Shablov, Bilyk, and Popov [Phys. Rev. A $65,$ 042719 (2002)] claimed to have analyzed the convergent close-coupling (CCC) method within the framework of the rigorous Coulomb scattering theory, but without electron exchange. They concluded that “… the amplitude obtained within the framework of this method in principle does not converge to the observable physical amplitude.” We correct a misunderstanding of the origins of the CCC equations, and show that no-exchange CCC calculations exhibit no ready convergence, off or on the energy shell while those with exchange show convergence, but only on the energy shell. Since all previously published comparisons of CCC with experiment utilized on-shell amplitudes from calculations which included exchange, we question the stated conclusion.

A simple scheme to realize quantum controlled dense coding with a bright tripartite entangled state light generated from nondegenerate optical parametric amplifiers is proposed in this paper. The quantum channel between Alice and Bob is controlled by Claire. As a local oscillator and balanced homodyne detector are not needed, the proposed protocol is easy to be realized experimentally.

We present a theoretical investigation of the stability properties of semiconductor lasers with strong feedback from dispersive extended cavities. Surprisingly, unstable behavior had been observed experimentally for chirped fiber grating lasers, where the stability of the laser was found to depend upon the orientation of the fiber grating. We reproduce this finding through a linear stability analysis, demonstrating that the presence of both the linewidth enhancement of the semiconductor diode, and a negative curvature of the phase of the external cavity reflection coefficient are necessary for instability to occur. In order to explain the role of the linewidth enhancement and phase curvature, we present a second approach based on more approximate model wherein the field evolution is found to be described by an equation that resembles the nonlinear Schrödinger equation (NLSE); the curvature of the phase then corresponds to the dispersion coefficient of a usual NLSE, and the linewidth enhancement factor corresponds to the nonlinear coefficient. We find an unstable regime analogous to the anomalous dispersion regime of the usual NLSE, where the boundary between normal and anomalous dispersion depends upon the width of the reflectivity spectrum. We also find that there is an additional unstable region that arises due to the carrier dynamics, and has no analogy in systems with an instantaneous nonlinearity. Further, for lasers with a negatively chirped grating, we find that oscillation tends to occur on the red side of the reflection spectrum peak.

We report on a measurement of a highly forbidden magnetic-dipole transition amplitude in ytterbium using the Stark-interference technique. This amplitude is important in interpreting a future parity nonconservation experiment that exploits the same transition. We find $|〈{5d6s}^3{D}_1|M1\mathrm{}|{6s}^{21}{S}_0〉|{=1.33\mathrm{}\left(6\right)\mathrm{}}_{\mathrm{Stat}}{\mathrm{}\left(20\right)\mathrm{}}_{\beta }\times {10}^{-4}{\mu }_0,$ where the larger uncertainty comes from the previously measured vector transition polarizability $\beta .$ The $M1\mathrm{}$ amplitude is small and should not limit the precision of the parity nonconservation experiment.

The density matrix $\rho$ for an n-level system is decomposed into the minimum number of pure states necessary to calculate physical observables. The corresponding physical system is first represented by a set $\mathcal{B}$ of n pure states $|{\beta }_i〉,$ together with their statistical weights. The time evolution of the system is therefore calculated as $\mathcal{B}\mathrm{}\left(t\right)=U\mathcal{B}{(t}_0),$ with the propagator U operating on each member of the set, in contrast to the more laborious $\rho \mathrm{}\left(t\right)=U\rho {(t}_0{)U}^{†}.$ At least one of the states can be eliminated from the set by reducing its weight to zero via a simple transformation of the density matrix. When there are degenerate weights, the transformation is applied using the weight with the largest degeneracy. Thus, even if none of the original statistical weights are equal to zero, so that $\mathrm{rank}\left(\rho \right)=n,$ the system can be described by a set of m states with $m<n.\mathrm{}$ The resulting minimal set provides a concommitant increase in calculational efficiency that depends on the fractional degeneracy. The independent evolution of the states comprising the set also provides an inherent parallelism that can be readily utilized in numerical computations. In addition to efficiency and conceptual simplification, this decomposition can provide physical insight, since the explicit contribution of each pure state to the dynamics of the system is readily apparent.

The lateral Casimir force acting between a sinusoidally corrugated gold plate and sphere was calculated and measured. The experimental setup was based on the atomic force microscope specially adapted for the measurement of the lateral Casimir force. The measured force oscillates sinusoidally as a function of the phase difference between the two corrugations. Both systematic and random errors are analyzed and a lateral force amplitude of $3.2\times {10}^{-13}\mathrm{N}$ was measured at a separation distance of 221 nm with a resulting relative error 24% at a 95% confidence probability. The dependence of the measured lateral force amplitude on separation was investigated and shown to be consistent with the inverse fourth power distance dependence. The complete theory of the lateral Casimir force is presented including finite conductivity and roughness corrections. The obtained theoretical dependence was analyzed as a function of surface separation, corrugation amplitudes, phase difference, and plasma wavelength of a metal. The theory was compared with the experimental data and shown to be in good agreement. The constraints on hypothetical Yukawa-type interactions following from the measurements of the lateral Casimir force are calculated. The possible applications of the lateral vacuum forces to nanotechnology are discussed.

A quantum theory of feedback of bosonic many-atom systems is formulated. The feedback-induced many-atom correlations are treated by use of a parametrized correlation function, for which closed equations of motion are derived. Therefrom the dynamics of any additive property of the system, i.e., properties derived from the reduced single-atom density operator, can be obtained. An example is given that indicates the correlation effects of feedback.

Quantum teleportation is possible because entanglement allows a definition of precise correlations between the noncommuting properties of a local system and corresponding noncommuting properties of a remote system. In this paper, the exact causality achieved by maximal entanglement is analyzed and the results are applied to the transfer of effects acting on the entanglement distribution channels to the teleported output state. In particular, it is shown how measurements performed on the entangled system distributed to the sender provide information on the teleported state while transferring the corresponding backaction to the teleported quantum state.

Solid-harmonic derivatives of solid-harmonic-Gaussian integrals are evaluated. Cross differentiation and the n-j generalized Gaunt coefficients are defined. The generalized Gaunt coefficients ensure that cross differentiation in uncoupled, n-center, solid-harmonic derivatives of rotationally invariant Gaussian matrix elements subtracts zero total angular momentum. This preserves the spherical-tensor properties of quantum-chemical matrix elements. The generalized Gaunt coefficients are (n-1)-dimensional objects because the sum of the azimuthal quantum numbers is zero, which facilitates their use in computer programs. The 4-j and higher number of centers generalized Gaunt coefficients are characterized by quantum numbers $k_{\mathrm{ij}},$ the angular momentum lost directly by cross differentiation from solid harmonics of differential operators about centers i and j. The generalized Gaunt coefficients vanish if the absolute value of the sum of the i and j azimuthal quantum numbers is greater than the sum of the two total angular momenta minus twice $k_{\mathrm{ij}}.$ This constraint further limits the number of coefficients that must be processed at one time. All coefficients for any given number of centers satisfy the same recurrence relations but differ, because different terms in any given recurrence relation vanish according to the various azimuthal-quantum-number bounds. Direct and recursive FORTRAN-90 code has been written for the 3-j to 6-j generalized Gaunt coefficients.

Angle-resolved energetic-ion-yield spectra were observed in the region of the O $1s$ to Rydberg excitation of ${\mathrm{CO}}_2$ with incident photon energy resolving power $E/\Delta E\simeq 14000.$ The assignments of the electronic states were established in accordance with the angular distribution data. All the observed vibrational structures were assigned to the antisymmetric stretching vibrations caused by the vibronic coupling between the nearly degenerate O $1s$ core-excited states. A Franck-Condon analysis was performed within the linear coupling model.

We have identified the dipole-forbidden $1s2s2p{}^4{P}_{5/2}\to {1s}^{2}2s{}^2{S}_{1/2}$ transition in lithiumlike ${\mathrm{Ar}}^{15+}$ in high-resolution K-shell x-ray emission spectra recorded at the Livermore EBIT-II electron-beam ion trap and the Princeton National Spherical Tokamak Experiment. Unlike other ${\mathrm{Ar}}^{15+}$ satellite lines, which can be excited by dielectronic recombination, the line is exclusively excited by electron-impact excitation. Its predicted radiative rate is comparable to that of the well-known $1s2p{}^3{P}_1\to {1s}^2{}^1{S}_0$ magnetic quadrupole transition in heliumlike ${\mathrm{Ar}}^{16+}.$ As a result, it can also only be observed in low-density plasma. We present calculations of the electron-impact excitation cross sections of the innershell excited ${\mathrm{Ar}}^{15+}$ satellite lines, including the magnetic sublevels needed for calculating the linear line polarization. We compare these calculations to the relative magnitudes of the observed $1s2s2\overrightarrow{p}{1s}^{2}2s$ transitions and find good agreement, confirming the identification of the lithiumlike $1s2s2p{}^4{P}_{5/2}\to {1s}^{2}2s{}^2{S}_{1/2}$ magnetic quadrupole line.

The formation of long-lived ${\mathrm{CO}}^{2+}$ from ${\mathrm{CO}}^{+}$ via photoionization in the energy range 25 to 45 eV was studied experimentally at high spectral resolution. All five allowed components of a Rydberg series with an electronically excited ${}^3{\Sigma }^{+}(\nu =0)$ core are identified. Four components of a second Rydberg series with a vibrationally excited ${}^3{\Sigma }^{+}(\nu =1)$ core and structure due to the initial vibrational state of the ion beam are also discernible. The total photoionization cross section was measured with a vibrationally relaxed ${\mathrm{CO}}^{+}$ ion beam. Franck-Condon factors from ground state ${\mathrm{CO}}^{+}$ to the relevant ${\mathrm{CO}}^{2+}$ states were calculated.

The breakup dynamics of ${\mathrm{H}}_3^{+}$ and ${\mathrm{D}}_3^{+}$ following dissociative recombination is studied using the combination of two-dimensional imaging and storage ring techniques. The vibrational distributions of the molecular ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ fragments produced in the two body fragmentation channel were measured, as well as the kinematical correlation between the hydrogen or deuterium atoms produced in the three body channel. For the latter, we find predominantly linear dissociation geometries. The data also show that the initial molecular ions ${\mathrm{H}}_3^{+}$ and ${\mathrm{D}}_3^{+},$ which were stored and electron cooled for up to 40 s prior to recombination, are still rotationally hot.

We report the generation of polarization-entangled photons by femtosecond-pulse-pumped spontaneous parametric down-conversion in a cascade of two type-I crystals. Highly entangled pulsed states were obtained by introducing a temporal delay between the two orthogonal polarization components of the pump field. They exhibited high-visibility quantum interference and a large concurrence value, without the need of postselection using narrow-bandwidth spectral filters. The results are well explained by the theory which incorporates the space-time dependence of interfering two-photon amplitudes if dispersion and birefringence in the crystals are appropriately taken into account. Such a pulsed entangled photon well localized in time domain is useful for various quantum communication experiments, such as quantum cryptography and quantum teleportation.

Relativistic configuration-interaction calculations yield lifetimes of 19.8, 32.2, 35.0, 48.2, and 84.9 s for Ne, Ar, Kr, Xe, and Rn. For Ar, Kr, and Xe the experimental values are 38, 39, and 43 s. This resolves the theoretical-experimental discrepancy, where theory could be as large as twice experiment (Xe). An accurate description of the $p^{5}d$ basis function is essential.

Silver and tin slab targets were irradiated by line-focused chirped pulse amplification glass laser light. In this experiment, the laser pulses consisted of two pulses with 4 ps duration, separated by 1.2 ns. Strong amplification in the nickel-like silver and tin x-ray lasers at the wavelengths of 13.9 and 12.0 nm was demonstrated with pumping energy of 12 and 14 J, respectively, and gain-saturation behavior could be seen. A hydrodynamics simulation coupled with a collisional-radiative model was performed under the present experimental conditions, and the calculated result was compared with the experimental results.

The R-matrix-Floquet technique for atoms in strong laser fields is extended to describe double photoionization by combining it with B-spline basis sets. One advantage of this extended R-matrix-Floquet approach is that the effects of the intense laser field on double photoionization can be taken into account nonperturbatively. The results agree with existing experimental and theoretical results for double photoionization of the ground state of He within 5% above a photon energy of 4.2 a.u. We further apply this approach to study double photoionization of the autoionizing ${2s}^2{}^1{S}^e$ state of He to demonstrate its capability in dealing with autoionization of the initial state and photoionization simultaneously.

We theoretically study rotational Raman coherence excited impulsively in a hydrogen-gas-filled hollow fiber by an fs pump pulse with an arbitrary ellipticity. The results of our simulations based on a rotationally invariant formalism show that the use of an elliptically polarized pulse leads to more efficient phonon excitation than that of a linearly or circularly polarized one. The phonon amplitude dramatically depends both on ellipticity and propagation distance. The passage of a probe pulse in this excited media leads to the generation of high-order Raman sidebands in the pulse.

We propose and discuss a scheme for robust and efficient generation of many-particle entanglement in an ensemble of Rydberg atoms with resonant dipole-dipole interactions. It is shown that in the limit of complete dipole blocking, the system is isomorphic to a multimode Jaynes-Cummings model. While dark-state population transfer is not capable of creating entanglement, other adiabatic processes are identified that lead to complex, maximally entangled states, such as the N-particle analog of the Greenberger-Horne-Zeilinger (GHZ) state in a few steps. The process is robust, works for even and odd particle numbers and the characteristic time for entanglement generation scales linearly with N.

What entanglement is present in naturally occurring physical systems at thermal equilibrium? Most such systems are intractable and it is desirable to study simple but realistic systems that can be solved. An example of such a system is the one-dimensional infinite-lattice anisotropic $\mathrm{XY}$ model. This model is exactly solvable using the Jordan-Wigner transform, and it is possible to calculate the two-site reduced density matrix for all pairs of sites. Using the two-site density matrix, the entanglement of formation between any two sites is calculated for all parameter values and temperatures. We also study the entanglement in the transverse Ising model, a special case of the $\mathrm{XY}$ model, which exhibits a quantum phase transition. It is found that the next-nearest-neighbor entanglement (though not the nearest-neighbor entanglement) is a maximum at the critical point. Furthermore, we show that the critical point in the transverse Ising model corresponds to a transition in the behavior of the entanglement between a single site and the remainder of the lattice.