The ionization and dissociation fractions of the CO${}_2$ molecule under the impact of 10–30-keV negative ions H${}^{-}$, C${}^{-}$, and O${}^{-}$ are studied. The four recoil ions originating from the target molecule CO${}_2$ (CO${}_2{}^{+}$, CO${}^{+}$, O${}^{+}$, and C${}^{+}$) are measured and identified in coincidence with projectiles in two final charge states ($q=0$ and $q=+1$) by using a time-of flight spectrometer. The ionization and dissociation fractions of CO${}_2$ are found to associate with the momentum of the impacting ions. We also analyze the fractions for ionization and dissociation from a physical point. No comparison is given since no other experimental and theoretical data exist in the investigated energy range.

In the preceding paper [Konovalov et al., Phys. Rev. A 84, 032707 (2011)] we used the $J$-matrix (JM) method to solve the $S$-wave $e$-He scattering problem within the frozen-core model of helium. In the present work we go beyond the frozen-core model and adopt a general configuration-interaction description of helium (within the $S$-wave model). As a result a more accurate description of the helium target states is achieved that allowed us to obtain highly accurate numerical solutions of the $e$-He scattering problem for the elastic, $2^{1,3}S$, $3^{1,3}S$ excitation and total ionization cross sections. The JM results are confirmed by the corresponding convergent-close-coupling (CCC) calculations. At energies above the ionization threshold some minor pseudoresonances are still evident in both the JM and CCC results despite the large bases used. The pseudoresonances are nearly completely eliminated by averaging a number of JM calculations creating a challenging benchmark for any current or future ab initio electron-atom scattering methods.

It is commonly accepted in the strong-laser physics community that the dynamical regime of atomic ionization is characterized by the Keldysh parameter $\gamma$. Two distinct cases, where $\gamma <1$ and $\gamma >1$, are associated with ionization mechanisms that are predominantly in the tunneling and in the multiphoton regimes, respectively. We perform fully three-dimensional quantum simulations for the ionization of the hydrogen atom by solving the time-dependent Schrödinger equation for a wide range of laser parameters encoded by the Keldysh parameter. We find that the meaning of the Keldysh parameter $\gamma$ changes when the laser frequency $\omega$ is changed, and demonstrate that $\gamma$ is useful in determining the dynamical ionization regime only when coupled with the scaled laser frequency $\Omega$ when a large range of laser frequencies and peak intensities are considered. The scaled frequency $\Omega$ relates the laser frequency $\omega$ to the classical Kepler frequency ${\omega }_K$ of the bound electron. Together with the Keldysh parameter, the pair ($\gamma ,\Omega$) refers to a more realistic picture of the dynamical ionization regime. We refer to final momentum distributions of the ionized electrons at several interesting points on the ($\gamma ,\Omega$) landscape in order to infer whether the tunneling or the multiphoton mechanism is dominant in these regions.

We study long-time spin diffusion of harmonically trapped lattice fermions in one dimension. Combining thermodynamic Bethe ansatz approach and local-density approximation, we calculate spin current and spin-diffusion coefficient driven by the population imbalance. We find spin current is driven by susceptibility effects rather than typical diffusion where magnetization would transport from regions of high magnetization to low. As expected, spin transport is zero through insulating regions and only present in the metallic regions. In the weak-coupling limit, the local spin-diffusion coefficient shows maxima at all the insulating regions. Further, we estimate damping rate of diffusion modes in the weak-coupling limit within the lower metallic portion of the cloud.

We introduce balanced-detection (BD) Raman-induced Kerr-effect (RIKE) as a powerful coherent Raman scattering (CRS) technique. RIKE relies on the Raman-induced birefringence that occurs when the pump-Stokes frequency detuning is in resonance with a vibrational transition. We show how a balanced-detection configuration, inspired by that applied in electro-optic sampling, suppresses both the linear and nonlinear backgrounds that affect other CRS techniques. BD-RIKE enables homodyne amplification of the signal and linear dependence on the concentration of the target oscillators, as well as cancellation of laser intensity noise. In addition, it allows straightforward selection of either the imaginary or the real part of the nonlinear vibrational response.

We study theoretically the effects of a plasmonic nanostructure on the linear absorption and dispersion spectrum of a four-level double- $\mathsf{V}$-type quantum system. In the quantum system under study one $\mathsf{V}$-type transition is influenced by the interaction with surface plasmons while the other $\mathsf{V}$-type transition interacts with free-space vacuum. As plasmonic nanostructure we consider a two-dimensional array of metal-coated dielectric nanospheres. We analyze the optical properties of a linearly polarized laser field that couples the lowest state with the upper states in the free-space transitions. We show that, due to the presence of the plasmonic nanostructure, effects of optical transparency are created. These effects are also complemented by the existence of slow light.

Increasing the information-carrying capacity of a single photon may be achieved by utilizing multiple degrees of freedom. We describe here an approach that utilizes two degrees of freedom to encode three qubits per photon: one in polarization and two in the spatial-parity symmetry of the transverse field. In this conception, a polarization-sensitive spatial light modulator corresponds to a three-qubit controlled-unitary gate with one control qubit (polarization) and two target (spatial-parity-symmetry) qubits. We describe the construction of controlled-not (cnot), $\sqrt[n]{cnot}$, controlled-phase, and Fredkin gates, and the preparation of one-photon, three-qubit Greenberger-Horne-Zeilinger (GHZ) and $W$ states. This approach enables simple optical implementations of few-qubit tasks in quantum information processing.

In this work, I consider the center-of-mass wave function for a homogenous sphere under the influence of the self-interaction due to Newtonian gravity. I solve for the ground state numerically and calculate the average radius as a measure of its size. For small masses, $M\lesssim {10}^{-17}$ kg, the radial size is independent of density, and the ground state extends beyond the extent of the sphere. For masses larger than this, the ground state is contained within the sphere and to a good approximation given by the solution for an effective radial harmonic-oscillator potential. This work thus determines the limits of applicability of the point-mass Newton Schrödinger equations for spherical masses. In addition, I calculate the fringe visibility for matter-wave interferometry and find that in the low-mass case, interferometry can in principle be performed, whereas for the latter case, it becomes impossible. Based on this, I discuss this transition as a possible boundary for the quantum-classical crossover, independent of the usually evoked environmental decoherence. The two regimes meet at sphere sizes $R\approx {10}^{-7}$ m, and the density of the material causes only minor variations in this value.

Quantal calculations are performed to determine the absorption profile of the broadened potassium resonance line $4p\leftarrow 4s$ in its far wings provoked by helium perturbers. First, the $X{}^2{\Sigma }^{+}$, $A{}^2\Pi ,$ and $B{}^2{\Sigma }^{+}$ potentials, as well as the transition dipole moments, are carefully computed through ab initio methods, based on state-averaged complete active space self-consistent field multireference configuration interaction (SA-CASSCF-MRCI) calculations involving the Davidson and basis-set superposition error (BSSE) corrections. The data are then used to generate the $\mathrm{KHe}$ photoabsorption spectra and to examine their behavior with temperature. The theoretical profile is dominated by the free-free transitions and exhibits, in the vicinity of the wavelength position of 693 nm, a satellite peak in the blue wing attributed to the $B\leftarrow X$ transitions. The results are compared with previous theoretical and experimental investigations and, in general, good agreement is found.

We present a quantum Monte Carlo (QMC) technique for calculating the exact finite-temperature properties of Bose-Fermi mixtures. The Bose-Fermi auxiliary-field quantum Monte Carlo (BFAFQMC) algorithm combines two methods, a finite-temperature AFQMC algorithm for bosons and a variant of the standard AFQMC algorithm for fermions, into one algorithm for mixtures. We demonstrate the accuracy of our method by comparing its results for the Bose-Hubbard and Bose-Fermi-Hubbard models against those produced using exact diagonalization for small systems. Comparisons are also made with mean-field theory and the worm algorithm for larger systems. As is the case with most fermion Hamiltonians, a sign or phase problem is present in the BFAFQMC algorithm. We discuss the nature of these problems in this framework and describe how they can be controlled with well-studied approximations to expand the BFAFQMC algorithm's reach. This algorithm can serve as an essential tool for answering many unresolved questions about many-body physics in mixed Bose-Fermi systems.

Motivated by recent experiments measuring the spin transport in ultracold unitary atomic Fermi gases [Sommer et al., Nature (London) 472, 201 (2011); Sommer et al., New J. Phys. 13, 055009 (2011)], we explore the theory of spin and heat transport in a three-dimensional spin-polarized atomic Fermi gas. We develop estimates of spin and thermal diffusivities and discuss magnetocaloric effects, namely the the spin Seebeck and spin Peltier effects. We estimate these transport coefficients using a Boltzmann kinetic equation in the classical regime and present experimentally accessible signatures of the spin Seebeck effect. We study an exactly solvable model that illustrates the role of momentum-dependent scattering in the magnetocaloric effects.

We show that the Bethe-Peierls (BP) boundary condition should be modified for ultracold atoms with spin-orbit (SO) coupling. Moreover, we derive a general form of the modified BP boundary condition, which is applicable to a system with an arbitrary kind of SO coupling. In the modified BP condition, an anisotropic term appears and the interatomic scattering length is normally SO-coupling dependent. For the special system in the current experiments, however, it can be proved that the scattering length is SO-coupling independent, and it takes the same value as in the case without SO coupling. Our result is helpful for the study of both few-body and many-body physics in SO-coupled ultracold gases.

Even at zero temperature, there exist phase fluctuations associated with an array of Bose-Einstein condensates confined in a one-dimensional optical lattice. We demonstrate a method to measure the phase fluctuations based on the Fourier spectrum of the atomic density for a condensate released from the optical lattice. The phase variance is extracted from the relative intensities of different peaks in the Fourier spectrum. This method works even for high lattice depth where interference peaks disappear in the atomic density distribution.

When interactions are turned off, the theory of interacting quantum and classical ensembles due to Hall and Reginatto is shown to suffer from a nonlocal signaling effect that is effectively action at a distance. This limits the possible applicability of the theory. In its present form, it is restricted to those situations in which interactions are always on, such as classical gravity interacting with quantized matter.

We study how multipartite entanglement evolves under the paradigm of separable operations, which include the local operations and classical communication as a special case. We prove that the average “decay” of entanglement induced by a separable operation is measure independent (among SL-invariant ones) and state independent: The ratio between the average output entanglement and the initial entanglement is solely a function of the separable operation, regardless of the input state and of the SL-invariant entanglement measure being used. We discuss the “disentangling power” of a quantum channel and show that it exhibits a similar state invariance as the average entanglement decay ratio. Our Rapid Communication significantly extends the bipartite results of  Horodecki and Horodecki, Quantum Inf. Comput. 10, 901 (2010), Konrad et al., Nat. Phys. 4, 99 (2008), and Tiersch et al., Phys. Rev. Lett. 101, 170502 (2008) as well as the multipartite one of Gour, Phys. Rev. Lett. 105, 190504 (2010), all of the previous work being restricted to one-sided or particular noise models.

We report on the observation of the collapse of a dipolar Bose-Einstein condensate (BEC) upon release from a one-dimensional optical lattice. In strong contrast with the typical behavior of quantum gases under time of flight (TOF), we show here that a strongly dipolar BEC, stabilized in the lattice, may become unstable during the TOF dynamics due to the combined effect of the anisotropy of the dipolar interaction and intersite coherence. Such TOF-triggered collapse is a peculiar case of a collapse induced by the deconfinement of a dipolar many-body system at a fixed two-body interaction strength.

Mean-field methods such as Hartree-Fock (HF) and Hartree-Fock-Bogoliubov (HFB) constitute the building blocks upon which more elaborate many-body theories are based. The HF and HFB wave functions are built out of independent quasiparticles resulting from a unitary linear canonical transformation of the elementary fermion operators. Here, we discuss the possibility of allowing the HF transformation to become nonunitary. The properties of such HF vacua are discussed, as well as the evaluation of matrix elements among such states. We use a simple ansatz to demonstrate that a nonunitary transformation brings additional flexibility that can be exploited in variational approximations to many-fermion wave functions. The action of projection operators on nonunitary-based HF states is also discussed and applied, in a variation-after-projection approach, to the one-dimensional Hubbard model with periodic boundary conditions.

We consider entropy in generalized nonsignalling theory (also known as box world) where the most common definition of entropy is the measurement entropy. In this setting, we completely characterize the set of allowed entropies for a bipartite state. We find that the only inequalities among these entropies are subadditivity and non-negativity. Surprisingly nonlocality does not play a role—in fact, any bipartite entropy vector can be achieved by separable states of the theory. This is in stark contrast to the case of the von Neumann entropy in quantum theory, where only entangled states satisfy $S\left(AB\right)<S\left(A\right)$.

Energy-angular distributions of electron-positron pair creation in collisions of a laser beam and a nonlaser photon are calculated using the $S$-matrix formalism. The laser field is modeled as a finite pulse, similar to the formulation introduced in our recent paper in the context of Compton scattering [Phys. Rev. A 85, 062102 (2012)]. The nonperturbative regime of pair creation is considered here. The energy spectra of created particles are compared with the corresponding spectra obtained using the modulated plane-wave approximation for the driving laser field. A very good agreement in these two cases is observed, provided that the laser pulse is sufficiently long. For short pulse durations, this agreement breaks down. The sensitivity of pair production to the polarization of a driving pulse is also investigated. We show that in the nonperturbative regime, the pair-creation yields depend on the polarization of the pulse, reaching their maximal values for the linear polarization. Therefore, we focus on this case. Specifically, we analyze the dependence of pair creation on the relative configuration of linear polarizations of the laser pulse and the nonlaser photon. Lastly, we investigate the carrier-envelope phase effect on angular distributions of created particles, suggesting the possibility of phase control in relation to the pair-creation processes.

We have measured absolute dissociative electron attachment (DEA) cross sections in methylacetylene (propyne, C${}_3$H${}_4$) and dimethylacetylene (but-2-yne, C${}_4$H${}_6$). The main feature in the low-energy DEA spectrum is a ${\pi }^{*}$ shape resonance giving rise to fragments at 3.4 eV in C${}_3$H${}_4$ and 4.0 eV in C${}_4$H${}_6$. The process C${}_3$H${}_4+e\to$ C${}_3$${\text{H}}_3^{-}$+H proceeds via abstraction of the acetylenic hydrogen which is mediated by effective vibronic coupling. The abstraction of methyl group hydrogen, which does not require symmetry lowering of the transient negative ion, was not observed; the spatial separation of the bond from the resonance decreases the cross section dramatically. The presence of the methyl group further influences the DEA cross sections via change of the resonance's position and via blocking one or both of the DEA active sites in acetylene. No cleavage of the C-methyl bond has been observed, and reasons for this effect are discussed. Additionally, several higher-lying resonances (6 to 15 eV) leading to production of an H${}^{-}$ fragment were observed and assigned.