Sputtering the surface of a TiAlN(C,O) specimen with a $14.5-{\mathrm{keV}\mathrm{}\mathrm{Cs}}^{+}$ ion beam produces negative molecular ions of ${\mathrm{N}}_2^{\mathrm{-}}$ and ${\mathrm{CO}}^{\mathrm{-}}.$ They are detected in a double-focusing mass spectrometer operated at a mass resolution of $\sim 13000;$ this provides a separation of interfering species in this mass region (e.g., ${\mathrm{Si}}^{\mathrm{-}},$ ${\mathrm{AlH}}^{\mathrm{-}}$) and a mass assignment at a precision of $\sim {10}^{-4}\mathrm{amu}.$ The flux of ${\mathrm{N}}_2^{\mathrm{-}}$ relative to the stable ${\mathrm{N}}_3^{\mathrm{-}}$ amounts to about 0.3. On average, one ${\mathrm{N}}_2^{\mathrm{-}}$ is formed for ${10}^7$ N atoms sputtered and a similar rate is observed for ${\mathrm{CO}}^{\mathrm{-}}.$ The flight time through the mass spectrometer ($\sim 10\mu \mathrm{s}$) sets a lower limit for the stability of these molecular anions. The abundant production of electronically excited molecules in sputtering is proposed to constitute the prerequisite for the formation of ${\mathrm{N}}_2^{\mathrm{-}}$ and ${\mathrm{CO}}^{\mathrm{-}}:$ these excited molecules may capture a surface electron forming thus a temporary negative-ion state (Feshbach resonance). By contrast, the upper limit for the emission of ${\mathrm{N}}^{\mathrm{-}}$ is found to be less than one ${\mathrm{N}}^{\mathrm{-}}$ per ${10}^{10}$ nitrogen atoms ejected, in agreement with the very short lifetimes of the excited ${\mathrm{N}}^{\mathrm{-}}$ anions.

We have measured the x-ray emission from high-$n$ spectator electrons following radiative stabilization in dielectronically excited highly charged barium. ${\mathrm{Ba}}^{q+}\hspace{0.5em}(40\mathrm{}<~q<~42)$ ions with open-shell configurations were produced with an electron beam ion trap tuned to an energy at which dielectronic capture into states with $n$=6-8 takes place. The highly excited spectator electron decays via radiative cascades, and the dominant signature of this process is an enhanced emission in regions of the barium spectra where transitions from higher $n$ to the $n=3\mathrm{}$ level are located.

We study the x-ray $L$-shell production from large krypton clusters submitted to ultrashort and intense laser pulses. The x-ray photon emission pattern appears to be isotropic and the absolute x-ray photon yields per laser pulse are measured as a function of the laser intensity and of the estimated mean cluster size in the supersonic expansion. In particular, up to $4\times {10}^6$ x-ray photons per laser shot are detected at intensities approaching $5\times {10}^{17}$ ${\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ This allows us to determine precisely a maximum conversion efficiency of $1.7\times {10}^{-8}$ between the incoming IR photon and the generated x-ray photon energies. Finally, the x-ray photon emission is understood as the result of highly stripped ion production with $L$-shell electron-impact ionization and excitation in laser-heated cluster-sized nanoplasmas.

In the present work we are interested in near-threshold photoionization experiments involving postcollision effects related to the Auger decay of a vacancy in the Ar $L$ shell. In particular, we have measured the photoelectron energy spectrum resulting from the above postcollision interaction effects and have observed electrons produced by the process of electron capture and reemission.

The collective excitations of Bose condensates in anisotropic axially symmetric harmonic traps are investigated in the hydrodynamic and Thomas-Fermi limit. We identify an additional conserved quantity, besides the axial angular momentum and the total energy, and separate the wave equation in elliptic coordinates. The solution is thereby reduced to the algebraic problem of diagonalizing finite-dimensional matrices. The classical quasiparticle dynamics in the local-density approximation for energies of the order of the chemical potential is shown to be chaotic.

Numerical simulations of high-order harmonic generation in the molecular ions ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_3^{2+}$ show that two-color intense $\left(I\simeq {10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2\right)$ short $(<\mathrm{}1-\mathrm{psec})$ laser pulse excitation produces harmonics at large internuclear distances with larger efficiencies (intensities) at ${6U}_p,$ ${8U}_p,$ and ${10U}_p$ energies than in single-atom single-frequency excitation. The two-color excitation scheme creates efficient high harmonics by first ionizing electrons with a high-frequency laser pulse and accelerating them with lower frequency towards neighboring atoms. Such two-color schemes should be useful for enhancing high-order harmonic generation in clusters.

We show that a certain phase feedback between output and input changes dramatically the characteristics of photorefractive two-beam coupling. The diffraction efficiency of the dynamic phase grating, $\eta$, reaches 1 or 0 (depending on the feedback sign) within a finite time for a wide range of experimental parameters. The spatial-temporal behavior of the wave amplitudes is strongly nonlinear and the grating fringes for $\eta =1\mathrm{}$ or $0$ are bended but not tilted. The theory elucidates a number of empirical observations and reveals the great potential of active stabilization for applications.

Quantum error-correcting codes are analyzed from an information-theoretic perspective centered on quantum conditional and mutual entropies. This approach parallels the description of classical error correction in Shannon theory, while clarifying the differences between classical and quantum codes. More specifically, it is shown how quantum information theory accounts for the fact that “redundant” information can be distributed over quantum bits even though this does not violate the quantum “no-cloning” theorem. Such a remarkable feature, which has no counterpart for classical codes, is related to the property that the ternary mutual entropy vanishes for a tripartite system in a pure state. This information-theoretic description of quantum coding is used to derive the quantum analog of the Singleton bound on the number of logical bits that can be preserved by a code of fixed length which can recover a given number of errors.

In a recent article [Phys. Rev. A 54, 1793 (1996)] Krenn and Zeilinger investigated the conditioned two-particle correlations for the subensemble of data obtained by selecting the results of the spin measurements by two observers 1 and 2 with respect to the result found in the corresponding measurement by a third observer. In this paper we write out explicitly the condition required in order for the selected results of observers 1 and 2 to violate Bell’s inequality for general measurement directions $e{\to }_1,$ $e{\to }_1^{\prime },$ $e{\to }_2,$ $e{\to }_2^{\prime },$ and $e{\to }_3.$ It is shown that there are infinitely many sets of directions giving the maximum level of violation. Further, we extend the analysis by the authors to the class of triorthogonal states $|\Psi 〉{=c}_1|z_1〉|z_2〉|z_3〉{+c}_2|-z_1〉|-z_2〉|-z_3〉.$ It is found that a maximal violation of Bell’s inequality occurs provided the corresponding three-particle state yields a direct (“all or nothing”) nonlocality contradiction.

The fine-structure interval of $P$ states in hydrogenlike systems can be determined theoretically with high precision, because the energy levels of $P$ states are only slightly influenced by the structure of the nucleus. Therefore a measurement of the fine structure may serve as an excellent test of QED in bound systems, or alternatively as a means of determining the fine-structure constant $\alpha$ with very high precision. In this paper an improved analytic calculation of higher-order binding corrections to the one-loop self-energy of 3$P$ and 4$P$ states in hydrogenlike systems with a low nuclear charge number $Z$ is presented. The method of calculation has been described earlier by Jentschura and Pachucki [Phys. Rev. A 54, 1853 (1996)], and is applied here to the excited $P$ states. Because of the more complicated nature of the wave functions and the bound-state poles corresponding to decay of the excited states, the calculations are more complex. Comparison of the analytic results to the extrapolated numerical data for high-$Z$ ions [Mohr and Kim, Phys. Rev. A 45, 2727 (1992)] serves as an independent test of the analytic evaluation. Theoretical values for the Lamb shift of the $P$ states and for the fine-structure splittings are given.

This paper describes an algorithm for selecting a consistent set within the consistent histories approach to quantum mechanics and investigates its properties. The algorithm uses a maximum information principle to select from among the consistent sets formed by projections defined by the Schmidt decomposition. The algorithm unconditionally predicts the possible events in closed quantum systems and ascribes probabilities to these events. A simple spin model is described and a complete classification of all exactly consistent sets of histories formed from Schmidt projections in the model is proved. This result is used to show that for this example the algorithm selects a physically realistic set. Other tentative suggestions in the literature for set selection algorithms using ideas from information theory are discussed.

One of the significant successes in the field of neutron interferometry has been the experimental observation of the phase shift of a neutron de Broglie wave due to the action of the Earth’s gravitational field. Past experiments have clearly demonstrated the effect and verified the quantum-mechanical equivalence of gravitational and inertial masses to a precision of about 1%. In this experiment the gravitationally induced phase shift of the neutron is measured with a statistical uncertainty of order 1 part in 1000 in two different interferometers. Nearly harmonic pairs of neutron wavelengths are used to measure and compensate for effects due to the distortion of the interferometer as it is tilted about the incident beam direction. A discrepancy between the theoretically predicted and experimentally measured values of the phase shift due to gravity is observed at the 1% level. Extensions to the theoretical description of the shape of a neutron interferogram as a function of tilt in a gravitational field are discussed and compared with experiment.

Classically forbidden reflection by a potential step is analyzed by matching WKB waves to solutions of a Schrödinger equation involving the tail of the potential on the up side of the step. Analytic expressions for the leading deviation of the reflection probability from unity and the phase of the reflection amplitude at the top of the step are derived for potentials decaying as an inverse power of the coordinate $V\left(x\right)\mathrm{}\propto -{1/x}^{\alpha },\hspace{0.5em}\alpha >2\mathrm{}$.

We propose a method for reconstruction of the density matrix from measurable time-dependent (probability) distributions of physical quantities. The applicability of the method based on least-squares inversion is—compared with other methods—very universal. It can be used to reconstruct quantum states of various systems, such as harmonic and anharmonic oscillators including molecular vibrations in vibronic transitions and damped motion. It also enables one to take into account various specific features of experiments, such as limited sets of data and data smearing owing to limited resolution. To illustrate the method, we consider a Morse oscillator and give a comparison with other state-reconstruction methods suggested recently.

It was recently predicted that any metallic beam splitter must have a small probability of splitting a single photon into a pair of secondary photons, conserving energy in the process. This nonlinear effect is a consequence of the quantization of the field and is somewhat analogous to other nonlinear effects in QED. The splitting of single photons by a metallic beam splitter has now been observed experimentally and is in good agreement with the theoretical predictions.

One way to solve the long-standing “measurement”problem in quantum mechanics is to modify Schrödinger’s equation so that the collapse of a state vector is the result of some intrinsic physical dynamics. The introduction of such collapse dynamics often results in energy nonconservation. In this paper, we first derive a general expression for the rate of change in the expectation value of energy in a general collapse model that supports a linear evolution of the density matrix. In particular, we show, under certain plausible assumptions, that energy nonconservation is an inevitable consequence of collapse. We then work with a specific model, namely, the nonrelativistic continuous spontaneous localization (CSL) model, to derive further consequences of collapse. In CSL, in a nonunitary evolution, a particle interacts with a fluctuating scalar field, leading to the collapse of the state vector. One consequence of this interaction is that a free electron can radiate spontaneously. We calculate the spectrum of this radiation. The result is then compared with the observed upper bound on spontaneous radiation in Ge and a constraint on the parameters of CSL is obtained.

If one analyzes the effects of electromagnetic vacuum fluctuations upon an electron interference pattern in an approximation in which the electrons follow classical trajectories, an ultraviolet divergence results. It is shown that this divergence is an artifact of the classical trajectory approximation, and is absent when the finite sizes of electron wave packets are accounted for. It is shown that the vacuum fluctuation effect has a logarithmic dependence upon the wave-packet size. However, at least in one model geometry, this dependence cancels when one includes both vacuum fluctuation and photon emission effects.

The photoabsorption spectra near the $1s$ threshold of N${}_2$ and CO are calculated by the multiscattering self-consistent-field method. Our calculated near-threshold spectra are in good agreement with the experimental data. All the excited states are treated in a unified manner in the framework of the quantum defect theory. The $Z+1\mathrm{}$ model potential with the relaxed core hole is adopted in our calculation.

The electronic structure of the hydrogen molecule is investigated for the parallel configuration. The ground states of the Σ manifold are studied for ungerade and gerade parity as well as singlet and triplet states covering a broad regime of field strengths from $B=0\mathrm{}$ up to $B=100\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{}$ A variety of interesting phenomena can be observed. For the ${}^1{\Sigma }_g$ state we found a monotonous decrease of the equilibrium distance and a simultaneous increase of the dissociation energy with growing magnetic-field strength. The ${}^3{\Sigma }_g$ state is shown to develop an additional minimum which has no counterpart in field-free space. The ${}^1{\Sigma }_u$ state shows a monotonous increase in the dissociation energy with a first increasing and then decreasing internuclear distance of the minimum. For this state the dissociation channel is ${\mathrm{H}}_2{\to \mathrm{H}}^{\mathrm{-}}{+\mathrm{H}}^{+}$ for magnetic field strengths $B\gtrsim 20\mathrm{a}.\mathrm{u}.\mathrm{}$ due to the existence of strongly bound ${\mathrm{H}}^{\mathrm{-}}$ states in strong magnetic fields. The repulsive ${}^3{\Sigma }_u$ state possesses a very shallow van der Waals minimum for magnetic-field strengths smaller than 1.0 a.u. within the numerical accuracy of our calculations. The ${}^1{\Sigma }_g$ and ${}^3{\Sigma }_u$ states cross as a function of $B$ and the ${}^3{\Sigma }_u$ state, which is an unbound state, becomes the ground state of the hydrogen molecule in magnetic fields $B\gtrsim 0.2\mathrm{a}.\mathrm{u}.\mathrm{}$ This is of particular interest for the existence of molecular hydrogen in the vicinity of white dwarfs. In superstrong fields the ground state is again a strongly bound state, the ${}^3{\Pi }_u$ state.

In a classical model, the dynamics of the hydrogen atom subjected to a circularly polarized microwave field and a magnetic field is shown to belong to the family of so-called biparametric quadratic Hamiltonians. The energy-level structure is studied in terms of the parametric bifurcations.