We study single collisions between spin-polarized positron beams and targets that result in the production of positronium and target ions. We consider unpolarized as well as polarized targets with $0,1$, and $2$ unpaired electrons. First, we use angular momentum coupling to calculate the spin scattering matrices in all three cases. Then, for some targets, we define an angle $\beta$ between the polarization vectors of the positron beam and the target, and calculate density matrices whose elements are functions of $\beta$. Finally, from the density matrices, we obtain the probabilities of forming para- and ortho-positronium atoms for specific spins of the target ions. We present ratios of para- to ortho-positronium yields as well as total positronium annihilation rates.

This paper reports a study of valence photoionization of the LiCl monomer and dimer. The behavior of the photoionization partial cross section for molecular valence orbitals was measured as a function of photon energy between $15$ and $35$ eV. A square-integrable-function method was used to model the ionization partial cross section in both the LiCl monomer and dimer.

The effect of strongly repulsive interactions on the tunneling amplitude of hard-sphere (HS) bosons confined in a simple cubic optical lattice plus tight external harmonic confinement in continuous space is investigated. The quantum variational Monte Carlo (VMC) and the variational path integral (VPI) Monte Carlo techniques are used at zero temperature. The effects of the lattice spacing on the tunneling amplitude are also considered. The occupancies of the lattice sites as a function of the repulsion between the bosons are further revealed. Our chief result is that for a small number of bosons ($N=8$) the overlap of the wave functions in neighboring wells practically does not change with an increase of the repulsive interactions and changes only minimally for a larger number of particles ($N=40$). The tunneling amplitude rises with a reduction in the lattice spacing. In addition, the occupancy of the center of the trap decreases in favor of a rise in the occupancy of the lattice sites at the edges of the trap with increasing HS repulsion. Further, it was found that the energy per particle at certain optical-lattice barrier heights is insensitive to the number of particles and variations in the HS diameter of the bosons. In order to support our results, we compare the VMC results with corresponding VPI results.

Multiphoton effects in down-conversion are investigated based on the full-quantum multimode formalism by considering a three-level system as a prototype nonlinear system. We analytically derive the three-photon output wave function for two input photons, where one of the two input photons is down-converted and the other one is not. Using this output wave function, we calculate the down-conversion probability, the purity, and the fidelity to evaluate the entanglement between a down-converted photon pair and a non-down-converted photon. It is shown that the saturation effect occurs by multiphoton input and that it affects both the down-conversion probability and the quantum correlation between the down-converted photon pair and the non-down-converted photon. We also reveal the necessary conditions for multiphoton effects to be strong.

We investigate quantum phase transitions in which a change in the type of entanglement from bound entanglement to either free entanglement or separability may occur. In particular, we present a theoretical method to construct a class of quantum spin-chain Hamiltonians that exhibit this type of quantum criticality. Given parameter-dependent two-site reduced density matrices (with prescribed entanglement properties), we lay out a reverse construction for a compatible pure state for the whole system, as well as a class of Hamiltonians for which this pure state is a ground state. This construction is illustrated through several examples.

The resonant charge transfer process in the collision of hydrogen anions with metal surfaces is described within a single-active-electron wave-packet propagation method. The ion-survival probability is found to be strongly enhanced at two different surface-specific perpendicular velocities of the ion. It is shown that, while the low-velocity enhancement is induced from a dynamical confinement of the ion level inside the band gap, the high-velocity enhancement is due to electron recapture from transiently populated image states. Results are presented for Li(110), Cu(111), and Pd(111) surfaces.

We consider the steady states of a harmonic oscillator coupled so strongly to a two-level system (a qubit) that the rotating wave approximation cannot be made. The Hamiltonian version of this model is known as the $E\otimes \beta$ Jahn-Teller model. The semiclassical version of this system exhibits a fixed-point bifurcation, which in the quantum model leads to a ground state with substantial entanglement between the oscillator and the qubit. We show that the dynamical bifurcation survives in a dissipative quantum description of the system, amidst an even richer bifurcation structure. We propose an experimental implementation of this model based on a superconducting cavity: a superconducting junction in the central conductor of a coplanar waveguide.

We describe single-shot readout of a trapped-ion multiqubit register using space- and time-resolved camera detection. For a single qubit we measure $0.9(3)\times {10}^{-4}$ readout error in $400$ $\mu$s exposure time, limited by the qubit’s decay lifetime. For a four-qubit register (a “qunybble”), we measure an additional error of only $0.1(1)\times {10}^{-4}$ per qubit, despite the presence of $4\%$ optical cross-talk between neighboring qubits. A study of the cross-talk indicates that the method would scale with a negligible loss of fidelity to $~$$10\hspace{0.5em}000$ qubits at a density of $\lesssim$$1$ qubit/$\mu$m${}^2$, with a readout time of $~1$ $\mu$s/qubit.

We consider a harmonically trapped Tonks-Girardeau gas of impenetrable bosons in the presence of a single embedded ion, which is assumed to be tightly confined in an RF trap. In an ultracold ion-atom collision the ion’s charge induces an electric dipole moment in the atoms which leads to an attractive $r^{-4}$ potential asymptotically. We treat the ion as a static deformation of the harmonic trap potential and model its short range interaction with the gas in the framework of quantum defect theory. The molecular bound states of the ionic potential are not populated due to the lack of any possible relaxation process in the Tonks-Girardeau regime. Armed with this knowledge we calculate the density profile of the gas in the presence of a central ionic impurity and show that a density bubble of the order of $1\mu$m occurs around the ion for typical experimental parameters. From these exact results we show that an ionic impurity in a Tonks gas can be described using a pseudopotential approximation, allowing for significantly easier treatment.

We propose a deterministic remote state preparation scheme for photon polarization qubit states, where entanglement, local operations, and classical communication are used. By consuming one maximally entangled state and two classical bits, an arbitrary (either pure or mixed) qubit state can be prepared deterministically at a remote location. We experimentally demonstrate the scheme by remotely preparing $12$ pure states and $6$ mixed states. The fidelities between the desired and achieved states are all higher than $0.99$ and have an average of $0.9947$.

It is shown that orbitally degenerate states in highly symmetric systems are split by Jahn-Teller forces which are of relativistic origin (that is, they arise from the spin-orbit coupling operator). For the example of tetrahedral systems, the relativistic Jahn-Teller Hamiltonians of orbitally degenerate electronic states with spin $1/2$ are derived. While both electrostatic and relativistic forces contribute to the Jahn-Teller activity of vibrational modes of $E$ and $T_2$ symmetry in ${}^2{T}_2$ states of tetrahedral systems, the electrostatic and relativistic Jahn-Teller couplings are complementary for ${}^{2}E$ states: The $E$ mode is Jahn-Teller active through electrostatic forces, while the $T_2$ mode is Jahn-Teller active through the relativistic forces. The relativistic Jahn-Teller parameters have been computed with ab initio relativistic electronic-structure methods. It is shown for the example of the tetrahedral cluster cations of the group V elements that the relativistic Jahn-Teller couplings can be of the same order of magnitude as the familiar electrostatic Jahn-Teller couplings for the heavier elements.

The population of excited three-electron states in carbon ions after single-electron capture in $0.5-1.1$ MeV/amu ${\mathrm{C}}^{4+}(1s2s\hspace{0.5em}{}^{3}S)$-He collisions is analyzed theoretically by combining different methods. While the two-center basis generator method is used to calculate capture amplitudes on the single-particle level, all-electron structure calculations for the relevant ${\mathrm{C}}^{3+}$ states and their radiative and Auger transition rates are performed on the multiconfiguration Dirac-Fock level. These data are then combined and fed into a set of classical rate equations for the decay dynamics. Total cross sections for the production of the $1s2s2p\hspace{0.5em}{}^{4}P$, $1s2s2p\hspace{0.5em}{}^2{P}_{-}$, and $1s2s2p\hspace{0.5em}{}^2{P}_{+}$ states are calculated and their ratios compared with recent experimental data and previous calculations [D. Strohschein et al., Phys. Rev. A 77, 022706 (2008)]. It is found that the relative intensities of the $1s2s2p\hspace{0.5em}{}^{4}P$ states are considerably larger than expected on the basis of pure spin statistics. The Auger transitions, which were not included in the previous calculations, have a significant effect on the final results in that they reduce the $1s2s2p\hspace{0.5em}{}^{2}P$ intensities. Although our extended computations explain a significant part of the production of the $1s2s2p\hspace{0.5em}{}^{4}P$ states, the experimentally observed enhancement of these states is still considerably larger than the theoretical one.

We present an improved nonlocal resonance model for electron-HCl collisions. The short-range part of the model is fitted to ab initio electron-scattering eigenphase sums calculated using the Schwinger multichannel method, while the long-range part is based on the ab initio potential-energy curve of the bound anion HCl${}^{-}$. This model significantly improves the agreement of nonlocal resonance calculations with recent absolute experimental data on dissociative electron attachment cross sections for HCl and DCl. It also partly resolves an inconsistency in the temperature effect in dissociative electron attachment to HCl present in the literature. Finally, the present model reproduces all qualitative structures observed previously in elastic scattering and vibrational-excitation cross sections.

We discuss optical pumping when (a) the collision rates of optically pumped atoms with atoms or molecules of the background gas are small enough that individual velocity groups can be preferentially excited by a monochromatic light beam, (b) the collision rates are still fast enough to partially transfer the spin polarization to other velocity groups, and (c) there are nonnegligible losses of polarization due to collisional spin relaxation and Larmor precession. These conditions lead to a strong correlation between the velocity and the spin polarization of the atoms—that is, to “spin-tagging” of the different velocity groups. This regime is similar to that of optically pumped ${}^{23}\mathrm{Na}$ atoms of the Earth’s upper atmosphere, but it is seldom encountered in laboratory experiments. For cooling and trapping experiments, the collision rates with background gas are negligible. For gas-cell experiments the velocity-changing rates are normally so fast compared to spin relaxation or Larmor precession rates that the atoms have a Maxwellian velocity distribution with negligible correlation between the spin polarization and the velocity. We analyze the limiting cases of strong and weak collisions, which change the velocity by a large or small fraction, respectively, of the mean thermal velocity. The Keilson-Storer model [J. Keilson and A. E. Storer, Q. Appl. Math. 10, 243 (1952)] is used to discuss strong collisions, with memory parameter $\alpha =0$ and weak collisions with $\alpha \to 1$. For weak collisions, the physics can be modeled by coupled Fokker-Planck equations, identical to those for forced diffusion in a harmonic-oscillator potential well. In this limit there are solutions analogous to the quantum mechanical coherent states of a harmonic oscillator.

A recently developed [Phys. Rev. A 79, 042707 (2009)] impact parameter coupled pseudostate approximation (CP) is applied to calculate triple differential cross sections for single ionization of He by ${\mathrm{C}}^{6+}$, ${\mathrm{Au}}^{24+}$, and ${\mathrm{Au}}^{53+}$ projectiles at impact energies of $100$ and 2 MeV/amu for ${\mathrm{C}}^{6+}$ and $3.6$ MeV/amu for ${\mathrm{Au}}^{24+}$ and ${\mathrm{Au}}^{53+}$. For ${\mathrm{C}}^{6+}$, satisfactory, but not perfect, agreement is found with experimental measurements in coplanar geometry, but there is substantial disagreement with data taken in a perpendicular plane geometry. The CP calculations firmly contradict a projectile-nucleus interaction model which has been used to support the perpendicular plane measurements. For ${\mathrm{Au}}^{24+}$ and ${\mathrm{Au}}^{53+}$, there is a complete lack of accord with the available experiments. However, for ${\mathrm{Au}}^{24+}$ the theoretical position appears to be quite firm with clear indications of convergence in the CP approximation and very good agreement between CP and the completely different three-distorted-waves eikonal-initial-state (3DW-EIS) approximation. The situation for ${\mathrm{Au}}^{53+}$ is different. At the momentum transfers at which the measurements were made, there are doubts about the convergence of the CP approximation and a factor of 2 difference between the CP and 3DW-EIS predictions. The discord between theory and experiment is even greater with the experiment giving cross sections a factor of 10 larger than the theory. A study of the convergence of the CP approximation shows that it improves rapidly with reducing momentum transfer. As a consequence, lower-order cross sections than the triple are quite well converged and present an opportunity for a more reliable test of the experiment.

The nonequilibrium dynamics of spin impurity atoms in a strongly interacting one-dimensional (1D) Bose gas under the gravity field is studied. We show that due to the nonequilibrium preparation of the initial state as well as the interaction between the impurity atoms and other bosons, a counterintuitive phenomenon may emerge: the impurity atoms could propagate upward automatically in the gravity field. The effects of the strength of interaction, the gradient of the gravity field, as well as the different configurations of the initial state are investigated by studying the time-dependent evolution of the 1D strongly interacting bosonic system using time-evolving block decimation (TEBD) method. A profound connection between this counterintuitive phenomenon and the repulsive bound pair is also revealed.

Based on the cesium $6S_{1/2}$-$6P_{3/2}$-$8S_{1/2}$ ladder-type atomic system, double-resonance optical pumping (DROP) spectra including electromagnetically induced transparency (EIT) effects have been investigated with a room-temperature cesium vapor cell. For both cases of the probe and the coupling laser beams passing through the cesium vapor cell with the counter-propagation (CTP) and co-propagation (CP) configurations, the DROP spectra measured in the experiment display explicitly different linewidths. Thanks to the EIT effect, the linewidth of the DROP spectrum is explicitly narrower for the CTP configuration than for the CP configuration. Experimental results agree with the theoretical analysis considering Doppler averaging. Furthermore, when the coupling laser has moderate power, the DROP spectrum for the CTP configuration clearly shows two components: the narrow part due to the EIT effect and the broad part caused by optical pumping (but these two different components are never seen in the CP configuration). Also, the effect of the intensity of the coupling and probe lasers on the DROP spectra is investigated.

We describe a computational and experimental study on Hanle electromagnetically induced transparency and absorption resonance line shapes with a Laguerre Gaussian (LG) beam. It is seen that the LG beam profile brings about a significant narrowing in the line shape of the Hanle resonance and ground-state Zeeman coherence in comparison to a Gaussian beam. This narrowing is attributed to the azimuthal mode index of the LG field.

The quantum renormalization group method is applied to study the quantum criticality and entanglement entropy of the ground state of the Ising chain in the presence of antisymmetric anisotropic couplings and alternating exchange interactions. The quantum phase transitions can be characterized by the discontinuity in the second derivative of the energy of the renormalized ground state. A phase diagram is obtained by the critical boundary line. The first derivative of entanglement entropy also diverges at the same critical points after enough iterations of the renormalization of the coupling constants. The antisymmetric anisotropy and alternating interaction can enhance the renormalized entanglement via the creation of quantum fluctuations. The scaling behavior of the derivative of the entropy around the critical points manifests the logarithm dependence on the size of the spin system.

We consider properties of states of many qubits, which arise after sending certain entangled states via various noisy channels (white noise, colored noise, local depolarization, dephasing, and amplitude damping). Entanglement of these states and their ability to violate certain classes of Bell inequalities are studied. States which violate them allow a higher than classical efficiency in solving related distributed computational tasks with constrained communication. This is a direct property of such states—not requiring their further modification via stochastic local operations and classical communication such as entanglement purification or distillation procedures. We identify families of multiparticle states which are entangled but nevertheless allow the local realistic description of specific Bell experiments. For some of them, the “gap” between the critical values for entanglement and violation of Bell inequality remains finite even in the limit of infinitely many qubits.