The multiconfiguration time-dependent Hartree-Fock approach for the description of correlated few-electron dynamics in the presence of strong laser fields is introduced and a comprehensive description of the method is given. Total ionization and electron spectra for the ground and first excited ionic channels are calculated for one-dimensional model systems with up to six active electrons. Strong correlation effects are found in the shape of photoelectron peaks and the dependence of ionization on molecule size.

Using the molecular strong-field approximation we consider the effects of molecular symmetry on the ionization of molecules by a strong, linearly polarized laser pulse. Electron angular distributions and total ionization yields are calculated as a function of the relative orientation between the molecule and the laser polarization. Our studies focus on ethylene $\left({\mathrm{C}}_2{\mathrm{H}}_4\right)$, benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$, fluorobenzene $\left({\mathrm{C}}_6{\mathrm{H}}_5\mathrm{F}\right)$, and ortho chlorofluorobenzene $(1,2{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{ClF})$, the molecules representing four different point groups. The results are compared with experiments, when available, and with the molecular tunneling theory appropriately extended to nonlinear polyatomic molecules. Our investigations show that the orientational dependence of ionization yields is primarily determined by the nodal surface structure of the molecular orbitals.

Control pulse cleaning (CPC) is the process of experimentally removing extraneous control field features in a closed-loop quantum dynamics optimization experiment. We demonstrate CPC in ionization processes in the model systems ${\mathrm{K}}_2$ and NaK by applying evolution strategies during the closed loop shaping of fs pulses. The CPC technique operates by applying genetic pressure on the spectral components with appropriate cost functions, and CPC is investigated in the case of weak $\left({\mathrm{K}}_2\right)$ and of strong cleaning (NaK). A Pareto-optimal curve is constructed for the latter case, which reveals the correlation of the two generally conflicting goals of meeting the physical objective versus cleaning the control pulse. At weak genetic pressure unnecessary pulse components could be removed with marginal influence on the ionization process, whereas at strong genetic pressure the optimal degree of ionization is affected and, interestingly, electronic transitions to particular vibrational states are exposed. Thus, employing CPC while seeking optimal controls allows for extracting information about the chosen dynamical pathways including the significant intermediate states.

A different model potential for the Rydberg molecule is used. The potential is the total average interaction energy of this electron with all the nuclei and other electrons. We find that the introduction of a core potential results in an extreme increase of the number of closed orbits as compared to the hydrogen atom. In order to remove the singularities of the classical motion of the Hamiltonian at the nucleus, we implement the Thiele-Burrau transformation. Making use of molecular closed-orbit theory and our model potential, we calculated the recurrence spectra of the ${\mathrm{H}}_2$ molecule in parallel electric and magnetic fields for different quantum defects. The modulations in the spectra can be analyzed in terms of the scattering of the excited electron on the molecular core. Unlike the case of the Rydberg atom where elastic scattering is predominant, modulations produced by inelastic scattering are also vital to the photoabsorption spectrum of the Rydbeg molecule. The Fourier-transformed spectrum of the ${\mathrm{H}}_2$ molecule has allowed direct comparison between peaks in the spectrum and the scaled action values of closed orbits of the excited electron in external fields. Our results are in good agreement with the quantum results, which suggests that our method is correct.

In this paper we study the response in time of ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{F}}_2$ to laser pulses having a wavelength of $390\mathrm{nm}$. We find single-ionization suppression in ${\mathrm{O}}_2$ and its absence in ${\mathrm{F}}_2$, in accordance with experimental results at $\lambda =800\mathrm{nm}$. Within our framework of time-dependent density functional theory we are able to explain deviations from the predictions of intense-field many-body $S$-matrix theory (IMST). We confirm the connection of ionization suppression with destructive interference of outgoing electron waves from the ionized electron orbital. However, the prediction of ionization suppression, justified within the IMST approach through the symmetry of the highest occupied molecular orbital (HOMO), is not reliable since it turns out that—e.g., in the case of ${\mathrm{F}}_2$—the electronic response to the laser pulse is rather complicated and does not lead to dominant depletion of the HOMO. Therefore, the symmetry of the HOMO is not sufficient to predict ionization suppression. However, at least for ${\mathrm{F}}_2$, the symmetry of the dominantly ionized orbital is consistent with the nonsuppression of ionization.

Two simple schemes for generating macroscopic (many-photon) continuous-variable entangled states by means of continuous interactions (rather than collisions) between solitons in optical fibers are proposed. First, quantum fluctuations around two time-separated single-component temporal solitons are considered. Almost perfect correlation between the photon-number fluctuations can be achieved after passing a certain distance, with a suitable initial separation between the solitons. The photon-number correlation can also be achieved in a pair of vectorial solitons with two polarization components. In the latter case, the photon-number-entangled pulses can be easily separated by a polarization beam splitter. These results offer possibilities to produce entangled sources for quantum communication and computation.

We initially consider a quantum system consisting of two qubits, which can be in one of two nonorthogonal states $\mid {\Psi }_0⟩$ and $\mid {\Psi }_1⟩$. We distribute the qubits to two parties, Alice and Bob. They each measure their qubits and then compare their measurement results to determine which state they were sent. This procedure is error-free, which implies that it must sometimes fail. In addition, no quantum memory is required; it is not necessary for one of the qubits to be stored until the result of the measurement on the other is known. We consider the cases in which, should failure occur, both parties receive a failure signal or only one does. In the latter case, if the two states share the same Schmidt basis, the states can be discriminated with the same failure probability that would be obtained if the qubits were measured together. This scheme is sufficiently simple that it can be generalized to multipartite qubit, and qudit, states. Applications to quantum secret sharing are discussed. Finally, we present an optical scheme to experimentally realize the protocol in the case of two qubits.

Positive-operator-value measures (POVM’s) are the most general class of quantum measurements. We propose a setup in which all possible POVM’s of a single-photon polarization state (corresponding to all possible sets of two-dimensional Kraus operators) can be implemented easily using linear optics elements. This method makes it possible to experimentally realize any projective orthogonal, projective nonorthogonal or nonprojective sets of any number of POVM operators. Furthermore, our implementation only requires vacuum ancillas and is deterministic rather than probabilistic. Thus it realizes every POVM with the correct set of output states. We give the settings required to implement two different well-known nonorthogonal projective POVM’s.

Hyperspherical coordinates are used to study electron correlations in the radial motion of a four-electron atom within the model of $s^4$ configurations. We identify groups of hyperspherical adiabatic potential curves that support singly, doubly, triply, and quadruply excited states. By examining the nodal structures of the channel functions, we show that it is possible to relate the number of nodal surfaces in the hyperangles and the principal quantum numbers used in the independent particle picture.

The present study reports the nonzero partial wave elastic cross sections together with $s$-wave results for the scattering of an antihydrogen atom off a gaseous helium target at thermal energies (up to ${10}^{-2}\mathrm{a.u.}$). We have used a nonadiabatic atomic orbital method having different basis sets to investigate the system. The consideration of all the significant partial waves (up to $J=24$) reduces the oscillatory nature present in the individual partial wave cross section. The added elastic cross section is almost constant up to ${10}^{-7}\mathrm{a.u.}$ and then decreases steadily and very slowly with increasing energy.

X-ray-emission threshold-electron coincidence spectroscopy has been applied to ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{N}}_2\mathrm{O}$. The main features of the spectra can be interpreted as conventional core level threshold electron spectra, free from postcollision interaction effects. The relative cross sections for adiabatic ionization of close-lying core hole states and vibrational substates are presented. The results are compared to theoretical predictions and state-of-the-art photoelectron spectra and are discussed in terms of direct threshold ionization and core vacancy rearrangement processes.

We present studies of strong coupling in single-photon photoassociation of cesium dimers using an optical dipole trap. A thermodynamic model of the trap depletion dynamics is employed to extract absolute rate coefficents. From the dependence of the rate coefficient on the photoassociation laser intensity, we observe saturation of the photoassociation scattering probability at the unitarity limit in quantitative agreement with the theoretical model by Bohn and Julienne [Phys. Rev. A 60, 414 (1999)]. Also the corresponding power broadening of the resonance width is measured. We could not observe an intensity-dependent light shift in contrast to findings for lithium and rubidium, which is attributed to the absence of a $p$- or $d$-wave shape resonance in cesium.

We theoretically investigate some of the observable properties of quasi-two-dimensional condensates. Using a variational model based on a Gaussian-parabolic trial wave function we calculate chemical potential, condensate size in time-of-flight, release energy, and collective excitation spectrum for varying trap geometries and atom numbers and find good agreement with recent published experimental results.

Using a Green tensor formalism, we perform a comprehensive investigation into the spontaneous emission decay rate and energy-level shift in a circular dielectric waveguide. The two-dimensional (2D) Green tensor for an embedded line source and 3D Green tensor for a point source are obtained by a combination of Fourier integral and multipole methods. We fully characterize resonant states, which include whispering gallery modes, and examine the associated cavitylike effects. The quality factor of resonant states increases with frequency, resulting in a strongly enhanced decay rate and a large anomalous level shift in both the 2D and 3D cases.

We study Bell’s inequality in relation to the Einstein-Podolsky-Rosen paradox in the relativistic regime. For this purpose, a relativistically covariant analysis is used in the calculation of the Bell’s inequality, which results in the maximally violated Bell’s inequality in any reference frame.

We present a theoretical study of small sodium clusters embedded in a rare-gas matrix. We describe the active electrons of the embedded cluster using a time-dependent density-functional-theory method, while the surrounding polarizable atoms are treated by classical molecular dynamics. We study the spectroscopic properties of ${\mathrm{Na}}_2$ and ${\mathrm{Na}}_8$ embedded in an Ar environment by means of real-time dynamics. It is found that the surrounding material has a strong influence on the optical properties of the embedded Na clusters. Ar polarization and core repulsion act in opposite directions, so that the actual line shift depends crucially on the details of matrix configuration.

Collective two-color photoassociation of a freely interacting ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate is theoretically examined, focusing on stimulated Raman adiabatic passage (STIRAP) from an atomic to a stable molecular condensate. In particular, Drummond et al. [Phys. Rev. A. 65, 063619 (2002)] have predicted that particle-particle interactions can limit the efficiency of collective atom-molecule STIRAP, and that optimizing the laser parameters can partially overcome this limitation. We suggest that the molecular conversion efficiency can be further improved by treating the initial condensate density as an optimization parameter.

In the preceding Comment, the authors suggest that the molecular conversion efficiency in atom-molecule stimulated Raman adiabatic passage can be improved by lowering the initial atomic density, which in turn requires longer pulse durations to maintain adiabaticity. Apart from the fact that the mean-field approximation becomes questionable at low densities, we point out that a low-density strategy with longer pulses has several problems. It generally requires higher pulse energies and increases radiative losses. We also show that even within the approximations used in the Comment, their example leads to no efficiency improvement compared to our high-density case. In a more careful analysis including radiative losses neglected in the Comment, the proposed strategy gives almost no conversion owing to the longer pulse durations required.

Exact analytic solutions are found to the Dirac equation in $2+1$ dimensions for a combination of an Aharonov-Bohm potential and the Lorentz three-vector and scalar Coulomb potentials. By means of the solutions obtained the relativistic quantum Aharonov-Bohm effect is studied for the free (in the presence of a Lorentz three-vector Coulomb potential) and bound fermion states. We obtain the total scattering amplitude in a combination of the Aharonov-Bohm and Lorentz three-vector Coulomb potentials as a sum of two scattering amplitudes. This modifies the expression for the standard Aharonov-Bohm cross section due to the interference of these two amplitudes with each other. We discuss that the observable quantities can be the phases of electron wave functions or the energies of bound states.

The quantum color coding scheme proposed by Korff and Kempe [e-print quant-ph/0405086] is easily extended so that the color coding quantum system is allowed to be entangled with an extra auxiliary quantum system. It is shown that in the extended scheme we need only $\sim 2\sqrt{N}$ quantum colors to order $N$ objects in large $N$ limit, whereas $\sim N∕e$ quantum colors are required in the original nonextended version. The maximum success probability has asymptotics expressed by the Tracy-Widom distribution of the largest eigenvalue of a random Gaussian unitary ensemble (GUE) matrix.