We present a method of solving the Schrödinger equation for two electrons confined in an almost arbitrary infinite-potential well of $D_{\infty h}$ symmetry. The boundary conditions for the wave function are imposed by devising a one-electron basis set that incorporates detailed information about the shape of the cavity. Two-electron integrals are evaluated using a fully separable expansion of the Coulomb operator similar to that developed earlier by the authors for a cylindrical cavity. To illustrate the capabilities of our approach, we present full configuration-interaction solutions for the lowest ${}^1{\Sigma }_g^{+}$ and ${}^3{\Sigma }_u^{+}$ states of two electrons in four axially symmetric cavities: a sphere, a cylinder, a bulged cylinder, and a capsule. By comparing the ground-state electronic energies for different cavities of equal volume $V$, we find that spherical confinement is optimal for $V\lesssim 1500a_0^3$; for larger volumes, the capsule-shaped system has a significantly lower energy. This switchover is driven by the same physical mechanism as Wigner crystallization and may serve as an indication of that process. The magnitude of the singlet-triplet gap strongly depends on the shape of the cavity and, for a fixed volume, decreases in the following order: bulged cylinder, sphere, cylinder, capsule.

We present experimental results for proton ionization of nucleobases (adenine, cytosine, thymine, and uracil) based on an event-by-event analysis of the different ions produced combined with an absolute target density determination. We are able to disentangle in detail the various proton ionization channels from mass-analyzed product ion signals in coincidence with the charge-analyzed projectile. In addition we are able to determine a complete set of cross sections for the ionization of these molecular targets by 20–150 keV protons including the total and partial cross sections and the direct-ionization and electron-capture cross sections.

We consider a mixture of two species of spin-1 atoms with interspecies spin exchange, which may cooperate or compete with intraspecies spin exchanges and thus dramatically affect the ground state. It represents a new class of bosonic gases differing from single-species spinor gases. We determine the exact ground states in several parameter regimes and study the composite structures using the generating function method generalized here to be applicable to a mixture of two species of spinor gases. The most interesting phase is the so-called entangled Bose-Einstein condensation (BEC), which is fragmented BEC with quantum entanglement between the two species and with both interspecies and intraspecies singlet pairs. For comparison, we also apply the generating function method to a mixture of two species of pseudospin-$\frac{1}{2}$ atoms, for which the total spin quantum number of each species is fixed as half of the atom number, in contrast with the case of spin $1$, for which it is a variable determined by energetics. Consequently, singlet pairs in entangled BEC of a pseudospin-$\frac{1}{2}$ mixture are all interspecies. Interspecies spin exchange leads to novel features beyond those of spinor BEC of a single species of atoms as well as mixtures without interspecies spin exchange.

We perform a numeric study (worm algorithm Monte Carlo simulations) of ultracold two-component bosons in two-dimensional optical lattices. We study how the Mott-insulator–to–superfluid transition is affected by the presence of a second superfluid bosonic species. We find that, at fixed interspecies interaction, the upper and lower boundaries of the Mott lobe are differently modified. The lower boundary is strongly renormalized even for relatively low filling factor of the second component and moderate (interspecies) interaction. The upper boundary, instead, is affected only for large enough filling of the second component. Whereas boundaries are renormalized we find evidence of polaron-like excitations. Our results are of interest for current experimental setups.

Secret sharing is a procedure for sharing a secret among a number of participants such that only the qualified subsets of participants have the ability to reconstruct the secret. Even in the presence of eavesdropping, secret sharing can be achieved when all the members are quantum. So what happens if not all the members are quantum? In this paper, we propose two semiquantum secret sharing protocols by using maximally entangled Greenberger-Horne-Zeilinger-type states in which quantum Alice shares a secret with two classical parties, Bob and Charlie, in a way that both parties are sufficient to obtain the secret, but one of them cannot. The presented protocols are also shown to be secure against eavesdropping.

Finite-temperature relativistic atomic structure calculations are employed to study the perturbation of energy levels and transition probabilities due to a dense plasma environment. We demonstrate that, even in the framework of the hard sphere model, energy-level crossings are predicted. The analysis discovered a dependence not only on orbital quantum number but also on spin quantum number. Detailed calculations have been carried out for He-like aluminum and the crossing between the levels $\mathit{nsms}({}^1{S}_0)$ and $\mathit{nsmp}({}^3{P}_2,{}^3{P}_1,{}^3{P}_0)$ is studied. The dependence on plasma density and temperature is demonstrated for a wide range of parameters.

Excitation energies of the $[\mathrm{Kr}]{\mathit{ns}}_{1/2}$, $[\mathrm{Kr}]{\mathit{np}}_j$, $[\mathrm{Kr}]{\mathit{nd}}_j$, and $[\mathrm{Kr}]{\mathit{nf}}_j$ ($n⩽9$ and $[\mathrm{Kr}]=(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^6$) in Sr ii are evaluated. First-order, second-order, third-order, and all-order Coulomb energies and first-order and second-order Coulomb-Breit energies are calculated. Reduced matrix elements, oscillator strengths, transition rates, and lifetimes are determined for the levels up to $n=7$. Electric-dipole ($5s_{1/2}-{\mathit{np}}_j$, $n=5–26$), electric-quadrupole ($5s_{1/2}-{\mathit{nd}}_j$, $n=4–26$), and electric-octupole ($5s_{1/2}-{\mathit{nf}}_j$, $n=4–26$) matrix elements are calculated to obtain the ground-state $E1$, $E2$, and $E3$ static polarizabilities. Scalar and tensor polarizabilities for the $5p_j-9p_j$ and $4d_j-8d_j$ excited states in Sr ii are also calculated. All the above-mentioned matrix elements are determined using the all-order method. We also investigate the hyperfine structure in ${}^{87}\mathrm{Sr}$${}^{+}$. The hyperfine $A$ values and $B$ values are determined for the first low-lying levels up to $n=7$. The quadratic Stark effect on hyperfine-structure levels of the ${}^{87}\mathrm{Sr}$${}^{+}$ ground state is investigated. The calculated shift for the $(F=5,M=0)\leftrightarrow (F=4,M=0)$ transition is found to be 0.120(1) Hz/(kV/cm)${}^2$. These calculations provide a theoretical benchmark for comparison with the experiment and theory. A careful study of uncertainty of our calculations is carried out for the transition-matrix elements, line strengths, transition rates, lifetimes, polarizabilities, and the Stark shift coefficient.

The absolute cross sections for the $e-{\mathrm{N}}_2$ and $p-{\mathrm{N}}_2$ collisions for the first negative $B^2{\Sigma }_u^{+}–X^2{\Sigma }_g^{+}$ and Meinel $A^2{\Pi }_u–X^2{\Sigma }_g^{+}$ bands have been measured in the energy region of $400–1500$ eV for electrons and $0.4–10$ keV for protons, respectively. Measurements are performed in the visible spectral region of $400–800$ nm by an optical spectroscopy method. The ratio of the cross sections of the Meinel band system to the cross section of the first negative band system ($0,0$) does not depend on the incident electron energy. The populations of vibrational levels corresponding to $A^2{\Pi }_u$ states are consistent with the Franck-Condon principle. The ratios of the cross sections of ($4,1$) to ($3,0$) bands and ($5,2$) to ($3,0$) bands exhibit slight dependence on the proton energy. A theoretical estimation within the quasimolecular approximation provides a reasonable description of the total cross section for the first negative band.

We calculate the photon energy distribution and the total cross section for the Compton scattering on the $K$ electrons for the case when the photon wave length is much smaller than the size of the $K$ shell. We show that at the energies of the order of the binding energy $I$ of the $K$ electron most part of the spectrum is governed by the low-energy behavior. The total cross section has a local maximum at the energies ($1.5–2$)$I$, reaching the values of the order $1$ barn. At higher photon energies the spectrum curves have two maxima, corresponding to low-energy ejected photons or electrons. The cross sections in the whole region are calculated. The actual calculations are carried out employing the nonrelativistic Coulomb functions, thus being valid for the single-electron atoms. However, the main features of the analysis are expected to be true for many-electron ions and neutral atoms. The results of the present analysis may be useful in calculations of the laser-induced and laser-assisted processes.

Molecular ${\mathrm{N}}_2$ fluorescence excited by laser filaments formed from laser pulses at 400 and 800 nm propagating in air is investigated. A comparison showed that, when excited with 400 nm photons, the fluorescence from the first negative band of ${\mathrm{N}}_2{}^{+}$ was enhanced by a factor of 6.4 while that of the second positive band of neutral ${\mathrm{N}}_2$ remained relatively constant. The enhanced ${\mathrm{N}}_2{}^{+}$ signal is attributed to a more efficient inner-shell multiphoton process (to the $B^2$${\Sigma }_u^{+}$ state) at 400 nm leaving a larger population of ${\mathrm{N}}_2{}^{+}$ ions in the excited state. On the other hand, the stable fluorescence from neutral ${\mathrm{N}}_2$ is due to the fact that the plasma density is more or less the same at both wavelengths. Using these results, a theoretical model is developed to determine the clamped intensities of the laser filaments at 400 and 800 nm.

We present an implementation of the time-dependent configuration-interaction singles (TDCIS) method for treating atomic strong-field processes. In order to absorb the photoelectron wave packet when it reaches the end of the spatial grid, we add to the exact nonrelativistic many-electron Hamiltonian a radial complex absorbing potential (CAP). We determine the orbitals for the TDCIS calculation by diagonalizing the sum of the Fock operator and the CAP using a flexible pseudospectral grid for the radial degree of freedom and spherical harmonics for the angular degrees of freedom. The CAP is chosen such that the occupied orbitals in the Hartree-Fock ground state remain unaffected. Within TDCIS, the many-electron wave packet is expanded in terms of the Hartree-Fock ground state and its single excitations. The virtual orbitals satisfy nonstandard orthogonality relations, which must be taken into consideration in the calculation of the dipole and Coulomb matrix elements required for the TDCIS equations of motion. We employ a stable propagation scheme derived by second-order finite differencing of the TDCIS equations of motion in the interaction picture and subsequent transformation to the Schrödinger picture. Using the TDCIS wave packet, we calculate the expectation value of the dipole acceleration and the reduced density matrix of the residual ion. The technique implemented will allow one to study electronic channel-coupling effects in strong-field processes.

We investigate a multilayer stack of dipolar Bose-Einstein condensates in terms of a simple Gaussian variational ansatz and demonstrate that this arrangement is characterized by the existence of several stationary states. Using a Hamiltonian picture we show that in an excited stack there is a coupled motion of the individual condensates by which they exchange energy. We find that for high excitations the interaction between the single condensates can induce the collapse of one of them. We furthermore demonstrate that one collapse in the stack can force other collapses, too. We discuss the possibility of experimentally observing the coupled motion and the relevance of the variational results found to full numerical investigations.

We propose a way for realizing a two-qubit controlled phase gate with superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. In this proposal, the two lowest levels of each SQUID serve as the logical states and two intermediate levels of each SQUID are used for the gate realization. We show that neither adjustment of SQUID level spacings during the gate operation nor uniformity in SQUID parameters is required by this proposal. In addition, this proposal does not require the adiabatic passage or a second-order detuning and thus the gate is much faster.

We investigate the effect of boundary conditions on spin amplification in spin chains. We show that the boundaries play a crucial role for the dynamics: A single additional coupling between the first and last spins can macroscopically modify the physical behavior compared to the open chain, even in the limit of infinitely long chains. We show that this effect can be understood in terms of a bifurcation in Hilbert space that can give access to different parts of Hilbert space with macroscopically different physical properties of the basis functions, depending on the boundary conditions. On the technical side, we introduce semiclassical methods whose precision increases with increasing chain length and allow us to analytically demonstrate the effects of the boundaries in the thermodynamic limit.

Applications of the Feshbach formalism to systems of more than two active electrons are very scarce due to practical limitations in the construction of the projection operators $\mathcal{P}$ and $\mathcal{Q}$ that are inherent to the theory. As a consequence, most previous applications rely on the use of approximate quasiprojection operators, whose theoretical justification is not yet clear. In this work, an implementation of the Feshbach formalism for three-electron atoms is presented that includes all the ingredients of the original formalism. Energy positions and autoionization widths of the lowest ${}^2{S}^e$, ${}^2{P}^o$, and ${}^2{D}^e$ autoionizing states of Li and Ne${}^{7+}$ have been evaluated. The results show that the use of quasiprojection operators is justified for the evaluation of resonant positions. However, for the ${}^2{S}^e$ states of Li, the use of quasiprojection operators can lead to errors in the autoionization widths of the order of $100\%$.

We report on a study of the emission spectra of Ag vapor in a vacuum (${10}^{-2}$ Torr) formed in ablation of an Ag metal target by a high-repetition rate (1.0 kHz) pulsed nanosecond ArF laser ($\lambda =193$ nm, output energy of 15 mJ). The time-resolved infrared emission spectrum of Ag was recorded in the 1300- to 3600-cm${}^{-1}$ spectral region using the Fourier transform infrared spectroscopy technique with a resolution of $0.02$ cm${}^{-1}$. The time profiles of the measured lines have maxima at 5–6 $\mu$s after a laser shot and display nonexponential decay with a decay time of 3–7 $\mu$s. The lines reported here are given with an uncertainty of 0.0005–0.016 cm${}^{-1}$. The line classification is performed using relative line strengths expressed in terms of transition dipole matrix elements calculated with the help of the Fues model potential; these calculations show agreement with the large experimental and calculated data sets available in the literature. In addition to these data we also calculate transition probabilities and line and oscillator strengths for a number of transitions in the 1300- to 5000-cm${}^{-1}$ range between $(4d^{10}){\mathit{nl}}_j$ states of Ag.

The electron capture in ${\mathrm{O}}^{6+}$-H collisions is studied by the two-center atomic orbital close-coupling (TC-AOCC) method when the interactions of charged particles are screened and have a Yukawa form. Atomic orbitals and eigenenergies of $n$ $⩽$ $6$ states on ${\mathrm{O}}^{5+}$ and $n$ $⩽$ $2$ states on H are calculated as a function of the interaction screening parameter and used in the AOCC dynamics scheme to calculate the electron capture cross sections in the energy range $1$–$200$ keV/u. It is shown that the degree of interaction screening reduces the electron binding energies, the number of open electron capture channels, and the strength of exchange couplings, thus affecting the entire collision dynamics and the magnitude and energy behavior of electron capture cross sections. The state-selective electron capture cross sections in this collision system, as well as the intensities of a number of charge exchange spectral lines, for a number of representative interaction screening strengths are presented and discussed.

Angular and energy distributions of fragment protons dissociated from HeH${}^{+}$ during grazing-angle scattering from a KCl $(001)$ surface are measured. The surface of KCl $(001)$ is heated at ${180}^{°}$C and the beam current is kept lower than 1 fA to prevent macroscopic surface charging. The angular distribution of the fragment protons shows a well-defined peak similarly to the grazing-angle scattering of atomic ions. The observed peak, however, is shifted from the specular angle toward larger scattering angles. The observed angular shift for the trailing proton is larger than that for the leading proton. These results clearly indicate that the motion of the fragment protons is affected by the surface track potential induced by the partner He ion.

We extend the coupled coherent-state (CCS) approach to simulate the strong-field ionization of helium atoms at long wavelengths. This approach uses a basis of trajectories guided by frozen Gaussian coherent states, sampled from a Monte Carlo distribution, as the initial states of the quantum time-dependent Schrödinger equations. The CCS trajectories move over averaged potentials, which can remove the Columbic singularities exactly. The low-energy structure is predicted by our CCS calculation and a “rescattering” event is clearly identified in the higher-energy regime. In addition, the nonsequential double ionization is also explored and the rescattering event can be identified as the major mechanism. Finally, we also study the electron angular distribution of helium. It is found that the maximum angle between the electron and electric field directions becomes smaller with increase in the laser intensity and wavelength.

We extend our previous application of the convergent close-coupling (CCC) and time-dependent close-coupling (TDCC) methods [Phys. Rev. A 81, 023418 (2010)] to describe energy and angular resolved double photoionization (DPI) of lithium at arbitrary energy sharing. By doing so, we are able to evaluate the recoil ion momentum distribution of DPI of Li and make a comparison with recent measurements of Zhu et al. [Phys. Rev. Lett. 103, 103008 (2009)].