Table of contents

When Rydberg states are excited in a dense atomic gas, the mean number of excited atoms reaches a stationary value after an initial transient period. We shed light on the origin of this steady state that emerges from a purely coherent evolution of a closed system. To this end, we consider a one-dimensional ring lattice and employ the perfect blockade model, i.e. the simultaneous excitation of Rydberg atoms occupying neighboring sites is forbidden. We derive an equation of motion that governs the system's evolution in excitation number space. This equation possesses a steady state that is strongly localized. Our findings show that this state is, to good accuracy, given by the density matrix of the microcanonical ensemble where the corresponding microstates are the zero-energy eigenstates of the interaction Hamiltonian. We analyze the statistics of the Rydberg atom number count, providing expressions for the number of excited Rydberg atoms and the Mandel Q-parameter in equilibrium.

We address the problem of interference using the Heisenberg picture and highlight some new aspects through the use of pre-selection, post-selection, weak measurements and modular variables. We present a physical explanation for the different behaviors of a single particle when the distant slit is open or closed; instead of having a quantum wave that passes through all slits, we have a localized particle with non-local interactions with the other slit(s). We introduce a Gedanken experiment to measure this non-local exchange. While the Heisenberg and Schrödinger pictures are equivalent formulations of quantum mechanics, nevertheless, the results discussed here support a new approach to quantum mechanics which has lead to new insights, new intuitions, new experiments and even the possibility of new devices that were missing from the old perspective.

The finite-temperature Casimir effect for a scalar field in the bulk region of two Randall–Sundrum models, RSI and RSII, is studied. We calculate the Casimir energy and the Casimir force for two parallel plates with separation a on the visible brane in the RSI model. High-temperature and low-temperature cases are covered. Attraction versus repulsion of the temperature correction to the force is discussed in the special cases of Dirichlet–Dirichlet, Neumann–Neumann and Dirichlet–Neumann boundary conditions at low temperature. The Abel–Plana summation formula is used, as this is found to be the most convenient. Some comments are made on the related contemporary literature.

In this paper transport through nanochannels is assessed, both of liquids and of dissolved molecules or ions. First, we review principles of transport at the nanoscale, which will involve the identification of important length scales where transitions in behavior occur. We also present several important consequences that a high surface-to-volume ratio has for transport. We review liquid slip, chemical equilibria between solution and wall molecules, molecular adsorption to the channel walls and wall surface roughness. We also identify recent developments and trends in the field of nanofluidics, mention key differences with microfluidic transport and review applications. Novel opportunities are emphasized, made possible by the unique behavior of liquids at the nanoscale.

The effluent of a microscale atmospheric pressure plasma jet (μ-APPJ) operated in helium with a small admixture of molecular oxygen (<1.6%) has been analyzed by means of two independent diagnostics, quantitative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7×10¹⁵ cm^-3 could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4×10¹⁵ cm^-3 and an O₂ depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O₂ admixture of 0.6%, whereas the ozone density continues to increase toward higher O₂ admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the μ-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.

The use of synchrotron-based spectroscopy has revolutionized the way we look at matter. X-ray absorption spectroscopy (XAS) using linear and circular polarized light offers a powerful toolbox of element-specific structural, electronic and magnetic probes that is especially well suited for complex materials containing several elements. We use the specific example of Zn_1−xCo_xO (Co:ZnO) to demonstrate the usefulness of combining these XAS techniques to unravel its intrinsic properties. We demonstrate that as long as phase separation or excessive defect formation is absent, Co:ZnO is paramagnetic. We can establish quantitative thresholds based on four reliable quality indicators using XAS; samples that show ferromagnet-like behaviour fail to meet these quality indicators, and complementary experimental techniques indeed prove phase separation. Careful analysis of XAS spectra is shown to provide quantitative information on the presence and type of dilute secondary phases in a highly sensitive, non-destructive manner.

We propose a link between logical independence and quantum physics. We demonstrate that quantum systems in the eigenstates of Pauli group operators are capable of encoding mathematical axioms and show that Pauli group quantum measurements are capable of revealing whether or not a given proposition is logically dependent on the axiomatic system. Whenever a mathematical proposition is logically independent of the axioms encoded in the measured state, the measurement associated with the proposition gives random outcomes. This allows for an experimental test of logical independence. Conversely, it also allows for an explanation of the probabilities of random outcomes observed in Pauli group measurements from logical independence without invoking quantum theory. The axiomatic systems we study can be completed and are therefore not subject to Gödel's incompleteness theorem.

Coherent control employing a broadband excitation is applied to a branching reaction in the excited state. In a weak field for an isolated molecule, a control objective is only frequency dependent. This means that phase control of the pulse cannot improve the objective beyond the best frequency selection. Once the molecule is put into a dissipative environment a new timescale emerges. In this study, we demonstrate that the dissipation allows us to achieve coherent control of branching ratios in the excited state. The model studied contains a nuclear coordinate and three electronic states: the ground and two coupled diabatic excited states. The influence of the environment is modeled by the stochastic surrogate Hamiltonian. The excitation is generated by a Gaussian pulse where the phase control introduced a chirp to the pulse. For sufficient relaxation, we find significant control in the weak field depending on the chirp rate. The observed control is rationalized by a timing argument caused by a focused wavepacket. The initial non-adiabatic crossing is enhanced by the chirp. This is followed by energy relaxation which stabilizes the state by having an energy lower than the crossing point.

The Hamiltonian control of n qubits requires precision control of both the strength and timing of interactions. Compensation pulses relax the precision requirements by reducing unknown but systematic errors. Using composite pulse techniques designed for single qubits, we show that systematic errors for n-qubit systems can be corrected to arbitrary accuracy given either two non-commuting control Hamiltonians with identical systematic errors or one error-free control Hamiltonian. We also examine composite pulses in the context of quantum computers controlled by two-qubit interactions. For quantum computers based on the XY interaction, single-qubit composite pulse sequences naturally correct systematic errors. For quantum computers based on the Heisenberg or exchange interaction, the composite pulse sequences reduce the logical single-qubit gate errors but increase the errors for logical two-qubit gates.

We show that the correlations in stochastic outputs of time-distributed weak measurements can be used to study the dynamics of an individual quantum object, with a proof-of-principle setup based on small Faraday rotation caused by a single spin in a quantum dot. In particular, the third-order correlation can reveal the 'true' spin decoherence, which would otherwise be concealed by the inhomogeneous broadening effect in the second-order correlations. The viability of such approaches lies in the fact that (i) in weak measurement the state collapse that would disturb the system dynamics occurs at a very low probability and (ii) a shot of measurement projecting the quantum object to a known basis state serves as a starter or stopper of the evolution without pumping or coherently controlling the system as otherwise required in conventional spin echo.

An early van der Waals density functional (vdW-DF) described layered systems (such as graphite and graphene dimers) using a layer-averaged electron density in the evaluation of nonlocal correlations. This early vdW-DF version was also adapted to approximate the binding of polycyclic aromatic hydrocarbons (PAHs) (Chakarova S D and Schröder E 2005 J. Chem. Phys.122 054102). In parallel to that PAH study, a new vdW-DF version (Dion M, Rydberg H, Schröder E, Langreth D C and Lundqvist B I 2004 Phys. Rev. Lett. 92 246401) was developed that provides accounts of nonlocal correlations for systems of general geometry. We apply here the latter vdW-DF version to aromatic dimers of benzene, naphthalene, anthracene and pyrene, stacked in sandwich (AA) structure, and the slipped-parallel (AB) naphthalene dimer. We further compare the results of the two methods as well as other theoretical results obtained by quantum-chemistry methods. We also compare calculations for two interacting graphene sheets in the AA and the AB structures and provide the corresponding graphene-from-graphite exfoliation energies. Finally, we present an overview of the scaling of the molecular–dimer interaction with the number of carbon atoms and with the number of carbon rings.

The intrinsic spin-Hall effects (SHEs) in p-doped semiconductors (Murakami et alScience301 1348) and two-dimensional electron gases with Rashba spin–orbit coupling (Sinova et al 2004 Phys. Rev. Lett.92 126603) have been the subject of many theoretical studies, but their driving mechanisms have yet to be described in a unified manner. The former effect arises from the adiabatic topological curvature of momentum space, from which holes acquire a spin-dependent anomalous velocity. The SHE in Rashba systems, on the other hand, results from momentum-dependent spin dynamics in the presence of an external electric field. Our motivation in this paper is to address the disparity between the two mechanisms and, in particular, to clarify whether there is any underlying link between the two effects. In this endeavor, we consider the explicit time dependence of SHE systems starting with a general spin–orbit model in the presence of an electric field. We find that by performing a gauge transformation of the general model with respect to time, a well-defined gauge field appears in time space which has the physical significance of an effective magnetic field. This magnetic field is shown to precisely account for the SHE in the Rashba system in the adiabatic limit. Remarkably, by applying the same limit to the equations of motion of the general model, this magnetic field is also found to be the underlying origin of the anomalous velocity due to the momentum-space curvature. Thus, our study unifies the two seemingly disparate intrinsic SHEs under a common adiabatic framework.

The problem of quantifying the difference between evolutions of an open quantum system (in particular, between the actual evolution of an open system and the ideal target operation on the corresponding closed system) is important in quantum control, especially in control of quantum information processing. Motivated by this problem, we develop a measure for evaluating the distance between unitary evolution operators of a composite quantum system that consists of a sub-system of interest (e.g. a quantum information processor) and environment. The main characteristic of this measure is the invariance with respect to the effect of the evolution operator on the environment, which follows from an equivalence relation that exists between unitary operators acting on the composite system, when the effect on only the sub-system of interest is considered. The invariance to the environment's transformation makes it possible to quantitatively compare the evolution of an open quantum system and its closed counterpart. The distance measure also determines the fidelity bounds of a general quantum channel (a completely positive and trace-preserving map acting on the sub-system of interest) with respect to a unitary target transformation. This measure is also independent of the initial state of the system and straightforward to numerically calculate. As an example, the measure is used in numerical simulations to evaluate fidelities of optimally controlled quantum gate operations (for one- and two-qubit systems), in the presence of a decohering environment. This example illustrates the utility of this measure for optimal control of quantum operations in the realistic case of open-system dynamics.

In this paper, we formulate an analytical theory that quantifies the first-order effect of a small random uncontrollable disorder that is due to limitations in the realization of periodic arrays of plasmonic nanoparticles. In particular, we show how the effect of a small disorder may be quantitatively taken into account when evaluating the guidance properties of these otherwise periodic chains, and how the main effect of the small disorder consists of additional radiation losses for the guided mode. Similar quantitative analyses may be extended to the general class of periodic metamaterials, providing an idea of how disorder affects their electromagnetic response, and which types of disorder have the most effect.

We present space- and time-resolved simulations and measurements of single-cycle terahertz (THz) waves propagating through two-dimensional (2D) photonic crystal structures embedded in a slab waveguide. Specifically, we use a plane wave expansion technique to calculate the band structure and a time-dependent finite-element method to simulate the temporal evolution of the THz waves. Experimentally, we measure the space–time evolution of the THz waves through a coherent time-resolved imaging method. Three different structures are laser machined in LiNbO₃ crystal slabs and analyzing the transmitted as well as the reflected THz waveforms allows determination of the bandgaps. Comparing the results with the calculated band diagrams and the time-dependent simulations shows that the experiments are consistent with 3D simulations, which include the slab waveguide geometry, the birefringence of the material, and a careful analysis of the excited modes within the band diagrams.

We derive an exact (classical and quantum) expression for the entropy production of a finite system placed in contact with one or several finite reservoirs, each of which is initially described by a canonical equilibrium distribution. Although the total entropy of system plus reservoirs is conserved, we show that system entropy production is always positive and is a direct measure of system–reservoir correlations and/or entanglements. Using an exactly solvable quantum model, we illustrate our novel interpretation of the Second Law in a microscopically reversible finite-size setting, with strong coupling between the system and the reservoirs. With this model, we also explicitly show the approach of our exact formulation to the standard description of irreversibility in the limit of a large reservoir.

The dynamics of neutral modes for fractional quantum Hall states is investigated for a quantum point contact geometry in the weak-backscattering regime. The effective field theory introduced by Fradkin–Lopez for edge states in the Jain sequence is generalized to the case of propagating neutral modes. The dominant tunnelling processes are identified also in the presence of non-universal phenomena induced by interactions. The crossover regime in the backscattering current between tunnelling of single-quasiparticles and of agglomerates of p-quasiparticles is analysed. We demonstrate that higher-order cumulants of the backscattering current fluctuations are a unique resource to study quantitatively the competition between different carrier charges. We find that propagating neutral modes are a necessary ingredient in order to explain this crossover phenomenon.

Electron acceleration, in vacuum, mediated by an intense echelon phase-modulated long Gaussian laser pulse, is investigated here. Theoretical and numerical analyses show that this pulse can accelerate an electron to much higher energies than an unmodulated pulse. The staircase-like phase structure of the laser field encourages electron trapping in the favorable wave phase, greatly increasing the effective acceleration distance. Also, the electron motion is now more in the laser propagation direction. The conditions for efficient acceleration are obtained. Since the slower electrons can also be efficiently accelerated to high energies and the accelerated electrons are more axially oriented, the average energy gain by an initially Maxwellian electron bunch can be more than 13 times that obtained without the echelon phase modulation. This is at the expense of somewhat increased spatial and thermal spread, and the accelerated bunch is also more collimated.

We study the collective behavior of non-equilibrium systems subjected to an external field with a dynamics characterized by the existence of non-interacting states. Aiming at exploring the generality of the results, we consider two types of model according to the nature of their state variables: (i) a vector model, where interactions are proportional to the overlap between the states, and (ii) a scalar model, where interactions depend on the distance between states. The phase space is numerically characterized for each model in a fully connected network and in random and scale-free networks. For both models, the system displays three phases: two ordered phases, one parallel to the field and another orthogonal to the field, and one disordered phase. By placing the particles on a small-world network, we show that an ordered phase in a state different from the one imposed by the field is possible because of the long-range interactions that exist in fully connected, random and scale-free networks. This phase does not exist in a regular lattice and emerges when long-range interactions are included in a small-world network.

We present results on the occurrence of ultrashort terahertz harmonic generation (THG) driven by a millimeter nonlinear chirped few-cycle laser pulse in a symmetric double quantum well. By solving the effective nonlinear Bloch equations, THG with a generic plateau and cutoff can be produced. The time–frequency characteristic of the ultrashort terahertz harmonic spectrum is analyzed in detail by means of the wavelet transform of induced dipole acceleration. Furthermore, an ultrabroad supercontinuum terahertz harmonic spectrum can be generated and an isolated ultrashort terahertz pulse can be obtained at the cutoff region by choosing the appropriate chirping rate parameters.

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experiment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolution in both domains simultaneously. We show that resonant absorption is important in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660–94).

Recent experimental advances have allowed electronic band structures to be investigated by angle-resolved photoemission in considerably more detail. A recent study of ferromagnetic iron finds the occupied bandwidth, for the two shallow bands observed, reduced by ∼30% as compared to the calculated ground state, rendering bcc iron comparable with the strongly correlated transition metal Ni. Fermi velocities were reported to deviate from the ground state and these deviations have been assigned entirely to electron correlation. We show that spin–orbit splitting, final-state transitions and final-state broadening significantly change the band dispersion as measured by a modern energy analyzer, and a simple model that accounts for their effects is introduced. Applying our model, we find for the occupied bandwidth a narrowing of the order of only 10% in agreement with the literature. Substantial renormalization of the Fermi velocities is confirmed but a significantly smaller fraction of it is attributed to correlation effects, namely many-electron interactions.

The influence of electron spin on the nonlinear propagation of whistler waves is studied in this paper. For this purpose, a recently developed electron two-fluid model, where the spin-up and spin-down populations are treated as different fluids, is adapted to the electron magnetohydrodynamic (MHD) regime. A nonlinear Schrödinger equation is then derived for the whistler waves and the coefficients of nonlinearity with and without spin effects are compared. The relative importance of spin effects depends on the plasma density and temperature as well as the external magnetic field strength and wave frequency. The significance of our results for various plasmas is discussed.

We investigate the influence of hole shape on the group delay of femtosecond laser pulses propagating through arrays of rectangular subwavelength holes in metal films. We find a pronounced dependence of the group delay on the aspect ratio of the holes in the arrays. The maximum group delay occurs near the cut-off frequency of the holes. These experimental results are found to be in good agreement with calculations. The slow propagation of light through the array gives rise to enhancement of the second harmonic generated in the structures. The observed behavior is consistent with the presence of a resonance at the cut-off frequency of the rectangular holes.

This paper presents the first study of the collision dynamics of an ultra-cold spin-polarized mixture of rubidium and metastable helium (He^*) atoms. Our experiment monitors ion production from the mixture for both magnetically polarized and unpolarized cases. In the unpolarized case, we observe an increase in our background ion rate. However, in the completely polarized sample the ion production is below the sensitivity of our experiment. Nonetheless, we determine an upper limit of 5×10⁻¹² cm³ s⁻¹ for the polarized rate constant (β_Rb-He^*), which is two orders of magnitude below the unpolarized rate constant. Such a suppression of the He^*–⁸⁷Rb polarized rate was not apparent a priori and opens the intriguing possibility of creating a dual Bose–Einstein condensate comprising an alkali ground-state atom and an excited-state noble-gas atom.

As primitives for entanglement generation, controlled phase gates have a central role in quantum computing. Especially in ideas realizing instances of quantum computation in linear optical gate arrays, a closer look can be rewarding. In such architectures, all effective nonlinearities are induced by measurements. Hence the probability of success is a crucial parameter of such quantum gates. In this paper, we discuss this question for controlled phase gates that implement an arbitrary phase with one and two control qubits. Within the class of post-selected gates in dual-rail encoding with vacuum ancillas, we identify the optimal success probabilities. We construct networks that allow for implementation using current experimental capabilities in detail. The methods employed here appear specifically useful with the advent of integrated linear optical circuits, providing stable interferometers on monolithic structures.

We analyze the effective Hamiltonian arising from a suitable power series expansion of the overlap integrals of Wannier functions for confined bosonic atoms in a one-dimensional (1D) optical lattice. For certain constraints between the coupling constants, we construct an explicit relationship between such an effective bosonic Hamiltonian and the integrable spin-S anisotropic Heisenberg model. The former results are therefore integrable by construction. The field theory is governed by an anisotropic nonlinear σ-model with singlet and triplet massive excitations; this result holds also in the generic non-integrable cases. The criticality of the bosonic system is investigated. The schematic phase diagram is drawn. Our study sheds light on the hidden symmetry of the Haldane type for 1D bosons.

We investigated the magnetoresistance of Permalloy (Ni₈₀Fe₂₀) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al₂O₃, MgO and SiO₂ substrates. Growth and transport measurements were carried out at 80 K in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties were ascertained during growth. With increasing thickness the films exhibited a gradual transition from tunnelling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains, as well as the film's transition from superparamagnetic to ferromagnetic behaviour. Using an analysis based on a theoretical model of island growth, we found that the observed evolution of the magnetoresistance in the tunnelling regime originated from changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunnelling magnetoresistance and anisotropic magnetoresistance were found. We attributed this to an increasingly pronounced island growth, and to a slower percolation process of Permalloy when comparing growth on SiO₂, MgO and Al₂O₃ substrates. The different growth characteristics resulted in a markedly earlier onset of both tunnelling magnetoresistance and anisotropic magnetoresistance for SiO₂. For Al₂O₃ in particular the growth mode results in a structure of the film containing two different contributions to ferromagnetism, which lead to two distinct coercive fields in the high thickness regime.