Table of contents

Plasmonics is one of the growing fields in modern photonics that has garnered increasing interest over the last few years. In this focus issue, the specific challenges concerning terahertz plasmonics have been addressed and most recent advances in this specific field have been highlighted. The articles demonstrate the diversity and the opportunities of this rich field by covering a variety of topics ranging from the propagation of surface plasmon polaritons (SPPs) on artificially structures surfaces, 2D manipulation of surface plasmons and SPPs, plasmonic focusing, plasmonic high-Q resonators for sensing applications, plasmonically enhanced terahertz antennas to terahertz field manipulation by use of plasmonic structures. The articles substantiate the impact of plasmonics and its great innovative potential for terahertz technology.

The rapid growth of quantum information sciences over the past few decades has fueled a corresponding rise in high profile applications in fields such as metrology, sensors, spintronics, and attosecond dynamics, in addition to quantum information processing. Realizing this potential of today's quantum science and the novel technologies based on this requires a high degree of coherent control of quantum systems. While early efforts in systematizing methods for high fidelity quantum control focused on isolated or closed quantum systems, recent advances in experimental design, measurement and monitoring, have stimulated both need and interest in the control of complex or large scale quantum systems that may also be coupled to an interactive environment or reservoir. This focus issue brings together new theoretical and experimental work addressing the formulation and implementation of quantum control for a broad range of applications in quantum science and technology today.

To harness the power of controllable quantum systems for information processing or quantum simulation, it is essential to be able to accurately characterise the system's Hamiltonian. Although in principle this requires determining less parameters than full quantum process tomography, a general and extendable method for reconstructing a general Hamiltonian has been elusive. In their recent paper, Wang et al (2015 New J. Phys.17 093017) apply dynamical decoupling to the problem of Hamiltonian tomography and show how to reconstruct a general many-body Hamiltonian comprised of arbitrary interactions between qubits.

Our common understanding of the physical world deeply relies on the notion that events are ordered with respect to some time parameter, with past events serving as causes for future ones. Nonetheless, it was recently found that it is possible to formulate quantum mechanics without any reference to a global time or causal structure. The resulting framework includes new kinds of quantum resources that allow performing tasks—in particular, the violation of causal inequalities—which are impossible for events ordered according to a global causal order. However, no physical implementation of such resources is known. Here we show that a recently demonstrated resource for quantum computation—the quantum switch—is a genuine example of 'indefinite causal order'. We do this by introducing a new tool—the causal witness—which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order. We show however that the quantum switch does not violate any causal inequality.

We investigate the dynamics of an ion sympathetically cooled by another laser-cooled ion or small ion crystal. To this end, we develop simple models of the cooling dynamics in the limit of weak Coulomb interactions. Experimentally, we create a two-ion crystal of Ca⁺ and Al⁺ by photo-ionization of neutral atoms produced by laser ablation. We characterize the velocity distribution of the laser-ablated atoms crossing the trap by time-resolved fluorescence spectroscopy. We observe neutral atom velocities much higher than the ones of thermally heated samples and find as a consequence long sympathethic cooling times before crystallization occurs. Our key result is a new technique for detecting the loading of an initially hot ion with energy in the eV range by monitoring the motional state of a Doppler-cooled ion already present in the trap. This technique not only detects the ion but also provides information about the dynamics of the sympathetic cooling process.

A formalism termed the interface conductance modal analysis (ICMA) method is presented, which allows for calculations of the modal contributions to thermal interface conductance within the context of molecular dynamics (MD) simulations, which inherently include anharmonicity to full order. The eigen modes of vibration in this study are calculated from harmonic lattice dynamics (LD) calculations, however the generality of ICMA formalism also allows for incorporation of anharmonic LD results into the calculations. ICMA can be implemented in both equilibrium MD (EMD) and non-equilibrium MD (NEMD) simulations and both methods show qualitative agreement as validated through study of a simple system of Lennard-Jones solids. The formalism itself is based on a modal decomposition of the heat flow across an interface, which is then substituted into expressions for the conductance either based on EMD or NEMD. As a MD based method, it not only includes anharmonicity, but also naturally includes the atomic level details of the interface quality. The ICMA method now enables more in-depth study of various effects such as temperature, anharmonicity, interdiffusion, roughness, imperfections, dislocations, stress, changes in crystal structure through a single unified model, as it can essentially treat any material or object where the atoms vibrate around equilibrium sites (e.g., ordered or disordered solids and molecules). This formalism therefore serves as an important step forward toward better understanding of heat flow at interfaces.

We develop a general nonlinear Luttinger liquid theory to describe the dynamics of one-dimensional quantum critical systems at low temperatures. To demonstrate the predictive power of our theory we compare results for the autocorrelation G(t) in the XXZ chain with numerical density-matrix renormalization group data and obtain excellent agreement. Our calculations provide, in particular, direct evidence that G(t) shows a diffusion-like decay, $G(t)\sim 1/\sqrt{t},$ in sharp contrast to the exponential decay in time predicted by conventional Luttinger liquid theory.

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal Li_xCoO₂ (x = 0.33, 0.43, 0.50, 0.53, 0.72, and 0.87) are reported. The room temperature optical absorption spectra for x values in the range of 0.33–0.72 exhibit three bands near 1.60, 3.35, and 5.20 eV. On the basis of first-principles calculations, the observed optical excitations were appropriately assigned. The charge-transfer absorption bands shift to higher energies in Li_0.87CoO₂ because of symmetry-breaking-induced distortions of the hybridized Co-O orbitals with shifted oxygen 2p states. Furthermore, two Raman-active phonon modes, which display E_g and A_1g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li_0.50CoO₂, large splitting of the E_g mode and a discontinuous change in the frequency of the A_1g mode were observed. These results highlight the importance of spin-phonon coupling in Li_0.50CoO₂.

Cold atmospheric pressure plasmas have proven to provide an alternative treatment of cancer by targeting tumorous cells while leaving their healthy counterparts unharmed. However, the underlying mechanisms of the plasma–cell interactions are not yet fully understood. Reactive oxygen species, and in particular hydroxyl radicals (OH), are known to play a crucial role in plasma driven apoptosis of malignant cells. In this paper we investigate the interaction of OH radicals, as well as H₂O₂ molecules and HO₂ radicals, with DNA by means of reactive molecular dynamics simulations using the ReaxFF force field. Our results provide atomic-scale insight into the dynamics of oxidative stress on DNA caused by the OH radicals, while H₂O₂ molecules appear not reactive within the considered time-scale. Among the observed processes are the formation of 8-OH-adduct radicals, forming the first stages towards the formation of 8-oxoGua and 8-oxoAde, H-abstraction reactions of the amines, and the partial opening of loose DNA ends in aqueous solution.

We study the Landau level (LL) spectrum using a multi-band ${\bf{k}}\cdot {\bf{p}}$ theory in monolayer transition metal dichalcogenide semiconductors. We find that in a wide magnetic field range the LL can be characterized by a harmonic oscillator spectrum and a linear-in-magnetic field term which describes the valley degeneracy breaking. The effect of the non-parabolicity of the band-dispersion on the LL spectrum is also discussed. Motivated by recent magnetotransport experiments, we use the self-consistent Born approximation and the Kubo formalism to calculate the Shubnikov–de Haas oscillations of the longitudinal conductivity. We investigate how the doping level, the spin-splitting of the bands and the broken valley degeneracy of the LLs affect the magnetoconductance oscillations. We consider monolayer MoS₂ and WSe₂ as concrete examples and compare the results of numerical calculations and an analytical formula which is valid in the semiclassical regime. Finally, we briefly analyze the recent experimental results (Cui et al 2015 Nat. Nanotechnol.10 534) using the theoretical approach we have developed.

The monoclinic double perovskite Sr₂YRuO₆ has recently gained a renewed interest in order to get a deeper insight into the exotic magnetic ground states associated with geometric frustration. Striking discrepancies between the spin order derived from the neutron diffraction refinements and the macroscopic magnetic and thermal responses is a major challenge that must be addressed. In this work, detailed neutron diffraction measurements as a function of temperature yield a completely different interpretation of the patterns. We show that at low temperatures a spin structure of the K₂NiF₄-type is an accessible configuration for the magnetic ground state. In the neighborhood of the magnetic transition, this configuration evolves into a canted superstructure. The deduced temperature dependence of the canting angle exhibits two closely spaced peaks, which are in excellent agreement with the double peaks in the magnetic contribution to the specific heat and in the thermal expansion coefficient. We explain these features in terms of reorientation of the net ferromagnetic moment of the noncollinear spin state, due to the local breaking of the inversion symmetry promoted by the monoclinic distortions, with structural changes acting as the driving force.

The impact of disorder on the superconducting (SC) pairing mechanism is the centre of much debate. Some evidence suggests a loss of phase coherence of pairs while others point towards the formation of a competing phase. In our work we show that the two perspectives may be different sides of the same coin. Using an extension of the perturbative renormalization group approach we compare the impact of different disorder-induced interactions on a SC ground state. We find that in the strongly disordered regime an interaction between paired fermions and their respective disordered environment replaces conventional Cooper pairing. For these unconventional Cooper pairs the phase coherence condition, required for the formation of a SC condensate, is not satisfied.

The E1M1 transition rate of the $2s2p{\ }^{3}{P}_{0}\to 2{s}^{2}{\ }^{1}{S}_{0}$ line in beryllium-like ions has been calculated within the framework of relativistic second-order perturbation theory. Both multiconfiguration and quantum-electrodynamical computations have been carried out independently to better understand and test for all major electron–electron correlation contributions in the representation of the initial, intermediate and final states. By comparing the results from these methods, which agree well for all ions along the beryllium isoelectronic sequence, the lifetime of the metastable 2s2p³P₀ level is found to be longer by about 2–3 orders of magnitude for all medium and heavy elements than was estimated previously. This makes the ³P₀ level of beryllium-like ions to one of the longest living (low-lying) electronic excitations of a tightly bound system with potential applications for atomic clocks and in astro physics and plasma physics.

We examine square and kagome artificial spin ice for colloids confined in arrays of double-well traps. Unlike magnetic artificial spin ices, colloidal and vortex artificial spin ice realizations allow creation of doping sites through double occupation of individual traps. We find that doping square and kagome ice geometries produces opposite effects. For square ice, doping creates local excitations in the ground state configuration that produce a local melting effect as the temperature is raised. In contrast, the kagome ice ground state can absorb the doping charge without generating non-ground-state excitations, while at elevated temperatures the hopping of individual colloids is suppressed near the doping sites. These results indicate that in the square ice, doping adds degeneracy to the ordered ground state and creates local weak spots, while in the kagome ice, which has a highly degenerate ground state, doping locally decreases the degeneracy and creates local hard regions.

By controlling electron injection into the second period of the laser-driven wakefield in a downward density ramp, a high-quality low-energy electron beam can be accelerated in a short segment of high-density plasma. After a second downward density ramp followed by a low-density plasma plateau, the pre-accelerated electron beam can be seeded into the first period of the laser-driven wakefield for cascaded acceleration at an optimized phase. A monoenergetic electron beam with peak energy of ∼1.2 GeV can be generated from plasma with a length of 12 mm and density of 9 × 10¹⁷ cm⁻³, driven by a laser pulse with peak power of 77 TW. By modifying the acceleration stage comprising several density-ascending plasma segments, the peak energy of the quasi-monoenergetic electron beam can be efficiently increased by about 50% via a quasi-phase-stable multiple-cascade acceleration scheme.

We propose a scheme for performing an entanglement-swapping operation within a quantum communications hub (a Bell like measurement) using an NV-centre's $| \pm 1\rangle \leftrightarrow | {A}_{2}\rangle$ optical transition. This is based on the heralded absorption of a photon resonant with that transition. The quantum efficiency of a single photon absorption is low but can be improved by placing the NV centre inside a micro cavity to boost the interaction time and further by recycling the leaked photon back into the cavity after flipping its phase and/or polarization. Throughout this process, the NV is repeatedly monitored via a QND measurement that heralds whether or not the photon absorption has succeeded. Upon success we know a destructive Bell measurement has occurred between that photon and NV centre. Given low losses and high per-pass absorption probability, this scheme should allow the total success probability to approach unity. With long electron spin coherence times possible at low temperatures, this component could be useful within a memory-based quantum repeater or relay.

Femtosecond vacuum ultraviolet pulses from a monochromated high harmonic generation source excite vibrational wavepackets in the ${B}^{1}{{\rm{\Sigma }}}_{{\rm{g}}}^{+}$ state of D₂. The wavepacket motion is measured through strong field ionization into bound and dissociative ion states yielding ${{\rm{D}}}_{2}^{+}$ and D⁺ products. The time dependence of the ${{\rm{D}}}_{2}^{+}$ and D⁺ ion signals provides a sensitive fingerprint of the quantum nuclear wavepacket, due to the different ionization rates for the two channels. The experiments are modelled with excitation and ionization processes included explicitly, with the results of the model showing a very good agreement with the experimental observations. The experiment demonstrates the level of detail attainable when studying ultrafast quantum nuclear dynamics using high harmonic sources.

A stochastic differential equation that describes the dynamics of single-domain magnetic particles at any temperature is derived using a classical formalism. The deterministic terms recover existing theory and the stochastic process takes the form of a mean-reverting random walk. In the ferromagnetic state diffusion is predominantly angular and the relevant diffusion coefficient increases linearly with temperature before saturating at the Curie point (T_c). Diffusion in the macrospin magnitude, while vanishingly small at room temperature, increases sharply as the system approaches T_c. Beyond T_c, in the paramagnetic state, diffusion becomes isotropic and independent of temperature. The stochastic macrospin model agrees well with atomistic simulations.

A new class of pattern forming systems is identified and investigated: anisotropic systems that are spatially inhomogeneous along the direction perpendicular to the preferred one. By studying the generic amplitude equation of this new class and a model equation, we show that branched stripe patterns emerge, which for a given parameter set are stable within a band of different wave numbers and different numbers of branching points (defects). Moreover, the branched patterns and unbranched ones (defect-free stripes) coexist over a finite parameter range. We propose two systems where this generic scenario can be found experimentally, surface wrinkling on elastic substrates and electroconvection in nematic liquid crystals, and relate them to the findings from the amplitude equation.

One of the most promising approaches for generating spin- and energy-entangled electron pairs is splitting a Cooper pair into the metal through spatially separated terminals. Utilizing hybrid systems with the energy-dependent barriers at the superconductor/normal metal (NS) interfaces, one can achieve a practically 100% efficiency outcome of entangled electrons. We investigate a minimalistic one-dimensional model comprising a superconductor and two metallic leads and derive an expression for an electron-to-hole transmission probability as a measure of splitting efficiency. We find the conditions for achieving 100% efficiency and present analytical results for the differential conductance and differential noise.

High symmetry epitaxial quantum dots (QDs) with three or more symmetry planes provide a very promising route for the generation of entangled photons for quantum information applications. The great challenge to fabricate nanoscopic high symmetry QDs is further complicated by the lack of structural characterization techniques able to resolve small symmetry breaking. In this work, we present an approach for identifying and analyzing the signatures of symmetry breaking in the optical spectra of QDs. Exciton complexes in InGaAs/AlGaAs QDs grown along the [111]B crystalline axis in inverted tetrahedral pyramids are studied by polarization resolved photoluminescence spectroscopy combined with lattice temperature dependence, excitation power dependence and temporal photon correlation measurements. By combining such a systematic experimental approach with a simple theoretical approach based on a point-group symmetry analysis of the polarized emission patterns of each exciton complex, we demonstrate that it is possible to achieve a strict and coherent identification of all the observable spectral patterns of numerous exciton complexes and a quantitative determination of the fine structure splittings of their quantum states. This analysis is found to be particularly powerful for selecting QDs with the highest degree of symmetry (C_3v and ${D}_{3h}$) for potential applications of these QDs as polarization entangled photon sources. We exhibit the optical spectra when evolving towards asymmetrical QDs, and show the higher sensitivity of certain exciton complexes to symmetry breaking.

Based on the Floquet scattering theory, we analytically investigate the topological spin pumping for an exactly solvable model. Floquet spin Chern numbers are introduced to characterize the periodically time-dependent system. The topological spin pumping remains robust both in the presence and in the absence of the time-reversal symmetry, as long as the pumping frequency is smaller than the band gap, where the electron transport involves only the Floquet evanescent modes in the pump. For the pumping frequency greater than the band gap, where the propagating modes in the pump participate in the electron transport, the spin pumping rate decays rapidly, marking the end of the topological pumping regime.

Topological objects are interesting topics in various fields of physics ranging from condensed matter physics to the grand unified and superstring theories. Among those, ultracold atoms provide a playground to study the complex topological objects. In this paper we present a proposal to realize an optical lattice with stable fractionalized topological objects. In particular, we generate the fractionalized topological fluxes and fractionalized skyrmions on two-dimensional optical lattices and fractionalized monopoles on three-dimensional optical lattices. These results offer a new approach to study the quantum many-body systems on optical lattices of ultracold quantum gases with controllable topological defects, including dislocations, topological fluxes and monopoles.

Many complex systems generate multifractal time series which are long-range cross-correlated. Numerous methods have been proposed to characterize the multifractal nature of these long-range cross correlations. However, several important issues about these methods are not well understood and most methods consider only one moment order. We study the joint multifractal analysis based on partition function with two moment orders, which was initially invented to investigate fluid fields, and derive analytically several important properties. We apply the method numerically to binomial measures with multifractal cross correlations and bivariate fractional Brownian motions without multifractal cross correlations. For binomial multifractal measures, the explicit expressions of mass function, singularity strength and multifractal spectrum of the cross correlations are derived, which agree excellently with the numerical results. We also apply the method to stock market indexes and unveil intriguing multifractality in the cross correlations of index volatilities.

We introduce a scheme that combines photon-assisted tunneling (PAT) by a moving optical lattice with strong Hubbard interactions, and allows for the quantum simulation of paradigmatic quantum many-body models. We show that, in a certain regime, this quantum simulator yields an effective Hubbard Hamiltonian with tunable bond–charge interactions, a model studied in the context of strongly-correlated electrons. In a different regime, we show how to exploit a correlated destruction of tunneling to explore Nagaoka ferromagnetism at finite Hubbard repulsion. By changing the photon-assisted tunneling parameters, we can also obtain a t-J model with independently controllable tunneling t, super-exchange interaction J, and even a Heisenberg–Ising anisotropy. Hence, the full phase diagram of this paradigmatic model becomes accessible to cold-atom experiments, departing from the region $t\gg J$ allowed by standard single-band Hubbard Hamiltonians in the strong-repulsion limit. We finally show that, by generalizing the PAT scheme, the quantum simulator yields models of dynamical Gauge fields, where atoms of a given electronic state dress the tunneling of the atoms with a different internal state, leading to Peierls phases that mimic a dynamical magnetic field.

We report on orbital angular momentum exchange between sound and matter mediated by a non-dissipative chiral scattering process. An experimental demonstration is made possible by irradiating a three-dimensional printed, spiral-shaped chiral object with an incident ultrasonic beam carrying zero orbital angular momentum. Chiral refraction is shown to impart a nonzero orbital angular momentum to the scattered field and to rotate the object. This result constitutes a proof of concept of a novel kind of acoustic angular manipulation of matter.

Unconventional strongly correlated phases of the repulsive Fermi–Hubbard model, which could be emulated by ultracold vapors loaded in optical lattices, are investigated by means of energy minimizations with quantum number projection before variation and without any assumed order parameter. Using a tube-like geometry of optical plaquettes to realize the four-leg ladder Hubbard Hamiltonian, we highlight the intertwining of spin-, charge-, and pair-density waves embedded in a uniform d-wave superfluid background. As the lattice filling increases, this phase emerges from homogenous states exhibiting spiral magnetism and evolves towards a doped antiferromagnet. A concomitant enhancement of long-ranged d-wave pairing correlations is also found. Numerical tests of the approach for two-dimensional clusters are carried out, too.

We show that the excitation of long-range Rydberg molecules in a three-dimensional optical lattice can be used as a position- and time-sensitive probe for doubly occupied sites in the system. To this end, we detect the ions which are continuously generated by the decay of the formed Rydberg molecules. While a superfluid gas shows molecule formation for all parameters, a Mott insulator with n = 1 filling reveals a strong suppression of the number of formed molecules. In the limit of weak probing, the technique can be used to probe the superfluid to Mott-insulator transition in real-time. Our method can be extended to higher fillings and has various applications for the real-time diagnosis and manipulation of ultracold lattice gases.

Fast entangling gates for trapped ion pairs offer vastly improved gate operation times relative to implemented gates, as well as approaches to trap scaling. Gates on a neighbouring ion pair only involve local ions when performed sufficiently fast, and we find that even a fast gate between a pair of distant ions with few degrees of freedom restores all the motional modes given more stringent gate speed conditions. We compare pulsed fast gate schemes, defined by a timescale faster than the trap period, and find that our proposed scheme has less stringent requirements on laser repetition rate for achieving arbitrary gate time targets and infidelities well below 10⁻⁴. By extending gate schemes to ion crystals, we explore the effect of ion number on gate fidelity for coupling two neighbouring ions in large crystals. Inter-ion distance determines the gate time, and a factor of five increase in repetition rate, or correspondingly the laser power, reduces the infidelity by almost two orders of magnitude. We also apply our fast gate scheme to entangle the first and last ions in a crystal. As the number of ions in the crystal increases, significant increases in the laser power are required to provide the short gate times corresponding to fidelity above 0.99.

Raman backscattering (RBS) in plasma is the basis of plasma-based amplifiers and is important in laser-driven fusion experiments. We show that saturation can arise from nonlinearities due to coupling between the fundamental and harmonic plasma wave modes for sufficiently intense pump and seed pulses. We present a time-dependent analysis that shows that plasma wave phase shifts reach a maximum close to wavebreaking. The study contributes to a new understanding of RBS saturation for counter-propagating laser pulses.

Entanglement is one of the most striking features of quantum mechanics, and yet it is not specifically quantum. More specific to quantum mechanics is the connection between entanglement and thermodynamics, which leads to an identification between entropies and measures of pure state entanglement. Here we search for the roots of this connection, investigating the relation between entanglement and thermodynamics in the framework of general probabilistic theories. We first address the question whether an entangled state can be transformed into another by means of local operations and classical communication. Under two operational requirements, we prove a general version of the Lo–Popescu theorem, which lies at the foundations of the theory of pure-state entanglement. We then consider a resource theory of purity where free operations are random reversible transformations, modelling the scenario where an agent has limited control over the dynamics of a closed system. Our key result is a duality between the resource theory of entanglement and the resource theory of purity, valid for every physical theory where all processes arise from pure states and reversible interactions at the fundamental level. As an application of the main result, we establish a one-to-one correspondence between entropies and measures of pure bipartite entanglement. The correspondence is then used to define entanglement measures in the general probabilistic framework. Finally, we show a duality between the task of information erasure and the task of entanglement generation, whereby the existence of entropy sinks (systems that can absorb arbitrary amounts of information) becomes equivalent to the existence of entanglement sources (correlated systems from which arbitrary amounts of entanglement can be extracted).

The Ising exchange interaction is a limiting case of strong exchange anisotropy and represents a key property of many magnetic materials. Here we find the necessary and sufficient conditions to achieve Ising exchange interaction for metal sites with unquenched orbital moments. Contrary to current views, the rules established here narrow much the range of lanthanide and actinide ions that can exhibit Ising exchange interaction. It is shown that the Ising interaction can be of two types: (i) coaxial, with magnetic moments directed along the anisotropy axes on the metal sites and (ii) non-coaxial, with arbitrary orientation of one of the magnetic moments. These findings will contribute to purposeful design of lanthanide- and actinide-based materials.

We investigate anomalous damping of the monopole mode of a non-degenerate 3D Bose gas under isotropic harmonic confinement as recently reported by the JILA TOP trap experiment (Lobser et al in preparation). Given a realistic confining potential, we develop a model for studying collective modes that includes the effects of anharmonic corrections to a harmonic potential. By studying the influence of these trap anharmonicities throughout a range of temperatures and collisional regimes, we find that the damping is caused by the joint mechanisms of dephasing and collisional relaxation. Furthermore, the model is complimented by Monte Carlo simulations which are in fair agreement with data from the JILA experiment.

We report electron counting experiments in a silicon metal-oxide-semiconductor quantum dot architecture which has been previously demonstrated to generate a quantized current in excess of 80 pA with uncertainty below 30 parts per million. Single-shot detection of electrons pumped into a reservoir dot is performed using a capacitively coupled single-electron transistor. We extract the full probability distribution of the transfer of n electrons per pumping cycle for $n=0,1,2,3,\mathrm{and}\;4.$ We find that the probabilities extracted from the counting experiment are in agreement with direct current measurements in a broad range of dc electrochemical potentials of the pump. The electron counting technique is also used to confirm the improving robustness of the pumping mechanism with increasing electrostatic confinement of the quantum dot.

We theoretically investigate the role of dissipation in excited state quantum phase transitions (ESQPT) within the Lipkin–Meshkov–Glick model. Signatures of the ESQPT are directly visible in the complex spectrum of an effective Hamiltonian, whereas they get smeared out in the time-dependence of system observables. In the latter case, we show how delayed feedback control can be used to restore the visibility of the ESQPT signals.

We study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.

Ion acceleration driven by the interaction of an ultraintense (2 × 10²⁰ W cm⁻²) laser pulse with an ultrathin ($\leqslant 40$ nm) foil target is experimentally and numerically investigated. Protons accelerated by sheath fields and via laser radiation pressure are angularly separated and identified based on their directionality and signature features (e.g. transverse instabilities) in the measured spatial-intensity distribution. A low divergence, high energy proton component is also detected when the heated target electrons expand and the target becomes relativistically transparent during the interaction. 2D and 3D particle-in-cell simulations indicate that under these conditions a plasma jet is formed at the target rear, supported by a self-generated azimuthal magnetic field, which extends into the expanded layer of sheath-accelerated protons. Electrons trapped within this jet are directly accelerated to super-thermal energies by the portion of the laser pulse transmitted through the target. The resulting streaming of the electrons into the ion layers enhances the energy of protons in the vicinity of the jet. Through the addition of a controlled prepulse, the maximum energy of these protons is demonstrated experimentally and numerically to be sensitive to the picosecond rising edge profile of the laser pulse.

In classical optics the Wolf function is the natural analogue of the quantum Wigner function and like the latter it may be negative in some regions. We discuss the implications this negativity has on the generalized ray interpretation of free-space paraxial wave evolution. Important examples include two classes of beams carrying optical orbital angular momentum—Laguerre–Gaussian (LG) and Bessel beams. We formulate their defining eigenfunction properties as phase–space symmetries of their Wolf functions, whose analytical form is shown, and discuss their interpretation in the ray picture. By moving to a more general picture of partly coherent fields, we find that new solutions displaying the same symmetries appear. In particular, we find that mixtures of Gaussian beams (thus fully describable using classical ray optics) can mimic the basic properties of LG beams without the need for negativity, and are not restricted to quantized values of angular momentum. The quantization of both the l and p parameters and negativity of the Wolf function are both inevitable and, indeed, arise naturally when a requirement on the purity of the solution is added. This work is supplemented by a set of computer animations, graphically illustrating the interpretative aspects of the described model.

Microbial colonies are experimental model systems for studying the colonization of new territory by biological species through range expansion. We study a generalization of the two-species Eden model, which incorporates local frequency-dependent selection, in order to analyze how social interactions between two species influence surface roughness of growing microbial colonies. The model includes several classical scenarios from game theory. We then concentrate on an expanding public goods game, where either cooperators or defectors take over the front depending on the system parameters. We analyze in detail the critical behavior of the nonequilibrium phase transition between global cooperation and defection and thereby identify a new universality class of phase transitions dealing with absorbing states. At the transition, the number of boundaries separating sectors decays with a novel power law in time and their superdiffusive motion crosses over from Eden scaling to a nearly ballistic regime. In parallel, the width of the front initially obeys Eden roughening and, at later times, passes over to selective roughening.

We theoretically explore the scattering properties in collisions between ${}^{6}{\rm{Li}}$ in the ground state and fermionic ${}^{171}{\rm{Yb}}$ in the metastable ${}^{3}{{\rm{P}}}_{2}$ state. Unlike the role of the electronic spin degree of freedom in collisions of two alkali-metal atoms, the orbital degrees of freedom from the p electron of the metastable atoms, which induce not only mixings between different partial waves but also couplings between channels in different fine-structure manifolds, introduce the anisotropic interactions for producing broad-enough Feshbach resonances. Our closed-coupling calculation shows that this mechanism is similar to that in highly magnetic atoms, but the resonances also suffer from a large inelastic rate at the magnitude of ${10}^{-10\;}{\mathrm{cm}}^{3}\;{{\rm{s}}}^{-1}$ to lower-lying fine-structure states or Zeeman sublevels, which will be an obstacle for associating ultracold polar molecules with both electron spin and electric dipole momentum. However, such a rich mixture of inelastic processes, and the experimental advantage of immunity from intraspecies inelastic collisions for metastable fermionic ${}^{171}{\rm{Yb}},$ allow for precise investigations on the interspecies scattering properties for systems with excited-state atoms included.

We investigate a general scheme for generating, either dynamically or in the steady state, continuous variable entanglement between two mechanical resonators with different frequencies. We employ an optomechanical system in which a single optical cavity mode driven by a suitably chosen two-tone field is coupled to the two resonators. Significantly large mechanical entanglement can be achieved, which is extremely robust with respect to temperature.

We have demonstrated efficient injection and trapping of a cold positron beam in a dipole magnetic field configuration. The intense 5 eV positron beam was provided by the NEutron induced POsitron source MUniCh facility at the Heinz Maier-Leibnitz Zentrum, and transported into the confinement region of the dipole field trap generated by a supported, permanent magnet with 0.6 T strength at the pole faces. We achieved transport into the region of field lines that do not intersect the outer wall using the ${\bf{E}}\times {\bf{B}}$ drift of the positron beam between a pair of tailored plates that created the electric field. We present evidence that up to 38% of the beam particles are able to reach the intended confinement region and make at least a 180° rotation around the magnet where they annihilate on an insertable target. When the target is removed and the ${\bf{E}}\times {\bf{B}}$ plate voltages are switched off, confinement of a small population persists for on the order of 1 ms. These results lend optimism to our larger aims to apply a magnetic dipole field configuration for trapping of both positrons and electrons in order to test predictions of the unique properties of a pair plasma.

We explore the possibility of exciting spin waves in insulating antiferromagnetic films by injecting spin current at the surface. We analyze both magnetically compensated and uncompensated interfaces. We find that the spin current induced spin-transfer torque can excite spin waves in insulating antiferromagnetic materials and that the chirality of the excited spin wave is determined by the polarization of the injected spin current. Furthermore, the presence of magnetic surface anisotropy can greatly increase the accessibility of these excitations.

We consider a two-component interacting bosonic condensate with dominating intra-species repulsive density–density interactions. We study the phase diagram of the system at finite temperature with rotation, using large-scale Monte Carlo simulations of a two-component Ginzburg–Landau model of the system. In the presence of rotation, the system features a competition between long-range vortex–vortex interactions and short-range density–density interactions. This leads to a rotation-driven 'mixing' phase transition in a spatially inhomogeneous state with a broken ${\rm{U}}(1)$ symmetry. Thermal fluctuations in this state lead to nematic two-component sheets of vortex liquids. At sufficiently strong inter-component interaction, we find that the superfluid and ${{\mathbb{Z}}}_{2}$ phase transitions split. This results in the formation of an intermediate state which breaks only ${{\mathbb{Z}}}_{2}$ symmetry. It represents two phase separated normal fluids with a difference in their densities.

We propose a protocol for countering the effects of dephasing in quantum state transfer over a noisy spin channel weakly coupled to the sender and receiver qubits. Our protocol, based on performing regular global measurements on the channel, significantly suppresses the nocuous environmental effects and offers much higher fidelities than the traditional no-measurement approach. Our proposal can also operate as a robust two-qubit entangling gate over distant spins. Our scheme counters any source of dephasing, including those for which the well established dynamical decoupling approach fails. Our protocol is probabilistic, given the intrinsic randomness in quantum measurements, but its success probability can be maximized by adequately tuning the rate of the measurements.

We explore tomographic mask-based Fourier transform x-ray holography with respect to the use of a thin slit as a reference wave source. This imaging technique exclusively uses the interference between the waves scattered by the object and the slit simplifying the experimental realization and ensuring high data quality. Furthermore, we introduce a second reference slit to rotate the sample around a second axis and to record a dual-axes tomogram. Compared to a single-axis tomogram, the reconstruction artifacts are decreased in accordance with the reduced missing data wedge. Two demonstration experiments are performed where test structures are imaged with a lateral resolution below 100 nm.

The formation of image-potential states at the interface between a graphene layer and a metal surface is studied by means of model calculations. An analytical one-dimensional model-potential for the combined system is constructed and used to calculate energies and wave functions of the image-potential states at the $\bar{{\rm{\Gamma }}}$-point as a function of the graphene–metal distance. It is demonstrated how the double series of image-potential states of freestanding graphene evolves into interfacial states that interact with both surfaces at intermediate distances, and finally into a single series of states resembling those of a clean metal surface covered by a monoatomic spacer layer. The model quantitatively reproduces experimental data available for graphene/Ir(111) and graphene/Ru(0001), systems which strongly differ in interaction strength and therefore adsorption distance. Moreover, it provides a clear physical explanation for the different binding energies and lifetimes of the first (n = 1) image-potential state in the valley and hill areas of the strongly corrugated moiré superlattice of graphene/Ru(0001).

We report a comprehensive approach to analysing continuous-output photon detectors. We employ principal component analysis to maximize the information extracted from output signals, followed by a novel noise-tolerant parameterized approach to the tomography of photon-number resolving detectors. We further propose a measure for rigorously quantifying a detector's photon-number-resolving capability. Our approach applies to all detectors with continuous-output signals. We illustrate our methods by applying them to experimental data obtained from a transition-edge sensor detector.

We consider a spin-1/2 fermionic ladder with spin–orbit coupling and a perpendicular magnetic field, which shares important similarities with topological superconducting wires. We fully characterize the symmetry-protected topological phase of this ladder through the identification of fractionalized edge modes and non-trivial spin winding numbers. We propose an experimental scheme to engineer such a ladder system with cold atoms in optical lattices, and we present two protocols that can be used to extract the topological signatures from density and momentum-distribution measurements. We then consider the presence of interactions and discuss the effects of a contact on-site repulsion on the topological phase. We find that such interactions could enhance the extension of the topological phase in certain parameters regimes.

The increasing transmission capacity needs in a future energy system raise the question of how associated costs should be allocated to the users of a strengthened power grid. In contrast to straightforward oversimplified methods, a flow tracing based approach provides a fair and consistent nodal usage and thus cost assignment of transmission investments. This technique follows the power flow through the network and assigns the link capacity usage to the respective sources or sinks using a diffusion-like process, thus taking into account the underlying network structure and injection pattern. As a showcase, we apply power flow tracing to a simplified model of the European electricity grid with a high share of renewable wind and solar power generation, based on long-term weather and load data with an hourly temporal resolution.

We consider a sonic black-hole scenario where an atom condensate flows through a subsonic–supersonic interface. We discuss several criteria that reveal the existence of non-classical correlations resulting from the quantum character of the spontaneous Hawking radiation (HR). We unify previous general work as applied to HR analogs. We investigate the measurability of the various indicators and conclude that, within a class of detection schemes, only the violation of quadratic Cauchy–Schwarz inequalities can be discerned. We show numerical results that further support the viability of measuring deep quantum correlations in concrete scenarios.

We investigate wave transport properties of parity–time (PT) symmetric lattices that are periodically modulated along the direction of propagation. We demonstrate that in the regime of unbroken PT-symmetry, the system Floquet–Bloch modes may interfere constructively leading to either controlled oscillations or power absorption and unlimited amplification occurring exactly at the phase-transition point. The differential power response is affected by the overlap of the gain and loss system distribution with wave intensity pattern that is formed through Rabi oscillations engaging the coupled Floquet–Bloch modes.

In this article, we show that eigenenergies and eigenstates of a system consisting of four one-dimensional hard-core particles with masses 6m, 2m, m, and 3m in a hard-wall box can be found exactly using Bethe Ansatz. The Ansatz is based on the exceptional affine reflection group ${\tilde{F}}_{4}$ associated with the symmetries and tiling properties of an octacube—a Platonic solid unique to four-dimensions, with no three-dimensional analogues. We also uncover the Liouville integrability structure of our problem: the four integrals of motion in involution are identified as invariant polynomials of the finite reflection group F₄, taken as functions of the components of momenta.

We numerically generate and then study the basic properties of dark soliton-like excitations in a dipolar gas confined in a quasi one-dimensional trap. These excitations, although very similar to dark solitons in a gas with contact interaction, interact with each other and can form bound states. During collisions these dipolar solitons emit phonons, losing energy but accelerating. Even after thousands of subsequent collisions they survive as gray solitons and finally reach dynamical equilibrium with background quasiparticles. Finally, in the frame of classical field approximation, we verified, that these solitons appear spontaneously in thermal samples, analogously to the type II excitations in a gas of atoms with contact interaction.