An approach is proposed to account for the general effect of the nuclei motion in the intensity of electronic transitions caused by fast electron collision with molecular targets, following a similar procedure that has recently been used by our group to determine the optical oscillator strength of symmetry-forbidden transition, and which consists of expanding the (squared) transition moment along the normal coordinates of vibration. It is shown that the profile of total generalized oscillator strength as a function of the squared transferred moment can be significantly changed by the inclusion of terms that depend explicitly on the normal coordinates of vibration. The generalized oscillator strength for the inner-shell C $1\overrightarrow{s}3s{\sigma }_g$ transition in ${\mathrm{CO}}_2$ is calculated within this approach and compared with experimental results.

Processes leading to possible anharmonic dipole-plasmon oscillations in alkali-metal clusters are investigated. The electronic excitation spectrum of sodium clusters including highly excited states with more than one plasmon is calculated with an approach based on the separation of center of mass and intrinsic motion of delocalized electrons. The spectrum is analyzed in terms of collective plasmon oscillations, intrinsic electron motion, and their interaction. The latter is of primary importance. It is responsible for the anharmonicity of giant dipole oscillations. The predicted nonlinear blueshift of the dipole resonance is comparable with its spreading width, and therefore ought to be observed in photoabsorption experiments.

We demonstrate that Rydberg atoms can be transferred to states of lower principal quantum number by exposing them to a frequency chirped microwave pulse. Specifically, we have transferred $n=75\mathrm{}$ atoms to $n=66\mathrm{}$ with a 400-ns pulse chirped from 7.8 to 11.8 GHz. In spite of the large number of coupled levels, using a simplified model we can describe the process reasonably well as a sequence of adiabatic rapid passages.

We present calculations, based on the solution of the time-dependent Schrödinger equation for the hydrogen atom in a discrete basis demonstrating the possibility of extracting the phase profile of a harmonic through its interference with the fundamental. We examine two aspects of the interference: the generated harmonic radiation and the photoelectron energy spectra (if the harmonic is above the ionization threshold). We have focused on the third and the eleventh harmonics (of a 1.5 eV fundamental), where our results demonstrate a reconstruction of the phase profile with deviations of a few percent, on the basis of the modulation of the radiation. In the case of the 11th harmonic, we have also examined the PES modulation, and the phase profile calculated can be of the same quality, although more sensitive.

We have performed precision microwave spectroscopy on ultracold ${}^{87}\mathrm{Rb}$ confined in a magnetic trap, both above and below the Bose-condensation transition. The cold collision frequency shifts for both normal and condensed clouds were measured, which allowed the intrastate and interstate density correlations (characterized by sometimes controversial “factors of 2”) to be determined. Additionally, temporal coherence of the normal cloud was studied, and the importance of mean-field and velocity-changing collisions in preserving coherence is discussed.

The question of an efficient multimode description of optical pulses is studied. We show that a relatively very small number of nonmonochromatic modes can be sufficient for a complete quantum description of pulses with Gaussian quadrature statistics. For example, a three-mode description was enough to reproduce the experimental data of photon number correlations in optical solitons [S. Spälter, N. Korolkova, F. König, A. Sizmann, and G. Leuchs, Phys. Rev. Lett. 81, 786 (1998)]. This approach is very useful for a detailed understanding of squeezing properties of soliton pulses with the main potential for quantum communication with continuous variables. We show how homodyne detection and/or measurements of photon number correlations can be used to determine the quantum state of the multimode field. We also discuss a possible way of physical separation of the nonmonochromatic modes.

We report the observation of antibunched emission from single self-assembled InAs quantum dots under various conditions. We have measured the correlation function of the photon emission pertaining to single emission lines from single quantum dots under continuous and pulsed laser excitation, as well as under continuous white light excitation. The measurements were performed under different excitation intensities at liquid helium temperatures on two samples with distinct structures. At higher temperatures (30 K), an antibunching dip was still observed. We have also observed antibunching on a second emission line in the quantum dot spectrum, attributed to the biexciton, demonstrating the possibility of generating photon pairs with a single quantum dot. Polarization correlations on the biexciton and exciton line were also measured in an attempt to generate entangled photon pairs.

We analyze the output of a degenerate optical parametric amplifier using a different approach to understanding and characterizing multimode squeezed light. This approach is based on the concept of eigenmodes of the squeezing and predicts the mode structure of the local oscillator that should be used in order to measure the smallest quadrature noise. Although the importance of mode matching is well known, we find that typical experimental setups are suboptimal and we predict that in such cases squeezed light measurements stand to be improved noticeably with appropriate shaping (in space and/or time) of the local oscillator field in accordance with the results of the theory. Under conditions of negligible or small phase mismatch, the optimal local oscillator pulse duration is found to be approximately equal to the geometric mean of the pump pulse duration and nonlinear response time. Also, we find that the effective number of squeezed modes can be altered by varying the pump pulse duration and/or phase-matching parameters.

A four-level system is proposed to produce large index of refraction accompanied by vanishing absorption in the ${\mathrm{Er}}^{3+}$-doped yttrium aluminum garnet crystal. It is found that the separation of the two absorption peaks and the maximum value of index of refraction with zero absorption can be adjusted by changing the coherent field and the incoherent pumping. It is shown that a higher index of refraction with zero absorption can be easily obtained when the coherent field is off resonance.

In earlier work, expressions have been constructed for the single-particle kinetic-energy functional $T_s\left[\rho \right]$ for independent fermions subject to harmonic confinement in low dimensions, with $\rho$ the particle density. Here, the differential equation for $\rho$ is first obtained in d dimensions for an arbitrary number of closed shells. Then, by using the known Euler-Lagrange equation, the functional derivative $\delta T_s/\delta \rho \left(\mathbf{r}\right)$ is constructed. $T_s\left[\rho \right]$ itself is proved to take the form of a linear combination of three pieces: (i) a von Weizsäcker inhomogeneity kinetic energy, but with the original coefficient reduced by a dimensionality factor $1/d,$ (ii) a Thomas-Fermi kinetic energy, and (iii) a truly nonlocal contribution that, however, is shown to involve only the density $\rho$ itself and its first derivative. Thus, for this model, which is currently highly relevant to the interpretation of experiments on the evaporative cooling of dilute, and hence almost noninteracting, fermions, a complete density-functional theory now exists.

We extend to finite temperature a Green’s-function method that was previously proposed to evaluate ground-state properties of mesoscopic clouds of noninteracting fermions moving under harmonic confinement in one dimension. By calculations of the particle and kinetic-energy density profiles, we illustrate the role of thermal excitations in smoothing out the quantum shell structure of the cloud and in spreading the particle spill out from quantum tunnel at the edges. We also discuss the approach of the exact density profiles to the predictions of a semiclassical model often used in the theory of confined atomic gases at finite temperature.

Entanglement shared between the two ends of a quantum-communication channel has been shown to be a useful resource in increasing both the quantum and classical capacities for these channels. The entanglement-assisted capacities were derived assuming an unlimited amount of shared entanglement per channel use. In this paper, bounds are derived on the minimum amount of entanglement required per use of a channel, in order to asymptotically achieve the capacity. This is achieved by introducing a class of entanglement-assisted quantum codes. Codes for classes of qubit channels are shown to achieve the quantum entanglement-assisted channel capacity when an amount of shared entanglement per channel given by, ${\mathcal{E}}_Q^{\mathrm{Random}}>~1-Q_E,$ is provided. It is also shown that for very noisy channels, as the capacities become small, the amount of required entanglement converges for the classical and quantum capacities.

Quantum search has been proved to be the best possible algorithm for the exhaustive search problem in the sense that the number of queries it requires cannot be reduced. However, the number of nonquery operations, and thus the total number of operations, can be reduced. The number of nonquery unitary operations can be reduced by a factor of $\log N/\alpha \log \left(\log N\right)$ while increasing the number of queries by a factor of only $[1+(\log {N)}^{-\alpha }].$ For example, by choosing α to be $O(\log N/\log (\log N\left)\right),$ the number of nonquery unitary operations can be reduced by 40% while increasing the number of queries by just two.

Positronium (Ps) decay to four- and five-photon final states has been measured. From a sample of $3.73\times {10}^9$ detected events of Ps annihilation, the QED-allowed branching ratios of para-Ps to four photons $R_4=1.14\mathrm{}\left(33\right)\mathrm{}\left(21\right)\mathrm{}\times {10}^{-6}$ (stat., sys., 68% C.L.), and ortho-Ps to five photons $R_5=1.67\mathrm{}\left(99\right)\mathrm{}\left(37\right)\mathrm{}\times {10}^{-6}$ (68% C.L.) were measured. No charge-conjugation symmetry violating events were observed, establishing a branching ratio limit for $(o-\mathrm{Ps}\to 4\gamma )$ of $R_4^{\mathbf{C}\mathbf{/}}<3.7\mathrm{}\times {10}^{-6}$ (90% C.L.) and a new limit for $(p-\mathrm{Ps}\to 5\gamma )$ of $R_5^{\mathbf{C}\mathbf{/}}<2.7\mathrm{}\times {10}^{-7}$ (90% C.L.). The experiment was performed using the Gammasphere array of Compton-suppressed high-purity germanium detectors. Gammasphere’s segmentation, high efficiency, and excellent energy resolution allowed event discrimination sufficient to observe the rare decay modes in the presence of substantial backgrounds.

An analytical expression for the external well of the ${\mathrm{Cs}}_2{0}_g^{-}{(6s+6P}_{3/2})$ double-well potential curve is derived, involving asymptotic parameters fitted on the spectrum obtained from photoassociation of ultracold cesium atoms and ultracold molecule formation [Fioretti et al., Europhys. J. D 5, 389 (1999)]. The results are compared to a previous Rydberg-Klein-Rees determination from our group, and may have consequences on the determination of cesium triplet scattering length. Values for radiative lifetimes ${\tau }_{3/2}=30.462\mathrm{}\left(3\right)\mathrm{}$ ns and ${\tau }_{1/2}=34.88\mathrm{}\left(2\right)\mathrm{}$ ns of the ${6P}_{3/2}$ and ${6P}_{1/2}$ atomic levels, respectively, are extracted with an accuracy better than previous determinations. The method of Derevianko and Porsev [Phys. Rev. A 65, 053403 (2002)], allows the derivation of an improved value for the van der Waals coefficient of ground-state cesium molecule, $C_6^{6s}=6828\mathrm{}\left(19\right)\mathrm{}$ a.u.

The photodetachment of the metastable ${\mathrm{He}}^{-}1s2s2p{}^4{P}^o$ state has been calculated in two photon energy regions of interest: the first, named here as energy region I, below the double photoionization threshold ${\mathrm{He}}^{+}\mathrm{}(n=1\mathrm{}),$ involving outer-shell ionization and doubly excited states of ${\mathrm{He}}^{-},$ and the second, named here as region II, above the ${\mathrm{He}}^{-}$ $1s$ ionization threshold and below the ${\mathrm{He}}^{+}\mathrm{}(n=2\mathrm{})\mathrm{}$ threshold, involving K-shell detachment and triply excited states of a ${\mathrm{He}}^{-}$ “hollow ion.” We have implemented an ab initio three-electron configuration-interaction method in the LS-coupling scheme combined with complex scaling to obtain resonance positions and widths and the photodetachment cross sections. We have revisited region I, although widely studied before, as a test of our method. Notwithstanding some small discrepancies, our complex scaling results compare well with the previously published results and also add new understanding to some features in the cross section. Our emphasis is given to K-shell photodetachment in photon energy region II, where comparison is made with two other recent theoretical calculations that use completely different methods, and that were in dispute. We also compare with a very recent experiment for the ${\mathrm{He}}^{-}$ K-shell photodetachment, which displays three major features; a broad nonresonant hump after the $\mathrm{He}2s2p{}^3{P}^o$ threshold and two other peaks. A complex scaling analysis of prominent structures in the photodetachment spectra in region II, previously claimed to be nonresonant structures, leads to a different conclusion; i.e., they are all true triply excited-state resonances, and two of them correspond to the peaks observed experimentally.

The electron-hydrogen scattering problem is studied in the Faddeev-Merkuriev integral equation approach. These integral equations possess compact kernels for all energies, therefore they are especially well suited to study this problem. The equations are solved by using the Coulomb-Sturmian separable expansion technique. We present S- and P-wave scattering and reactions cross sections up to the $\mathrm{H}\mathrm{}(n=4\mathrm{})\mathrm{}$ threshold.

In the modeling of the astrophysical plasmas, the relative elemental abundance inferred from solar and stellar upper atmosphere can be affected by as much as a factor of 5 due to the uncertainties in the current dielectronic recombination (DR) rate coefficients used to analyze the spectra [Savin and Laming, Astrophys. J. 566, 1166 (2002)]. DR rate coefficients for oxygenlike ions have been identified as the most urgent needs for the astrophysical applications. In this work, we report on the calculations of DR rate coefficients for Mg V, Si VII, S IX, and Fe XIX ions which are important for the modeling of the astrophysical plasmas. The calculations are carried out in isolated resonance and distorted-wave approximations. The relevant atomic data are calculated using the multiconfigurational Dirac-Fock method. We include $2s-2p,$ ${2p}_{1/2}-{2p}_{3/2},$ $2\mathcal{l}-3{\mathcal{l}}^{\prime },$ and $1s-2p$ excitations and cover temperatures ranging from 0.001 eV to 10 000 eV. For low temperatures, it is essential to have accurate DR resonance energies and to include fine-structure excitations in order to obtain reliable DR rate coefficients. Good agreement with experiment has been found for Fe XIX. For Mg V, Si VII, and S IX, significant discrepancies are noted between this work and recommended rate coefficients.

The photodissociation of ${\mathrm{H}}_2^{+}$ and ${\mathrm{HD}}^{+}$ by an intense laser pulse is investigated by solving the close-coupling equations without discretization. For the case of ${\mathrm{H}}_2^{+}$ the photodissociation spectra are calculated under the condition mimicking the experimental one, and a fairly good agreement with the experiment is obtained. The uncertainty in the relative phases of initial states is found to lead to somewhat smoothing of the spectra, depending on the pulse length. It is also found that Raman-type transitions via intermediate dissociation continuum play an important role in determining the photodissociation spectra. This leads to a population increase of lower vibrational states and deforms the spectral profile. Dissociation from the lower vibrational states due to the bond softening is not strong enough. Photodissociation spectra and angular distribution are calculated also for ${\mathrm{HD}}^{+}$ under the same conditions as in the ${\mathrm{H}}_2^{+}$ case. The dipole transitions lead to additional structures in the energy spectra and angular distribution. There is a noticeable difference in the peak positions of dissociation spectrum for particles dissociated by the direct electronic-dipole transition and by the transitions via intermediate bound states. The photodissociation dynamics is further clarified by using the three-dimensional plots of the spectra as a function of the field intensity and frequency.

Coherent carrier control in quantum nanostructures is studied within the framework of optimal control. We develop a general solution scheme for the optimization of an external control (e.g., lasers pulses), which allows to channel the system’s wave function between two given states in its most efficient way; physically motivated constraints, such as limited laser resources or population suppression of certain states, can be accounted for through a general cost functional. Using a generic three-level scheme for the quantum system, we demonstrate the applicability of our approach and identify the pertinent calculation and convergence parameters.