We investigate the effect of the pulse length on the two-photon double ionization (DI) of H${}_2$ in the direct domain, for a femtosecond (fs) laser with a polarization vector oriented along the molecular axis. In the fixed-nuclei approximation, we find that the doubly excited $Q_1{}^1{\Sigma }_u^{+}$ states manifest themselves as resonances in the angle-integrated cross sections if the laser interaction lasts longer than about 3 fs. Decay into single-ionization channels does not significantly affect the shape of the angular distribution. A sharp rise in the probability for DI, due to virtual sequential ionization, occurs when the photon energy approaches the threshold for sequential DI.

Following a “bottom-up approach” in understanding many-particle effects and dynamics we provide a systematic ab initio study of the dependence of the breathing dynamics of ultracold bosons in a one-dimensional (1D) harmonic trap on the number of bosons ranging from few to many. To this end, we employ the multilayer multiconfiguration time-dependent Hartree method for bosons (ML-MCTDHB) which has been developed very recently [Krönke, Cao, Vendrell, and Schmelcher, New J. Phys. 15, 063018 (2013)]. The beating behavior for two bosons is found numerically and consequently explained by an analytical approach. Drawing on this, we show how to compute the complete breathing mode spectrum in this case. We examine how the two-mode breathing behavior of two bosons evolves to the single-frequency behavior of the many-particle limit when adding more particles. In the limit of many particles, we numerically study the dependence of the breathing mode frequency on both the interaction strength as well as on the particle number. We provide an estimate for the parameter region where the mean-field description provides a valid approximation.

We show that coupling among multiple resonances can be conveniently introduced and controlled by boundary wave scattering. This is demonstrated in optical microcavities of quasicircular shapes, where the couplings of multiple modes are determined by the scattering from harmonic boundary deformations. We analyze these couplings using a perturbation theory, which gives an intuitive understanding of the lowest-order and higher-order scattering processes. Different scattering paths between two boundary waves can either enhance or reduce their coupling strength. The effect of controlled multimode coupling is most pronounced in the direction of output from an open cavity, as the coupling can cause a dramatic change of the external cavity field distribution.

We investigate the controllable generation of robust photon entanglement with a circuit cavity electromechanical system, consisting of two superconducting coplanar waveguide cavities capacitively coupled by a nanoscale mechanical resonator. We show that, with this electromechanical system, two-mode continuous-variable entanglement of cavity photons can be engineered deterministically either via coherent control on the dynamics of the system, or through a dissipative quantum-dynamical process. The first scheme, operating in the strong-coupling regime, explores the excitation of the cavity Bogoliubov modes and is insensitive to the initial thermal noise. The second one is based on the reservoir-engineering approach, which exploits the mechanical dissipation as a useful resource to perform ground-state cooling of two delocalized cavity Bogoliubov modes. The achieved amount of entanglement in both schemes is determined by the relative ratio of the effective electromechanical coupling strengths, which thus can be tuned and made much larger than that in previous studies.

A particular family of time- and space-dependent discrete-time quantum walks (QWs) is considered in one-dimensional physical space. The continuous limit of these walks is defined through a procedure discussed here and computed in full detail. In this limit, the walks coincide with the propagation of a massless Dirac fermion in an arbitrary gravitational field. A QW mimicking the radial propagation of a fermion outside and inside the event horizon of a Schwarzschild black hole is explicitly constructed and simulated numerically. Thus, the family of QWs considered in our manuscript provides an analog system to study experimentally coherent quantum propagation in curved spacetime.

We study the expansion of harmonically trapped bosons in a two-dimensional lattice after suddenly turning off the confining potential. We show that, in the presence of multiple occupancies per lattice site and strong interactions, the system exhibits a clear dynamical separation into slowly and rapidly expanding clouds. We discuss how this effect can be understood within a simple picture by invoking doublons and Bose enhancement. This picture is corroborated by an analysis of the momentum distribution function in the regions with slowly and rapidly expanding bosons.

It is shown how the quantum mechanical mass flux and the electromagnetic fields of an electron Bessel vortex mode generate its intrinsic linear momentum and angular momentum properties. Although the corresponding volume density vectors due to the mass flux contain transverse vector components, their volume integrals are shown by explicit analysis to yield null results. The total linear and angular momenta are thus purely axial vectors. There are additional contributions associated with the vortex electric and magnetic fields and these too are shown to be purely axial vectors. Order of magnitude estimates are made in the context of a suggested experiment on the rotation of an optically levitated nanoparticle subject to an electron vortex.

We study an experimentally realizable paradigm of complex many-body quantum systems, a two-band Wannier-Stark model, for which diffusion in Hilbert space as well as many-body Landau-Zener processes can be engineered. A crossover between regular and quantum chaotic spectra is found within the many-body avoided crossings at resonant tunneling conditions. The spectral properties are shown to determine the evolution of states across a cascade of Landau-Zener events. We apply the obtained spectral information to study the nonequilibrium dynamics of our many-body system in different parameter regimes.

Using as a prototype example the harmonic oscillator we show how losing self-adjointness of the Hamiltonian $H$ changes drastically the related functional structure. In particular, we show that even a small deviation from strict self-adjointness of $H$ produces two deep consequences, not well understood in the literature: First of all, the original orthonormal basis of $H$ splits into two families of biorthogonal vectors. These two families are complete but, contrarily to what often claimed for similar systems, none of them is a basis for the Hilbert space $\mathcal{H}$. Second, the so-called metric operator is unbounded, as well as its inverse. In the second part of the paper, after an extension of some previous results on the so-called $\mathcal{D}$ pseudobosons, we discuss some aspects of our extended harmonic oscillator from this different point of view.

We address the dynamics of quantum correlations in continuous-variable open systems and analyze the evolution of bipartite Gaussian states in independent noisy channels. In particular, we introduce the notion of dynamical path through a suitable parametrization for symmetric states and focus attention on phenomena that are common to Markovian and non-Markovian Gaussian maps under the assumptions of weak coupling and the secular approximation. We find that the dynamical paths in the parameter space are universal, that is, they depend only on the initial state and on the effective temperature of the environment, with non-Markovianity that manifests itself in the velocity of running over a given path. This phenomenon allows one to map non-Markovian processes onto Markovian ones and may reduce the number of parameters needed to study a dynamical process, e.g., it may be exploited to build constants of motions valid for both Markovian and non-Markovian maps. Universality is also observed in the value of Gaussian discord at the separability threshold, which itself is function of the initial conditions only, in the limit of high temperature. We also prove the existence of excluded regions in the parameter space, i.e., sets of states that cannot be linked by any Gaussian dynamical map.

We consider quantum state transfer in a fully connected spin network, in which the results indicate that it is impossible to achieve high fidelity by free dynamics. However, the addition of certain kinds of noise can be helpful for this purpose. In fact, we introduce a model of Gaussian white noise affecting the spin-spin couplings (edges), except those linked to the input and output node, and prove that it enhances the fidelity of state transfer. The observed noise benefit is scale free as it applies to a quantum network of any size. The amount of the fidelity enhancement, depending on the noise strength as well as on the number of edges to which it is applied, can be so high as to take the fidelity close to one.

We have performed ab initio QED calculations of the ${\left(1s\right)}^2{\left(2s\right)}^{2}2p_{3/2}-{\left(1s\right)}^2{\left(2s\right)}^{2}2p_{1/2}$ fine-structure splitting along the boron isoelectronic sequence for all ions with $17\le Z\le 100$. This level splitting was evaluated within the extended Furry picture and by making use of four different screening potentials in order to estimate the effects of interelectronic correlations. The accuracy of the predicted transition energies has been improved significantly when compared with previous computations.

The deuteron quadrupole moment $Q_d$ affects the hyperfine splitting of energy levels in the D${}_2^{+}$ molecular ion. The hyperfine coupling constants are calculated within the framework of the spin-dependent Breit-Pauli Hamiltonian for rovibrational states with the vibrational quantum number $v=0–4$ and the rotational quantum number $L=1–2$. This is a complete ab initio calculation at the level of order $R_{\infty }{\alpha }^2$ corrections based on the the precise variational solutions of the three-body Schrödinger equation. Finally, the current uncertainty in $Q_d$ contributes at the level of about 1–3 Hz to the hyperfine splitting.

We performed calculations of the optimized structure, harmonic vibrational frequencies, and dissociation energies of RbOH and its anion, and investigate the interactions between Rb and OH${}^{-}$ leading to possible associative detachment pathways. The electron affinity of RbOH was computed to be 0.2890 eV, with a bond energy of Rb+OH${}^{-}$ half that of Rb+OH. To determine other possible charge loss pathways, the Rb+OH and Rb+OH${}^{-}$ dissociation curves were computed using coupled-cluster methods along all possible collisional angles. An adiabatic curve crossing between the neutral and charged molecule was found at the inner wall of the molecular potential curve for linear geometries. Associative detachment rates were estimated using the Langevin ion capture cross section for hydroxide. We find for $v\ge 2$ an associative detachment rate of $>2\times {10}^{-9}$ cm${}^3$s${}^{-1}$, while for $v=0$ and 1 no appreciable rate exists. This strong dependence on vibrational level suggests the ability to control the associative detachment rate directly.

We studied blocking effects on ion guiding through nanocapillaries in highly insulating polyethylene terephthalate (PET). The experiments were initiated in view of a previous study with capillaries in polycarbonate (PC) for which strong blocking effects were observed, whereas for PET these effects could not be detected. The aim was to find out whether the different results are caused by differences in the PET and PC materials or by differences in the areal densities of the capillaries. Transmission experiments of 3-keV Ne${}^{7+}$ ions were performed for a variety of PET samples with an areal capillary density ranging from 3 to $60\times {10}^6$ cm${}^{-2}$. The tilt angles were close to zero degree because previous blocking effects were found to be largest at small angles. Our results clearly show that blocking effects also exist for PET and that they are sensitively dependent on the areal capillary density, i.e., the mean capillary distance. The potential produced by the charges accumulated in neighboring capillaries is calculated showing that it plays an important role in the ion blocking. In addition, model calculations are performed providing expressions to determine the direct ion transmission for small tilt angles and divergent ion beams.

We propose a source for high-brightness ion and electron beams based on the ionization of an effusive atomic beam which is transversely laser cooled and compressed. The very low transverse temperature (mK range) and the relative low density of the starting atomic sample ensure excellent initial conditions for obtaining bright and monochromatic charge sources. In contrast to the standard photoionization techniques used by similar sources, we utilize field ionization of Rydberg atoms. This approach allows a substantial reduction of the required laser power and copes differently with the problems of the energy spread created during the ionization process and of the stochastic space-charge effect. Theoretical modeling and prospective ideas of this emerging technology are given.

A common goal of quantum control is to maximize a physical observable through the application of a tailored field. The observable value as a function of the field constitutes a quantum-control landscape. Previous papers have shown, under specified conditions, that the quantum-control landscape should be free of suboptimal critical points. This favorable landscape topology is one factor contributing to the efficiency of climbing the landscape. An additional complementary factor is the landscape structure, which constitutes all nontopological features. If the landscape's structure is too complex, then climbs may be forced to take inefficient convoluted routes to find optimal controls. This paper provides a foundation for understanding control-landscape structure by examining the linearity of gradient-based optimization trajectories through the space of control fields. For this assessment, a metric $R\ge 1$ is defined as the ratio of the path length of the optimization trajectory to the Euclidean distance between the initial control field and the resultant optimal control field that takes an observable from the bottom to the top of the landscape. Computational analyses for simple model quantum systems are performed to ascertain the relative abundance of nearly straight control trajectories encountered when optimizing a state-to-state transition probability. The distribution of $R$ values is found to be centered near remarkably low values upon sampling large numbers of randomly chosen initial control fields. Additionally, a stochastic algorithm is used to locate many distinct initial control fields, each of which corresponds to the start of an almost straight control trajectory with $R\simeq 1.0$. The collected results indicate that quantum-control landscapes have very simple structural features. The favorable topology and the complementary simple structure of the control landscape provide a basis for understanding the generally observed ease of optimizing a state-to-state transition probability.

The partial and total photoionization cross sections of the Mg-like S${}^{4+}$ ion for the ground $3s^2$ ${}^1{S}^e$ and the excited $3s3p$ ${}^3{P}^o$ and $3s3p$ ${}^1{P}^o$ initial states, leaving the residual S${}^{5+}$ 3$l$ states, have been calculated in both the dipole length and velocity forms using a noniterative eigenchannel $R$-matrix method combined with multichannel quantum-defect theory at the $R$-matrix surface. Noticeable features in the cross sections are found for the autoionizing Rydberg series converging to the S${}^{5+}$ 3$l$ thresholds, owing to the effect of interlopers which converge to the other threshold limits. The resonance parameters for low members of these autoionizing levels are identified and characterized. Our calculational results are compared with the available previous results, and the ionization thresholds are in better agreement with NIST data than Opacity Project (OP) results. The present photoionization cross sections are in overall agreement with the OP results, but some discrepancies are found.

The semiclassical dynamics of atoms, when the atoms are confined inside a standing-wave high-finesse resonator, is theoretically studied. The atoms are cooled by scattering processes in which the photons of a transverse laser are coherently scattered into the cavity mode. We derive a Fokker-Planck equation for the atomic center-of-mass variables which allows us to determine the equations of motion in the semiclassical limit for any value of the intensity of the laser field. We extract its prediction for the dynamics when the resonator is essentially in the vacuum state and the atoms are cooled by scattering photons into the cavity mode, which then decays. Its predictions for the stationary atomic distribution are compared with the ones of the Fokker-Planck equation by Domokos et al. [J. Phys. B 34, 187 (2001)], which has been derived under different assumptions. We find full agreement in the considered parameter regime.

We present an accurate ab initio tight-binding model, capable of describing the dynamics of Dirac points in tunable honeycomb optical lattices following a recent experimental realization [Tarruell et al., Nature (London) 483, 302 (2012)]. Our scheme is based on first-principle maximally localized Wannier functions for composite bands. The tunneling coefficients are calculated for different lattice configurations, and the spectrum properties are well reproduced with high accuracy. In particular, we show which tight-binding description is needed in order to accurately reproduce the position of Dirac points and the dispersion law close to their merging, for different laser intensities.