Table of contents

The quantum theory of the damped harmonic oscillator has been a subject of continual investigation since the 1930s. The obstacle to quantization created by the dissipation of energy is usually dealt with by including a discrete set of additional harmonic oscillators as a reservoir. But a discrete reservoir cannot directly yield dynamics such as Ohmic damping (proportional to velocity) of the oscillator of interest. By using a continuum of oscillators as a reservoir, we canonically quantize the harmonic oscillator with Ohmic damping and also with general damping behaviour. The dynamics of a damped oscillator is determined by an arbitrary effective susceptibility that obeys the Kramers–Kronig relations. This approach offers an alternative description of nano-mechanical oscillators and opto-mechanical systems.

Using high-sensitivity magneto-optical imaging, we find evidence for a jump in local vortex density associated with a vortex liquid to vortex solid phase transition just above the lower critical field in a single crystal of Bi₂Sr₂CaCu₂O₈. We find that the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field–temperature vortex phase diagram, we identify phase boundaries demarcating a dilute vortex liquid phase and the vortex solid phase. The phase diagram also identifies a coexistence regime of the dilute vortex liquid and solid phases and shows the effect of pinning on the vortex liquid to vortex solid phase transition line. We find that the phase boundary lines can be fitted to the theoretically predicted expression for the low-field portion of the phase boundary delineating a dilute vortex solid from a vortex liquid phase. We show that the same theoretical fit can be used to describe the pinning dependence of the low-field phase boundary lines provided that the dependence of the Lindemann number on pinning strength is considered.

We investigate power propagation in a metal-coated tapered optical fiber. We analyze in detail the conversion from the fiber core guided mode to a surface plasmon polariton (SPP) confined at the tip apex. To this aim, we adapt coupled local-mode theory to include lossy modes. Two distinct regimes are identified. In the case of thin metal coating, a strong coupling regime occurs between a core guided mode and a SPP with good conversion efficiency. In the case of thick metal coating, a very weak coupling occurs. Finally, energy confinement and the role of Joule losses are discussed in the near-infrared and visible ranges. Moreover, the coupled equations derived for local lossy modes are not limited to plasmonic systems but also apply to any absorbing or leaky optical waveguide of arbitrary shape, so this formalism could find applications in various areas.

We formulated an effective theory for a single interlayer exciton in a bilayer quantum antiferromagnet, in the limit when the holon and doublon are strongly bound onto one interlayer rung by the Coulomb force. Upon using a rung linear spin-wave approximation of the bilayer Heisenberg model, we calculated the spectral function of the exciton for a wide range of the interlayer Heisenberg coupling α = J_⊥/Jz. In the disordered phase at large α, a coherent quasi-particle peak appears, representing free motion of the exciton in a spin singlet background. In the Néel phase, which applies to more realistic model parameters, a ladder spectrum arises due to Ising confinement of the exciton. The exciton spectrum is visible in measurements of the dielectric function, such as c-axis optical conductivity measurements.

A mechanism to generate a spin-polarized current in a two-terminal zigzag silicene nanoribbon is predicted. When a weak local exchange field that is parallel to the surface of silicene is applied on one of edges of the silicene nanoribbon, a gap is opened in the corresponding edge states but another pair of gapless edge states with opposite spin are still protected by the time-reversal symmetry. Hence, a spin-polarized current can be induced in the gap opened by the local exchange field in this two-terminal system. What is important is that the spin-polarized current can be obtained even in the absence of Rashba spin–orbit coupling and in the case of the very weak exchange field. That is to say, the mechanism to generate the spin-polarized currents can be easily realized experimentally. We also find that the spin-polarized current is insensitive to weak disorder.

The shapes and alignment of elastic vesicles similar to red blood cells (RBCs) in cylindrical capillary flow are investigated by mesoscopic hydrodynamic simulations. We study the collective flow behavior of many RBCs, where the capillary diameter is comparable to the diameter of the RBCs. Two essential control parameters are the RBC volume fraction (the tube hematocrit, H_T), and the suspension flow velocity. Depending on H_T, flow velocity and capillary radius, the RBC suspension exhibits a disordered phase and two distinct ordered phases, consisting of a single file of parachute-shaped cells and a zigzag arrangement of slipper-shaped cells, respectively. We argue that thermal fluctuations, included in the simulation method, coupled to hydrodynamic flows are important contributors to the RBC morphology. We examine the changes to the phase structures when the capillary diameter and the material properties (bending rigidity κ and stretching modulus μ) of the model RBCs are varied, constructing phase diagrams for each case. We focus on capillary diameters, which range from about 1.0 to about 1.4 times the RBC long diameter. For the smallest capillary diameter, the single-file arrangement dominates; for the largest diameter, the ordered zigzag arrangement begins to loose its stability and alternates with an asymmetric structure with two lanes of differently oriented cells. In simulations with long capillaries, the coexistence of different phases can be observed.

Using scanning tunnelling microscopy (STM), it is possible to observe detailed structure of the molecular orbitals (MOs) of fullerene anions C⁻₆₀. However, understanding the experimental observations is not straightforward because of the inherent presence of Jahn–Teller (JT) interactions, which (in general) split the MOs in one of a number of equivalent ways. Tunnelling between equivalent distortions means that any observed STM image will be a superposition of images arising from the individual configurations. Interactions with the surface substrate must also be taken into account. We will show how simple ideas involving a symmetry analysis and Hückel molecular orbital theory can be used to understand observed STM images without need for the more usual but more complicated density functional calculations. In particular, we will show that when the fullerene ion is adsorbed with a pentagon, hexagon or double-bond facing the surface, STM images involving the lowest unoccupied molecular orbital (LUMO) can be reproduced by adding together just two images of squares of components of the LUMO, in ratios that depend on the strength of the JT effect and the surface interaction. It should always be possible to find qualitative matches to observed images involving any of these orientations by simply looking at images of the components, without doing any detailed calculations. A comparison with published images indicates that the JT effect in the C⁻₆₀ ion favours D_3d distortions.

The interaction between super-intense coherent x-ray light and nuclei is studied theoretically. One of the main difficulties in driving nuclear transitions arises from the very narrow nuclear excited state widths, which limit the coupling between lasers and nuclei. In the context of direct laser–nucleus interaction, we consider the nuclear width broadening that occurs when in solid targets, when the excitation caused by a single photon is shared by a large number of nuclei, forming a collective excited state. Our results show that cooperative effects mostly contribute with a modest increase to the nuclear excited state population except in the case of ⁵⁷Fe, where the enhancement can reach almost two orders of magnitude. Additionally, an update is given of previous estimates of the nuclear excited state population and signal photons for x-ray lasers interacting with solid-state and ion beam nuclear targets taking into account the experimental advances of x-ray coherent light sources. The presented values are an improvement by orders of magnitude and are encouraging as to the future prospects of nuclear quantum optics.

Cooling of the mechanical motion of a GaAs nano-membrane using the photothermal effect mediated by excitons was recently demonstrated by some of the authors (Usami et al 2012 Nature Phys.8 168) and provides a clear example of the use of thermal forces to cool down mechanical motion. Here, we report on a single-free-parameter theoretical model to explain the results of this experiment which matches the experimental data remarkably well.

We analyse frequency spectra of absolute optical instruments and show that they have very specific properties: the eigenfrequencies form tight groups that are almost equidistantly spaced. We prove this by theoretical analysis and demonstrate by numerically calculated spectra of various examples of absolute instruments. We also show that in rotationally and spherically symmetric absolute instruments a source, its image and the centre of the device must lie on a straight line.

The Doppler-shift spectra of the γ-rays from positron annihilation in molecules were determined by using the momentum distribution of the annihilation electron–positron pair. The effect of the positron wavefunction on spectra was analysed in a recent paper (Green et al 2012 New J. Phys.14 035021). In this companion paper, we focus on the dominant contribution to the spectra, which arises from the momenta of the bound electrons. In particular, we use computational quantum chemistry models (Hartree–Fock with two basis sets and density functional theory (DFT)) to calculate the wavefunctions of the bound electrons. Numerical results are presented for noble gases and small molecules such as H₂, N₂, O₂, CH₄ and CF₄. The calculations reveal relatively small effects on the Doppler-shift spectra from the level of inclusion of electron correlation energy in the models. For atoms, the difference in the full-width at half-maximum of the spectra obtained using the Hartree–Fock and DFT models does not exceed 2%. For molecules the difference can be much larger, reaching 8% for some molecular orbitals. These results indicate that the predicted positron annihilation spectra for molecules are generally more sensitive to inclusion of electron correlation energies in the quantum chemistry model than the spectra for atoms are.

We explore a systematic approach to studying the dynamics of evolving networks at a coarse-grained, system level. We emphasize the importance of finding good observables (network properties) in terms of which coarse-grained models can be developed. We illustrate our approach through a particular social network model: the 'rise and fall' of a networked society (Marsili M et al 2004 Proc. Natl Acad. Sci. USA101 1439). We implement our low-dimensional description computationally using the equation-free approach and show how it can be used to (i) accelerate simulations and (ii) extract system-level stability/bifurcation information from the detailed dynamic model. We discuss other system-level tasks that can be enabled through such a computer-assisted coarse-graining approach.

Robust synchronization is essential to ensure the stable operation of many complex networked systems such as electric power grids. Increasing energy demands and more strongly distributing power sources raise the question of where to add new connection lines to the already existing grid. Here we study how the addition of individual links impacts the emergence of synchrony in oscillator networks that model power grids on coarse scales. We reveal that adding new links may not only promote but also destroy synchrony and link this counter-intuitive phenomenon to Braess's paradox known for traffic networks. We analytically uncover its underlying mechanism in an elementary grid example, trace its origin to geometric frustration in phase oscillators, and show that it generically occurs across a wide range of systems. As an important consequence, upgrading the grid requires particular care when adding new connections because some may destabilize the synchronization of the grid—and thus induce power outages.

We report the realization of a silicon three-dimensional photonic crystal nanocavity containing self-assembled germanium-island emitters. The three-dimensional woodpile photonic crystal was assembled layer by layer by micromanipulation using silicon plates grown by molecular beam epitaxy. An optical nanocavity was formed in the center of the photonic crystal by introducing a point defect into one of the plates. Measurements of the filtered spontaneous emission from the Ge islands in the active plate through the localized modes of the structure directly reveal information on the evolution of the frequency and Q-factor as upper cladding plates are sequentially added. An exponential increase of the cavity-Q is observed when the number of upper cladding plates is increased up to a maximum of ten. The emission of germanium-islands within the cavity reveals several strongly polarized cavity modes with quality-factors up to ≈13 600. The emission intensity of the cavity modes is enhanced by large factors up to ≈58× as compared with the active plate outside the photonic environment.

We formulate macroscopic quantum electrodynamics in the most general linear, absorbing media. In particular, Onsager reciprocity is not assumed to hold. The field quantization is based on the source-quantity representation of the electromagnetic field in terms of the dyadic Green's tensor. For media with a nonlocal response, a description in terms of a complex conductivity tensor is employed. As an alternative description, we introduce the permittivity, permeability and magnetoelectric susceptibilities to obtain an explicitly duality-invariant scheme. We find that duality invariance only holds as a continuous symmetry when nonreciprocal responses are allowed for.

An extensive debate on quantum non-demolition (QND) measurement, reviewed in Grangier et al (1998 Nature396 537), finds that true QND measurements must have both non-classical state-preparation capability and non-classical information-damage tradeoff. Existing figures of merit for these non-classicality criteria require direct measurement of the signal variable and are thus difficult to apply to optically-probed material systems. Here we describe a method to demonstrate both criteria without need for to direct signal measurements. Using a covariance matrix formalism and a general noise model, we compute meter observables for QND measurement triples, which suffice to compute all QND figures of merit. The result will allow certified QND measurement of atomic spin ensembles using existing techniques.

The perfect drain for the Maxwell fish eye (MFE) is a non-magnetic dissipative region placed in the focal point to absorb all the incident radiation without reflection or scattering. The perfect drain was recently designed as a material with complex permittivity that depends on frequency. However, this material is only a theoretical material, so it cannot be used in practical devices. The perfect drain has been claimed as necessary for achieving super-resolution (Leonhardt 2009 New J. Phys.11 093040), which has increased the interest in practical perfect drains suitable for manufacturing. Here, we present a practical perfect drain that is designed using a simple circuit (made of a resistance and a capacitor) connected to the coaxial line. Moreover, we analyze the super-resolution properties of a device equivalent to the MFE, known as a spherical geodesic waveguide, loaded with this perfect drain. The super-resolution analysis for this device is carried out using COMSOL Multiphysics. The results of simulations predict a super-resolution of up to λ/3000.

Metallic gyroid metamaterials are formed by a combination of nanoplasmonic helices leading to unique and complex optical characteristics. To unravel this inherent complexity we set up an analytic tri-helical metamaterial model that reveals the underlying physical properties. This analytic tri-helical model is complete in the sense that it is only dependent on the structure's geometric and material parameters. It allows us to elucidate the characteristic transverse and longitudinal modes of the metal nano-gyroid as well as explain the surprisingly small optical chirality of gyroid metamaterials that is observed in experiments. We argue that this behaviour originates from the interconnection of multiple helices of opposing handedness.

It is shown that a laser wakefield driven in the highly nonlinear regime or the wave-breaking regime in underdense plasma can emit ultra-broad-band bright extreme ultraviolet (XUV) radiation pulses, which are as short as a few hundreds of attoseconds in one-dimensional simulations and a few femtoseconds in multi-dimensional cases. The emission is caused by a transverse current sheet co-moving with the laser pulse. This current sheet is formed by an electron density spike with trapped electrons in the wakefield with a certain transverse kinetic momentum of electrons left behind the laser pulse. This residual momentum appears when the laser pulse undergoes a strong self-modulation and subsequent pulse steepening at the front. In the multi-dimensional simulations, the XUV emission is found only near the laser axis with a much smaller spot size than the laser pulse due to its transverse ponderomotive force effects. The present scheme provides an alternative method for producing single bright ultrashort XUV pulses for ultrafast applications.

We present a generalized method for calculating the k-shell structure of weighted networks. The method takes into account both the weight and the degree of a network, in such a way that in the absence of weights we resume the shell structure obtained by the classic k-shell decomposition. In the presence of weights, we show that the method is able to partition the network in a more refined way, without the need of any arbitrary threshold on the weight values. Furthermore, by simulating spreading processes using the susceptible-infectious-recovered model in four different weighted real-world networks, we show that the weighted k-shell decomposition method ranks the nodes more accurately, by placing nodes with higher spreading potential into shells closer to the core. In addition, we demonstrate our new method on a real economic network and show that the core calculated using the weighted k-shell method is more meaningful from an economic perspective when compared with the unweighted one.

We studied the dynamics of an initially inverted atom in a semi-infinite waveguide, in the presence of a single propagating photon. We show that atomic relaxation is enhanced by a factor of 2, leading to maximal bunching in the output field. This optimal irreversible stimulated emission is a novel phenomenon that can be observed with state-of-the-art solid-state atoms and waveguides. When the atom interacts with two one-dimensional electromagnetic environments, the preferential emission in the stimulated field can be exploited to efficiently amplify a classical or a quantum state.

The nanoplasmonic field enhancement effects in the energetic electron emission from few-nm-sized silver clusters exposed to intense femtosecond dual pulses are investigated by high-resolution double differential electron spectroscopy. For moderate laser intensities of 10¹⁴ W cm⁻², the delay-dependent and angular-resolved electron spectra show laser-aligned emission of electrons up to keV kinetic energies, exceeding the ponderomotive potential by two orders of magnitude. The importance of the nanoplasmonic field enhancement due to resonant Mie-plasmon excitation observed for optimal pulse delays is investigated by a direct comparison with molecular dynamics results. The excellent agreement of the key signatures in the delay-dependent and angular-resolved spectra with simulation results allows for a quantitative analysis of the laser and plasmonic contributions to the acceleration process. The extracted field enhancement at resonance verifies the dominance of surface-plasmon-assisted re-scattering.

We discuss the interaction of ultrashort near-infrared laser pulses with sharp metal tips at moderate nominal intensities (I₀ ∼ 10¹¹ W cm⁻²). As external electric fields are strongly enhanced at such tips (enhancement factor ∼10) our system turns out to be an ideal miniature laboratory to investigate strong-field effects at solid surfaces. We analyse the electron-energy spectra as a function of the strength of the laser field and the static extraction field and present an intuitive model for their interpretation. The size of the effective field acting on the metal electrons can be determined from the electron spectra. The latter are also reproduced by time-dependent density functional theory (TDDFT) simulations.

Half-integer topological defects in polariton condensates can be regarded as magnetic charges, with respect to built-in effective magnetic fields present in microcavities. We show how an integer topological defect can be separated into a pair of half-integer ones, paving the way for flows of magnetic charges: spin currents or magnetricity. We discuss the corresponding experimental implementation within microwires (with half-solitons) and planar microcavities (with half-vortices).

In this paper, we propose a new approach to characterize time series with noise perturbations in both the time and frequency domains by combining Granger causality and complex networks. We construct directed and weighted complex networks from time series and use representative network measures to describe their physical and topological properties. Through analyzing the typical dynamical behaviors of some physical models and the MIT-BIH⁷ human electrocardiogram data sets, we show that the proposed approach is able to capture and characterize various dynamics and has much potential for analyzing real-world time series of rather short length.

Using a specifically tailored experimental approach, we revisit the exemplary effect of photoemission from quasi-free electronic states in crystals. Applying a momentum microscope, we measure photoelectron momentum patterns emitted into the complete half-space above the sample after excitation from a linearly polarized laser light source. By the application of a fully three-dimensional (3D) geometrical model of direct optical transitions, we explain the characteristic intensity distributions that are formed by the photoelectrons in k-space under the combination of energy conservation and crystal momentum conservation in the 3D bulk as well as at the two-dimensional (2D) surface. For bismuth surface alloys on Cu(111), the energy-resolved photoelectron momentum patterns allow us to identify specific emission processes in which bulk excited electrons are subsequently diffracted by an atomic 2D surface grating. The polarization dependence of the observed intensity features in momentum space is explained based on the different relative orientations of characteristic reciprocal space directions with respect to the electric field vector of the incident light.

We use THz time-domain spectroscopy to investigate the far-infrared properties of vanadium dioxide thin films, strain-engineered through epitaxial growth on (100)_R TiO₂ substrates. The films exhibit a large uniaxial tensile strain along the rutile c-axis. X-ray diffraction measurements reveal a structural transition temperature of 340 K, whereas independent THz conductivity measurements yield a metal–insulator transition temperature of 365 K along c_R. Analysis of these results suggests a Mott–Hubbard behavior along the c_R-axis. Along c_R the conductivity is approximately 5500 (Ω cm)⁻¹, comparable to bulk single crystals. The tensile strain leads to remarkably uniform cracking oriented along the rutile c-axis, resulting in a large conductivity anisotropy in our single-crystal epitaxial thin films. We discuss our results in the context of previous measurements and calculations of the properties of VO₂, under different strain conditions. This work demonstrates the potential of strain engineering to tune the properties of complex materials while also serving as a powerful discriminatory tool for probing microscopic responses.

Proximity-effect and resistance magneto-fluctuation measurements in submicron Nd_1.2Ba_1.8Cu₃O_z (NBCO) nano-loops are reported to investigate coherent charge transport in the non-superconducting state. We find an unexpected inhibition of Cooper pair transport, and a destruction of the induced superconductivity, by lowering the temperature from 6 K to 250 mK. This effect is accompanied by a significant change in the conductance-voltage characteristics and the zero bias conductance response to the magnetic field, pointing to the activation of a strong pair-breaking mechanism at lower temperature. The data are discussed in the framework of mesoscopic effects specific to superconducting nanostructures, proximity effect and high temperature superconductivity.

We describe a system composed of two coupled optical cavity modes with a coupling modulated by a bulk mechanical resonator. In addition, one of the cavity modes is irreversibly coupled to a single photon source. Our scheme is an opto-mechanical realization of the Jaynes–Cummings model where the qubit is a dual rail optical qubit while the bosonic degree of freedom is a matter degree of freedom realized as the bulk mechanical excitation. We show the possibility of engineering phonon number states of the mechanical oscillator in such a system by computing the conditional state of the mechanics after successive photon counting measurements.

The interaction between a high-frequency dilational mode of a thin dielectric film and an optical cavity field is studied theoretically in the membrane-in-the-middle setup. A derivation from first principles leads to a multi-mode optomechanical Hamiltonian where multiple cavity modes are coupled by the thickness variation of the membrane. For membrane thicknesses of the order of 1 μm, the frequency of this dilational mode is in the GHz range. This can be matched to the free spectral range of the optical cavity, such that the mechanical oscillator will resonantly couple cavity modes at different frequencies. Furthermore, such a large mechanical frequency also means that the quantum ground state of motion can be reached with conventional refrigeration techniques. Estimation of the coupling strength with realistic parameters suggests that optomechanical effects can be observable with this dilational mode. It is shown how this system can be used as a quantum limited optical amplifier. The dilational motion can also lead to quantum correlations between cavity modes at different frequencies, which is quantified with an experimentally accessible two-mode squeezing spectrum. Finally, an explicit signature of radiation pressure shot noise in this system is identified.

We report a simple and practical approach for generating high-quality polarization entanglement in a fully guided-wave fashion. Both deterministic pair separation into two adjacent telecommunication channels and the paired photons' temporal walk-off compensation are achieved using standard fiber components. Two-photon interference experiments are performed, both for quantitatively demonstrating the relevance of our approach and for manipulating the produced state between bosonic and fermionic symmetries. The compactness, versatility and reliability of this configuration makes it a potential candidate for quantum communication applications.

A suspended, doubly clamped single-wall carbon nanotube is characterized at cryogenic temperatures. We observe specific switching effects in dc-current spectroscopy of the embedded quantum dot. These have been identified previously as nano-electromechanical self-excitation of the system, where positive feedback from single-electron tunneling drives mechanical motion. A magnetic field suppresses this effect, by providing an additional damping mechanism. This is modeled by eddy current damping, and confirmed by measuring the resonance quality factor of the radio-frequency-driven nano-electromechanical resonator in an increasing magnetic field.

Using functionalized tips, the atomic resolution of a single organic molecule can be achieved by noncontact atomic force microscopy (nc-AFM) operating in the regime of short-ranged repulsive Pauli forces. To theoretically describe the atomic contrast in such AFM images, we propose a simple model in which the Pauli repulsion is assumed to follow a power law as a function of the probed charge density. As the exponent in this power law is found to be largely independent of the sample molecule, our model provides a general method for simulating atomically resolved AFM images of organic molecules. For a single perylene-tetracarboxylic-dianhydride (PTCDA) molecule imaged with a CO-terminated tip, we find excellent agreement with the experimental data. Our model eliminates the need to take into account the full tip and sample system and therefore reduces computational cost by three orders of magnitude.

In this paper, we show that a fast switch is able to lead a complex dynamical system to being asymptotically stable, although this system is completely unstable in every switch duration and even the associated connection matrices are randomly selected. Importantly, we define some new exponents by which we can figure out the essential patterns that guarantee the stability of fast switching systems, and besides, their calculations need little computational cost. More interestingly, we show the efficiency of some random switches in inducing stability through a comparison of the systems with different switch connection matrices and switch durations, and we give a design method for obtaining higher efficient random switch rules. We also investigate the generalization of the obtained results to a more realistic case where the switch obeys some renewal process.

We consider a case when a weak value is introduced as a physical quantity rather than as an average of weak measurements. The case we treat is a time evolution of a particle by the 1 + 1-dimensional Dirac equation. In particular, in a spontaneous pair production via a supercritical step potential, a quantitative explanation can be given by a weak value for the group velocity of the particle. We also show that the condition for the pair production (supercriticality) corresponds to the condition when the weak value takes a strange value (superluminal velocity).

The growth of a thin elastic sheet imposes constraints on its geometry such as its Gaussian curvature K_G. In this paper, we construct the shapes of sympetalous bell-shaped flowers with a constant Gaussian curvature. Minimizing the bending energies of both the petal and the veins, we are able to predict quantitatively the global shape of these flowers. We discuss two toy problems where the Gaussian curvature is either negative or positive. In the former case, the axisymmetric pseudosphere turns out to mimic the correct shape before edge curling; in the latter case, singularities of the mathematical surface coincide with strong veins. Using a variational minimization of the elastic energy, we find that the optimal number for the veins is either four, five or six, a number that is deceptively close to the statistics on real flowers in nature.

The nano-scale structure of cytoskeletal biopolymers as well as sophisticated superstructures determine the versatile cellular shapes and specific mechanical properties. One example is keratin intermediate filaments in epithelial cells, which form thick bundles that can further organize in a cross-linked network. To study the native structure of keratin bundles in whole cells, high-resolution techniques are required, which do at the same time achieve high penetration depths. We employ scanning x-ray diffraction using a nano-focused x-ray beam to study the structure of keratin in freeze-dried eukaryotic cells. By scanning the sample through the beam we obtain x-ray dark-field images with a resolution of the order of the beam size, which clearly show the keratin network. Each individual diffraction pattern is further analyzed to yield insight into the local sample structure, which allows us to determine the local structure orientation. Due to the small beam size we access the structure in a small sample volume without performing the ensemble average over one complete cell.

The African trypanosome is a single flagellated micro-organism that causes the deadly sleeping sickness in humans and animals. We study the locomotion of a model trypanosome by modeling the spindle-shaped cell body using an elastic network of vertices with additional bending rigidity. The flagellum firmly attached to the model cell body is either straight or helical. A bending wave propagates along the flagellum and pushes the trypanosome forward in its viscous environment, which we simulate with the method of multi-particle collision dynamics. The relaxation dynamics of the model cell body due to a static bending wave reveals the sperm number from elastohydrodynamics as the relevant parameter. Characteristic cell body conformations for the helically attached flagellum resemble experimental observations. We show that the swimming velocity scales as the root of the angular frequency of the bending wave reminiscent of predictions for an actuated slender rod attached to a large viscous load. The swimming velocity for one geometry collapses on a single master curve when plotted versus the sperm number. The helically attached flagellum leads to a helical swimming path and a rotation of the model trypanosome about its long axis as observed in experiments. The simulated swimming velocity agrees with the experimental value.

Recent experiments have demonstrated an open system realization of the Dicke quantum phase transition in the motional degrees of freedom of an optically driven Bose–Einstein condensate in a cavity. Relevant collective excitations of this light–matter system are polaritonic in nature, allowing access to the quantum critical behavior of the Dicke model through light leaking out of the cavity. This opens the path to using photodetection-based quantum optical techniques to study the dynamics and excitations of this elementary quantum critical system. We first discuss the photon flux observed at the cavity face and find that it displays a different scaling law near criticality than that obtained from the mean-field theory for the equivalent closed system. Next, we study the second-order correlation measurements of photons leaking out of the cavity. Finally, we discuss a modulation technique that directly captures the softening of polaritonic excitations. Our analysis takes into account the effect of the finite size of the system, which may result in an effective symmetry-breaking term.

We show that ablation features in poly-methyl methacrylate (PMMA) induced by a single femtosecond laser pulse are imposed by light polarization. The ablation craters are elongated along the major axis of the polarization vector and become increasingly prominent as the pulse energy is increased above the threshold energy. We demonstrate ∼40% elongation for linearly and elliptically polarized light in the fluence range of 4–20 J cm⁻², while circularly polarized light produced near circular ablation craters irrespective of pulse energies. We also show that irradiation with multiple pulses erases the polarization-dependent elongation of the ablation craters. However, for line ablation the orientation of the electric field vector is imprinted in the form of quasi-periodic structures inside the ablated region. Theoretically, we show that the polarization dependence of the ablation features arises from a local field enhancement during light–plasma interaction. Simulations also show that in materials with high nonlinearities such as doped PMMA, in addition to conventional explosive boiling, sub-surface multiple filamentation can also give rise to porosity.

This paper presents a coupled channel model for transport in two-dimensional semiconductor Majorana nanowires coupled to normal leads. When the nanowire hosts a zero-mode pair, conspicuous signatures of the linear conductance are predicted. An effective model in second quantization allowing a fully analytical solution is used to clarify the physics. We also discuss the nonlinear current response (dI/dV ).

The 7.8 eV nuclear isomer transition in ²²⁹thorium has been suggested as a clock transition in a new type of optical frequency standard. Here we discuss the construction of a 'solid-state nuclear clock' from thorium nuclei implanted into single crystals transparent in the vacuum ultraviolet range. We investigate crystal-induced line shifts and broadening effects for the specific system of calcium fluoride. At liquid nitrogen temperatures, the clock performance will be limited by decoherence due to magnetic coupling of the thorium nuclei to neighboring nuclear moments, ruling out the commonly used Rabi or Ramsey interrogation schemes. We propose clock stabilization based on a fluorescence spectroscopy method and present optimized operation parameters. Taking advantage of the large number of quantum oscillators under continuous interrogation, a fractional instability level of 10⁻¹⁹ might be reached within the solid-state approach.

The electron emission of highly charged ions has been reanalyzed with the goal of separating the magnetic and retardation contributions to the electron–electron (e–e) interaction from the static Coulomb repulsion in strong fields. A remarkable change in the electron angular distribution due to the relativistic terms in the e–e interaction is found, especially for the autoionization of beryllium-like projectiles, following a 1s → 2p_3/2 Coulomb excitation in collision with some target nuclei. For low-energetic, high-Z projectiles with T_p ≲ 10 MeV u⁻¹, a diminished (electron) emission in the forward direction as well as oscillations in the electron angular distribution due to the magnetic and retarded interactions are predicted for the autoionization of the 1s2s²2p_3/2³P₂ resonance into the 1s²2s ²S_1/2 ground and the 1s²2p ²P_1/2 excited levels of the finally lithium-like ions, and in contrast to a pure Coulomb repulsion between the bound and emitted electrons. The proposed excitation–autoionization process can be observed at existing storage rings and will provide a novel insight into the dynamics of electrons in strong fields.

We apply the vortex path model of critical currents to a comprehensive analysis of contemporary data on defect-engineered superconductors, showing that it provides a consistent and detailed interpretation of the experimental data for a diverse range of materials. We address the question of whether electron mass anisotropy plays a role of any consequence in determining the form of this data and conclude that it does not. By abandoning this false interpretation of the data, we are able to make significant progress in understanding the real origin of the observed behavior. In particular, we are able to explain a number of common features in the data including shoulders at intermediate angles, a uniform response over a wide angular range and the greater discrimination between individual defect populations at higher fields. We also correct several misconceptions including the idea that a peak in the angular dependence of the critical current is a necessary signature of strong correlated pinning, and conversely that the existence of such a peak implies the existence of correlated pinning aligned to the particular direction. The consistency of the vortex path model with the principle of maximum entropy is introduced.

The processes underlying crater formation by energetic nanoparticle impact are investigated using molecular dynamics simulations. Both metallic and van-der-Waals-bonded targets are studied. We find a transition from crater formation by melt flow at small impact energies to an evaporation (gas flow) mechanism at higher energies. The transition occurs gradually at impact energies per atom of a few tens of the cohesive energy of the target. van-der-Waals-bonded solids do not exhibit the melt flow cratering regime, in agreement with the narrow liquid zone in their phase diagram. We find that the size of the target region heated above the critical temperature roughly corresponds to the crater volume. The transition shows up most clearly in the increase of the volume of ejected material relative to the crater volume. Finally, we demonstrate the punching of dislocations below the crater for high-velocity impact into ductile targets, leading to a contribution of plastic flow to the crater volume.

We use dynamic scanning capacitance microscopy to image compressible and incompressible strips at the edge of a Hall bar in a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime. This method gives access to the complex local conductance, G_ts, between a sharp metallic tip scanned across the sample surface and ground, comprising the complex sample conductance. Near integer filling factors we observe a bright stripe along the sample edge in the imaginary part of G_ts. The simultaneously recorded real part exhibits a sharp peak at the boundary between the sample interior and the stripe observed in the imaginary part. The features are periodic in the inverse magnetic field and consistent with compressible and incompressible strips forming at the sample edge. For currents larger than the critical current of the QHE break-down the stripes vanish sharply and a homogeneous signal is recovered, similar to zero magnetic field. Our experiments directly illustrate the formation and a variety of properties of the conceptually important QHE edge states at the physical edge of a 2DEG.

The optimization of two-dimensional (2D) lattice ion trap geometries for trapped ion quantum simulation is investigated. The geometry is optimized for the highest ratio of ion–ion interaction rate to decoherence rate. To calculate the electric field of such array geometries a numerical simulation based on a 'Biot–Savart like law' method is used. In this article we will focus on square, hexagonal and centre rectangular lattices for optimization. A method for maximizing the homogeneity of trapping site properties over an array is presented for arrays of a range of sizes. We show how both the polygon radii and separations scale to optimize the ratio between the interaction and decoherence rate. The optimal polygon radius and separation for a 2D lattice is found to be a function of the ratio between radio-frequency (rf) voltage and drive frequency applied to the array. We then provide a case study for ¹⁷¹Yb⁺ ions to show how a 2D quantum simulator array could be designed.

The second-order correlation function g⁽²⁾(τ = 0), input–output curves and pulse duration of the emission from a microcavity exciton–polariton system subsequent to picosecond-pulsed excitation are measured for different temperatures. At low temperatures a two-threshold behaviour emerges, which has been attributed to the onset of polariton lasing and conventional lasing at the first and the second threshold, respectively. We observe that polariton lasing is stable up to temperatures comparable with the exciton binding energy. At higher temperatures a single threshold displays the direct transition from thermal emission to photon lasing.

The coherent manipulation of wave packets is an important tool in many areas of physics. We demonstrate the experimental realization of quasi-free wave packets of ultra-cold atoms bound by an external harmonic trap. The wave packets are produced by modulating the intensity of an optical lattice containing a Bose–Einstein condensate. The evolution of these wave packets is monitored in situ and their six-photon reflection at a band gap is observed. In direct analogy with pump–probe spectroscopy, a probe pulse allows for the resonant de-excitation of the wave packet into states localized around selected lattice sites at a long, controllable distance of more than 100 lattice sites from the main component. This precise control mechanism for ultra-cold atoms thus enables controlled quantum state preparation and splitting for quantum dynamics, metrology and simulation.

We report the first experimental observation of terahertz (THz) radiation from the rear surface of a solid target while interacting with an intense laser pulse. Experimental and two-dimensional particle-in-cell simulations show that the observed THz radiation is mostly emitted at large angles to the target normal. Numerical results point out that a large part of the emission originates from a micron-scale plasma sheath at the rear surface of the target, which is also responsible for the ion acceleration. This opens a perspective for the application of THz radiation detection for on-site diagnostics of particle acceleration in laser-produced plasmas.

Photoionization of trapped atoms is a recent technique for creating ion beams with low transverse temperature. The temporal behavior of the current that can be extracted from such an ultracold ion source is measured when operating in the pulsed mode. A number of experimental parameters are varied to find the conditions under which the time-averaged current is maximized. A dynamic model of the source is developed that agrees quite well with the experimental observations. The radiation pressure exerted by the excitation laser beam is found to substantially increase the extracted current. For a source volume with a typical root-mean-square radius of 20 μm, a maximum peak current of 88 pA is observed, limited by the available ionization laser power of 46 mW. The optimum time-averaged current is 13 pA at a 36% duty cycle. Particle-tracking simulations show that stochastic heating strongly reduces the brightness of the ion beam at higher current for the experimental conditions.

A state selected at random from the Hilbert space of a many-body system is overwhelmingly likely to exhibit highly non-classical correlations. For these typical states, half of the environment must be measured by an observer to determine the state of a given subsystem. The objectivity of classical reality—the fact that multiple observers can agree on the state of a subsystem after measuring just a small fraction of its environment—implies that the correlations found in nature between macroscopic systems and their environments are exceptional. Building on previous studies of quantum Darwinism showing that highly redundant branching states are produced ubiquitously during pure decoherence, we examine the conditions needed for the creation of branching states and study their demise through many-body interactions. We show that even constrained dynamics can suppress redundancy to the values typical of random states on relaxation timescales, and prove that these results hold exactly in the thermodynamic limit.

We propose and experimentally demonstrate a near-optimal discrimination scheme for the quadrature phase shift keying (QPSK) protocol. We show in theory that the performance of our hybrid scheme is superior to the standard scheme—heterodyne detection—for all signal amplitudes and underpin the predictions with our experimental results. Furthermore, our scheme provides hitherto the best performance in the domain of highly attenuated signals. The discrimination is composed of a quadrature measurement, a conditional displacement and a threshold detector.

We investigate a two-electron double quantum dot with both spin and valley degrees of freedom as they occur in graphene, carbon nanotubes or silicon and regard the 16-dimensional space with one electron per dot as a four-qubit logic space. In the spin-only case, it is well known that the exchange coupling between the dots combined with arbitrary single-qubit operations is sufficient for universal quantum computation. The presence of valley degeneracy in the electronic band structure alters the form of the exchange coupling and, in general, leads to spin–valley entanglement. Here, we show that universal quantum computation can still be performed by exchange interaction and single-qubit gates in the presence of an additional (valley) degree of freedom. We present an explicit pulse sequence for a spin-only controlled-NOT consisting of the generalized exchange coupling and single-electron spin and valley rotations. We also propose state preparations and projective measurements with the use of adiabatic transitions between states with (1,1) and (0,2) charge distributions similar to the spin-only case, but with the additional requirement of controlling the spin and valley Zeeman energies by an external magnetic field. Finally, we demonstrate a universal two-qubit gate between a spin and a valley qubit, allowing universal gate operations on the combined spin and valley quantum register.

A major challenge of today's quantum communication systems lies in the transmission of quantum information with high rates over long distances in the presence of unavoidable losses. Thereby the achievable quantum communication rate is fundamentally limited by the amount of energy that can be transmitted per use of the channel. It is hence vital to develop quantum communication protocols that encode quantum information as energy efficiently as possible. To this aim we investigate continuous-variable quantum teleportation as a method of distributing quantum information. We explore the possibility to encode information on multiple optical modes and derive upper and lower bounds on the achievable quantum channel capacities. This analysis enables us to benchmark single-mode versus multi-mode entanglement resources. Our research reveals that multiplexing does not only feature an enhanced energy efficiency, leading to an exponential increase in the achievable quantum communication rates in comparison to single-mode coding, but also yields an improved loss resilience. However, as reliable quantum information transfer is only achieved for entanglement values above a certain threshold a careful optimization of the number of coding modes is needed to obtain the optimal quantum channel capacity.

In this paper, we studied the strategies to enhance synchronization on directed networks by manipulating a fixed number of links. We proposed a centrality-based manipulating (CBM) method, where the node centrality is measured by the well-known PageRank algorithm. Extensive numerical simulation on many modeled networks demonstrated that the CBM method is more effective in facilitating synchronization than the degree-based manipulating method and the random manipulating method for adding or removing links. The reason is that the CBM method can effectively narrow the incoming degree distribution and reinforce the hierarchical structure of the network. Furthermore, we apply the CBM method to the links rewiring procedure where at each step one link is removed and one new link is added. The CBM method helps to decide which links should be removed or added. After several steps, the resulting networks are very close to the optimal structure from the theoretical analysis and the evolutionary optimization algorithm. The numerical simulations on the Kuramoto model further demonstrate that our method has an advantage in shortening the convergence time to synchronization on directed networks.

Linear chains of spins acting as quantum wires are a promising approach for achieving scalable quantum information processors. Nuclear spins in apatite crystals provide an ideal test bed for the experimental study of quantum information transport, as they closely emulate a one-dimensional spin chain, while magnetic resonance techniques can be used to drive the spin chain dynamics and probe the accompanying transport mechanisms. Here we demonstrate initialization and readout capabilities in these spin chains, even in the absence of single-spin addressability. These control schemes enable preparing desired states for quantum information transport and probing their evolution under the transport Hamiltonian. We further optimize the control schemes by a detailed analysis of ¹⁹F NMR lineshape.

We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding and impurity Green's function approaches. Density-functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three sp²σ dangling bonds adjacent to the vacancy introduce localized states (Vσ) in the mid-gap region, which split due to the crystal field and a Jahn–Teller distortion, while the p_zπ states introduce a sharp resonance state (Vπ) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the σ and the π states, so that these bands are clearly identifiable in the calculated band structure. As to the magnetic moment of the vacancy, the Hund's rule coupling aligns the spins of the four localized Vσ₁↑↓, Vσ₂↑ and Vπ↑ electrons, resulting in an S = 1 state, with a magnetic moment of 2μ_B, which is reduced by about 0.3μ_B due to the anti-ferromagnetic spin polarization of the π band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of 1.7μ_B. Using the Lippmann–Schwinger equation, we reproduce the well-known ∼1/r decay of the localized Vπ wave function with distance, and in addition, find an interference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the Vπ wave function is a unique feature of the graphene vacancy and we suggest that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature.

We present comparative measurements of the charge occupation and conductance of a GaAs/AlGaAs quantum dot (QD). The dot charge is measured with a capacitively coupled quantum point-contact sensor. In the single-level Coulomb blockade regime near equilibrium, charge and conductance signals are found to be proportional to each other. We conclude that in this regime, the two signals give equivalent information about the QD system. Out of equilibrium, we study the inelastic-cotunneling regime. We compare the measured differential dot charge with an estimate assuming a dwell time of transmitted carriers on the dot given by h/E, where E is the blockade energy of first-order tunneling. The measured signal is of similar magnitude as the estimate, compatible with a picture of cotunneling as transmission through a virtual intermediate state with a short lifetime.

The Uhrig dynamical decoupling sequence achieves high-order decoupling of a single system qubit from its dephasing bath through the use of bang–bang Pauli pulses at appropriately timed intervals. High-order decoupling of single and multiple qubit systems from baths causing both dephasing and relaxation can also be achieved through the nested application of Uhrig sequences, again using single-qubit Pauli pulses. For the three-qubit decoherence free subsystem (DFS) and related subsystem encodings, Pauli pulses are not naturally available operations; instead, exchange interactions provide all required encoded operations. Here we demonstrate that exchange interactions alone can achieve high-order decoupling against general noise in the three-qubit DFS. We present decoupling sequences for a three-qubit DFS coupled to classical and quantum baths and evaluate the performance of the sequences through numerical simulations.

We report on in-lab free space quantum key distribution (QKD) experiments over 40 cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2 kbit s⁻¹ (35.4 kbit s⁻¹) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g⁽²⁾(0) value of 0.35 (0.49) at moderate (high) excitation. The red emitting diodes generate sifted keys at a rate of 95.0 kbit s⁻¹ at a QBER of 4.1% and a g⁽²⁾(0) value of 0.49. This first successful proof of principle QKD experiment based on electrically operated semiconductor single-photon sources can be considered as a major step toward practical and efficient quantum cryptography scenarios.

The time-dependent surface flux (t-SURFF) method is extended to single and double ionization of two-electron systems. Fully differential double emission spectra by strong pulses at extreme UV and infrared wavelengths are calculated using simulation volumes that only accommodate the effective range of the atomic binding potential and the quiver radius of free electrons in the external field. For a model system, we found a pronounced dependence of shake-up and non-sequential double ionization on the phase and duration of the laser pulse. The extension to fully three-dimensional calculations is discussed.

We explore the question of state estimation for a qubit restricted to the x–z-plane of the Bloch sphere, with the trine measurement. In our earlier work (H K Ng and B-G Englert 2012 Int. J. Quantum Inf.11 1250038), similarities between quantum tomography and the tomography of a classical die motivated us to apply a simple modification of the classical estimator for use in the quantum problem. This worked very well. In this article, we adapt a different aspect of the classical estimator to the quantum problem. In particular, we investigate the mean estimator, where the mean is taken with a weight function identical to that in the classical estimator but now with quantum constraints imposed. Among such mean estimators, we choose an optimal one with the smallest worst-case error—the minimax mean estimator—and compare its performance with that of other estimators. Despite the natural generalization of the classical approach, this minimax mean estimator does not work as well as one might expect from the analogous performance in the classical problem. While it outperforms the often-used maximum-likelihood estimator in having a smaller worst-case error, the advantage is not significant enough to justify the more complicated procedure required to construct it. The much simpler adapted estimator introduced in our earlier work is still more effective. Our previous work emphasized the similarities between classical and quantum state estimation; in contrast, this paper highlights how intuition gained from classical problems can sometimes fail in the quantum arena.

Starting from a four-partite photonic hyper-entangled Dicke resource, we report the full tomographic characterization of three-, two- and one-qubit states obtained by projecting out part of the computational register. The reduced states thus obtained correspond to fidelities with the expected states larger than 87%, therefore certifying the faithfulness of the entanglement-sharing structure within the original four-qubit resource. The high quality of the reduced three-qubit state allows for the experimental verification of the Koashi–Winter relation for the monogamy of correlations within a tripartite state. We show that, by exploiting the symmetries of the three-qubit state obtained upon projection over the four-qubit Dicke resource, such a relation can be experimentally fully characterized using only five measurement settings. We highlight the limitations of such an approach and sketch an experimentally oriented way of overcoming them.

We analyze the behavior of estimation errors evaluated by two loss functions, namely the Hilbert–Schmidt distance and infidelity, in one-qubit state tomography with finite data. We show numerically that there can be a large gap between the estimation errors and those predicted by an asymptotic analysis. The origin of this discrepancy is the existence of a boundary in the state space imposed by the requirement that density matrices be non-negative (positive semidefinite). We derive the explicit form of a function reproducing the behavior of estimation errors with high accuracy by introducing two approximations: a Gaussian approximation of the multinomial distributions of outcomes and a linearization of the boundary. This function gives us an intuition of the behavior of the expected losses for finite data sets. We show that this function can be used to determine the amount of data necessary for the estimation to be treated reliably with the asymptotic theory. We give an explicit expression for this amount, which exhibits strong sensitivity to the true quantum state as well as the choice of measurement.

We describe two related methods for reconstructing multi-scale entangled states from a small number of efficiently-implementable measurements and fast post-processing. Both methods only require single-particle measurements and the total number of measurements is polynomial in the number of particles. Data post-processing for state reconstruction uses standard tools, namely matrix diagonalization and conjugate gradient method, and scales polynomially with the number of particles. Both methods prevent the build-up of errors from both numerical and experimental imperfections. The first method is conceptually simpler but requires unitary control. The second method circumvents the need for unitary control but requires more measurements and produces an estimated state of lower fidelity.

We introduce and experimentally demonstrate a technique for performing quantum state tomography (QST) on multiple-qubit states despite incomplete knowledge about the unitary operations used to change the measurement basis. Given unitary operations with unknown rotation angles, our method can be used to reconstruct the density matrix of the state up to local $\hat \sigma _z$ rotations as well as recover the magnitude of the unknown rotation angle. We demonstrate high-fidelity self-calibrating tomography on polarization-encoded one- and two-photon states. The unknown unitary operations are realized in two ways: using a birefringent polymer sheet—an inexpensive smartphone screen protector—or alternatively a liquid crystal wave plate with a tuneable retardance. We explore how our technique may be adapted for QST of systems such as biological molecules where the magnitude and orientation of the transition dipole moment is not known with high accuracy.

We reconstruct the polarization sector of a bright polarization squeezed beam starting from a complete set of Stokes measurements. Given the symmetry that underlies the polarization structure of quantum fields, we use the unique SU(2) Wigner distribution to represent states. In the limit of localized bright states, the Wigner function can be approximated by an inverse three-dimensional Radon transform. We compare this direct reconstruction with the results of a maximum likelihood estimation, thus finding excellent agreement.

We address the quantum characterization of photon counters based on transition-edge sensors (TESs) and present the first experimental tomography of the positive operator-valued measure (POVM) of a TES. We provide the reliable tomographic reconstruction of the POVM elements up to 11 detected photons and M = 100 incoming photons, demonstrating that it is a linear detector.