The strong field ionization of ${\mathrm{Cl}}_2$ molecules is investigated by using an ultrashort pulse $\mathrm{Ti}$:sapphire laser. A spatial imaging technique is used in such measurements to reduce the effect of spatial integration. ${\mathrm{Cl}}_2$ shows strong ionization suppression as do other diatomic molecules having valence orbitals with antibonding symmetry $\left({\mathrm{O}}_2{\mathrm{,S}}_2\right)$ when compared with the field ionization of atoms with nearly identical ionization potential. A more general molecular tunneling ionization model is proposed, and the calculations are in reasonable agreement with the measurements. Our results support that antibonding leads to ionization suppression, a trend that only ${\mathrm{F}}_2$ goes against and that needs to be further investigated.

In this work, a derivative of spectrum of polarization resonances in $5S_{1∕2}\text{−}5D_{5∕2}$ transitions of $\mathrm{Rb}$ atoms at different values of longitudinal magnetic field, which occurs due to the Faraday effect, was observed in a temperature controlled and magnetically shielded cell. The amplitude and spectral shape of Faraday resonances were investigated as a function of external magnetic field and the experimental results are described by a simple theoretical model.

We discuss the long-distance transmission of qubits encoded in optical coherent states. Through absorption, these qubits suffer from two main types of errors, namely the reduction of the amplitude of the coherent states and accidental application of the Pauli $Z$ operator. We show how these errors can be fixed using techniques of teleportation and error-correcting codes.

We investigate the relationship between mixedness and entanglement for Gaussian states of continuous variable systems. We introduce generalized entropies based on Schatten $p$ norms to quantify the mixedness of a state and derive their explicit expressions in terms of symplectic spectra. We compare the hierarchies of mixedness provided by such measures with the one provided by the purity (defined as $\mathrm{tr}{\varrho }^2$ for the state $\varrho$) for generic $n$-mode states. We then review the analysis proving the existence of both maximally and minimally entangled states at given global and marginal purities, with the entanglement quantified by the logarithmic negativity. Based on these results, we extend such an analysis to generalized entropies, introducing and fully characterizing maximally and minimally entangled states for given global and local generalized entropies. We compare the different roles played by the purity and by the generalized $p$ entropies in quantifying the entanglement and the mixedness of continuous variable systems. We introduce the concept of average logarithmic negativity, showing that it allows a reliable quantitative estimate of continuous variable entanglement by direct measurements of global and marginal generalized $p$ entropies.

Double-electron transfer and two-step single-electron transfer in collisions of proton with potassium-metal target are measured in the collision energy from $0.3\text{to}4\mathrm{keV}$ by using a charge-inversion mass spectrometry. Two prominent ${\mathrm{H}}^{-}$ ion peaks are observed with different values of the energy loss and show different target density dependences. The peak with larger energy loss is identified as double-electron transfer and the other as two-step successive single-electron transfer from the analysis of the target density dependence. The two-step single-electron transfer is considered to occur as the process accompanying spontaneous Ly-$\alpha$ emission, followed by negative ${\mathrm{H}}^{-}$ formation. A theoretical analysis is also carried out, and the single-electron transfer cross section obtained is found to be in excellent agreement with the present measurement, while the present measurement for double-electron transfer is found to be much smaller than those evaluated earlier and the present theory.

We report a detailed study of the electronic structure, energies, and the nature of bonding in various $\mathrm{Al}\text{−}\mathrm{Li}$ clusters: ${\mathrm{Al}}_n{\mathrm{Li}}_n(n=1–11)$, ${\mathrm{Al}}_2^{-}$, ${\mathrm{Al}}_2^{2-}$, ${\mathrm{Al}}_2\mathrm{Li}$, ${\mathrm{Al}}_2{\mathrm{Li}}^{-}$, and ${\mathrm{Al}}_6{\mathrm{Li}}_8$. We use the standard Born-Oppenheimer molecular dynamics within the framework of density-functional theory. The growth structure in these clusters is found to occur in two broad categories: the first (for $n=2–4$) has the structure of a bent rhombus, and the second $(n=7–9,11)$ has the structure of a pentagonal ring. A substantial charge transfer between the atoms is observed in nearly all the clusters with the exception of ${\mathrm{Al}}_2{\mathrm{Li}}_2$. In this cluster, a hybridization of the $2s$ orbital of $\mathrm{Li}$ with the $3p$ orbital of $\mathrm{Al}$ is observed. In clusters with more than six $\mathrm{Al}$ atoms, the eigenvalue spectrum is divided into two main groups: at the lower end of the energy spectrum, jelliumlike orbitals are observed, while at the higher end, localized bonds are seen. The localized bond formation appears to result from the interaction between $p$ electrons on each of the $\mathrm{Al}$ atoms. We also have found a tetravalent structure for $\mathrm{Al}$ atoms within the $\mathrm{Al}\text{−}\mathrm{Li}$ cluster arising due to a charge transfer from the $\mathrm{Li}$ atoms to the $\mathrm{Al}$ atoms.

Infrared quadrupole modes (IRQM) of the valence electrons in light deformed sodium clusters are studied by means of the time-dependent local-density approximation. IRQM are classified by angular momentum components $\lambda \mu =20$, 21, and 22 whose $\mu$ branches are separated by cluster deformation. In light clusters with a low spectral density, IRQM are unambiguously related to specific electron-hole excitations, thus giving access to the single-electron spectrum near the Fermi surface. Most of the IRQM are determined by cluster deformation and so can serve as a sensitive probe of the deformation effects in the mean field. The IRQM branch $\lambda \mu =21$ is coupled with the magnetic scissors mode, which enables us to detect the latter. We discuss two-photon processes, Raman scattering, stimulated emission pumping, and stimulated adiabatic Raman passage as the relevant tools to observe IRQM. An alternative method to detect the IRQM population in clusters is proposed.

We study the nonlinear process of $e^{-}{e}^{+}$ pair creation by a nucleus which moves at a relativistic energy in the laboratory frame and collides with an intense x-ray laser beam. The collision system under consideration is chosen in such a way that the simultaneous absorption of at least two photons from the laser wave is required in order to exceed the energy threshold of the reaction. We calculate total and differential rates for both free-free and bound-free pair production. In the case of free-free pair creation we demonstrate the effect of the laser polarization on the spectra of the produced particles, and we show that at very high intensities the total rate exhibits features analogous to those well known from above-threshold ionization rates for atoms. In the case of bound-free pair creation a singularity is found in the laboratory frame angular distribution of the produced positron. This singularity represents a distinct characteristic of the bound-free pair production and allows one to separate this process from free-free pair creation even without detecting a bound state of the captured electron. For both types of pair creation we consider the dependences of the total rates on the collision parameters, give the corresponding scaling laws, and discuss the possibility to observe these nonlinear processes in a future experiment.

Our recent paper reports the experimental realization of a one-atom laser in a regime of strong coupling [J. McKeever, A. Boca, A. D. Boozer, J. R. Buck, and H. J. Kimble, Nature (London) 425, 268 (2003)]. Here we provide the supporting theoretical analysis relevant to the operating regime of our experiment. By way of a simplified four-state model, we investigate the passage from the domain of conventional laser theory into the regime of strong coupling for a single intracavity atom pumped by coherent external fields. The four-state model is also employed to exhibit the vacuum-Rabi splitting and to calculate the optical spectrum. We next extend this model to incorporate the relevant Zeeman hyperfine states as well as a simple description of the pumping processes in the presence of polarization gradients and atomic motion. This extended model is employed to make quantitative comparisons with our earlier measurements for the intracavity photon number versus pump strength and for the photon statistics as expressed by the intensity correlation function $g^{\left(2\right)}\left(\tau \right)$.

The second-order intensity correlation function for the light transmitted from a driven optical cavity containing identical two-level atoms is calculated in the weak-field limit by using the equations for expectation values of operators. Atomic nonradiative dephasing is taken into account in addition to cavity-field damping and atomic radiative decay. The correlation function including the atomic nonradiative dephasing rate is numerically calculated. An analytic expression for the correlation function is also derived in the case of purely radiative atomic decay.

Close to converged energies and expectation values for $e^{+}\mathrm{Li}$ are computed using a ground state wave function consisting of 1200 explicitly correlated Gaussians. The best estimate of the $e^{+}\mathrm{Li}$ energy was $-\mathrm{7.532\hspace{0.17em}895\hspace{0.17em}5}$ hartree which has a binding energy of 0.002 482 hartree against dissociation into $\mathrm{Ps}+{\mathrm{Li}}^{+}$. The $2\gamma$ annihilation rate for the spin singlet state was $6.996\times {10}^9{\mathrm{s}}^{-1}$. The annihilation rate for the triplet state, taking into account core annihilation and the $3\gamma$ decay, was $9.36\times {10}^6{\mathrm{s}}^{-1}$.

Recent work on $\Lambda$ resonances in alkali-metal vapors (E. Mikhailov, I. Novikova, Yu. V. Rostovtsev, and G. R. Welch, e-print quant-ph/0309171, and references therein) investigated a type of electromagnetically induced absorption resonance that occurs in three-level systems under specific conditions normally associated with electromagnetically induced transparency. In this note, we show that these resonances have a direct analog in nonlinear magneto-optics, and support this conclusion with a calculation for a $J=1\to J^{\prime }=0$ system interacting with a single, nearly circularly polarized light field in the presence of a weak longitudinal magnetic field.

We present a systematic study on a class of stable vortex solutions in a two-component Bose-Einstein condensate. They are nontopological in nature and possess continuous angular momenta. When coupled to external fields, nontopological vortices exhibit “spin” dynamical behavior and develop a Berry’s phase under the adiabatic change of external fields which can be directly measured. We explain a method to create them experimentally by using phase imprinting.

We discuss the relationship between entropic uncertainty relations and entanglement. We present two methods for deriving separability criteria in terms of entropic uncertainty relations. In particular, we show how any entropic uncertainty relation on one part of the system results in a separability condition on the composite system. We investigate the resulting criteria using the Tsallis entropy for two and three qubits.

The dynamics of two two-level dipole-dipole interacting atoms coupled to a common electro-magnetic bath and closely located inside a lossy cavity, is reported. Initially injecting only one excitation in the two-atom cavity system, loss mechanisms asymptotically drive the matter sample toward a stationary maximally entangled state. The role played by the closeness of the two atoms, with respect to such a cooperative behavior, is carefully discussed. Stationary radiation trapping effects are found and transparently interpreted.

We present a model for the correlation hole to partially correct the errors present in short-range correlation when the hole is expanded in a one-particle basis set. Results for several two-electron test systems are presented and discussed. In general, the short-range correlation hole model improves the total energy substantially, reaching true chemical accuracy of one milliHartree with a modest basis set.

An ab initio calculation of the electron capture cross sections for collisions of ground and metastable states of ${\mathrm{N}}^{2+}$ with $\mathrm{H}\left(1s\right)$ is presented. Total cross sections are evaluated for ${}^{14}\mathrm{N}$ impact energies from $2\times {10}^{-3}\text{to}300\mathrm{keV}$, using both quantal and semiclassical treatments. The results are compared with experimental and previous theoretical data, and are used to check the presence of metastable ions in the beams employed in the experiments. Partial cross sections are also presented and related to the collision mechanism.

We investigate the electronic structure of the beryllium atom subjected to a strong magnetic field in the regime $0⩽\gamma ⩽10\mathrm{a.u.}$. The ground as well as many excited states of spin-singlet, -triplet, and -quintet multiplicity covering the magnetic quantum numbers $\mid M\mid =0,1,3,6$ for both positive and negative $z$ parity are discussed and analyzed. Total and one-particle ionization energies are presented. Transition wavelengths as a function of the field strengths for allowed dipole transitions are provided.

Real-time impulsive resonant Raman scattering induced by a $4\text{−}\mathrm{fs}$ ultrashort laser pulse was spectrally resolved with a multichannel lock-in amplifier. An optical and vibrational two-dimensional spectrum was introduced to separate the effects of electronic and vibrational broadening in the power spectrum of the vibrations. Spectrally resolved real-time oscillations can identify the wave packet motion associated with the stretching vibrational mode of the carbon-carbon double and triple bonds of the polymer polydiacetylene. Calculated nonlinear polarizations which take into account the ground-state wave packets reproduced the observed dynamics fairly well. It is confirmed that the ground-state vibrations play a significant role in the dynamics of the wave packet even far below the absorption edge, as well as the excited-state vibrations.

It is shown that coherent superpositions of two oppositely polarized $n$-photon states can be created by postselecting the transmission of $n$ independently generated photons into a single-mode transmission line. It is thus possible to generate highly nonclassical $n$-photon polarization states using only the bunching effects associated with the bosonic nature of photons. The effects of mode-matching errors are discussed and the possibility of creating $n$-photon entanglement by redistributing the photons into $n$ separate modes is considered.