We report theoretical close-coupling calculations for electron-impact excitation cross sections of He-like Ti XXI ions using the R-matrix method, including relativistic and radiation damping effects. The cross sections are in good agreement with experimental measurements from the electron-beam ion trap [Chantrenne et al., Phys. Rev. Lett. 69, 265 (1992)] for the four prominent x-ray transitions: 1${\mathit{s}}^2$${(}^1$${\mathit{S}}_0$)→1s2s${(}^3$${\mathit{S}}_1$), 1s2p${(}^3$${\mathit{P}}_{1,2}^{\mathit{o}}$), 1s2p${(}^1$${\mathit{P}}_1^{\mathit{o}}$), indicating that both the relativistic and the resonance effects, with radiative damping, may affect the electron-impact excitation process significantly in highly charged ions.

A technique to cancel the frequency shift due to cold collisions of laser-cooled atoms is proposed. Results for future clocks based on ${}_{}{}^{135}{}_{}{}^{}\mathrm{Cs}$ and ${}_{}{}^{137}{}_{}{}^{}\mathrm{Cs}$ are presented. We find a ${}_{}{}^{137}{}_{}{}^{}\mathrm{Cs}$ fountain clock can be operated with no cold-collision shift by adjusting the population ratios in the fountain, and it may also be possible for ${}_{}{}^{135}{}_{}{}^{}\mathrm{Cs}$. This technique can also be applied to other phase-sensitive measurements using cold atoms, including atom interferometers.

Let the density matrix of an arbitrary state be expressed in terms of number states as ρ^=${\sum }_{\mathit{n}=0\mathrm{}}^{\mathrm{\infty }}$${\sum }_{\mathit{m}=0\mathrm{}}^{\mathrm{\infty }}$ρ(n,m)‖n〉〈m mU. We prove the following theorem: If ρ(0,0)=0, then the state is as nonclassical as possible according to the measure introduced by Lee [Phys. Rev. A 44, R2775 (1991)].

The "incoming" interaction picture found by Xu and Tsai [Phys. Rev. A 41, 53 (1990)], used to develop a convenient formulation of the closed-time-path approach to Green's function theory, is shown to be a particular case of a more general set of solutions.

Controversial points in this Comment concerning the general picture transformations which may give rise to a variety of interaction pictures are analyzed.

In a recent article [Phys. Rev. A 50, R2803 (1994)] De Caro and Garuccio state that many parametric-down-conversion sources cannot be used in tests of Bell's inequalities because they produce quantum-mechanical product states. Here we point out that not only have suitable sources already been proposed in the literature, but experiments not suffering from the problems cited by the authors have also been reported.

From the hypotheses of realism and stationarity, the latter related to the Leggett-Garg assumption of non-invasive measurability, we derive inequalities, involving three autocorrelation functions, to be fulfilled by any two-state stochastic process. We seek to test a broad class of realist theories against quantum mechanics for a system consisting of a Rydberg atom interacting with a single quantized mode in a superconducting resonant cavity. Departures from quantum predictions are enhanced when the temperature is decreased to about 0.5 K and the state of the cavity approximates a Fock state.

In crossed electric and magnetic fields the hydrogen atom undergoes a transition to chaotic scattering associated with a critical point in the Hamiltonian flow. The stability of the critical point is determined and leads to an accurate prediction of the transition to scattering that is independent of the magnetic-field strength. Nevertheless, observed variations in the apparent ionization threshold with magnetic-field strength are explained.

We report close-coupling calculations in a basis of molecular states, modified with a common translation factor, and augmented with a set of pseudostates, for ${\mathrm{H}}^{+}$ + ${\mathrm{He}}^{+}$ collisions for impact energies from 9 to 300 keV/amu.

By using a combination of laser photodetachment and resonant ionization spectroscopy, the binding energy of the $1s^{2}2s2p^2{}_{}{}^4{}_{}{}^{}P$ state in ${\mathrm{Be}}^{-}$ is determined with much improved accuracy to be 290.99±0.10 meV. This value is considerably larger than the recent measurement of 261±10 meV, but only slightly above the new theoretical value of 289.1±1.0 meV. The transition wavelength of $1s^{2}2s2p^2{}_{}{}^4{}_{}{}^{}P_{\frac{3}{2}-1\mathrm{}{s}^{2}2p^3{}_{}{}^4{}_{}{}^{}S_{\frac{3}{2}}^{}{}_{}{}^{}}^{}{}_{}{}^{}$ is determined by laser excitation to be at 2653.18±0.08 Å, yielding the binding energy of the $1s^{2}2p^3{}_{}{}^4{}_{}{}^{}S$ state in ${\mathrm{Be}}^{-}$ to be 295.49±0.25 meV, in good agreement with the newest theoretical result of 295.0±0.7 meV.

We study the ionization of the ${\mathrm{H}}_2^{+}$ molecular ion in intense, short-pulse laser fields by numerically solving the three-dimensional time-dependent Schrödinger equation as a function of internuclear distance $R$. Anomalously high ionization for the molecular ion at large internuclear separations is observed for orientations parallel to the linearly polarized laser field. The ionization rate is found to exhibit maxima at large $R$, exceeding the atom limit by an order of magnitude. This is attributed to transitions between pairs of chargeresonant states which are strongly coupled by the field in diatomic molecular ions. The effect is shown to also occur in higher odd-charge diatomic molecular ions and can be attributed to field-induced nonadiabatic transitions between the charge-resonant states and electron tunneling suppression by the instantaneous Stark field of the laser.

We describe a method for loading paramagnetic atoms or molecules into a magnetic trap. A ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ buffer gas is employed to thermalize atoms or molecules to a temperature of approximately 240 mK, lower than the depth of the trap. A model is described that indicates an initial loading density of approximately ${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Once loading has taken place the buffer gas is removed by cryopumping. Evaporative cooling can then be applied to further lower the temperature and increase the density of the trapped sample.

It is shown that, as a result of recoil during spontaneous emission, optical activity in a vapor can be induced under the action of resonant pump and probe fields. For field geometries that exclude other mechanisms for rotation of the probe field's plane of polarization, the recoil accompanying spontaneous emission provides the needed coupling between the spatial and internal atomic degrees of freedom to produce an optically active medium. A transition between ground and excited states, each having angular momentum $J=\frac{1}{2}$, is analyzed in detail. The effect can be interpreted as the result of pump field scattering off a spatial polarization grating of ground-state Zeeman coherence.

It is shown that the main part of the differences between classical and quantum predictions concerning the microwave ionization of H-atom threshold frequency dependence, commonly explained as a manifestation of quantum localization, originates from the enhanced, in classical simulations, role of the Coulomb singularity. When the Coulomb potential is softened, classical simulations reproduce quantum predictions and experimental data satisfactorily. No interference, intrinsically important for localization phenomena, is necessary.

Two-color excitation of a hydrogen atom has been studied by solving the time-dependent Schrödinger equation. We have chosen the two laser frequencies to be the fundamental and its third harmonic with an adjustable phase between the two laser fields. This has been proposed and successfully implemented in the past for "coherent control" of atomic and chemical processes. Here we have extended the study to the high-intensity domain and well into the tunneling and the stabilization regimes. We show that in the tunneling regime the two-color excitation can enhance the intensity of harmonics by more than two orders of magnitude compared to a single color at the same effective intensity. In the stabilization regime we confirm that two-color excitation can be used to generate higher harmonics.

Photons incident on a periodic medium at the Bragg angle are dynamically diffracted and exhibit the pendellösung phenomenon familiar from x-ray and neutron-diffraction experiments. The entangled two-photon state incident from the positive and negative Bragg angle exhibits a pendellösung length that is half that for the single-photon case. In our experiment the photon pairs were produced via parametric down-conversion and the periodic medium was a crystal whose refractive index was periodically altered by an acoustic wave. The predicted Pendellösung-length behavior was experimentally confirmed.

In a time-symmetric form, classical relativistic formulations involve not only a reference time but also relatively advanced and retarded times. This work examines a simple model problem having this structure with the equations of motion having closed-form solutions. The complexity of more realistic models is avoided while the general features are retained. The determinism and casuality of the theory are clearly demonstrated. Some problems associated with the quantization of power series representations using conventional procedures are illustrated and discussed.

The pseudomomentum is conserved for a single charge in any constant (over space and time) electromagnetic field. It is also conserved for a neutral two-body system, not only in a pure (constant) magnetic field, but also in a pure (constant) electric field, and explicit wave equations can be written. A neutral two-body system in a constant electric field is treated in a way that parallels our previous approach to this system in a constant magnetic field. Relative motion is separated, but the relative time now survives, whereas a spacelike degree of freedom gets eliminated. In other cases, there is, in general, no evidence of a closed form of the wave equations, but conservation of the pseudomomentum can be required as a reasonable condition for implicitly determining the so-called ‘‘three-body terms’’ in the wave equation.

Instantaneous Galilean invariance is used to derive from first principles the expression for the Hamiltonian of an electron with a position-dependent effective mass, as well as the adequate boundary conditions for the wave function in the case of abrupt heterojunctions. A very elementary model sustaining these results in the envelope-function approximation is also proposed.