Double and transfer ionization processes are theoretically investigated for collisions of He atoms with bare energetic projectile ions. Two-electron transition amplitudes are described by linear combinations of products of one-electron transition amplitudes, which enables one to incorporate the effects of electron correlation in the asymptotic channels. A collision mechanism in which the projectile interacts separately with both electrons gives a realistic account of the processes only at medium impact energies, while the final state correlation and the shake-off mechanism together with correlations in the initial state are found to be important for the lower and higher collision energies, respectively.

The interaction of a wave packet (and in particular the wave front) with a mass barrier is investigated in one dimension. We discuss the main features of the wave packet that are inherent to two-dimensional wave packets, such as compression during reflection, penetration in the case when the energy is lower than the height of the barrier, waving tails, precursors, and the retardation of the reflected and penetrated wave packets. These features depend on the wave-packet envelope function which we demonstrate by considering the case of a rectangular wave packet with sharp front and trailing edges and a smooth Gaussian wave packet. The method of Fourier integral for obtaining the nonstationary solutions is used.

Simulation methods based on stochastic realizations of state vector evolutions are commonly used tools to solve open quantum system dynamics, both in the Markovian and non-Markovian regimes. Here, we address the question of the waiting time distribution (WTD) of quantum jumps for non-Markovian systems. We generalize Markovian quantum trajectory methods in the sense of deriving an exact analytical WTD for non-Markovian quantum dynamics and show explicitly how to construct this distribution for certain commonly used quantum optical systems.

In this paper we present an introduction to the theory of anyons and discuss how basis sets and matrix representation of the interaction terms can be obtained, using non-Abelian Fibonacci anyons. We show that the change of basis matrix for certain fusion trees called pentagons is given by a certain unitary matrix which is introduced in Sec. I, and denoted $F_{\tau }^{\tau ,\tau ,\tau }$ throughout the remainder of the paper. We perform a quantum walk with the Fibonacci anyons governed by the unitary matrix $F_{\tau }^{\tau ,\tau ,\tau }$ and obtain the limiting distribution of the walk.

We present an analysis of transfer of quantum information between the collective spin degrees of freedom of a large ensemble of two-level systems and a single central qubit. The coupling between the central qubit and the individual ensemble members may be varied and thus provides access to more than a single storage mode. Means to store and manipulate several independent qubits are derived for the case where the variation in coupling strengths does not allow addressing orthogonal modes of the ensemble. While our procedures and analysis may apply to a number of different physical systems, for concreteness, we study the transfer of quantum states between a single electron spin and an ensemble of nuclear spins in a quantum dot.

We study the magnetic-field dependence of the entanglement entropy in quantum phase transition induced by a quench of the $XX$, $XXX,$ and Lipkin-Meshkov-Glick (LMG) models. The entropy for a block of $L$ spins with the rest follows a logarithmic scaling law where the block size $L$ is restricted due to the dependence of the prefactor on the quench time. Within this restricted region the entropy undergoes a renormalization group (RG) flow. From the RG flow equation we have analytically determined the magnetic field dependence of the entropy. The anisotropy parameter dependence of the entropy for the $XY$ and the LMG models has also been studied in this framework. The results are found to be in excellent agreement with that obtained by other authors from numerical studies without any quench.

Bound states of ${}^{40}$Ca-${}^n$He${}_2$, $n=3,4$, triatomic complexes are investigated. The potential-energy surface, represented as the addition of atomic-pair interactions, is that recently used to study these systems by Gou and Li, Phys. Rev. A 85, 012510 (2012). The results obtained from three different methods, in fair agreement, profoundly disagree with those reported in the reference above. In addition, we address the feasibility of two Ca-He interactions existing in the literature by analyzing the characteristics of their ground and excited vibrorotational states. To this end, simulated absorption spectra in the region of microwaves are also presented and discussed.

With the Hylleraas-Gaussian basis set, in which the term of $r_{12}^1$ is expanded approximately in Gaussian-type geminals, a full configuration-interaction (CI) method is applied to calculate the lowest ${}^1{\Sigma }_g$ and ${}^1{\Sigma }_u$ states of the hydrogen molecule in magnetic fields up to 2.35 × 10${}^7$ T. In the absence of magnetic field, the total energies of the lowest ${}^1{\Sigma }_g$ and ${}^1{\Sigma }_u$ states in our calculation are $-$1.174 447 7(4) at the equilibrium distance of $R$ $=$ 1.40 a.u and $-$0.756 613 4(6) at $R$ $=$ 2.43 a.u., respectively. Compared to the CI method with Gaussian basis set, a significant improvement in the precision of the total energies and the dissociation energies at corresponding equilibrium distances has been achieved. The $z_1$-$z_2$ probability density distributions in different field regions are calculated and analyzed.

We have calculated cross sections for electron impact ionization (EII) of P-like Fe${}^{11+}$ forming Si-like Fe${}^{12+}$. We have used the flexible atomic code (FAC) and a distorted-wave (DW) approximation. Particular attention has been paid to the ionization through the $3l\to n{l}^{\prime }$ and $2l\to n{l}^{\prime }$ excitation autoionization (EA) channels. We compare our results to previously published FAC DW results and recent experimental results. We find that the previous discrepancy between theory and experiment at the EII threshold can be accounted for by the $3l\to n{l}^{\prime }$ EA channels which were not included in the earlier calculations. At higher energies the discrepancy previously seen between theory and experiment for the magnitude of the $2l\to n{l}^{\prime }(n\ge 4)$ EA remains, though the difference has been reduced by our newer results. The resulting Maxwellian rate coefficient derived from our calculations lies within 11$\%$ of the experimentally derived rate coefficient in the temperature range where Fe${}^{11+}$ forms in collisional ionization equilibrium.

Ab initio first-order Born calculations of the fivefold differential cross section (FDCS) and transverse spin asymmetry in FDCS for the $K$-shell double ionization of atoms by transversely polarized incident electron are reported. The FDCS and transverse spin asymmetry in FDCS are found to be sensitive to atomic number $Z$, ejection angle, and energy-sharing ratio of the ejected electrons. FDCS is decomposed into longitudinal and transverse contributions and their effects on the angular profile of FDCS such as shifting in the binary peak are reported. Hitherto, all $(e,3e)$ experiments have been performed in the nonrelativistic energy regime without consideration of spin polarization. We hope that the present calculation of FDCS and spin asymmetry in FDCS may induce quantum mechanically complete $(e,3e)$ experiments in the relativistic energy regime wherein spin-dependent interaction becomes prominent.

The ionization of a model acetylene molecule (C${}_2$H${}_2$) by an intense laser pulse is studied by numerically solving the time-dependent Hartree-Fock equations. It is shown that when the C–H bond is stretched to twice the value of the equilibrium internuclear distance, the ionization probability is strongly enhanced. The physical mechanism is identified to be similar to the enhanced ionization found previously in H${}_2^{+}$, with the two HOMOs in acetylene playing the same role as the charge resonance states $1{\sigma }_g$ and $1{\sigma }_u$ in H${}_2^{+}$. Electrons are ejected in a two-step process. First, the electron density is transferred to one of the peripheral H atom sites, and then it is ejected from there by tunneling. The same process is effective also in C${}_2$H${}_2$${}^{2+}$. The mechanism is further clarified by employing both a simplified two-state model and the single-active-orbital approximation. In the enhancement of the ionization of C${}_2$H${}_2$, the elongation of the C–H distance plays a decisive role, while the variation of the C–C distance gives only a secondary contribution. In addition to the two highest occupied molecular orbitals, electrons in the third highest molecular orbital are also found to be ejected efficiently. The role of the third highest molecular orbital in the ionization dynamics is investigated by introducing projection operators.

We investigated the nuclear dynamics of diatomic molecular ions in intense laser fields by analyzing their fragment kinetic-energy release (KER) spectra as a function of the pump-probe delay $\tau$. Within the Born-Oppenheimer (BO) approximation, we calculated ab initio adiabatic potential-energy curves and their electric dipole couplings, using the quantum chemistry code gamess. By comparing simulated KER spectra as a function of either $\tau$ or the vibrational quantum-beat frequency for the nuclear dynamics on both individual and dipole-coupled BO potential curves with measured spectra, we developed a scheme for identifying electronic states that are relevant for the dissociation dynamics. We applied this scheme to investigate the nuclear dynamics in O${}_2$${}^{+}$ ions that are produced by ionization of neutral O${}_2$ molecules in an ultrashort infrared (IR) pump pulse and dissociate due to the dipole coupling of molecular potential curves in a delayed IR probe laser field.

Three-level atomic gradient echo memory ($\Lambda$-GEM) is a proposed candidate for efficient quantum storage and for linear optical quantum computation with time-bin multiplexing [Hosseini et al., Nature (London) 461, 241 (2009)]. In this paper we investigate the spatial multimode properties of a $\Lambda$-GEM system. Using a high-speed triggered CCD, we demonstrate the storage of complex spatial modes and images. We also present an in-principle demonstration of spatial multiplexing by showing selective recall of spatial elements of a stored spin wave. Using our measurements, we consider the effect of diffusion within the atomic vapor and investigate its role in spatial decoherence. Our measurements allow us to quantify the spatial distortion due to both diffusion and inhomogeneous control field scattering and compare these to theoretical models.

Lately, much interest has been directed towards designing setups that achieve decisive tests of local realism. Here we present Bell tests with measurements based on linear optical displacements and single-photon detection. The scheme displays good tolerance to loss. In particular, for entangled squeezed states, we find thresholds compatible with current efficiencies of detectors and sources. Furthermore, the scheme is easily extendible to any number of observers, allowing observation of multipartite nonlocality for a single photon shared among multiple modes. We also consider the case of atom-photon entanglement, where the loss threshold can be lowered further, as well as local filters compensating transmission and coupling inefficiencies at the source.

The intensities of the $X$ and $A$ valence photoelectron lines of N${}_2$ have been found to display Fano line shapes as a function of photon energy around the N $1s\to$ Rydberg excitations. The vibrational intensity distributions of these photoelectron lines change at the N $1s\to 3s\sigma$ and $3p\pi$ resonances. These effects indicate interference between direct and resonant photoionization channels. Our numerical simulations reproduce quite well the experimental results.

We report diamagnetic spectra of sodium atoms observed in a strong magnetic field up to 3 T and compare them with theoretical calculations based on the formulation of an exact quantum-defect theory. We focus on one-photon absorption processes from the ground $3s$ state to the excited $np$ Rydberg states whose magnetic quantum numbers are determined by the polarization of the laser field. The calculations employ $B$-spline basis expansion and the complex spatial coordinate rotation technique, and their results agree well with the observed ones when atomic core effects are appropriately taken into account. When graphed against magnetic field, level anticrossings and core-induced interaction of the $np$ states provide clear signatures for studying the effects of quantum defect on the diamagnetic spectra.

We theoretically investigate the generation of elliptically polarized isolated attosecond pulses from an oriented CO gas medium using a linearly polarized driving pulse. Our results reveal that a largely elliptically polarized modulated supercontinuum at the microscopic level can be generated from a properly oriented CO molecule. Moreover, it is found that the nonsymmetric structure of the highest occupied molecular orbital of the oriented CO molecule is responsible for the generation of the largely elliptical harmonics. In addition, propagation effects are also employed to macroscopically select the short quantum path. Then a smooth elliptically polarized harmonic spectrum can be obtained at the macroscopic level. Finally, an isolated attosecond pulse with elliptical polarization can be directly obtained, which might open additional avenues for applications in probing ultrafast electron dynamics.

We analyze the role played by system-environment correlations in the emergence of non-Markovian dynamics. By working within the framework developed in Breuer et al. [Phys. Rev. Lett. 103, 210401 (2009)], we unveil a fundamental connection between non-Markovian behavior and dynamics of system-environment correlations. We derive an upper bound to the rate of change of the distinguishability between different states of the system that explicitly depends on the establishment of correlations between system and environment. We illustrate our results using a fully solvable spin-chain model, which allows us to gain insight into the mechanisms triggering non-Markovian evolution.

We suggest a source of polarization-entangled photon pairs based on a cross-shaped plasmonic nanoantenna driven by a single quantum dot. The integration of the nanoantenna with a metal mirror overcomes the fundamental tradeoff between the spontaneous emission (SE) enhancement and the extraction efficiency typical of microcavity- and nanowire-based architectures. With a very high extraction efficiency of entangled photons ($\approx$$90\%$) at $1.55$ $\mu$m and large SE enhancement ($\approx$90) over a broad 330-nm spectral range, the proposed design will pave the way toward reliable integrated sources of nonclassical light.