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The full text is available in the PDF provided.

The full text is available in the PDF provided.

The full text is available in the PDF provided.

We perform a detailed study of pulse sequences in a photoassociation via adiabatic passage (PAP) process to transfer population from an ensemble of ultracold atomic clouds to a vibrationally cold molecular state. We show that an appreciable final population of ultracold NaCs molecules can be achieved with optimized pulses in either the 'counter-intuitive' (t_P > t_S) or 'intuitive' (t_P < t_S) PAP pulse sequences, with t_P and t_S denoting the temporal centers of the pump and Stokes pulses, respectively. By investigating the dependence of the reactive yield on pulse sequences, in a wide range of t_P–t_S, we show that there is not a fundamental preference to either pulse sequence in a PAP process. We explain this no-sequence-preference phenomenon by analyzing a multi-bound model so that an analogy can be drawn to the conventional stimulated Raman adiabatic passage.

We study a model for itinerant, strongly interacting fermions where a judicious tuning of the interactions leads to a supersymmetric Hamiltonian. On the triangular lattice this model is known to exhibit a property called superfrustration, which is characterized by an extensive ground state entropy. Using a combination of numerical and analytical methods we study various ladder geometries obtained by imposing doubly periodic boundary conditions on the triangular lattice. We compare our results to various bounds on the ground state degeneracy obtained in the literature. For all systems we find that the number of ground states grows exponentially with system size. For two of the models that we study we obtain the exact number of ground states by solving the cohomology problem. For one of these, we find that via a sequence of mappings the entire spectrum can be understood. It exhibits a gapped phase at 1/4 filling and a gapless phase at 1/6 filling and phase separation at intermediate fillings. The gapless phase separates into an exponential number of sectors, where the continuum limit of each sector is described by a superconformal field theory.

We present ion beam erosion experiments on Si(001) with simultaneous sputter co-deposition of steel at 660 K. At this temperature, the sample remains within the crystalline regime during ion exposure and pattern formation takes place by phase separation of Si and iron-silicide. After an ion fluence of F ≈ 5.9 × 10²¹ ions m⁻², investigations by atomic force microscopy and scanning electron microscopy identify sponge, segmented wall and pillar patterns with high aspect ratios and heights of up to 200 nm. Grazing incidence x-ray diffraction and transmission electron microscopy reveal the structures to be composed of polycrystalline iron-silicide. The observed pattern formation is compared to that in the range of 140–440 K under otherwise identical conditions, where a thin amorphous layer forms due to ion bombardment.

Large-bandgap semiconductor microwires constitute an advantageous alternative to planar microcavities in the context of the room temperature strong coupling regime between excitons and light. In this work, we demonstrate that in the undoped half of a single GaN microwire, the strong coupling regime is achieved up to room temperature with a large Rabi splitting of 115 meV. The demonstration relies on a method that does not require any knowledge a priori of the energy of the uncoupled whispering gallery modes in the microwire, i.e. the details of the microwire cross-section shape. The other half of the microwire is heavily n-doped. Thus, within the same microwire, the exciton oscillator strength transits from its nominal value to zero within a region of 2 μm length. Using this property, we can observe the dispersion properties of a given whispering gallery mode in both the strong and weak coupling regimes.

The possibility of controlling and directing a complex system's behavior at will is rooted in its interconnectivity and can lead to significant advances in disparate fields, ranging from nationwide energy saving to therapies that involve multiple targets. In this work, we address complex network controllability from the perspective of the minimum dominating set (MDS). Our theoretical calculations, simulations using artificially generated networks as well as real-world network analyses show that the more heterogeneous a network degree distribution is, the easier it is to control the entire system. We demonstrate that relatively few nodes are needed to control the entire network if the power-law degree exponent is smaller than 2, whereas many nodes are required if it is larger than 2.

The transport of slightly deformable chiral objects in a uniform shear flow is investigated. Depending on the equilibrium configuration one finds up to four different asymptotic states that can be distinguished by lateral drift velocity of their center of mass, rotational motion about the center of mass and deformations of the object. These deformations influence the magnitudes of the principal axes of the second moment tensor of the considered object and also modify a scalar index characterizing its chirality. Moreover, the deformations induced by the shear flow are essential for the phenomenon of dynamical symmetry breaking: objects that are achiral under equilibrium conditions may dynamically acquire chirality and consequently experience a drift in the lateral direction.

We study quantum transport properties of an open Heisenberg XXZ spin 1/2 chain driven by a pair of Lindblad jump operators satisfying a global 'micro-canonical' constraint, i.e. conserving the total magnetization. We will show that this system has an additional discrete symmetry that is specific to the Liouvillean description of the problem. Such symmetry reduces the dynamics even more than would be expected in the standard Hilbert space formalism and establishes existence of multiple steady states. Interestingly, numerical simulations of the XXZ model suggest that a pair of distinct non-equilibrium steady states becomes indistinguishable in the thermodynamic limit, and exhibit sub-diffusive spin transport in the easy-axis regime of anisotropy Δ > 1.

The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers has been investigated as a function of the soft layer thickness (t_Fe = 2, 3.5, 5 nm) combining magnetization loops at variable angle, magnetic domain analysis by magnetic force microscopy and numerical micromagnetic simulations. The analytical model proposed in the literature can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and remanence by increasing the soft layer thickness, but cannot satisfactorily describe the magnetization process of real systems. We showed that for a thickness of the soft layer exceeding the FePt exchange length (∼2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. Indeed, just above the coercive field, the magnetization reversal does not proceed in single step switching, as predicted by the analytical model, but according to a more complex process: evolution of nucleated magnetic domains whose magnetization is approximately along the surface normal in the hard layer and slightly out of the film plane in the soft layer, followed by rotation of Fe moments along the field direction.

Fast moving ions travel great distances along channels between low-index crystallographic planes, slowing through collisions with electrons, until finally they hit a host atom initiating a cascade of atomic displacements. Statistical penetration ranges of incident particles are reliably used in ion-implantation technologies, but a full, necessarily quantum-mechanical, description of the stopping of slow, heavy ions is challenging and the results of experimental investigations are not fully understood. Using a self-consistent model of the electronic structure of a metal, and explicit treatment of atomic structure, we find by direct simulation a resonant accumulation of charge on a channelling ion analogous to the Okorokov effect but originating in electronic excitation between delocalized and localized valence states on the channelling ion and its transient host neighbours, stimulated by the time-periodic potential experienced by the channelling ion. The charge resonance reduces the electronic stopping power on the channelling ion. These are surprising and interesting new chemical aspects of channelling, which cannot be predicted within the standard framework of ions travelling through homogeneous electron gases or by considering either ion or target in isolation.

The properties of ion bunches stored in an electrostatic ion beam trap (EIBT) have been investigated using the Cryogenic Trap for Fast ion beams (CTF). The extremely high vacuum used rendered the main ion loss mechanism, namely collisions with the rest gas, negligible. Aluminum dimer anions were photo-detached by a pulsed laser to measure the longitudinal ion distribution in the bunch, which for the first time revealed the presence of a dc ion beam component co-existing with the oscillating ion bunch after several hundreds of revolutions. Bunches stabilized by the so-called self-bunching mode of operation have been observed for times as long as 12 s (a factor of 100 longer than previous room-temperature experiments) using N⁺₂ and Al⁻₂ bunches at 6–7.1 keV beam energies after which the bunch abruptly decayed. The decay of the bunch was observed to be intensity dependent and is well reproduced by a model that includes the expansion of the bunch along the beam axis, intrabeam scattering and collisional losses between the bunch and the dc component. Radio-frequency bunching of the ions resulted in the extension of the bunch observation time to 600 s, placing upper limits on all other EIBT ion bunch and trap losses as well as supporting the newly developed decay model and EIBT-adapted bunch dynamics.

We discuss the unitary quantum dynamics of the Dicke model (spin and oscillator coupled). A suitable quasi-probability representing the quantum state turns out to obey a Fokker–Planck equation, with drift terms representing the underlying classical Hamiltonian flow and diffusion terms describing quantum fluctuations. We show (by projecting the dynamics onto a co-moving Poincaré section) how the interplay of deterministic drift and quantum diffusion generates equilibration to the microcanonical density, under conditions of global classical chaos. The pertinent photon statistics reveals macroscopic quantum fluctuations.

Recent advances in quantum information processing with trapped ions have demonstrated the need for new ion trap architectures capable of holding and manipulating chains of many (>10) ions. Here we present the design and detailed characterization of a new linear trap, microfabricated with scalable complementary metal-oxide-semiconductor (CMOS) techniques, that is well-suited to this challenge. Forty-four individually controlled dc electrodes provide the many degrees of freedom required to construct anharmonic potential wells, shuttle ions, merge and split ion chains, precisely tune secular mode frequencies, and adjust the orientation of trap axes. Microfabricated capacitors on dc electrodes suppress radio-frequency pickup and excess micromotion, while a top-level ground layer simplifies modeling of electric fields and protects trap structures underneath. A localized aperture in the substrate provides access to the trapping region from an oven below, permitting deterministic loading of particular isotopic/elemental sequences via species-selective photoionization. The shapes of the aperture and radio-frequency electrodes are optimized to minimize perturbation of the trapping pseudopotential. Laboratory experiments verify simulated potentials and characterize trapping lifetimes, stray electric fields, and ion heating rates, while measurement and cancellation of spatially-varying stray electric fields permits the formation of nearly-equally spaced ion chains.

We describe how the notion of optical beam shifts (including the spatial and angular Goos–Hänchen shift and Imbert–Federov shift) can be understood as a classical analogue of a quantum measurement of the polarization state of a paraxial beam by its transverse amplitude distribution. Under this scheme, complex quantum weak values are interpreted as spatial and angular shifts of polarized scalar components of the reflected beam. This connection leads us to predict an extra spatial shift for beams with a radially-varying phase dependance.

In 2008 we presented the first images obtained with a new type of matter wave microscope: NEutral Helium Atom MIcroscopy (NEMI). The main features in NEMI are the low energy of the atoms (<0.1 eV) and the fact that they are neutral. This means that fragile and/or insulating samples can be imaged without surface damage and charging effects. The ultimate resolution limit is given by the de Broglie wavelength (about 0.06 nm for a room-temperature beam), but reaching a small focus spot is still a major challenge. The best result previously was about 2 μm. The main result of this paper is the focusing of a helium atom beam to a diameter below 1 μm. A particular challenge for neutral helium microscopy is the optical element for focusing. The most promising option is to manipulate neutral helium via its de Broglie wavelength, which requires optical elements structured to nanometre precision. Here we present an investigation of the helium focusing properties of nanostructured Fresnel zone-plates. Experiments were performed by varying the illuminated area and measuring the corresponding focused spot sizes and focused beam intensities. The results were fitted to a theoretical model. There is a deviation in the efficiency of the larger zone plate, which indicates a distortion in the zone-plate pattern, but nevertheless there is good agreement between model and experiments for the focus size. This together with the demonstration of focusing to below 1 μm is an important step towards nanometre resolution neutral helium microscopy.

The nature of the effective spin Hamiltonian and magnetic order in the honeycomb iridates is explored by considering a trigonal crystal field effect and spin–orbit (SO) coupling. Starting from a Hubbard model, an effective spin Hamiltonian is derived in terms of an emergent pseudo-spin-1/2 moment in the limit of large trigonal distortions and SO coupling. The present pseudo-spins arise from a spin–orbital locking and are different from the j_eff = 1/2 moments that are obtained when the SO coupling dominates and trigonal distortions are neglected. The resulting spin Hamiltonian is anisotropic and frustrated by further neighbour interactions. Mean-field theory suggests a ground state with four-sublattice zigzag magnetic order in a parameter regime that can be relevant to the honeycomb iridate compound Na₂IrO₃, where a similar magnetic ground state has recently been observed. Various properties of the phase, the spin-wave spectrum and experimental consequences are discussed. The present approach contrasts with the recent proposals to understand iridate compounds starting from the strong SO coupling limit and neglecting non-cubic lattice distortions.

The simple reflection of a light beam of finite transverse extent from a homogeneous interface gives rise to a surprisingly large number of subtle shifts and deflections which can be seen as diffractive corrections to the laws of geometrical optics (Goos–Hänchen shifts) and manifestations of optical spin–orbit coupling (Imbert–Fedorov shifts), related to the spin Hall effect of light. We develop a unified linear algebra approach to dielectric reflection which allows for a simple calculation of all these effects and lends itself to an interpretation of beam shifts as weak values in a classical analogue to a quantum weak measurement. We present a systematic study of the shifts for the whole beam and its polarization components, finding symmetries between input and output polarizations and predicting the existence of material independent shifts.

We report an observation and detail mapping of exciton-polaritons (polaritons hereafter) on both lower-polariton and upper-polariton branches in ZnO microcrystallines using second-harmonic generation. Our investigations reveal that those polaritons can be well described by the dispersion curves of nanowires at certain diameter. The polaritons in the tree-like microcrystallines evolve as a function of incident wavelength in a similar fashion as that in a single nanowire. Hence, the origin of polaritons generated in the wire-based microcrystallines is the coupling between the excitons and the nanowire-cavities. The resonance profile of the polaritons reveals that the enhancement is due to the strong coupling between second-harmonic photons and the A, B and C excitons in ZnO. The high photon–polariton conversion efficiency suggests a new strategy for harvesting solar energy.

In a quantum electrodynamics (QED) system of arrayed two-level atoms interacting with light, because the energy of the photon is around the spacing between two atomic energy levels, the photon will be absorbed and is not in the propagating mode but the attenuated mode. Our theoretical study shows that coherent phasing of atomic states occurs when the photons are in the attenuated mode. The existence of this ordered state is a result of a quantum phase transition induced by the mediation of the attenuated photonic field. By tuning the intensity of the light, the ordered state can be manipulated so that two-level atoms can be in an arbitrary uniform linear combination of the single-atom ground state and the excited state. Potential applications of this phenomenon are quantum computation, lasing physics, optical lattices and related subjects in nanotechnology. Similar phase transitions induced by the mediation of transmuted intermediate boson-like phonons, magnons or other fields may also occur in condensed-matter systems.

We use angle-resolved photoemission spectroscopy to study twinned and detwinned iron pnictide compound NaFeAs. Distinct signatures of electronic reconstruction are observed to occur at the structural (T_S) and magnetic (T_SDW) transitions. At T_S, C₄ rotational symmetry is broken in the form of an anisotropic shift in the orthogonal d_xz and d_yz bands. The magnitude of this orbital anisotropy rapidly develops to near completion upon approaching T_SDW, at which temperature band folding occurs via the antiferromagnetic ordering wave vector. Interestingly, the anisotropic band shift onsetting at T_S develops in such a way as to enhance the nesting conditions in the C₂ symmetric state, and hence is intimately correlated with the long-range collinear antiferromagnetic (AFM) order. Furthermore, similar behaviors of the electronic reconstruction in NaFeAs and Ba(Fe_1−xCo_x)₂As₂ suggest that this rapid development of large orbital anisotropy between T_S and T_SDW is likely a general feature of the electronic nematic phase in the iron pnictides, and the associated orbital fluctuations may play an important role in determining the ground state properties.

We present a novel method for transporting ultracold atoms in a focused optical lattice over macroscopic distances of many Rayleigh ranges. With this method ultracold atoms were transported over 5 cm in 250 ms without significant atom loss or heating. By translating the interference pattern together with the beam geometry, the trap parameters are maintained over the full transport range. Thus, the presented method is well suited for tightly focused optical lattices that have sufficient trap depth only close to the focus. Tight focusing is usually required for far-detuned optical traps or traps that require high laser intensity for other reasons. The transport time is short and thus compatible with the operation of an optical lattice clock in which atoms are probed in a well-designed environment spatially separated from the preparation and detection region.

Simple analytical parameterizations for the ground-state energy of the one-dimensional repulsive Hubbard model are developed. The charge dependence of energy is parameterized using exact results extracted from the Bethe-ansatz (BA). The resulting parameterization is shown to be in better agreement with highly precise data obtained from a fully numerical solution to the BA equations than previous expressions (Lima et al 2003 Phys. Rev. Lett.90 146402). Unlike these earlier proposals, the present parameterization correctly predicts a positive Mott gap at half filling for any U > 0. The construction is extended to spin-dependent phenomena by parameterizing the magnetization dependence of the ground-state energy using further exact results and numerical benchmarking. Lastly, the parameterizations developed for the spatially uniform model are extended by means of a simple local-density-type approximation to spatially inhomogeneous models, e.g. in the presence of impurities, external fields or trapping potentials. The results are shown to be in excellent agreement with independent many-body calculations, at a fraction of the computational cost.

We determine the quantum states and measurements that optimize the accessible information in a reference frame alignment protocol associated with the groups U(1), corresponding to a phase reference, and $\mathbb {Z}_M$, the cyclic group of M elements. Our result provides an operational interpretation of the G-asymmetry which is information-theoretic and which was thus far lacking. In particular, we show that in the limit of many copies of the bounded-size quantum reference frame, the accessible information approaches the Holevo bound. This implies that the rate of alignment of reference frames, measured by the (linearized) accessible information per system, is equal to the regularized, linearized G-asymmetry. The latter quantity is equal to the number variance in the case where G = U(1). Quite surprisingly, for the case where $G=\mathbb {Z}_{M}$ and M ⩾ 4, it is equal to a quantity that is not additive in general, but instead can be super-additive under tensor product of two distinct bounded-size reference frames. This remarkable phenomenon is purely quantum and has no classical analogue.

We study the implementation of one-, two- and three-qubit quantum gates for interacting qubits using optimal control. Markovian and non-Markovian environments are compared and efficient optimization algorithms utilizing analytic gradient expressions and quasi-Newton updates are given for both cases. The performance of the algorithms is analysed for a large set of problems in terms of the fidelities attained and the observed convergence behaviour. New notions of success rate and success speed are introduced and density plots are utilized to study the effects of key parameters, such as gate operation times, and random variables such as the initial fields required to start the iterative algorithm. Core characteristics of the optimal fields are analysed statistically. Substantial differences between Markovian and non-Markovian environments in terms of the possibilities for control and the control mechanisms are uncovered. In the non-Markovian case, gate fidelities improve substantially when the details of the system bath coupling are taken into account, although imperfections such as field leakage can be a significant problem. In the Markovian case, computation time is saved if the fields are pre-optimized neglecting the environment, while including the latter generally does not significantly improve gate fidelities.

We study the order parameter of a quasi-two-dimensional (quasi-2D) gas of ultracold atoms trapped in an optical potential in the presence of controllable disorder. Our results show that disorder drives phase fluctuations without significantly affecting the amplitude of the quasi-condensate order parameter. This is evidence that disorder can drive phase fluctuations in 2D systems, relevant to the phase-fluctuation mechanism for the superconductor-to-insulator phase transition (SIT) in disordered 2D superconductors.

High-temperature superconductivity remains arguably the greatest enigma of condensed matter physics. The discovery of iron-based high-temperature superconductors [1, 2] has renewed the importance of understanding superconductivity in materials susceptible to magnetic order and fluctuations. Intriguingly, they show magnetic fluctuations reminiscent of superconducting (SC) cuprates [3], including a 'resonance' and an 'hourglass'-shaped dispersion [4], which provides an opportunity to gain new insights into the coupling between spin fluctuations and superconductivity. In this paper, we report inelastic neutron scattering data on Fe_1+yTe_0.7Se_0.3 using excess iron concentration to tune between an SC (y = 0.02) and a non-SC (y = 0.05) ground state. We find incommensurate spectra in both the samples but discover that in the one that becomes SC, a constriction toward a commensurate hourglass-shape develops well above T_c. Conversely, a spin gap and a concomitant spectral weight shift happen below T_c. Our results imply that the hourglass-shaped dispersion is most likely a prerequisite for superconductivity, whereas the spin gap and shift of spectral weight are the consequences of superconductivity. We explain this observation by pointing out that an inward dispersion toward the commensurate wave vector is needed for the opening of a spin gap to lower the magnetic exchange energy and hence provide the necessary condensation energy for the SC state to emerge.

Nonlinearity and disorder in discrete systems give rise to fascinating dynamics in various fields of physics. Photonic lattices allow investigation of them in an optical context. The very nature of discrete propagation allows perfect reconstruction of arbitrary initial wave packets by introducing phase shifts to specific lattice sites. We investigate, both numerically and experimentally, the interplay of nonlinearity with this so-called segmentation imaging in the presence of disorder. We find that whereas in the linear regime perfect imaging is achieved for arbitrary amounts of coupling disorder, the onset of nonlinear self-focusing generally destroys imaging. Interestingly, the influence of Anderson localization in strongly disordered lattices renders the imaging significantly more susceptible to nonlinear perturbations.

We study the efficiency of exciton transport as a function of the typical reorganization time scale of the environment using the hierarchy of equations of motion. As a model system, we choose the Fenna–Matthews–Olson (FMO) complex. An environment in which the dynamics is not much faster than the system leads to prolonged quantum coherent transport, even at room temperature. We find that this does not make the transport process more efficient for standard FMO parameters, but does increase the efficiency in the case when exciton decay competes with trapping at the reaction center. We furthermore find that initial correlations do not influence population oscillations.

The complete magnetic and multiferroic phase diagram of Mn_1−xCo_xWO₄ single crystals is investigated by means of magnetic, heat capacity and polarization experiments. We show that the ferroelectric (FE) polarization $\skew3\vec{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x_c1 = 0.075, $\skew3\vec{P}$ rotates from the b-axis into the a–c-plane and at x_c2 = 0.15 it flips back to the b-axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the FE polarization is derived by taking into account the intrachain (c-axis) as well as the interchain (a-axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1 ⩽ x ⩽ 0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO₄.

The ATLAS and CMS collaborations report indications of a Higgs boson at M_h ∼ 125 GeV. In addition, CMS data show a tenuous bump in the ZZ channel, at about 320 GeV. We make the bold assumption that it might be the indication of a secondary line corresponding to the heaviest scalar Higgs boson of minimal supersymmetry, H, and discuss the viability of this hypothesis. We discuss also the case of a heavier H. The relevance of the $b \bar b$ decay channel is underlined.

Colloidal particles dispersed in a liquid crystal (LC) lead to distortions of the director field. The distortions are responsible for long-range effective colloidal interactions whose asymptotic behaviour is well understood. The short-distance behaviour depends on the structure and dynamics of the topological defects nucleated near the colloidal particles and a full nonlinear theory is required to describe it. Spherical colloidal particles with strong planar degenerate anchoring nucleate a pair of antipodal surface topological defects, known as boojums. We use the Landau–de Gennes theory to resolve the mesoscopic structure of the boojum cores and to determine the pairwise colloidal interactions. We compare the results in three (3D) and two (2D) spatial dimensions for spherical and disc-like colloidal particles, respectively. The corresponding free energy functionals are minimized numerically using finite elements with adaptive meshes. Boojums are always point-like in 2D, but acquire a rather complex structure in 3D, which depends on the combination of the anchoring potential, the radius of the colloid, the temperature and the LC elastic anisotropy. We identify three types of defect cores in 3D that we call single, double and split-core boojums, and investigate the associated structural transitions. The split-core structure is favoured by low temperatures, strong anchoring and small twist to splay or bend ratios. For sufficiently strong anchoring potentials characterized by a well-defined uniaxial minimum, the split-core boojums are the only stable configuration. In the presence of two colloidal particles, we observe substantial re-arrangements of the inner defects in both 3D and 2D. These re-arrangements lead to qualitative changes in the force-distance profile when compared to the asymptotic quadrupole–quadrupole interaction. In line with the experimental results, the presence of the defects prevents coalescence of the colloidal particles in 2D, but not in 3D systems.

We design the interactions between oscillators communicating via variably delayed pulse coupling to guarantee their synchronization on arbitrary network topologies. We identify a class of response functions and prove convergence to network-wide synchrony from arbitrary initial conditions. Synchrony is achieved if the pulse emission is unreliable or intentionally probabilistic. These results support the design of scalable, reliable and energy-efficient communication protocols for fully distributed synchronization as needed, e.g., in mobile phone networks, embedded systems, sensor networks and autonomously interacting swarm robots.

Force and conductance were simultaneously measured during the formation of Cu–C₆₀ and C₆₀–C₆₀ contacts using a combined cryogenic scanning tunneling and atomic force microscope. The contact geometry was controlled with submolecular resolution. The maximal attractive forces measured for the two types of junctions were found to differ significantly. We show that the previously reported values of the contact conductance correspond to the junction being under maximal tensile stress.

We consider a model of active Brownian particles (ABPs) with velocity alignment in two spatial dimensions with passive and active fluctuations. Here, active fluctuations refers to purely non-equilibrium stochastic forces correlated with the heading of an individual active particle. In the simplest case studied here, they are assumed to be independent stochastic forces parallel (speed noise) and perpendicular (angular noise) to the velocity of the particle. On the other hand, passive fluctuations are defined by a noise vector independent of the direction of motion of a particle, and may account, for example, for thermal fluctuations. We derive a macroscopic description of the ABP gas with velocity-alignment interaction. Here, we start from the individual-based description in terms of stochastic differential equations (Langevin equations) and derive equations of motion for the coarse-grained kinetic variables (density, velocity and temperature) via a moment expansion of the corresponding probability density function. We focus here on the different impact of active and passive fluctuations on onset of collective motion and show how active fluctuations in the active Brownian dynamics can change the phase-transition behaviour of the system. In particular, we show that active angular fluctuations lead to an earlier breakdown of collective motion and to the emergence of a new bistable regime in the mean-field case.

The propagation of a relativistic electron with initial energy ≳100 MeV in a number of simple one-dimensional laser field configurations with circular polarization is studied by solving the relativistic equation of motion in the Landau–Lifschitz approach to account for the radiation friction force. The radiation back-reaction on the electron dynamics becomes visible at dimensionless field amplitudes a ≳ 10 at these high particle energies. Analytical expressions are derived for the energy and the longitudinal momentum of the electron, the frequency shift of the light scattered by the electron and the particle trajectories. These findings are compared with the numerical solutions of the basic equations. A strong radiation damping effect results in reduced light scattering, forming at the same time a broad quasi-continuous spectrum. In addition, the electron dynamics in the strong field of a quasistationary laser piston is investigated. Analytical solutions for the electron trajectories in this complex field pattern are obtained and compared with the numerical solutions. The radiation friction force may stop a relativistic electron after propagation over several laser wavelengths at high laser field strengths, which supports the formation of a stable piston.

When members of a population engage in dyadic interactions reflecting a prisoner's dilemma game, the evolutionary dynamics depends crucially on the population structure, described by means of graphs and networks. Here, we investigate how selection pressure contributes to change the fate of the population. We find that homogeneous networks, in which individuals share a similar number of neighbors, are very sensitive to selection pressure, whereas strongly heterogeneous networks are more resilient to natural selection, dictating an overall robust evolutionary dynamics of coordination. Between these extremes, a whole plethora of behaviors is predicted, showing how selection pressure can change the nature of dilemmas populations effectively face. We further show how the present results for homogeneous networks bridge the existing gap between analytic predictions obtained in the framework of the pair-approximation from very weak selection and simulation results obtained from strong selection.

The dynamics of an interacting Fermi gas of atoms at sufficiently high temperatures can be efficiently studied via a numerical simulation of the Boltzmann equation. In this paper, we describe in detail the setup we used recently to study the oscillations of two spin-polarized fermionic clouds in a trap. We focus here on the evaluation of interparticle interactions. We compare different ways of choosing the phase space coordinates of a pair of atoms after a successful collision and demonstrate that the exact microscopic setup has no influence on the macroscopic outcome.

It is well known that the evolution of resistance of microorganisms to a range of different antibiotics presents a major problem in the control of infectious diseases. Accordingly, new bactericidal 'agents' are in great demand. Using a cold atmospheric pressure (CAP) plasma dispenser operated with ambient air, a more than five orders of magnitude inactivation or reduction of Methicillin-resistant Staphylococcus aureus (MRSA; resistant against a large number of the tested antibiotics) was obtained in less than 10 s. This makes CAP the most promising candidate for combating nosocomial (hospital-induced) infections. To test for the occurrence and development of bacterial resistance against such plasmas, experiments with Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Enterococcus mundtii) were performed. The aim was to determine quantitative limits for primary (naturally) or secondary (acquired) resistance against the plasma treatment. Our results show that E. coli and E. mundtii possess no primary resistance against the plasma treatment. By generating four generations of bacteria for every strain, where the survivors of the plasma treatment were used for the production of the next generation, a lower limit to secondary resistance was obtained. Our results indicate that CAP technology could contribute to the control of infections in hospitals, in outpatient care and in disaster situations, providing a new, fast and efficient broad-band disinfection technology that is not constrained by bacterial resistance mechanisms.

We present a theoretical analysis of the selective darkening method for implementing quantum controlled-NOT (CNOT) gates. This method, which we have recently proposed and demonstrated, consists of driving two transversely coupled quantum bits (qubits) with a driving field that is resonant with one of the two qubits. For specific relative amplitudes and phases of the driving field felt by the two qubits, one of the two transitions in the degenerate pair is darkened or, in other words, becomes forbidden by effective selection rules. In these driving conditions, the evolution of the two-qubit state realizes a CNOT gate. The gate speed is found to be limited only by the coupling energy J, which is the fundamental speed limit for any entangling gate. Numerical simulations show that at gate speeds corresponding to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and increases further for lower gate speeds. In addition, the effect of higher-lying energy levels and weak anharmonicity is studied, as well as the scalability of the method to systems of multiple qubits. We conclude that in all these respects this method is competitive with existing schemes for creating entanglement, with the added advantages of being applicable for qubits operating at fixed frequencies (either by design or for the exploitation of coherence sweet-spots) and having the simplicity of microwave-only operation.

We discuss theoretically the acoustic resonant transmission and zeros of transmission between two substrates connected by sub-wavelength pillars. The features of the transmission coefficient are explained in terms of the coupling of the incident waves with the Fabry–Perot oscillations inside the pillars and with the surface waves of both substrates. We discuss the dependence of the selective and zero transmission frequencies, in particular Fano resonances resulting from the proximity of a resonance to a zero of transmission, on the geometrical and physical parameters of the materials constituting the pillars and the substrate. These phenomena are studied in both one- and two-dimensional periodicities where the substrates are connected by a series of parallel plates or by a square lattice of cylindrical pillars, respectively. Finally, the calculation is extended to a periodic stacking of slabs and pillars that constitute a type of three-dimensional phononic crystal.

The interaction of trimethyl methylcyclopentadienyl platinum (MeCpPtMe₃) with a fully hydroxylated SiO₂ surface has been explored by means of ab initio calculations. A large slab model (3 × 3 × 4 supercell) cut out from the hydroxylated β-cristobalite SiO₂ (111) surface was chosen to simulate a silica surface. Density functional theory calculations were performed to evaluate the energies of MeCpPtMe₃ adsorption to the SiO₂ surface. Our results show that the physisorption of the molecule is dependent on both (i) the orientation of the adsorbate and (ii) the adsorption site on the substrate. The most stable configuration was found with the MeCp and Me₃ groups of the molecule oriented toward the surface. Finally, we observe that van der Waals corrections are crucial for the stabilization of the molecule on the surface. We discuss the relevance of our results for the growth of Pt-based nanostructured materials via deposition processes such as electron beam-induced deposition.

We theoretically study the implementation of two-qubit gates in a system of two coupled superconducting qubits. In particular, we analyze two-qubit gate operations under the condition that the coupling strength is comparable with or even larger than the anharmonicity of the qubits. By numerically solving the time-dependent Schrödinger equation under the assumption of negligible decoherence, we obtain the dependence of the two-qubit gate fidelity on the system parameters in the case of both direct and indirect qubit–qubit coupling. Our numerical results can be used to identify the 'safe' parameter regime for experimentally implementing two-qubit gates with high fidelity in these systems.

Emerging possibilities for creating and studying novel plasma regimes, e.g. relativistic plasmas and dense systems, in a controlled laboratory environment also require new modeling tools for such systems. This motivates theoretical studies of the kinetic theory governing the dynamics of plasmas for which both relativistic and quantum effects occur simultaneously. Here, we investigate relativistic corrections to the Pauli Hamiltonian in the context of a scalar kinetic theory for spin-1/2 quantum plasmas. In particular, we formulate a quantum kinetic theory for the collective motion of electrons that takes into account effects such as spin–orbit coupling and Zitterbewegung. We discuss the implications and possible applications of our findings.

A multi-species Monte Carlo (MC) model, combined with an analytical surface model, has been developed in order to investigate the general plasma processes occurring during the sputter deposition of complex oxide films in a dual-magnetron sputter deposition system. The important plasma species, such as electrons, Ar⁺ ions, fast Ar atoms and sputtered metal atoms (i.e. Mg and Al atoms) are described with the so-called multi-species MC model, whereas the deposition of Mg_xAl_yO_z films is treated by an analytical surface model. Target–substrate distances for both magnetrons in the dual-magnetron setup are varied for the purpose of growing stoichiometric complex oxide thin films. The metal atoms are sputtered from pure metallic targets, whereas the oxygen flux is only directed toward the substrate and is high enough to obtain fully oxidized thin films but low enough to avoid target poisoning. The calculations correspond to typical experimental conditions applied to grow these complex oxide films. In this paper, some calculation results are shown, such as the densities of various plasma species, their fluxes toward the targets and substrate, the deposition rates, as well as the film stoichiometry. Moreover, some results of the combined model are compared with experimental observations. Note that this is the first complete model, which can be applied for large and complicated magnetron reactor geometries, such as dual-magnetron configurations. With this model, we are able to describe all important plasma species as well as the deposition process. It can also be used to predict film stoichiometries of complex oxide films on the substrate.

A formalism to extract and quantify unknown quantities such as sample deformation, the viscosity of the sample and surface energy hysteresis in amplitude modulation atomic force microscopy is presented. Recovering the unknowns only requires the cantilever to be accurately calibrated and the dissipative processes occurring during sample deformation to be well modeled. The theory is validated by comparison with numerical simulations and shown to be able to provide, in principle, values of sample deformation with picometer resolution.

A method is presented for improving the brilliance of laser-produced soft x-ray sources that are based on pulsed gas jets as the targets. The conversion efficiency of laser energy into soft x-ray radiation is enhanced by locally increasing the particle density of the target species. This is achieved by applying a small background pressure to the supersonic flow emanating from a nozzle. In this manner, a supersonic jet with a so-called barrel shock system is formed. On passing the shocks, particles become locally concentrated, forming high-density regions that are used as the targets. An estimate of possible increases in particle densities is provided. The jet flow is then analyzed experimentally by Schlieren imaging, thus visualizing the spatial shock structure. Additionally, a quantitative measurement of the gas density is made using a Hartmann–Shack wavefront sensor. The beneficial effect of the applied background gas on plasma generation is clearly more prominent than its absorbing effect on the photons originating from the plasma. This is shown for a nitrogen target with helium as the background gas. A plasma, generated behind the barrel shock in the nitrogen jet, emits monochromatic photons at a wavelength of 2.88 nm. The peak brilliance of the source is increased by an order of magnitude, resulting in 3.15 × 10¹⁶ photons (mm² mrad² s)⁻¹.

The Shannon dimensionality of orbital-angular-momentum (OAM) entanglement produced in spontaneous parametric down-conversion can be probed by using multi-sector phase analysers [1]. We demonstrate a spatial light modulator-based implementation of these analysers, and use it to measure a Schmidt number of about 50.

We demonstrate double electromagnetically induced transparency (double-EIT) and double four-wave mixing (double-FWM) based on a new scheme of non-degenerate four-wave mixing (FWM) involving five levels of a cold ⁸⁵Rb atomic ensemble, in which the double-EIT windows are used to transmit the probe field and enhance the third-order nonlinear susceptibility. The phase-matching conditions for both four-wave mixings can be satisfied simultaneously. The frequency of one component of the generated bichromatic field is less than the other by ground-state hyperfine splitting (3 GHz). This specially designed experimental scheme for simultaneously generating different nonlinear wave-mixing processes is expected to find applications in quantum information processing and cross-phase modulation. Our results agree well with the theoretical simulation.

Topological phases can be defined in terms of local equivalence: two systems are in the same topological phase if it is possible to transform one into the other by a local reorganization of its degrees of freedom. The classification of topological phases therefore amounts to the classification of long-range entanglement. Such local transformation could result, for instance, from the adiabatic continuation of one system's Hamiltonian to the other. Here, we use this definition to study the topological phase of translationally invariant stabilizer codes in two spatial dimensions, and show that they all belong to one universal phase. We do this by constructing an explicit mapping from any such code to a number of copies of Kitaev's code. Some of our results extend to some two-dimensional (2D) subsystem codes, including topological subsystem codes. Error correction benefits from the corresponding local mappings. In particular, it enables us to use decoding algorithm developed for Kitaev's code to decode any 2D stabilizer code and subsystem code.

We study the non-equilibrium dynamics of a spinful single-orbital quantum dot with an incorporated quantum mechanical spin-1/2 magnetic impurity. Due to the spin degeneracy, double occupancy is allowed, and Coulomb interaction together with the exchange coupling of the magnetic impurity influence the dynamics. By extending the iterative summation of real-time path integrals (ISPI) to this coupled system, we monitor the time-dependent non-equilibrium current and the impurity spin polarization to determine features of the time-dependent non-equilibrium dynamics. We particulary focus on the deep quantum regime, where all time and energy scales are of the same order of magnitude and no small parameter is available. We observe a significant influence of the non-equilibrium decay of the impurity spin polarization both in the presence and in the absence of Coulomb interaction. The exponential relaxation is faster for larger bias voltages, electron–impurity interactions and temperatures. We show that the exact relaxation rate deviates from the corresponding perturbative result. In addition, we study in detail the impurity's back action on the charge current and find a reduction of the stationary current for increasing coupling to the impurity. Moreover, our approach allows us to systematically distinguish mean-field Coulomb and impurity effects from the influence of quantum fluctuations and flip-flop scattering, respectively. In fact, we find a local maximum of the current for a finite Coulomb interaction due to the presence of the impurity.

We show that the presence of an interaction in the quantum walk of two atoms leads to the formation of a stable compound, a molecular state. The wave function of the molecule decays exponentially in the relative position of the two atoms; hence it constitutes a true bound state. Furthermore, for a certain class of interactions, we develop an effective theory and find that the dynamics of the molecule is described by a quantum walk in its own right. We propose a setup for the experimental realization as well as sketch the possibility to observe quasi-particle effects in quantum many-body systems.

Neutral atoms trapped by laser light are among the most promising candidates for storing and processing information in a quantum computer or simulator. The application certainly calls for a scalable and flexible scheme for addressing and manipulating the atoms. We have now made this a reality by implementing a fast and versatile method to dynamically control the position of neutral atoms trapped in optical tweezers. The tweezers result from a spatial light modulator (SLM) controlling and shaping a large number of optical dipole-force traps. Trapped atoms adapt to any change in the potential landscape, such that one can rearrange and randomly access individual sites within atom-trap arrays.

The magnetic and superconducting properties of an Eu(Fe_0.81Co_0.19)₂As₂ single crystal are investigated by means of ac magnetic susceptibility, dc magnetization, specific heat, transverse resistivity and Hall effect measurements in magnetic fields up to 9 T, applied parallel and perpendicular to the c-axis. The compound exhibits the coexistence of magnetism and superconductivity (SC), characterized by structural distortion (SD) and/or spin-density-wave (SDW) ordering at T_SD/SDW = 78 ± 4 K, canted-antiferromagnetic (C-AF) ordering at the Néel temperature T_N = 16.5 ± 0.5 K and SC at the critical temperature T_c = 5.3 ± 0.2 K at zero field. Upon applying fields both the C-AF and SC states evolve in an unconventional manner. Magnetic field distinctly affects the spin canting, resulting in separation of the C-AF into two new phases: the C-AF and ferromagnetic (F) ones. The unusual behavior of the SC state produces field-induced SC in the H⊥c configuration as an outcome of the weakening orbital pair-breaking effect. From the experimental data we derive the field-temperature phase diagrams for Eu(Fe_0.81Co_0.19)₂As₂. A comparison of experimental results is made with theory developed for type II superconductors and then some important thermodynamic parameters characteristic of the superconducting state of Eu(Fe_0.81Co_0.19)₂As₂ are deduced such as the specific heat jump at T_c, ΔC_p(T_c)/γ_nT_c, the electron–phonon coupling constant λ_e–ph, the upper critical field H_c2, coherence length ξ, the Fermi wave-vector k_F, effective mass m*, Hall mobility μ_H, magnetic penetration depth λ and the Ginzburg–Landau parameter κ.

A mass-ejection model in a time-dependent random environment with both temporal and spatial correlations is introduced. When the environment has a finite correlation length, individual particle trajectories are found to diffuse at large times with a displacement distribution that approaches a Gaussian. The collective dynamics of diffusing particles reaches a statistically stationary state, which is characterized in terms of a fluctuating mass density field. The probability distribution of density is studied numerically for both smooth and non-smooth scale-invariant random environments. Competition between trapping in the regions where the ejection rate of the environment vanishes and mixing due to its temporal dependence leads to large fluctuations of mass. These mechanisms are found to result in the presence of intermediate power-law tails in the probability distribution of the mass density. For spatially differentiable environments, the exponent of the right tail is shown to be universal and equal to −3/2. However, at small values, it is found to depend on the environment. Finally, spatial scaling properties of the mass distribution are investigated. The distribution of the coarse-grained density is shown to possess some rescaling properties that depend on the scale, the amplitude of the ejection rate and the Hölder exponent of the environment.

The Karlsruhe Tritium Neutrino (KATRIN) experiment will determine the mass of the electron neutrino with a sensitivity of 0.2 eV (90% CL) via a measurement of the β-spectrum of gaseous tritium near its endpoint of E₀ = 18.57 keV. An ultra-low background of about b = 10 mHz is among the requirements on reaching this sensitivity. In the KATRIN main beam line, two spectrometers of MAC-E filter type are used in tandem configuration. This setup, however, produces a Penning trap, which could lead to increased background. We have performed test measurements showing that the filter energy of the pre-spectrometer can be reduced by several keV in order to diminish this trap. These measurements were analyzed with the help of a complex computer simulation, modeling multiple electron reflections from both the detector and the photoelectric electron source used in our test setup.

Few-femtosecond to attosecond electron pulses are required for advancing ultrafast diffraction and microscopy to the regime of electrons in motion. Here, we report the combination of a single-electron source with a microwave cavity for pulse compression. In such an arrangement, the electron pulses can become significantly shorter than the laser pulses used for electron generation. This comes at the expense of an increase in energy spread. We report the use of an energy analyzer for characterizing microwave-compressed single-electron pulses. Phase effects, linearity, focal distances, incoming pulse durations and laser–microwave jitter are measured for three different synchronization approaches. The results demonstrate the applicability of a microwave cavity in the single-electron regime and identify jitter as the current limitation on the way to few-femtosecond, eventually attosecond pulses of single electrons.

We study the dynamics of a spin–orbit (SO)-coupled Schrödinger particle with two internal degrees of freedom moving in a one-dimensional random potential. Numerical calculation of the density of states reveals the emergence of a Dyson-like singularity at zero energy when the system approaches the quasi-relativistic limit of the random-mass Dirac model for large SO coupling. Simulations of the expansion of an initially localized wave-packet show a crossover from an exponential (Anderson) localization to an anomalous power-law behavior reminiscent of the zero-energy (mid-gap) state of the random-mass Dirac model. We discuss conditions under which the crossover is observable in an experiment and derive the zero-energy state, thus proving its existence under proper conditions. Finally we describe a possible experimental realization using an ensemble of cold ⁸⁷Rb-atoms interacting with external control lasers and speckle fields.

We theoretically analyse the Bragg spectroscopic interferometer of two spatially separated atomic Bose–Einstein condensates that was experimentally realized by Saba et al (2005 Science307 1945) by continuously monitoring the relative phase evolution. Even though atoms in the light-stimulated Bragg scattering interact with intense coherent laser beams, we show that the phase is created by quantum measurement-induced backaction on the homodyne photocurrent of the lasers, opening the possibilities for quantum-enhanced interferometric schemes. We identify two regimes of phase evolution: a running phase regime observed in the experiment of Saba et al, which is sensitive to an energy offset and suitable for an interferometer, and a trapped phase regime, which can be insensitive to the applied forces and detrimental to interferometric applications.

A series of experiments dedicated to probing the phenomenon of lane formation in binary complex plasmas over a broad range of parameters has been performed with the PK-3 Plus laboratory on board the International Space Station (ISS) under microgravity conditions. In the experiments, bunches of small particles were driven through a background of big particles. We show that the dynamics of lane formation varies considerably with the density of the background and the size ratio between small and big particles. For consecutive injections of small particles a memory effect of the previous penetration was discovered for the first time. This memory effect was investigated quantitatively with respect to the structure formation and the penetration speed. We show that the memory effect in lane formation is linear. In addition, we studied the crossover from lane formation to phase separation driven by the nonadditive interactions between small and big particles. We found that during this transition the small penetrating particles effectively cage the background particles.

We study the relaxation dynamics of the one-dimensional Tomonaga–Luttinger model after an interaction quench, paying particular attention to the momentum dependence of the two-particle interaction. Several potentials of different analytical forms are investigated that all lead to universal Luttinger liquid (LL) physics in equilibrium. The steady-state fermionic momentum distribution shows universal behavior in the sense of the LL phenomenology. For generic regular potentials, the large time decay of the momentum distribution function toward the steady-state value is characterized by a power law with a universal exponent that depends only on the potential at zero momentum transfer. The commonly employed ad hoc procedure fails to give this exponent. In addition to quenches from zero to positive interactions, we also consider the abrupt changes in the interaction between two arbitrary values. Additionally, we discuss the appearance of a factor of two between the steady-state momentum distribution function and that obtained in equilibrium at equal two-particle interaction.

We study under-barrier tunneling for a pair of energetically bound bosonic atoms in an optical lattice with a barrier. We identify the conditions under which this exotic molecule tunnels as a point particle with the coordinate given by the bound pair center of mass and discuss the atomic co-tunneling beyond this regime. In particular, we quantitatively analyze resonantly enhanced co-tunneling, where two interacting atoms penetrate the barrier with higher probability than a single atom.

We analyze the recently developed folding algorithm (Bañuls et al 2009 Phys. Rev. Lett.102 240603) for simulating the dynamics of infinite quantum spin chains and we relate its performance to the kind of entanglement produced under the evolution of product states. We benchmark the accomplishments of this technique with respect to alternative strategies using Ising Hamiltonians with transverse and parallel fields, as well as XY models. Also, we evaluate its capability of finding ground and thermal equilibrium states.

Stochastic exclusion processes play an integral role in the physics of non-equilibrium statistical mechanics. These models are Markovian processes, described by a classical master equation. In this paper, a quantum mechanical version of a stochastic hopping process in one dimension is formulated in terms of a quantum master equation. This allows the investigation of coherent and stochastic evolution in the same formal framework. The focus lies on the non-equilibrium steady state. Two stochastic model systems are considered: the totally asymmetric exclusion process and the fully symmetric exclusion process. The steady-state transport properties of these models are compared to the case with additional coherent evolution, generated by the XX Hamiltonian.

Single-particle momentum spectra for a dynamically evolving one-dimensional Bose gas are analysed in the semi-classical wave limit. Representing one of the simplest correlation functions, these provide information on a possible universal scaling behaviour. Motivated by the previously discovered connection between (quasi-) topological field configurations, strong wave turbulence and non-thermal fixed points of quantum field dynamics, soliton formation is studied with respect to the appearance of transient power-law spectra. A random-soliton model is developed for describing the spectra analytically, and the analogies and differences between the emerging power laws and those found in a field theory approach to strong wave turbulence are discussed. The results open a new perspective on solitary wave dynamics from the point of view of critical phenomena far from thermal equilibrium and the possibility of studying this dynamics by experiment without the need for detecting solitons in situ.

We consider the non-equilibrium dynamics of the interacting Lieb–Liniger gas after instantaneously switching the interactions off. The subsequent time evolution of the space- and time-dependent correlation functions is computed exactly. Different relaxation behavior is observed for different correlation functions. The long time average is compared with the predictions of several statistical ensembles. The generalized Gibbs ensemble restricted to a fixed number of particles is shown to give correct results at large times for all length scales.

We report a theoretical study of the many-body effects of electron–electron interaction on the ground-state and spectral properties of double-layer graphene. Using a projector-based renormalization method we show that if a finite-voltage difference is applied between the graphene layers, electron–hole pairs can be formed and—at very low temperatures—an excitonic instability might emerge in a double-layer graphene structure. The single-particle spectral function near the Fermi surface exhibits a prominent quasiparticle peak, different from neutral (undoped) graphene bilayers. Away from the Fermi surface, we find that the charge carriers strongly interact with plasmons, thereby giving rise to a broad plasmaron peak in the angle-resolved photoemission spectrum.

4f core level photoelectron spectroscopy has been performed on negatively charged lead clusters, in the size range of 10–90 atoms. We deploy 4.7 nm radiation from the free-electron laser FLASH, yielding sufficiently high photon flux to investigate mass-selected systems in a beam. A new photoelectron detection system based on a hemispherical spectrometer and a time-resolving delayline detector makes it possible to assign electron signals to each micro-pulse of FLASH. The resulting 4f binding energies show good agreement with the metallic sphere model, giving evidence for a fast screening of the 4f core holes. By comparing the present work with previous 5d and valence region data, the paper presents a comprehensive overview of the energetics of lead clusters, from atoms to bulk. Special care is taken to discuss the differences of the valence- and core-level anion cluster photoionizations. Whereas in the valence case the escaping photoelectron interacts with a neutral system near its ground state, core-level ionization leads to transiently highly excited neutral clusters. Thus, the photoelectron signal might carry information on the relaxation dynamics.

The Rayleigh–Taylor instability (RTI) is a fundamental fluid instability that occurs when a light fluid is accelerated into a heavier one. While techniques for observing the RTI in classical fluids continue to improve, the instability has not been demonstrated in quantum fluids. Here, we exploit the formal equivalence between condensed matter and coherent nonlinear optics to observe the superfluid-like instability directly in the optical system. For the RTI, an initial refractive index gradient sets the acceleration, while self-induced nonlinear interactions lead to velocity differences and shear. The experimental observations show that density fingering is always accompanied by vortex generation, with perturbation modes following a hybrid dynamics: horizontal modes (along the interface) propagate as an incompressible fluid, but the vertical length scale (mixing length) is set by compressible shock dynamics. The growth rate, obtained analytically, shows that inhibition due to diffraction has the same spectral form as viscosity and diffusion, despite the fact that the system is dispersive rather than dissipative. This gives rigorous support for the observation that turbulence in quantum fluids has the same scaling as turbulence in normal fluids. The results hold for any Schrödinger flow, e.g. superfluids and quantum plasma, and introduce a new class of fluid-inspired instabilities in nonlinear optics.

Waveform-controlled light fields offer the possibility of manipulating ultrafast electronic processes on sub-cycle timescales. The optical lightwave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95 nm diameter SiO₂ nanoparticles in few-cycle laser fields with a well-defined waveform. Projections of the three-dimensional (3D) electron momentum distributions were obtained via single-shot velocity-map imaging (VMI), where phase tagging allowed retrieving the laser waveform for each laser shot. The application of this technique allowed us to efficiently suppress background contributions in the data and to obtain very accurate information on the amplitude and phase of the waveform-dependent electron emission. The experimental data that are obtained for 4 fs pulses centered at 720 nm at different intensities in the range (1–4) × 10¹³ W cm⁻² are compared to quasi-classical mean-field Monte-Carlo simulations. The model calculations identify electron backscattering from the nanoparticle surface in highly dynamical localized fields as the main process responsible for the energetic electron emission from the nanoparticles. The local field sensitivity of the electron emission observed in our studies can serve as a foundation for future research on propagation effects for larger particles and field-induced material changes at higher intensities.

We study the spatial coherence of polariton condensates subjected to coherent modulation by a one-dimensional tunable acoustic potential. We use an interferometric technique to measure the amplitude and phase of the macroscopic condensate wavefunction. By increasing the acoustic modulation amplitude, we track the transition from the extended wavefunction of the unperturbed condensate to a regime where the wavefunction is spatially modulated and then to a fully confined regime, where independent condensates form at the minima of the potential with negligible particle tunneling between adjacent sites.

Electron–positron clusters are studied using a quantum hydrodynamic model that includes Coulomb and exchange interactions. A variational Lagrangian method is used to determine their stationary and dynamical properties. The cluster static features are validated against existing Hartree–Fock calculations. In the linear response regime, we investigate both dipole and monopole (breathing) modes. The dipole mode is reminiscent of the surface plasmon mode usually observed in metal clusters. The nonlinear regime is explored by means of numerical simulations. We show that, by exciting the cluster with a chirped laser pulse with slowly varying frequency (autoresonance), it is possible to efficiently separate the electron and positron populations on a timescale of a few tens of femtoseconds.

It is analytically shown that the asymptotic correlations following a quantum quench in exactly solvable models can sometimes look essentially thermal provided the initial coupling between the system eigenmodes induces a large gap. We study this phenomenon using simple models, which also illustrate the relationship between the entanglement spectrum of the initial state and the generalized Gibbs ensemble describing the long-time correlations after the quench. We also show that the effective temperature characterizing the correlations is not related to the energy fluctuations after the quench, and therefore does not have thermodynamic meaning. The latter observation implies a breakdown of the fluctuation–dissipation theorem.

One of the main milestones in the study of opto- and electro-mechanical systems is to certify entanglement between a mechanical resonator and an optical or microwave mode of a cavity field. In this work, we show how a suitable time-periodic modulation can help to achieve large degrees of entanglement, building upon the framework introduced in Mari and Eisert (2009 Phys. Rev. Lett.103 213603). It is demonstrated that with suitable driving, the maximum degree of entanglement can be significantly enhanced, in a way exhibiting a nontrivial dependence on the specifics of the modulation. Such time-dependent driving might help to experimentally achieve entangled mechanical systems also in situations when quantum correlations are otherwise suppressed by thermal noise.

Microelectromechanical systems (MEMS) have been applied to many measurement problems in physics, chemistry, biology and medicine. In parallel, cavity optomechanical systems have achieved quantum-limited displacement sensitivity and ground state cooling of nanoscale objects. By integrating a novel cavity optomechanical structure into an actuated MEMS sensing platform, we demonstrate a system with high-quality-factor interferometric readout, electrical tuning of the optomechanical coupling by two orders of magnitude and a mechanical transfer function adjustable via feedback. The platform separates optical and mechanical components, allowing flexible customization for specific scientific and commercial applications. We achieve a displacement sensitivity of 4.6 fm Hz^−1/2 and a force sensitivity of 53 aN Hz^−1/2 with only 250 nW optical power launched into the sensor. Cold-damping feedback is used to reduce the thermal mechanical vibration of the sensor by three orders of magnitude and to broaden the sensor bandwidth by approximately the same factor, to above twice the fundamental frequency of ≈40 kHz. The readout sensitivity approaching the standard quantum limit is combined with MEMS actuation in a fully integrated, compact, low-power, stable system compatible with Si batch fabrication and electronics integration.

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump–probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump–probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump–probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron–ion recombination.

Based on trajectory calculations of xenon clusters up to ≈6000 atoms irradiated by laser pulses (peak intensities I_M = 10¹⁴–10¹⁶ W cm⁻², Gaussian pulse lengths τ = 10–230 fs and frequency 0.35 fs⁻¹), we have analyzed the interrelation between outer ionization and ion kinetic energies. The following three main categories have been identified. (A) For short pulses (τ = 10 fs) of higher intensity I_M = 10¹⁶ W cm⁻², the outer ionization level leads to a sufficiently high positive cluster charge, which confines the remaining nanoplasma electrons to the cluster center. In this case, ion energies can be reasonably well accounted for by a multi-charge state lychee model, according to which outer ionization is vertical and the nanoplasma can be described by a non-expanding neutral cluster interior, causing a zero-energy component in the ion kinetic energy distribution and an expanding electron-free cluster periphery. (B) For a very low outer ionization level, which is realized for short pulses of low intensity (I_M = 10¹⁴ W cm⁻²) and/or large clusters, a slow gradual evaporation of nanoplasma electrons under laser-free conditions on the picosecond time scale is observed, making the entire outer ionization process highly non-vertical despite the short laser pulse. Accordingly, ions are accelerated only by a gradual buildup of the total cluster charge. (C) For long pulses (τ = 230 fs), the cluster expansion during the laser pulse is large and outer ionization is non-vertical. The nanoplasma electrons attain high kinetic energies by resonance heating and are distributed over the entire ion framework without a neutral cluster interior. Consequently, a zero-energy component in the ion energy distribution is missing.

We give an overview of the coherence properties of exciton–polariton condensates generated by optical parametric scattering. Different aspects of the first-order coherence (g⁽¹⁾) have been investigated. The spatial coherence extension of a two-dimensional (2D) polariton system, below and at the parametric threshold, demonstrates the development of a constant phase coherence over the entire condensate, once the condensate phase transition takes place. The effect on coherence of the photonic versus excitonic nature of the condensates is also examined. The coherence of a quasi-1D trap, composed of a line defect, is studied, showing the detrimental effect of reduced dimensionality on the establishment of the long range order. In addition, the temporal coherence decay, $g^{(1)}(\tau)$, reveals a fast decay in contrast with the 2D case. The situation of a quasi-1D condensate coexisting with a 2D one is also presented.

We investigate thermodynamic properties of a two-dimensional photon gas confined by a dye-filled optical microcavity. A thermally equilibrated state of the photon gas is achieved by radiative coupling to a heat bath that is realized with dye molecules embedded in a polymer at room temperature. The chemical potential of the gas is freely adjustable. The optical microcavity consisting of two curved mirrors induces both a non-vanishing effective photon mass and a harmonic trapping potential for the photons. While previous experiments of our group have used liquid dye solutions, the measurements described here are based on dye molecules incorporated into a polymer host matrix. The solid state material allows a simplified operation of the experimental scheme. We furthermore describe studies of fluorescence properties of dye-doped polymers, and verify the applicability of Kennard–Stepanov theory in this system. In the future, dye-based solid state systems hold promise for the realization of single-mode light sources in thermal equilibrium based on Bose–Einstein condensation of photons, as well as for solar energy concentrators.

We introduce a dissipation term in the Gross–Pitaevskii equation that describes the stimulated relaxation of condensed bosons due to scattering with different types of particles. This situation applies to Bose–Einstein condensates of quasi-particles in the solid state, such as magnons and excitons. Our model is compatible with the phenomenology of superfluidity: supercurrents are stable up to a critical speed and decay when they are faster. We apply our model to a description of the relaxation of polariton condensates in a shallow trap.

Ionization mechanisms in water irradiated with bandwidth-limited and temporally asymmetric femtosecond laser pulses are investigated via ultrafast spectral interferometry. By using a novel common-path interferometer with an enlarged temporal measurement window, we directly observe the dynamics of free-electron plasma generated by shaped pulses. We found that a temporally asymmetric pulse and its time-reversed counterpart address multiphoton and avalanche ionization mechanisms in a different fashion. Positive third-order dispersion shaped pulses produce a much higher free-electron density than negative ones at the same fluence, instantaneous frequency and focusing conditions. From the experimental data obtained after irradiation with bandwidth-limited and shaped pulses the multiphoton and avalanche coefficients were determined using a generic rate equation. We conclude that temporal tailored femtosecond pulses are suitable for manipulation of the initial steps in laser processing of high band gap materials.

Positron clouds are compressed following accumulation in a Surko-type two-stage buffer gas trap using an asymmetric rotating wall electric field. An analytic theory used to describe measurements of the rate of compression is discussed. Furthermore, we describe measurements taken without the rotating wall applied and with the rotating wall compression present during accumulation of the positron cloud. This has enabled total loss rates for the positrons via annihilation and collisional-induced radial transport to be isolated, with the latter mechanism found to be dominant. We have shown that the application of the rotating wall at a resonant frequency virtually eliminates radial transport, such that the positron loss is caused by annihilation in the gas.

In a system of ac-driven condensed bosons we study a new type of Josephson effect occurring between states sharing the same region of space and the same internal atom structure. We first develop a technique to calculate the long-time dynamics of a driven interacting many-body system. For resonant frequencies, this dynamics can be shown to derive from an effective time-independent Hamiltonian which is expressed in terms of standard creation and annihilation operators. Within the subspace of resonant states, and if the undriven states are plane waves, a locally repulsive interaction between bosons translates into an effective attraction. We apply the method to study the effect of interactions on the coherent ratchet current of an asymmetrically driven boson system. We find a wealth of dynamical regimes which includes Rabi oscillations, self-trapping and chaotic behavior. In the latter case, a full many-body calculation deviates from the mean-field results by predicting large quantum fluctuations of the relative particle number.

We investigate a network of coupled superconducting transmission line resonators, each of them made nonlinear with a capacitively shunted Josephson junction coupling to the odd flux modes of the resonator. The resulting eigenmode spectrum shows anticrossings between the plasma mode of the shunted junction and the odd resonator modes. Notably, we find that the combined device can inherit the complete nonlinearity of the junction, allowing for a description as a harmonic oscillator with a Kerr nonlinearity. Using a dc SQUID instead of a single junction, the nonlinearity can be tuned between 10 kHz and 4 MHz while maintaining resonance frequencies of a few gigahertz for realistic device parameters. An array of such nonlinear resonators can be considered a scalable superconducting quantum simulator for a Bose–Hubbard Hamiltonian. The device would be capable of accessing the strongly correlated regime and be particularly well suited for investigating quantum many-body dynamics of interacting particles under the influence of drive and dissipation.

The one-body density matrix of weakly interacting condensed bosons in external potentials is calculated using inhomogeneous Bogoliubov theory. We determine the condensate deformation caused by weak external potentials at the mean-field level. The momentum distribution of quantum fluctuations around the deformed ground state is obtained analytically and finally, the resulting quantum depletion is calculated. The depletion due to the external potential, or potential depletion for short, is a small correction to the homogeneous depletion, validating our inhomogeneous Bogoliubov theory. Analytical results are derived for weak lattices and spatially correlated random potentials, with simple, universal results in the Thomas–Fermi limit of very smooth potentials.

In this paper, we address the question of the origin of the charge density wave instability in 1T-TiSe₂. We develop a model considering the direct Coulomb interaction between electrons and holes in the valence and conduction bands near the Fermi energy. Using the Bethe–Salpeter equation, we calculate the electron–hole correlator, which reveals an instability at low temperature leading to a transition toward a commensurate superstructure mediated by the electron–phonon coupling, in agreement with experiments. On the basis of this correlator, the electron self-energies are then calculated and the corresponding photoemission spectra are compared with the experimental ones, revealing good agreement. The signature of electron–hole fluctuations in photoemission is emphasized. Furthermore, we calculate the spectral function of the phonon mode, whose softening is experimentally observed at the transition.

We show that the spin–charge separation predicted for correlated fermions in one dimension could be observed using polarized photons propagating in a nonlinear optical waveguide. Using coherent control techniques and employing a cold atom ensemble interacting with the photons, large nonlinearities in the single-photon level can be achieved. We show that the latter can allow for the simulation of a strongly interacting gas, which is made of stationary dark-state polaritons of two species and is shown to form a Luttinger liquid of effective fermions for the right regime of interactions. The system can be tuned optically to the relevant regime where the spin–charge separation is expected to occur. The characteristic features of the separation as demonstrated in different spin and charge densities and velocities can be efficiently detected via optical measurements of the emitted photons with current optical technologies.

Artificial spin ices are nanoscale geometrically engineered systems that mimic the behavior of bulk spin ices at room temperature. We describe the nanoscale magnetic interactions in a square spin ice lattice by an experimentally verified model that accounts for the correct shape of the magnetic islands. Magnetic force microscopy measurements on lithographically fabricated lattices are compared to Monte Carlo simulations of the reversal process of two lattices with different lattice spacings. Lattice node statistics and correlations show significant differences in the reversal mechanism for lattices with different spacings. The effect of structural variations is also compared for the two lattice reversals.