We report on an experimental observation of the qualitative signatures of symmetry-induced quantum-interference effects in above-threshold-ionization spectra of ${\mathrm{O}}_2$ (antibonding ${\pi }_g$ symmetry) and ${\mathrm{N}}_2$ (bonding ${\sigma }_g$ symmetry). It confirms the corresponding predictions of intense-field S-matrix theory.

New insight into the nature of Bohm’s quantum force is presented. Based on our numerical study, we conclude that, for the kicked pendulum, Bohm’s quantum-force time-series for nonstationary states is typically or generically non-Gaussian stable distributed with a flat power spectrum. For fixed system parameters and initial wave function, the stable parameters and the constant value of the power spectrum are independent of the initial Bohmian angle. We conjecture that these properties of the quantum-force time-series are also typical or generic for other classically chaotic Hamiltonian dynamical systems since the kicked pendulum is a prototypical member of this class of systems. A new method of calculating the quantum probability density of a particle’s position implied by these quantum-force properties is described.

By limiting the resolution of quantum measurements, measurement-induced changes of the quantum state can be reduced, permitting subsequent measurements of variables that do not commute with the initially measured property. It is then possible to determine correlations between noncommuting variables experimentally. The application of this method to the polarization statistics of entangled photon pairs reveals that negative conditional probabilities between nonorthogonal polarization components are responsible for the violation of Bell’s inequalities. Such negative probabilities can also be observed in finite-resolution measurements of the polarization of a single photon. The violation of Bell’s inequalities therefore originates from local properties of the quantum statistics of single-photon polarization.

This paper explores an application of quantum entanglement. The problem treated here is the quantum channel identification problem: given a parametric family $\{{\Gamma }_{\theta }{\}}_{\theta }$ of quantum channels, find the best strategy of estimating the true value of the parameter θ. As a simple example, we study the estimation problem of the isotropic depolarization parameter θ for a two-level quantum system $\mathcal{H}\simeq {\mathbf{C}}^2.$ In the framework of noncommutative statistics, it is shown that the optimal input state on $\mathcal{H}\otimes \mathcal{H}$ to the channel exhibits a transitionlike behavior according to the value of the parameter θ.

We discuss the problem of transfering a qubit from Alice to Bob using a noisy quantum channel and only finite resources. As the basic protocol for the transfer we apply quantum teleportation. It turns out that, for a certain quality of the channel, direct teleportation combined with qubit purification is superior to entanglement purification of the channel. If, however, the quality of the channel is rather low, one should simply apply an estimation-preparation scheme.

We address the question of whether the entanglement of formation is additive over tensor products of full matrix algebras, and derive a necessary and sufficient condition for optimality of vector states. By discussing two particular cases, we suggest that such a condition may be used either to prove or to find a counterexample to additivity.

Photoionization cross sections and asymmetry $\left(\beta \right)$ parameters for Kr $3s,$ $3p,$ and $3d$ subshells have been measured and calculated in the 300–1300-eV photon energy range. Good agreement between experiment and theory is found for both cross-section branching ratios and $\beta$ parameters. Interchannel coupling among the channels arising from $3s,$ $3p,$ and $3d$ subshells is found to be necessary for quantitative accuracy of the theory. This shows that the interchannel coupling phenomenology far above threshold, found previously for outer shells of Ne and Ar, is also operative for inner atomic shells.

Through an extensive numerical study, we find that the low-lying, quasidegenerate eigenenergies of weakly interacting trapped N bosons with total angular momentum L are given in the case of small $L/N$ and sufficiently small L by $E=Lħ\omega +g\left[N\right(N-L/2\mathrm{}-1)+1.59n(n-1)/2],$ where $\omega$ is the frequency of the trapping potential and g is the strength of the repulsive contact interaction. The last term arises from the pairwise repulsive interaction among n octupole excitations and describes the lowest-lying excitation spectra from the yrast line. In this case, the quadrupole modes do not interact with themselves and, together with the octupole modes, exhaust the low-lying spectra that are separated from others by energy gaps linear in N.

We formulate a first-principles description of the behavior of a bidirectional ring cavity containing a saturable absorbing medium, and driven by an external coherent field. The setting of interest to this study is an extension of the ordinary optically bistable system, where a ring resonator supports only one of the two possible directions of propagation of the cavity field. In this more general case and in the uniform-field limit, we show that the dynamics of the fields-atoms system is described by an infinite set of coupled equations that can be readily solved by standard numerical means after appropriate truncation of the number of atomic variables. With the help of two different approaches, we find unexpected, long-time stationary solutions such that the forward and backward fields oscillate with different carrier frequencies: the first is based upon the time-dependent equations and the second on a set of nonlinear-algebraic equations describing the steady states. As an added confirmation, we carry out a linear-stability analysis of the unidirectional steady state for the purpose of identifying the conditions under which a backward field can grow from an initial fluctuation. By this approach we also suggest a possible strategy for the experimental observation of stationary, frequency-shifted output fields.

A class of coupled-channel cavity QED models describing two-photon resonant Raman processes in multiple-Λ configuration are solved analytically by an algebraic method that introduces an angular momentum algebra for this multiwave mixing system.

We show a general initial value form of the semiclassical propagator. Similar to cellular dynamics, this formulation involves only the nearby orbits approximation: the evolution of nearby orbits is approximated by linearized dynamics. This phase space smearing formulation keeps the accuracy of the original Van Vleck–Gutzwiller propagator. As an illustration, we present a simple initial value form of the semiclassical propagator. It is nonsingular everywhere and is efficient for numeric implementation.

Including the Breit interaction leads to a 0.6% reduction in the magnitude of the $6s-7s$ parity-nonconservation (PNC) amplitude in ${}^{133}\mathrm{Cs},$ confirming a result recently obtained by A. Derevianko [Phys. Rev. Lett. 85, 1618 (2000)]. A revised value of the theoretical PNC amplitude for ${}^{133}\mathrm{Cs}$ is given; the corresponding value of the weak charge shows no noticeable deviation from the standard model.

Analytical relations between kinetic-energy and particle densities are set out here, and used for numbers of harmonically confined independent fermions up to 100 particles. The use of positive definite kinetic-energy density is advocated in the present context, and this is then shown to tend to the von Weizsäcker inhomogeneity kinetic-energy density near the tunneling regime for the harmonic potential barrier, the example of 50 fermions being studied specifically.

The energy loss of the fragments resulting from the dissociation of fast ${\mathrm{N}}_2^{+}$ molecular ions when traversing amorphous carbon foils has been calculated. In addition to using the dielectric formalism to describe the electronic excitations of the stopping medium and the Coulomb repulsion to account for the variation of internuclear distances, we have also taken into account the transient charge states of each fragment. The inclusion of this effect was crucial in order to obtain a satisfactory comparison of our calculations with the available experimental data.

Damping factors arise from static as well as dynamic perturbations. These must appear with constant sign in the perturbation theory denominators for compatibility with quantum mechanics, conformity with time-reversal symmetry, and agreement with experiment.

We show that damping factors must not be incorporated in the perturbation of the ground state by a static electric field. If they are included, as in the theory of Stedman et al. [preceding Comment, Phys. Rev. A 63, 047801 (2001)], then there would be an electric dipole in the y direction induced in a hydrogen atom in the $M_S=+\frac{1}{2}$ state by a static electric field in the x direction. Such a dipole is excluded by symmetry.

Quantum degeneracy modifies light scattering and spontaneous emission. For fermions, Pauli blocking leads to a suppression of both processes. In contrast, in a weakly interacting Bose-Einstein condensate, we find spontaneous emission to be enhanced, while light scattering is suppressed. This difference is attributed to many-body effects and quantum interference in a Bose-Einstein condensate.

A magnetic waveguide structure allows switching of neutral atoms between two guides. The switch consists of lithographically patterned current-carrying wires on a sapphire substrate. By selectively sending current through a particular set of wires, we select the desired output port of an incoming beam. We utilize two different magnetic-guiding schemes to adiabatically manipulate the atom trajectory.

A bipartite spin-1/2 system having the probabilities $\mathrm{}(1+3x)/4\mathrm{}$ of being in the Einstein-Podolsky-Rosen (EPR) entangled state $|{\Psi }^{-}〉\equiv (1/\sqrt{2})\left(|\uparrow {〉}_A|\downarrow {〉}_B-|\downarrow {〉}_A|\uparrow {〉}_B\right)$ and $3\left(1-x\right)/4\mathrm{}$ of being orthogonal is known to admit a local realistic description if and only if $x<\mathrm{}1/3\mathrm{}$ (Peres criterion). We consider here a more general case where the probabilities of being in the entangled states $|{\Phi }^{\pm }〉\equiv (1/\sqrt{2})(|\uparrow {〉}_A|\uparrow {〉}_B\pm |\downarrow {〉}_A|\downarrow {〉}_B)$ and $|{\Psi }^{\pm }〉\equiv (1/\sqrt{2})(|\uparrow {〉}_A|\downarrow {〉}_B\pm |\downarrow {〉}_A|\uparrow {〉}_B)$ (Bell basis) are given, respectively, by $\left(1\mathrm{}-x\right)/4,$ $\left(1\mathrm{}-y\right)/4,$ $\left(1\mathrm{}-z\right)/4,$ and $\mathrm{}(1+x+y+z)/4.\mathrm{}$ Following Abe and Rajagopal, we use the nonextensive entropic form $S_q\equiv \left(1\mathrm{}-\mathrm{Tr}{\rho }^q\right)/\left(q-1\right)$$(q\in \mathcal{R}{;S}_1=-\mathrm{Tr}\rho \ln \rho )$ which has enabled a current generalization of Boltzmann-Gibbs statistical mechanics, and determine the entire region in the $\mathrm{}(x,y,z)\mathrm{}$ space where the system is separable. For instance, in the vicinity of the EPR state, separability occurs if and only if $x+y+z<\mathrm{}1,$ which recovers Peres’ criterion when $x=y=z.\mathrm{}$ In the vicinity of the other three states of the Bell basis, the situation is identical. These results illustrate the computational power of this nonextensive-quantum-information procedure. In addition to this, a critical-phenomenon-like scenario emerges which enrichens the discussion.