Single-electron-detachment (SED) cross sections for 8–30 keV singly charged negative copper ions in collision with helium, neon, and argon are measured. The SED cross sections for ${\mathrm{Cu}}^{-}+\mathrm{Ne}$ and ${\mathrm{Cu}}^{-}+\mathrm{Ar}$ are found to be of similar magnitude and energy dependence, and are much larger than that for ${\mathrm{Cu}}^{-}+\mathrm{He}.$ A general discussion of the results is given.

We analyze the vortex dynamics and vortex-vortex interactions in Bose-Einstein condensates confined in toroidal traps. We show that this particular geometry strongly distorts the vortex dynamics. The numerically calculated vortex trajectories are well explained by an analytical calculation based on the image method and conformal mapping. Finally, the dissipation effects are discussed.

We study the ground-state magnetic properties of ferromagnetic spinor Bose-Einstein condensates confined in deep optical lattices. In the Mott-insulator regime, the “minicondensates” at each lattice site behave as mesoscopic spin magnets that can interact with neighboring sites through both the static magnetic dipolar interaction and the light-induced dipolar interaction. We show that such an array of spin magnets can undergo a ferromagnetic or antiferromagnetic phase transition under the magnetic dipolar interaction depending on the dimension of the confining optical lattice. The ground-state spin configurations and related magnetic properties are investigated in detail.

Time-resolved fragmentation measurements have been performed on stored, size-selected gold cluster ions ${\mathrm{Au}}_n^{+}$ $(n=17\mathrm{}–21)$ that have been excited up to 15 eV by multiphoton absorption. These excitation energies are far above the clusters’ dissociation thresholds and initiate multistep sequential unimolecular dissociation by evaporation of neutral monomers. The measurements allow for the determination of a combination of kinetic-energy release and radiative cooling of the excited clusters. Also, previously determined model-independent values of the cluster dissociation energies are confirmed by the present measurements. The data are consistent with thermal values of the kinetic-energy release with no significant radiative cooling of the excited clusters.

We consider the linear density response of a two-component (superfluid) Fermi gas of atoms when the perturbation is caused by laser light. We show that various types of laser excitation scheme can be transformed into linear density perturbations; however, a Bragg spectroscopy scheme is needed for transferring energy and momentum into a collective mode. This makes other types of laser probing schemes insensitive for collective excitations and therefore well suited for the detection of the superfluid order parameter. We show that, for the special case when laser light is coupled between the two components of the Fermi gas, density response is always absent in a homogeneous system.

To elucidate the effects of overall molecular shape upon the electronic response properties of molecules and nanoclusters we recently have considered various jellium cluster models for the mean excitation energy. Here we apply similar models to investigate the influence of gross molecular shape on Bragg’s rule. We find a direct way of expressing deviations entirely in terms of geometrical features of the molecule and its constituent atoms.

We propose a method for addressing qubits using a method that combines spatial and spectral selectivity. The result is a design for quantum computation that provides the potential for a high density of quantum information storage and processing. Specifically, this method uses an ensemble of spectrally resolved atoms in a spectral hole burning solid. The quantum coupling is provided by strong atom-cavity interaction. Using a thin disk of diamond containing nitrogen-vacancy color centers as an example, we present an explicit model that may yield up to 300 coupled qubits in a single spot. We show how about 100 operations may take place in parallel, yielding close to $4\times {10}^4$ operations before decoherence.

We give a proof that entanglement purification, even with noisy apparatus, is sufficient to disentangle an eavesdropper (Eve) from the communication channel. In the security regime, the purification process factorizes the overall initial state into a tensor-product state of Alice and Bob, on one side, and Eve on the other side, thus establishing a completely private, albeit noisy, quantum communication channel between Alice and Bob. The security regime is found to coincide for all practical purposes with the purification regime of a two-way recurrence protocol. This makes two-way entanglement purification protocols, which constitute an important element in the quantum repeater, an efficient tool for secure long-distance quantum cryptography.

We have demonstrated that the large disagreement between the frequency of the velocity-changing collisions in the ${\mathrm{H}}_2-\mathrm{Ar}$ system expected from theory and that obtained by Farrow et al. [Phys. Rev. Lett. 63, 746 (1989)] from the analysis of the $Q\left(1\right)\mathrm{}$ Raman line of ${\mathrm{H}}_2$ perturbed by Ar can be explained using the speed-dependent billiard ball profile (SDBBP). Using the SDBBP, we have estimated values of the “hardness” parameter corresponding to the ${\mathrm{H}}_2$ vibration lines perturbed by various gases and compared with those reported by Hoang, Joubert, and Robert [Phys. Rev. A 65, 012507 (2002)].

We show how polarization measurements on the output fields generated by parametric down conversion will reveal a violation of multiparticle Bell inequalities, in the regime of both low- and high-output intensity. In this case, each spatially separated system, upon which a measurement is performed, is comprised of more than one particle. In view of the formal analogy with spin systems, the proposal provides an opportunity to test the predictions of quantum mechanics for spatially separated higher spin states. Here the quantum behavior possible even where measurements are performed on systems of large quantum (particle) number may be demonstrated. Our proposal applies to both vacuum-state signal and idler inputs, and also to the quantum-injected parametric amplifier as studied by De Martini et al. The effect of detector inefficiencies is included, and weaker Bell-Clauser-Horne inequalities are derived to enable realistic tests of local hidden variables with auxiliary assumptions for the multiparticle situation.

A scheme is presented for protecting one-qubit quantum information against decoherence due to a general environment and local exchange interactions. The scheme operates essentially by distributing information over two pairs of qubits and through error-prevention procedures. In the scheme, quantum information is encoded through a decoherence-free subspace for collective phase errors and exchange errors affecting the qubits in pairs; leakage out of the encoding space due to amplitude damping is reduced by quantum Zeno effect. In addition, how to construct decoherence-free states for n-qubit information against phase and exchange errors is discussed.

A statistical theory of Fano resonances is presented for highly excited atomic and molecular systems with a classical chaotic limit. The probability distribution of the Fano parameter, which characterizes the asymmetry profile of a single resonance, is derived both for the case when the quantum system obeys time-reversal symmetry and when time-reversal symmetry is broken. Applications of the theory to photodissociation spectra of molecules and autoionization of Rydberg atoms are discussed.

What interactions are sufficient to simulate arbitrary quantum dynamics in a composite quantum system? Dodd et al. [Phys. Rev. A 65, 040301(R) (2002)] provided a partial solution to this problem in the form of an efficient algorithm to simulate any desired two-body Hamiltonian evolution using any fixed two-body entangling N-qubit Hamiltonian, and local unitaries. We extend this result to the case where the component systems are qudits, that is, have D dimensions. As a consequence we explain how universal quantum computation can be performed with any fixed two-body entangling N-qudit Hamiltonian, and local unitaries.

Consider a situation in which a quantum system is secretly prepared in a state chosen from the known set of states. We present a principle that gives a definite distinction between the operations that preserve the states of the system and those that disturb the states. The principle is derived by alternately applying a fundamental property of classical signals and a fundamental property of quantum ones. The principle can be cast into a simple form by using a decomposition of the relevant Hilbert space, which is uniquely determined by the set of possible states. The decomposition implies the classification of the degrees of freedom of the system into three parts depending on how they store the information on the initially chosen state: one storing it classically, one storing it nonclassically, and the other one storing no information. Then the principle states that the nonclassical part is inaccessible and the classical part is read-only if we are to preserve the state of the system. From this principle, many types of no-cloning, no-broadcasting, and no-imprinting conditions can easily be derived in general forms including mixed states. It also gives a unified view on how various schemes of quantum cryptography work. The principle helps one to derive optimum amount of resources (bits, qubits, and ebits) required in data compression or in quantum teleportation of mixed-state ensembles.

Resonant photoemission involving dissociative core excited states has been the subject of a great number of experimental and theoretical investigations in recent time. The resonant decay of such dissociating systems has been shown to lead to semiatomic Auger electron emission spectra, with particular angular behavior. In the present paper a detailed theoretical analysis of dissociative resonant photoemission spectra of homonuclear diatomic molecules is presented. The theory addresses both fixed in space and randomly oriented homonuclear molecules and emphasizes the Doppler effect and the role of the interference between channels referring to the Doppler split atomic fragments. It is shown that peaks originating from decay in the atomic fragments can be asymmetric and structured due to the Doppler interference effect. The predicted strong non-Lorentzian behavior of the substructure on the top of the Doppler broadened atomiclike contribution is traced to the interplay between decay channels leading to gerade and ungerade final states. Simulations based on wave-packet theory are compared with experimental data for molecular oxygen. Our numerical simulations of the atomiclike resonance of fixed in space molecules show that the spectral profile is very sensitive to the shape of interatomic potentials of core excited and final states. It is shown that the Doppler effect in the decay spectra depends upon the symmetry of the core excited state.

The role of multiparameter entanglement in quantum interference from collinear Type-II spontaneous parametric down-conversion is explored using a variety of aperture shapes and sizes, in regimes of both ultrafast and continuous-wave pumping. We have developed and experimentally verified a theory of down-conversion which considers a quantum state that can be concurrently entangled in frequency, wavevector, and polarization. In particular, we demonstrate deviations from the familiar triangular interference dip, such as asymmetry and peaking. These findings improve our capacity to control the quantum state produced by spontaneous parametric down-conversion, and should prove useful to those pursuing the many proposed applications of down-converted light.

High-resolution photoion yield spectra of ${\mathrm{N}}_2$ are measured near the K-shell ionization threshold. Previously unresolved multiple excitations are distinguished in the Σ- and Π-symmetry-resolved spectra, which were obtained from the ion yield spectra recorded at 0° and 90° relative to the polarization direction. The three ${}^1{\Pi }_u$ and two ${}^1{\Sigma }_u^{+}$ doubly excited states are clearly identified. Furthermore, a weak Π-symmetry feature just at the ${\sigma }^{*}$-shape resonance position (∼419 eV) is definitely resolved and assigned to the lowest ${}^1{\Pi }_u$ triple excitation with the help of quantum chemical calculations.

Recombination spectra of lithiumlike Be ions, taken at the ion storage ring CRYRING, have been investigated with high-energy resolution. The absolute recombination rate coefficient is measured in the center-of-mass energy range 0–5 eV that covers all $\Delta n=0\mathrm{}$ core excitations. The results are compared with ab initio calculations and show very good agreement in the resonance energy positions. They also agree in the resonance strengths, except close to the ${2s}_{1/2}\to {2p}_{1/2,3/2}$ core excitation limit and at the zero center-of-mass energy limit. In the first case the calculated rate coefficient exceeds that of the experiment by up to a factor of 1.7 and in the second it undervalues the experiment by 3.1. Interference effects of direct radiative recombination and recombination via doubly excited states are discussed.

In conditions where the interaction between an atom and a short high-frequency extreme ultraviolet laser pulse is a perturbation, we show that a simple theoretical approach, based on Coulomb-Volkov-type states, can make reliable predictions for ionization. To avoid any additional approximation, we consider here a standard case: the ionization of hydrogen atoms initially in their ground state. For any field parameter, we show that the method provides accurate energy spectra of ejected electrons, including many above threshold ionization peaks, as long as the two following conditions are simultaneously fulfilled: (i) the photon energy is greater than or equal to the ionization potential, (ii) the ionization process is not saturated. Thus, ionization of atoms or molecules by the high order harmonic laser pulses which are generated at present may be addressed through this Coulomb-Volkov treatment.

We analyze the generation of solitonlike solutions in a single-component Fermi gas of neutral atoms at zero and finite temperatures with the phase imprinting method. By using both the numerical and analytical calculations, we find the conditions when the quasisolitons, which apparently resemble the properties of solitons in nonlinear integrable equations, do exist in a noninteracting Fermi gas. We present the results for both spatially homogeneous and trapped cases, and emphasize the importance of the Fermi statistics and the absence of the interaction for the existence of such solutions.