Table of contents

We would like to note that most of the results published in Li et al 2008 New J. Phys.10 043007 agree with the results of Müller and Kern 1996 Appl. Surf. Sci.102 6 that do not appear to be widely known.

We reply to the comment on our paper made by Müller and Kern 2009 New J. Phys.11 018001.

We report on a device capable of imaging second-order spatio-temporal correlations g⁽²⁾(x, τ) between photons. The imager is based on a monolithic array of single-photon avalanche diodes (SPADs) implemented in CMOS technology and a simple algorithm to treat multiphoton time-of-arrival distributions from different SPAD pairs. It is capable of 80 ps temporal resolution with fluxes as low as 10 photons s⁻¹ at room temperature. An important application might be the local imaging of g⁽²⁾ as a means of confirming the presence of true Bose–Einstein macroscopic coherence (BEC) of cavity exciton polaritons.

Thanks to a new focused array of piezoelectric transducers, experimental results are reported here to evidence helical acoustical shock waves resulting from the nonlinear propagation of acoustical vortices (AVs). These shock waves have a three-dimensional spiral shape, from which both the longitudinal and azimuthal components are studied. The inverse filter technique used to synthesize AVs allows various parameters to be varied, especially the topological charge which is the key parameter describing screw dislocations. Firstly, an analysis of the longitudinal modes in the frequency domain reveals a wide cascade of harmonics (up to the 60th order) leading to the formation of the shock waves. Then, an original measurement in the transverse plane exhibits azimuthal behaviour which has never been observed until now for acoustical shock waves. Finally, these new experimental results suggest interesting potential applications of nonlinear effects in terms of acoustics spanners in order to manipulate small objects.

The wave attenuation and energy transfer mechanisms of a metamaterial having a negative effective mass density are studied. The metamaterial considered is represented by a lattice system consisting of mass-in-mass units. The attenuation of wave amplitude for frequencies in the stop band is studied from the energy transfer point of view. It is found that most of the work done by the external force on the lattice system is stored by the internal mass if the forcing frequency is close to the local resonance frequency. However, the energy stored in the internal mass is only temporary; it is taken out by the external force in the form of negative work in a cyclic manner. This behavior is utilized to design metamaterials for preventing stress waves from passing them.

Classical molecular dynamics (MD) is a frequently used technique in the study of radiation damage cascades because it provides information on very small time and length scales inaccessible to experiment. In a radiation damage process, energy transfer from ions to electrons may be important, yet there is continued uncertainty over how to accurately incorporate such effects in MD. We introduce a new technique based on the quantum mechanical Ehrenfest approximation to evaluate different methods of accounting for electronic losses. Our results suggest that a damping force proportional to velocity is sufficient to model energy transfer from ions to electrons in most low energy cascades. We also find, however, that a larger rate of energy transfer is seen when the ionic kinetic energy is confined to a focused sequence of collisions. A viscous damping coefficient dependent on the local atomic environment is shown to be an excellent model for electronic energy losses in low energy cascades in metals.

We report on correlated real-time detection of individual electrons in an InAs nanowire double quantum dot (DQD). Two self-aligned quantum point contacts (QPCs) in an underlying two-dimensional electron gas material serve as highly sensitive charge detectors for the DQD. Tunnel processes of individual electrons and all tunnel rates are determined by simultaneous measurements of the correlated signals of the QPCs.

A generalization of the Bell states and Pauli matrices to dimensions which are powers of 2 is considered. A basis of maximally entangled multidimensional bipartite states (MEMBS) is chosen very similar to the standard Bell states and constructed of only symmetric and antisymmetric states. This special basis of MEMBS preserves all basic properties of the standard Bell states. We present a recursive and non-recursive method for the construction of MEMBS and discuss their properties. The antisymmetric MEMBS possess the property of rotationally invariant exclusive correlations which is a generalization of the rotational invariance of the antisymmetric singlet Bell state.

A key step in the use of diamond nitrogen vacancy (NV) centers for quantum computational tasks is a single shot quantum non-demolition measurement of the electronic spin state. Here, we propose a high fidelity measurement of the ground state spin of a single NV center, using the effects of cavity quantum electrodynamics. The scheme we propose is based in the one-dimensional atom or Purcell regime, removing the need for high Q cavities that are challenging to fabricate. The ground state spin of the NV center has a splitting of ≈6–10 μeV, which can be resolved in a high-resolution absorption measurement. By incorporating the center in a low-Q and low volume cavity we show that it is possible to perform single shot readout of the ground state spin using a weak laser with an error rate of ≈7×10⁻³, when realistic experimental parameters are considered. Since very low levels of light are used to probe the state of the spin we limit the number of florescence cycles, which dramatically reduces the measurement induced decoherence approximating a non-demolition measurement of ground state spin.

We introduce and demonstrate a technique for generating a range of novel multi-photon entangled states. Adjusting a simple experimental parameter allows the preparation of pure states with an arbitrary level of W-class entanglement, from a fully separable state to the maximally robust W state, enabling full control over this entanglement class in our system. Furthermore, the generated states exhibit a highly symmetric entanglement distribution that we show is optimally robust against qubit loss. The ability to prepare entanglement in robust configurations is particularly relevant to many emerging quantum technologies where entanglement is a valuable resource. We achieve a high quality experimental realization for the three-photon case, including a W state fidelity of 0.90±0.03. In addition, we present a new technique for characterizing quantum states in the laboratory in the form of iterative tomography.

We experimentally demonstrate coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable by changing the amplitude of the flux pulse. The demonstrated operation mode could allow quantum gates to be realized in less than 100 ps, which is much shorter than gate times attainable in other superconducting qubits. Another advantage of this type of qubit is its immunity to both thermal and magnetic field fluctuations.

We demonstrate that a highly frustrated anisotropic Josephson junction array (JJA) on a square lattice exhibits a zero-temperature jamming transition, which shares much in common with those in granular systems. Anisotropy of the Josephson couplings along the horizontal and vertical directions plays roles similar to normal load or density in granular systems. We studied numerically static and dynamic response of the system against shear, i.e. injection of external electric current at zero temperature. Current–voltage curves at various strength of the anisotropy exhibit universal scaling features around the jamming point much as do the flow curves in granular rheology, shear-stress versus shear-rate. It turns out that at zero temperature the jamming transition occurs right at the isotropic coupling and anisotropic JJA behaves as exotic fragile vortex matter: it behaves as a superconductor (vortex glass) in one direction, whereas it is a normal conductor (vortex liquid) in the other direction even at zero temperature. Furthermore, we find a variant of the theoretical model for the anisotropic JJA quantitatively reproduces universal master flow-curves of the granular systems. Our results suggest an unexpected common paradigm stretching over seemingly unrelated fields—the rheology of soft materials and superconductivity.

The interaction of two laser pulses in an underdense plasma has been proven to be able to inject electrons into plasma waves, thus providing a stable and tunable source of electrons. Whereas previous works focused on the 'beatwave' injection scheme in which two lasers with the same polarization collide in a plasma, this present paper studies the effect of polarization and more specifically the interaction of two colliding cross-polarized laser pulses. It is shown both theoretically and experimentally that electrons can also be preaccelerated and injected by the stochastic heating occurring at the collision of two cross-polarized lasers and thus, a new regime of optical injection is demonstrated. It is found that injection with cross-polarized lasers occurs at higher laser intensities.

In evolutionary game dynamics, reproductive success increases with the performance in an evolutionary game. If strategy A performs better than strategy B, strategy A will spread in the population. Under stochastic dynamics, a single mutant will sooner or later take over the entire population or go extinct. We analyze the mean exit times (or average fixation times) associated with this process. We show analytically that these times depend on the payoff matrix of the game in an amazingly simple way under weak selection, i.e. strong stochasticity: the payoff difference Δπ is a linear function of the number of A individuals i, Δπ=u i+v. The unconditional mean exit time depends only on the constant term v. Given that a single A mutant takes over the population, the corresponding conditional mean exit time depends only on the density dependent term u. We demonstrate this finding for two commonly applied microscopic evolutionary processes.

We apply a one-dimensional (1D) optical lattice, formed by two laser beams with a wavelength of 852 nm, to a 3D ⁸⁷Rb Bose–Einstein condensate (BEC) in a shallow magnetic trap. We use Kapitza–Dirac scattering to determine the depth of the optical lattice. A qualitative change in behavior of the BEC is observed at a lattice depth of 30E_rec, where the quantum gas undergoes a reversible transition from a superfluid state to a state that lacks well-to-well phase coherence. Our observations are consistent with a 1D Mott insulator transition, but could also be explained by mean-field effects.

The recovery of classical nonlinear and chaotic dynamics from quantum systems has long been a subject of interest. Furthermore, recent work indicates that quantum chaos may well be significant in quantum information processing. In this paper, we discuss the quantum to classical crossover of a superconducting quantum interference device (SQUID) ring. Such devices comprise a thick superconducting loop enclosing a Josephson weak link and are currently strong candidates for many applications in quantum technologies. The weak link brings with it a nonlinearity such that semiclassical models of this system can exhibit nonlinear and chaotic dynamics. For many similar systems an application of the correspondence principle together with the inclusion of environmental degrees of freedom through a quantum trajectories approach can be used to effectively recover classical dynamics. Here we show (i) that the standard expression of the correspondence principle is incompatible with the ring Hamiltonian and we present a more pragmatic and general expression which finds application here and (ii) that practical limitations to circuit parameters of the SQUID ring prevent arbitrarily accurate recovery of classical nonlinear dynamics.

The controlled generation of nano-particles has been an important issue for the nano-structure formation in processing plasmas. We observed that the particle growth under UV irradiation was enhanced due to electric charge reduction of the particles, suggesting that the variation of particle charges could be a control parameter for the particle growth. The particle growth variation by UV irradiation is well described by the particle coagulation model with time-dependent particle charges in consideration, where predator particles grow by adsorbing a few nanometer-sized proto-particles.

Phase-modulated femtosecond laser pulses are used to study the spectral response of a non-resonant two photon excitation from the Cu(111) Shockley surface state (SS). Controlled variations in the spectral phase of the laser pulse were introduced using a tuneable Fork prism phase modulator and resulted in a shift in the peak-position (of up to 110 meV), variations in the spectral width (up to 88 meV) and changes in the asymmetry of the SS peak as detected by two-photon photoemission. A satisfactory quantitative model of the experimental results can only be achieved if the complete spectral phase up to the third-order dispersion terms is taken into account. Of particular note, we find that a consistent description of this two photon absorption process does not require coupling of the excitation to an intermediate copper bulk state, which contradicts the previous results of Petek et al (1997 Phys. Rev. Lett. 79 4649).

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogen-vacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting ³E excited state, which is invaluable for the development of diamond-based quantum information processing.

We show that a large entangled current can be produced from a very simple passive device: a cluster of three resonant quantum dots, tunnel coupled to one input lead and two output leads. Through a rapid first-order resonant process within the cluster, entangled electron pairs are emitted into separate leads. We show that the process is remarkably robust to variations in systems parameters. The simplicity and robustness should permit experimental demonstration in the immediate future. Applications include quantum repeaters and unconditionally secure interfaces.

A multi-objective genetic algorithm is applied to optimize picosecond laser fields, driving vibrational quantum processes. Our examples are state-to-state transitions and unitary transformations. The approach allows features of the shaped laser fields and of the excitation mechanisms to be controlled simultaneously with the quantum yield. Within the parameter range accessible to the experiment, we focus on short pulse durations and low pulse energies to optimize preferably robust laser fields. Multidimensional Pareto fronts for these conflicting objectives could be constructed. Comparison with previous work showed that the solutions from Pareto optimizations and from optimal control theory match very well.

Using three-dimensional (3D) numerical discretization of the Ginzburg–Landau (GL) equations, we investigate the superconducting state of a sphere with a piercing hole in the presence of a magnetic field. In the case of samples with central perforation, in axially applied homogeneous magnetic field, we realized unconventional vortex states of broken symmetry due to complex, 3D competing interactions, which depend on the GL parameter κ. For certain sizes of the sample, non-hysteretic multi-vortex entry and exit is predicted with the non-existence of some vorticities as stable states. In a tilted magnetic field, we studied the gradual transformation of 3D flux patterns into 1D vortex chains, where vortices align along the perforation, and the evolvement of the multi-vortex entry as well. We analyze the flux-guiding ability of the hole in a tilted field, which leads to fractional flux response in magnetization M(H) curves.

The control of magnetic properties by means of an electric field is an important aspect in magnetism and magnetoelectronics. We here utilize magnetoelastic coupling in ferromagnetic/piezoelectric hybrids to realize a voltage control of magnetization orientation at room temperature. The samples consist of polycrystalline nickel thin films evaporated onto piezoelectric actuators. The magnetic properties of these multifunctional hybrids are investigated at room temperature as a function of the voltage controlled stress exerted by the actuator on the Ni film. Ferromagnetic resonance spectroscopy shows that the magnetic easy axis in the Ni film plane is rotated by 90° upon changing the polarity of the voltage V_p applied to the actuator. In other words, the in-plane uniaxial magnetic anisotropy of the Ni film can be inverted via the application of an appropriate voltage V_p. Using superconducting quantum interference device (SQUID) magnetometry, the evolution of the magnetization vector is recorded as a function of V_p and of the external magnetic field. Changing V_p allows to reversibly adjust the magnetization orientation in the Ni film plane within a range of approximately 70°. All magnetometry data can be quantitatively understood in terms of the magnetic free energy determined from the ferromagnetic resonance experiments. These results demonstrate that magnetoelastic coupling in hybrid structures is indeed a viable option to control magnetization orientation in technologically relevant ferromagnetic thin films at room temperature.

We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) with the sandwich configuration of La_0.67Sr_0.33MnO₃ (LSMO)/π-conjugated polymer regio-random poly(3-hexyl thiophene)/cobalt (Co), showing giant magnetoresistance (GMR) response. Different deposition rates of Co at the top electrode resulted in two types of devices: one with lower device resistance and linear current–voltage (I–V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I–V characteristics. We observed an asymmetric dc bias dependence of magnetoresistance (MR) in devices with more Co inclusion, while for the other type of device, bias dependence was more symmetric. At higher bias, %MR of both types of device showed no significant difference (5–10%), but at low dc bias it ranged between 50 and 160% MR. This can be attributed to the higher tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other. Magnetic tunnel junction-like features are observed in the devices with greater Co inclusions. Anomalous MR peaks were also observed in these devices and their origin was explained in terms of presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of PSVs showed a monotonic decrease in MR with temperature at high bias currents.

We investigate the extrinsic spin Hall effect (SHE) in the electron gas model due to magnetic impurities, by focusing on Ce- and Yb-impurities. In the dilute limit, the skew scattering term dominates the side jump term. For Ce-impurities, the spin Hall angle α_SH due to skew scattering is given by -8πδ₂/7, where δ₂ (≪1) is the phase shift for the d (l=2) partial wave. Since α_SH reaches O(10⁻¹) if δ₂≳0.03, considerably large SHE is expected to emerge in metals with rare-earth impurities. The present study provides a highly efficient way to generate a spin current.

We present a novel concept for precision laser spectroscopy on a single ion confined in a Penning trap. This concept circumvents the need for detection of fluorescence photons. Instead, changes in motional frequencies of the trapped ion are used to determine atomic transitions of interest with relative accuracies better than 10⁻¹⁰. We discuss the application to a measurement of forbidden transitions in highly charged ions, making stringent tests of bound-state quantum electrodynamics (QED) calculations, including the nuclear recoil contribution, possible. The method may also be used to 'weigh' optical excitations in light ions by the relativistic frequency shift.

The pinning properties of a superconducting thin film with a square array of blind holes are studied using the nonlinear Ginzburg–Landau theory. Although blind holes provide a weaker pinning potential than holes (also called antidots), several novel vortex structures are predicted for different size and thickness of the blind holes. Orientational dimer and trimer vortex states as well as concentric vortex shells can nucleate in the blind holes. In addition, we predict the stabilization of giant vortices that may be located both in the pinning centers and/or at the interstitial sites, as well as the combination of giant vortices with sets of individual vortices. For large blind holes, local vortex shell structures inside the blind holes may transfer their symmetry to interstitial vortices as well. The subtle interplay of shell formation and traditional Abrikosov vortex lattices inside the blind holes is also studied for different numbers of trapped vortices.

Naturally available optical materials are known to provide a wide variety of electric responses, spanning from positive to negative permittivity values. In contrast, owing to drastically modified conduction properties at the microscopic level, at such high frequencies magnetism and conductivity are very challenging to realize. This implies that extreme (high or low) values of permittivity, although highly desirable for a wide range of optical applications, are difficult to realize in practice. Here, we suggest the design of an engineered resonant nanoparticle composed of two conjoined hemispheres, whose optical response may be changed at will from an ideal electric conductor to an ideal magnetic conductor. Near the nanoparticle internal resonant frequency, we derive a closed-form solution that describes the electromagnetic response of this nanoparticle, showing how its light interaction may become dramatically dependent on the local field polarization, passing through all possible impedance values (from zero to infinity) by a simple mechanical or polarization rotation. Considering realistic frequency dispersion and loss in optical materials, we further show that these concepts may be applied to different geometries, with possibility for future experimental feasibility. We forecast various applications of this geometry as an optical nanoswitch, a novel nanocircuit element and as a building block for novel optical metamaterials.

We report direct numerical magnetohydrodynamic simulations at low magnetic Prandtl numbers of a turbulent two-cell flow in a bounded, spherical geometry, driven by a constant body force. The flow amplifies infinitesimal magnetic perturbations if the magnetic Reynolds number Rm is larger than a threshold Rm_c, resulting in a self-excited equatorial magnetic dipole. However, finite amplitude perturbations to the magnetic field can trigger dynamo action below Rm_c: a hysteresis cycle has been found that can sustain dynamo action in an interval Rm₀ < Rm < Rm_c. The instability is therefore governed by a subcritical bifurcation. This hysteretic behaviour is associated with changes in the turbulent velocity field caused by the finite amplitude magnetic field. It is then shown that the dynamo state can be accessed by transiently applying a magnetic field from an external source. Finally, a dynamo state with characteristics different from the self-excited case is found in the vicinity of Rm₀.

We observe dressed states and quantum interference effects in a strongly driven three-level quantum dot ladder system. The effect of a strong coupling field on one dipole transition is measured by a weak probe field on the second dipole transition using differential reflection. When the coupling energy is much larger than both the homogeneous and inhomogeneous linewidths an Autler–Townes splitting is observed. Significant differences are observed when the transitions resonant with the strong and weak fields are swapped, particularly when the coupling energy is nearly equal to the measured linewidth. This result is attributed to quantum interference: destructive or constructive interference with modest visibility is observed depending on the pump/probe geometry. The data demonstrate that coherence of both the bi-exciton and the exciton is maintained in this solid-state system, even under intense illumination, which is crucial for prospects in quantum information processing and nonlinear optical devices.

We consider the elementary processes associated with time refraction in non-stationary plasmas. We describe the frequency shifts of longitudinal and transverse wave propagation in isotropic plasmas, and their corresponding energy variation, which is a direct consequence of non-stationarity. We also show the appearance of reflected waves, propagating with the same (time-varying) frequency, but in the opposite direction, as a direct consequence of time refraction. The case of an expanding (or contracting) plasma bubble is discussed, and will be applied to an expanding universe, and to sonoluminescence.

We study Bragg spectroscopy of a strongly interacting Bose–Einstein condensate using time-dependent Hartree–Fock–Bogoliubov theory. We include approximatively the effect of the momentum-dependent scattering amplitude which is shown to be the dominant factor in determining the spectrum for large momentum Bragg scattering. The condensation of the Bragg scattered atoms is shown to alter significantly the observed excitation spectrum by creating a novel pairing channel of mobile pairs.

We present an efficiently pumped single photon source based on single quantum dots embedded in photonic crystal nanocavities. Resonant excitation of a single quantum dot via a higher order cavity mode is shown to result in a 100× reduced optical power at the saturation onset of the photoluminescence, compared with excitation at the same frequency, after the cavity mode is shifted. We demonstrate that this excitation scheme leads to selective excitation only of quantum dots that are coupled to the cavity by comparing photoluminescence and auto-correlation spectra for the same excitation wavelength, with and without spectral resonance with the cavity mode. This excitation technique provides cleaner conditions for single photon generation and a method to select a subset of quantum dots that are spatially coupled to the nanocavity mode.

We observe quantum interference of photons emitted by two continuously laser-excited single ions, independently trapped in distinct vacuum vessels. High contrast two-photon interference is observed in two experiments with different ion species, Ca⁺ and Ba⁺. Our experimental findings are quantitatively reproduced by Bloch equation calculations. In particular, we show that the coherence of the individual resonance fluorescence light field is determined from the observed interference.

Nonlinear transport through diluted magnetic semiconductor nanostructures is investigated. We have considered a II–VI multiquantum well nanostructure whose wells are selectively doped with Mn. The response to a dc voltage bias may be either a stationary or an oscillatory current. We have studied the transition from stationary to time-dependent current as a function of the doping density and the number of quantum wells. Analysis and numerical solution of a nonlinear spin transport model shows that the current in a structure without magnetic impurities is stationary, whereas current oscillations may appear if at least one well contains magnetic impurities. For long structures having two wells with magnetic impurities, a detailed analysis of nucleation of charge dipole domains shows that self-sustained current oscillations are caused by repeated triggering of dipole domains at the magnetic wells and motion towards the collector. Depending on the location of the magnetic wells and the voltage, dipole domains may be triggered at both wells or at only one. In the latter case, the well closer to the collector may inhibit domain motion between the first and the second well inside the structure. Our study could allow design of oscillatory spin-polarized current injectors.

Being able to quantify the level of coherent control in a proposed device implementing a quantum information processor (QIP) is an important task for both comparing different devices and assessing a device's prospects with regards to achieving fault-tolerant quantum control. We implement in a liquid-state nuclear magnetic resonance QIP the randomized benchmarking protocol presented by Knill et al (2008 Phys. Rev. A 77 012307). We report an error per randomized π/2 pulse of 1.3±0.1×10⁻⁴ with a single-qubit QIP and show an experimentally relevant error model where the randomized benchmarking gives a signature fidelity decay which is not possible to interpret as a single error per gate. We explore and experimentally investigate multi-qubit extensions of this protocol and report an average error rate for one- and two-qubit gates of 4.7±0.3×10⁻³ for a three-qubit QIP. We estimate that these error rates are still not decoherence limited and thus can be improved with modifications to the control hardware and software.

We present a systematic study of the Rashba-type spin–orbit interaction at the (0001) surfaces of rare-earth metals and their surface monoxides, specifically of Tb metal and the O/Tb, O/Lu and O/Y surfaces. By means of photoemission experiments and ab initio band-structure calculations, we uncover the influence of this interaction on the surface electronic structure. In turn, the dramatic impact of the charge-density distribution of the surface/interface states on the strength of the Rashba-type spin splitting is demonstrated. We discuss the Rashba effect at magnetic and non-magnetic rare-earth surfaces, and compare with cases where it is negligible. The difference between the Rashba effect and magnetic linear dichroism in photoemission is pointed out to help avoid possible confusion in connection with the simultaneous appearance of these two effects at a magnetic surface.

We report a point-contact spectroscopy investigation of aluminium atomic-size contacts produced with the help of the mechanically controllable break-junction technique at low temperature. The derivatives of the differential conductance traces (d²I/dV² versus V) display point-symmetric structures at certain voltage values, which move upon stretching the contact. These excitations may give rise to either enhancements or reductions of the conductance, indicating that they couple to highly or weakly transmissive electronic channels, respectively. By analyzing the sign of the conductance change and the strain dependence of the amplitudes and the energies we suggest that transverse as well as longitudinal vibronic modes are detected in the transport through single-atom contacts. Our findings are in agreement with symmetry arguments for the coupling of electronic and vibronic excitations.

With the help of the Floquet theory, we study the transport properties of symmetric Λ-type coupled triple quantum dots driven by an ac electric field. Under appropriate conditions, coherent destruction of tunneling (CDT) and photon-assisted Fano resonance (FR) appear because of quantum interference effects. As a consequence of competition between the super-exchange interaction and the CDT there is a drastic competition between the CDT and the FR. The phenomena represent two different ways of coherent electron trapping by which we can control the behavior of electrons and they provide a convenient way to fabricate a quantum switch.

We have studied the magnetic structure that forms in a Fe/native Fe oxide multilayer by nuclear resonant scattering of synchrotron radiation and polarized neutron reflectometry. Magnetic field-dependent experiments revealed a non-collinear magnetic arrangement of the adjacent metallic layers which is mediated by an antiferromagnetically ordered oxide layer. Despite its antiferromagnetic (AFM) order, the oxide exhibits a small net magnetization attributed to the presence of metallic Fe within the AFM matrix that aligns parallel to the external field. The presence of a strong uniaxial anisotropy prevents the system from forming small magnetic domains in remanence. The canting angle between the two magnetic sublattices remains close to 90° throughout the magnetization reversal on the hard axis. The results and the influence of the uniaxial anisotropy are discussed in the framework of the proximity magnetism model.

Acoustic transparency is studied in two-dimensional sonic crystals consisting of hexagonal distributions of cylinders with continuously varying properties. The transparency condition is achieved by selectively closing the acoustic bandgaps, which are governed by the structure factor of the cylindrical scatterers. It is shown here that cylindrical scatterers with the proposed continuously varying properties are physically realizable by using metafluids based on sonic crystals. The feasibility of this proposal is analyzed by a numerical experiment based on multiple scattering theory.

Basin boundaries are the boundaries between the basins of attraction of coexisting attractors. When one of the attractors breaks up and becomes a transient repelling structure the basin boundary also disappears. Nevertheless, it is possible to distinguish the two types of dynamics in phase space and to define and identify a remnant of the basin boundary, the edge of chaos. We here demonstrate the concept using a two-dimensional (2D) map, and discuss properties of the edge of chaos and its invariant subspaces, the edge states. The discussion is motivated and guided by observations on certain shear flows like pipe flow and plane Couette flow where the laminar profile and a transient turbulent dynamics coexist for certain parameters, and where the notions of edge of chaos and edge states proved to be useful concepts to characterize the transition to chaos. As in those cases we use the lifetime, i.e. the number of iterations needed to approach the laminar state, as an indicator function to track the edge of chaos and to identify the invariant edge states. The 2D map captures many of the features identified in laboratory experiments and direct numerical simulations of hydrodynamic flows. It illustrates the rich dynamical behavior in the edge of chaos and of the edge states, and it can be used to develop and test further characterizations.

An interesting aspect in the research of complex (dusty) plasmas is the experimental study of the interaction of micro-particles with the surrounding plasma for diagnostic purposes. Local electric fields can be determined from the behaviour of particles in the plasma, e.g. particles may serve as electrostatic probes. Since in many cases of applications in plasma technology it is of great interest to describe the electric field conditions in front of floating or biased surfaces, the confinement and behaviour of test particles is studied in front of floating walls inserted into a plasma as well as in front of additionally biased surfaces. For the latter case, the behaviour of particles in front of an adaptive electrode, which allows for an efficient confinement and manipulation of the grains, has been experimentally studied in terms of the dependence on the discharge parameters and on different bias conditions of the electrode. The effect of the partially biased surface (dc and rf) on the charged micro-particles has been investigated by particle falling experiments. In addition to the experiments, we also investigate the particle behaviour numerically by molecular dynamics, in combination with a fluid and particle-in-cell description of the plasma.

Volumes of sub-wavelength electromagnetic elements can act like homogeneous materials: metamaterials. In analogy, sheets of optical elements such as prisms can act ray-optically like homogeneous sheet materials. In this sense, such sheets can be considered to be metamaterials for light rays (METATOYs). METATOYs realize new and unusual transformations of the directions of transmitted light rays. We study here, in the ray-optics and scalar-wave limits, the wave-optical analog of such transformations, and we show that such an analog does not always exist. Perhaps, this is the reason why many of the ray-optical possibilities offered by METATOYs have never before been considered.

We have observed the magnetic reversal of an exchange bias model system on both sides of the ferromagnet/antiferromagnet (FM/AFM) interface via nuclear resonant scattering of synchrotron radiation from ⁵⁷Fe sensor layers. This method yields the spin direction and the fraction of uncompensated moments with nm depth resolution. The reversal of the ferromagnet along the easy axis proceeds via the formation of a domain structure that extends across the FM/AFM interface. This is responsible for archetypal exchange bias characteristics like the small magnitude of the bias and the asymmetric shape of the hysteresis loop.

We analyze the transmission of continuous-wave and pulsed squeezed vacuum through rubidium vapor under the conditions of electromagnetically induced transparency. Our analysis is based on a full theoretical treatment for a squeezed state of light propagating through temporal and spectral filters and detected using time and frequency-domain homodyne tomography. A model based on a three-level atom allows us to evaluate the linear losses and extra noise that degrade the nonclassical properties of the squeezed vacuum during the atomic interaction and eventually predict the quantum states of the transmitted light with a high precision.

An optical waveguide experiment was used to study the influence of dc electric fields on a hybrid aligned nematic liquid crystal cell. This dc switching differed from ac switching in two ways: first, the equilibrium states depended on the sign of the applied voltage, and second, there was transient activity over long (∼100 ms) timescales. To understand both of these, a numerical model of the cell's dynamics, which included both the Ericksen–Leslie theory and a drift-diffusion model of mobile ions, has been developed. Comparing modelling with observations, we find that the transients are caused by the motion of tiny concentrations of ionic impurities, and that the sign dependence is caused by an asymmetric distribution of surface charge, rather than the flexoelectric effect.

One century after Mie's original paper, Mie scattering is still a fertile field of scientific endeavor. We show that the Mie scattering distinguishes the topological charge of light beams with phase dislocations. We experimentally and numerically study the scattering of highly focused Laguerre–Gaussian beams by dielectric and metal spheres, and show that the scattered field is sensitive to the modulus and to the sign of the topological charge. The implications for position detection systems are also discussed.

The ground state of solid ⁴He is studied using the diffusion Monte Carlo method and a new trial wave function able to describe the supersolid. This wave function is symmetric under the exchange of particles and used as a guiding function in the method allows for reproducing the experimental equation of state. The use of this zero-temperature technique overcomes the conceptual ambiguity of finite-temperature methods in the search of a supersolid. Results for the one-body density matrix show the existence of off-diagonal long-range order with a very small condensate fraction ∼ 10^-4, the specific value being not fully independent of the trial wave function due to the remaining bias in the extrapolated estimator. The superfluid density of the commensurate system is below our resolution threshold, ρ_s/ρ< 10^-5. This zero-temperature result is incompatible with recent experimental measures of superfluidity in solid ⁴He showing that the origin of the experimental findings is not that of a supersolid in a perfect crystal. Introducing in the system a 1% concentration of vacancies the superfluid density is manifestly larger, ρ_s/ρ=3.2(1)×10^{- 3}.

Vibrational real-time spectra of poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) were measured in a 5 fs pump–probe experiment simultaneously at 128 probe wavelengths with a multichannel detection system. The spectral dependence of the coherent vibrational amplitudes obtained from the Fourier transform (FT) on the probe wavelength detected was found to be given by the sum of the ground-state absorption spectrum and its first and second derivatives. This indicates that the change of the transition probability caused by a wave packet motion can be explained as induced by both the non-Condon effect (non-Condon (NC) mechanism) and the time-dependent Franck–Condon factor (Frank-Condon (FC) mechanism). The FC mechanism can contribute to the first and second derivatives' dependence. On the other hand, the NC mechanism is dominant in the zeroth-order derivative. This result proves that the 1¹B_u exciton is strongly coupled with the excited ¹ A_g state, which is known to be essential in third-order optical nonlinearity. The amounts of shift of the absorption peaks and changes in the bandwidth due to the wave packet motions were determined for the four most prominent modes in the FT power spectra. The shift due to the FC mechanism was about 1.3–1.1% of the peak transition energy and the broadening of the vibronic transition due to the NC mechanism was about 8.0–7.6% of the bandwidth for the four modes. A novel ultrafast optical switch utilizing the modulation of electronic transition probability by molecular vibration through vibronic coupling and the interference of the wave packets is proposed.

Storage and retrieval of parametric down-conversion (PDC) photons are demonstrated with electromagnetically induced transparency (EIT). Extreme frequency filtering is performed for the THz order of broadband PDC light and the frequency bandwidth of the light is reduced to the MHz order. Storage and retrieval procedures are carried out for frequency-filtered PDC photons. Since the filtered bandwidth (full width at half-maximum (FWHM)=9 MHz) is within the EIT window (FWHM=12.6 MHz), the flux of the PDC light is successfully stored and retrieved. The nonclassicality of the retrieved light is confirmed by using the photon counting method, where the classical inequality that is only satisfied for classical light fields is introduced. Since PDC photons can be utilized for producing a single-photon state conditionally, storage and retrieval procedures are also performed for conditional single photons. The anticorrelation parameter used for checking the property of the single-photon state shows a value of less than 1, which means that the retrieved light is in a nonclassical region.

The Loschmidt echo (or quantum fidelity) is investigated in the context of the many-body electron dynamics in a nonparabolic quantum well, modeled by the self-consistent Wigner–Poisson system. The quantum fidelity drops abruptly after a quiescent period, as was observed for other self-interacting systems. A unifying interpretation of this phenomenon is given in terms of trajectory separation and the Ehrenfest time. The effects of Planck's constant and environment-induced decoherence are also studied.

We study the dependence of the electronic structure of iron pnictides on the angle formed by the arsenic–iron bonds. Within a Slater–Koster tight binding model which captures the correct symmetry properties of the bands, we show that the density of states and the band structure are sensitive to the distortion of the tetrahedral environment of the iron atoms. This sensitivity is extremely strong in a two-orbital (d_xz, d_yz) model due to the formation of a flat band around the Fermi level. Inclusion of the d_xy orbital destroys the flat band while keeping considerable angle dependence in the band structure.

We have studied ionization of alkali-metal Rydberg atoms by blackbody radiation (BBR). The results of theoretical calculations of ionization rates of Li, Na, K, Rb and Cs Rydberg atoms are presented. The calculations have been performed for nS, nP and nD states for principal quantum numbers n=8–65 at ambient temperatures of 77, 300 and 600 K. The calculations take into account the contributions of BBR-induced redistribution of population between Rydberg states prior to photoionization and field ionization by extraction electric field pulses. The obtained results show that these phenomena affect both the magnitude of the measured ionization rates and their n dependence. A Cooper minimum for BBR-induced transitions between bound Rydberg states of Li has been found. The calculated ionization rates are compared with our earlier measurements of BBR-induced ionization rates of Na nS and nD Rydberg states with n=8–20 at 300 K. Good agreement for all states except nS with n>15 is observed. Useful analytical formulae for quick estimates of BBR ionization rates of Rydberg atoms are presented. Application of BBR-induced ionization signal to measurements of collisional ionization rates is demonstrated.

We present a theoretical investigation of a lattice Tonks–Girardeau gas that is created by inelastic, instead of elastic interactions. An analytical calculation shows that in the limit of strong two-body losses, the dynamics of the system is effectively that of a hard-core boson gas. We also derive an analytic expression for the effective loss rate. We find good agreement between these analytical results and results from a rigorous numerical calculation. The hard-core character of the particles is visible both in a reduced effective loss rate and in the momentum distribution of the gas.

The laser-induced modification of a fundamental process of quantum electrodynamics, the conversion of a high-energy gamma photon in the Coulomb field of a nucleus into an electron–positron pair, is studied theoretically. Although the employed formalism allows for the general case where the gamma photon and laser photons cross at an arbitrary angle, we here focus on a theoretically interesting and numerically challenging setup, where the laser beam and gamma photon counterpropagate and impinge on a nucleus at rest. For a peak laser field smaller than the critical Schwinger field and gamma photon energy larger than the field-free threshold, the total cross section is verified to be almost unchanged with respect to the field-free case, whereas the differential cross section is drastically modified by the laser field. The modification of the differential cross section is explained by classical arguments. We also find the laser-dependent maximal energy of the produced pair and point out several interesting features of the angular spectrum.

We study the zero and finite temperature Casimir force acting on a perfectly conducting piston with arbitrary cross section moving inside a closed cylinder with infinitely permeable walls. We show that at any temperature, the Casimir force always tends to move the piston away from the walls and toward its equilibrium position. In the case of a rectangular piston, exact expressions for the Casimir force are derived. In the high-temperature regime, we show that the leading term of the Casimir force is linear in temperature and therefore the Casimir force has a classical limit. Due to duality, all these results also hold for an infinitely permeable piston moving inside a closed cylinder with perfectly conducting walls.

We present an analysis of the relativistic electronic structure of the 1×1 H/W(110) surface. Our calculations reproduce in detail the experimentally determined Fermi surface and spin polarization of the surface electron states. A projected spin-polarized density of states is defined in order to simulate the photoemission spectra exhibiting very good agreement with existing spin-resolved measurements. Our calculations confirm that the spin-split states, S₁ and S₂, are approximately circularly spin polarized with respect to the high-symmetry point . Moreover, we present the precise shape and magnitude of the spin polarization of the spin–orbit split S₁ and S₂ states, through the entire Brillouin zone.

The origin of the appearance of superconductivity in the high-transition-temperature (high-T_c) copper oxides has remained a subject of active inquiry. In order to address this issue, we chose La_2−xSr_xCuO₄ as a model system and succeeded in fine controlling the hole concentration around the composition where T_c appears. T_c did not emerge smoothly at a critical hole concentration, x_c. T_c and the absolute value of the Meissner signal below T_c increased progressively with Sr content x>x_c. Measurements of magnetization versus temperature under hydrostatic pressure (P) are also reported for La_2−xSr_xCuO₄, which has no charge reservoir. A dT_c/dP> 0 found in 0.05 <x< 0.22 peaks out at x≈0.07. Comparison between these results and predictions from existing models for the high-T_c superconductivity has been made.

A network can be analyzed at different topological scales, ranging from single nodes to motifs, communities, up to the complete structure. We propose a novel approach which extends from single nodes to the whole network level by considering non-overlapping subgraphs (i.e. connected components) and their interrelationships and distribution through the network. Though such subgraphs can be completely general, our methodology focuses on the cases in which the nodes of these subgraphs share some special feature, such as being critical for the proper operation of the network. The methodology of subgraph characterization involves two main aspects: (i) the generation of histograms of subgraph sizes and distances between subgraphs and (ii) a merging algorithm, developed to assess the relevance of nodes outside subgraphs by progressively merging subgraphs until the whole network is covered. The latter procedure complements the histograms by taking into account the nodes lying between subgraphs, as well as the relevance of these nodes to the overall subgraph interconnectivity. Experiments were carried out using four types of network models and five instances of real-world networks, in order to illustrate how subgraph characterization can help complementing complex network-based studies.

In view of current research effort on semiconducting photonic-crystal defect microcavities, we consider the dynamics of an array of coupled optical cavities, each containing an ensemble of qubits. By concentrating on the strong coupling regime, we analytically prove that the nonlinearity inherent in the dynamics of each ensemble coupled to the respective cavity field allows the formation of solitons. We further show how the use of the Holstein–Primakoff transformation and the large-detuning limit with the cavity allows one to recover the Bose–Hubbard model.

We present a multiscale approach for a molecular nanoelectromechanical shuttle, which takes into account the discrete energy levels and realistic orbitals of a specific molecular system. We deploy a Cu₁₃ cluster as a central island and describe the electrodes within the jellium model. The shuttling features are then examined for a wide spectrum of bias voltages. The microscopic data of the cluster are obtained with density functional theory (DFT) while the macroscopic part combines stochastic charge dynamics, based on the microscopically evaluated tunnelling rates, with Newtonian dynamics. We find four transport regimes within the analysed bias voltage interval: a blocked regime (V_bias⩽ 2 V), a pure shuttling regime (3 V ⩽V_bias⩽ 4 V), a mixed regime (5 V ⩽V_bias⩽ 6 V) and a direct tunnelling regime (V_bias⩾7 V). In the two latter cases, the shuttling motion leads to a reduced current as compared to an island with a fixed position. The transport characteristics are strongly coupled to the availability of energetically allowed channels and the system therefore depends strongly on the microscopic details and bias voltages. For the setup at hand, however, the shuttling transport of one electron is ubiquitous and stable for all but the blocked regime.

We present a universal scheme of pulsed operations suitable for the IBM oscillator-stabilized flux qubit comprising the controlled-σ_z (cphase) gate, single-qubit preparations and measurements. Based on numerical simulations, we argue that the error rates for these operations can be as low as about 0.5% and that noise is highly biased, with phase errors being stronger than all other types of errors by a factor of nearly 10³. In contrast, the design of a controlled-σ_x (cnot) gate for this system with an error rate of less than about 1.2% seems extremely challenging. We propose a special encoding that exploits the noise bias allowing us to implement a logicalcnot gate where phase errors and all other types of errors have nearly balanced rates of about 0.4%. Our results illustrate how the design of an encoding scheme can be adjusted and optimized according to the available physical operations and the particular noise characteristics of experimental devices.

Tetracene (Tc) and rubrene (Rub) are two prototype fluorescent molecules. Both molecules exhibit the same 'fluorescent backbone', but due to the additional phenyl groups, the backbone of Rub is twisted, whereas it is planar for Tc. In agreement with earlier investigations, optical spectroscopy of the respective solutions reveals that the S₀→S₁ transition in Rub is red-shifted with respect to Tc by ∼2000 cm⁻¹ and that Rub exhibits a considerably larger Stokes shift. In order to unravel the physical origin of these differences, we have performed a detailed normal coordinate analysis and frequency calculations using density functional theory (DFT) in conjunction with linear response time-dependent DFT (TD-DFT) energy scan calculations. The calculations yield dimensionless normal coordinate displacements of the excited-state origin that were employed for the calculation of the vibrational finestructure of the absorption and fluorescence spectra of Tc and Rub. The purely theoretical displacements were subsequently refined through fitting to the experimental spectra using the time-dependent theory of electronic spectroscopy. The analysis reveals that the ∼2000 cm⁻¹ red shift of the 0–0 vibronic band of Rub relative to Tc is mainly caused by the inductive effect of the phenyl substituents that leads to destabilization of the donor molecular orbital (MO) (the highest occupied molecular orbital (HOMO)). The large Stokes shift of 820 cm⁻¹ observed for Rub is found to originate mainly from unresolved vibrational progressions involving low-frequency modes that are characterized by appreciable displacements in the excited state. The analysis shows that the spectra of Rub are strongly subject to temperature induced broadening, whereas for Tc this is much less significant.

We have designed, fabricated and characterized integrated directional couplers capable of converting the mode of an optical dielectric waveguide into a long-range plasmon propagating along a thin metal stripe. We demonstrate that the coupling between the two types of waveguides is generally very weak unless specific conditions are met. This sensitivity could be potentially exploited in sensing applications or for developing novel active photonic components.

Systematic measurements of the photoluminescence lifetime of the 1.54 μm transition of erbium implanted at different energies in SiO₂ films with different metallic overlayers are reported. The lifetime shows a strong reduction up to a factor of 20 with decreasing distance between the erbium and the metal overlayer. The reduction of lifetime is mainly due to a near-field interaction between the erbium ions and the metal overlayers through generation of surface plasmon polaritons at the metal/SiO₂ interface and direct generation of heat in the metal. These experiments combined with rigorous theoretical modeling demonstrate that a high degree of control over the radiative properties of erbium can be achieved in erbium-implanted materials in a wide range of implantation energies. The experiments also allow us to determine the radiative efficiency of erbium in bulk SiO₂.