Raman adiabatic passage scheme. Our approach is based on the consideration that any enhancement of the flow of energy between two light beams involved in the SRS process is related to an increase in atomic population transfer between the energy levels of interest. One can thus profit from techniques developed in quantum optics to maximize such atomic population transfer for enhancing the sensitivity and robustness of optical imaging techniques.

We discuss the characterization of a π-phase shift quantum gate acting on a qubit encoded in superpositions of coherent states. We adopt a technique relying on some a priori knowledge about the physics underlying the functioning of the device. A parameter summarizing the global quality of the quantum gate is obtained by 'virtually' processing an entangled state. With such an approach, we can facilitate the characterization of our gate, focusing on the useful subspace rather than on the entire phase space.

We present a theoretical analysis of the ac transport property of quantum wires with Coulomb interaction included using the Luttinger liquid theory. We make experimentally testable predictions of a sensitive dependence of the ac conductance on the wire orientation. The ac conductance displays oscillating behavior as the frequency of the external voltage increases. The oscillation period provides us with a new way of measuring the relative strength of the Rashba and Dresselhaus spin–orbit interactions.

Individuals accepting an idea may intentionally or unintentionally impose influences in a certain neighborhood area, making it less likely or even impossible for other individuals within the area to accept competing ideas. Depending on whether such influences strictly prohibit neighborhood individuals from accepting other ideas or not, we classify them into exclusive and non-exclusive influences, respectively. Our study reveals, for the first time, the rich and complex dynamics of two competing ideas with neighborhood influences in scale-free social networks: depending on whether they have exclusive or non-exclusive influences, the final state varies from multiple co-existence to founder control to exclusion, with different sizes of population accepting each of the ideas, respectively. Such results provide helpful insights for better understanding of the spread (and the control of the spread) of ideas in human society.

We present the first experimental results of a photonic crystal (PC) structure-mediated charged particle beam velocity modulation and energy exchange. Our structure was based on two photonic lattices working at 9.532 GHz: a modulation lattice (ML) driven by a 2.5–6 W signal to velocity-modulate an electron beam of dc voltage from 15 to 30 kV and current from 50 to 150 μA, and an excitation lattice (EL) to exchange energy with the modulated beam, similar to a two-cavity klystron. Experimental results successfully demonstrated high spectral purity from signals excited by the velocity-modulated beam in the EL, with power level in excellent agreement with conventional theories.

Several materials, such as ferromagnets, spinor Bose–Einstein condensates and some topological insulators, are now believed to support knotted structures. One of the most successful base-models having stable knots is the Faddeev–Skyrme model and it is expected to be contained in some of these experimentally relevant models. The taxonomy of knotted topological solitons (Hopfions) of this model is known. In this paper, we describe some aspects of the dynamics of Hopfions and show that they indeed behave like particles: during scattering the Hopf charge is conserved and bound states are formed when the dynamics allows it. We have also investigated the dynamical stability of a pair of Hopfions in stacked or side-by-side configurations, whose theoretical stability has recently been discussed by Ward.

The influences of optical fields on the group delay of chiral tunneling in graphene are investigated in real time using the finite-difference time-domain method. The group delay of tunneling electrons irradiated by an optical field is significantly different from that observed in traditional quantum tunneling. We found that when the barrier width increases, the group delay becomes constant for the reflected wave packet, but increases linearly for the transmitted wave packet. This peculiar tunneling effect can be attributed to current leakage in a time-dependent barrier generated via the optical Stark effect.

We introduce a mapping between graphs and pure quantum bipartite states and show that the associated entanglement entropy conveys non-trivial information about the structure of the graph. Our primary goal is to investigate the family of random graphs known as complex networks. In the case of classical random graphs, we derive an analytic expression for the averaged entanglement entropy $\bar S$ while for general complex networks we rely on numerics. For a large number of nodes n we find a scaling $\bar {S} \sim c \log n +g_{\mathrm {e}}$ where both the prefactor c and the sub-leading O(1) term g_e are characteristic of the different classes of complex networks. In particular, g_e encodes topological features of the graphs and is named network topological entropy. Our results suggest that quantum entanglement may provide a powerful tool for the analysis of large complex networks with non-trivial topological properties.

Gliding arc discharge has been investigated in recent years as an innovative physicochemical technique for contaminated water treatment at atmospheric pressure and ambient temperature. In this study we tested a gas–liquid gliding arc discharge reactor, the bacterial suspension of which was treated circularly. When the bacterial suspension was passed through the electrodes and circulated at defined flow rates, almost 100% of the bacteria were killed in less than 3.0 min. Experimental results showed that it is possible to achieve an abatement of 7.0 decimal logarithm units within only 30 s. Circulation flow rates and types of feeding gas caused a certain impact on bacteria inactivation, but the influences are not obvious. So, under the promise of sterilization effect, industrial applications can select their appropriate operating conditions. All inactivation curves presented the same three-phase profile showing an apparent sterilization effect. Analysis of the scanning electron microscope images of bacterial cells supports the speculation that the gas–liquid gliding arc discharge plasma is acting under various mechanisms driven essentially by oxidation and the effect of electric field. These results enhance the possibility of applying gas–liquid gliding arc discharge decontamination systems to disinfect bacterial-contaminated water. Furthermore, correlational research indicates the potential applications of this technology in rapid sterilization of medical devices, spacecraft and food.

A flat box, almost completely filled with a mixture of granulate, is rotated slowly about its horizontal central axis. In this experiment, a regular vortex flow of the granular material is observed in the cell plane. These vortex structures have a superficial analogy to convection rolls in dissipative structures of ordinary liquids. Whereas in the latter, the origin of the convection can often be attributed to gradients e.g. of densities or surface tensions, there is no trivial explanation at present for the convection of the granulate in the rotating container. Despite the simplicity of the experiment, the underlying mechanisms for convection and segregation are difficult to extract. Here, we present a comprehensive experimental study of the patterns under various experimental conditions and propose a mechanism for the convection.

A continuous projective measurement of a quantum system often leads to a suppression of the dynamics, known as the Zeno effect. Alternatively, generalized nonprojective, so-called 'weak' measurements can be carried out. Such a measurement is parameterized by its strength parameter that can interpolate continuously between the ideal strong measurement with no dynamics—the strict Zeno effect, and a weak measurement characterized by almost free dynamics but blurry observations. Here we analyze the stochastic properties of this uncertainty component in the resulting observation trajectory. The observation uncertainty results from intrinsic quantum uncertainty, the effect of measurement on the system (backaction) and detector noise. It is convenient to separate the latter, system-independent contribution from the system-dependent uncertainty, and this paper shows how to accomplish this separation. The system-dependent uncertainty is found in terms of a quasi-probability, which, despite its weaker properties, is shown to satisfy a weak positivity condition. We discuss the basic properties of this quasi-probability with special emphasis on its time correlation functions as well as their relationship to the full correlation functions along the observation trajectory, and illustrate our general results with simple examples. We demonstrate a violation of classical macrorealism using the fourth-order time correlation functions with respect to the quasi-probability in the two-level system.

We investigate trions, paired states and quantum phase transitions in one-dimensional SU(3) attractive fermions in external fields by means of the Bethe ansatz formalism. Analytical results for the ground state energy, critical fields and complete phase diagrams are obtained for the weak coupling regime. Higher-order corrections for these physical quantities are presented in the strong attractive regime. Numerical solutions of the dressed energy equations allow us to examine how the different phase boundaries are modified by varying the inter-component coupling throughout the whole attractive regime. The pure trionic phase existing in the strong coupling regime decreases smoothly with a decrease in this coupling, until the weak limit is reached. In this weak regime, a pure Bardeen–Cooper–Schrieffer (BCS)-like paired phase can be sustained under certain nonlinear Zeeman splittings.

Periodic bursts, also called the geyser effect, previously observed in the vacuum expansion of pure and <1% ³He-doped solid ⁴He, are shown to occur also in solid ³He–⁴He alloys. Results are reported for ³He_0.54⁴He_0.46 for the temperature range of 1.73–2.13 K and pressures up to 92 bar, and then compared with the results for pure solid ⁴He. The geyser period is found to increase with pressure much faster in the mixed solid than in pure solid ⁴He. A direct measurement of the internal pressure exerted on the lateral walls of the source chamber provides the first information on the Poisson ratio of polycrystalline pure and mixed solids.

We performed Langevin dynamics simulations for the ac-driven flux lines in a type II superconductor with random point-like pinning centers. Scaling properties of flux-line velocity with respect to an instantaneous driving force of small frequency and around the critical dc depinning force are revealed successfully, which provides precise estimates on dynamic critical exponents. From the scaling function, we derive a creep law associated with activation by regular shaking. The effective energy barrier vanishes at the critical dc depinning point in a square-root way when the instantaneous driving force increases. The frequency plays a similar role to temperature in conventional creep motions, but in a nontrivial way governed by the critical exponents. We have also performed systematic finite-size scaling analysis for flux-line velocity in transient processes with dc driving, which provide estimates on critical exponents in good agreement with those derived with ac driving. The scaling law is checked successfully.

We employ surface acoustic waves (SAWs) to control the transfer of photo-generated carriers between interconnected quantum wells and quantum wires (QWRs) grown on pre-patterned (311)A GaAs substrates. Optical studies, carried out under remote acoustic excitation of a single QWR, have shown sharp photoluminescence lines and antibunched photons with tunable emission energy. These features are attributed to recombination of acoustically transported carriers in potential inhomogeneities within the wire. The origin of the photon antibunching is discussed in terms of a 'bottleneck' in the number of carriers trapped in the QWR, which restricts the number of recombination events per SAW cycle. We propose a model for antibunching based on the trapping of carriers induced by the SAW piezoelectric field in states at the interface between the GaAs QWR and the AlGaAs barriers. Non-classical light is emitted during the subsequent release of the trapped carriers into the recombination centers within the wire. The spatial distribution of the emitting recombination centers is estimated using time-resolved measurements.

We present ultrafast x-ray diffraction (UXRD) experiments on different photoexcited oxide superlattices. All data are successfully simulated by dynamical x-ray diffraction calculations based on a microscopic model, that accounts for the linear response of phonons to the excitation laser pulse. Some Bragg reflections display a highly nonlinear strain dependence. The origin of linear and two distinct nonlinear response phenomena is discussed in a conceptually simpler model using the interference of envelope functions that describe the diffraction efficiency of the average constituent nanolayers. The combination of both models facilitates rapid and accurate simulations of UXRD experiments.

Using on-demand coherence conversion via optical locking, a dynamic coherent control of the collective atom phase has been demonstrated for longer photon storage beyond the critical constraint of spin phase decay time, where the storage time can be extended up to hours in a rare-earth-doped solid. Coherent transient phenomena such as photon echoes have been investigated for frozen phase decay via coherent population transfer using a simple deshelving optical pulse pair. Unlike the rephasing halt applied to two-pulse photon echoes, where optical decoherence is accelerated by spin inhomogeneous broadening, a completely atom phase-locked coherence conversion has been observed in three-pulse photon echoes, resulting in spin dephasing-free coherence control. Here, the mechanism of the atom phase-locked coherence conversion via optical locking has been investigated in a solid medium whose optical transition is imperfect, where partial coherence is lost via optical depth-dependent imperfect population transfer. The relationship between coherence loss and optical depth is analyzed, where nearly perfect photon echo efficiency can be obtained for ultralong photon storage in an optically thick medium.

A kinetic model of partially ionized complex plasmas is employed for the numerical analysis of low-frequency longitudinal modes for typical laboratory plasmas. The approach self-consistently includes the effects of plasma particle absorption on dust, collisions with neutrals and electron impact ionization. In addition to the typical dust acoustic mode, the results reveal the existence of a novel long-wavelength mode, attributed to the interplay between the mechanisms of plasma production and loss. The main properties of mode dispersions are investigated through their dependence on plasma and dust parameters.

With quantum calculations, we have investigated the multiphoton nonsequential double ionization of helium atoms in intense laser fields at ultraviolet wavelengths. Very surprisingly, we found a so-far unobserved double-circle structure in the correlated electron momentum spectra. The double-circle structure essentially reveals multiphoton Rabi oscillations of two electrons, which are strongly supported by the oscillating population of a certain doubly excited state and by the oscillating double ionization signals. This two-electron multiphoton Rabi effect provides a profound understanding of electronic correlations and complicated multiphoton phenomena and is expected to be a new tool for broad applications, such as quantum coherent control.