Using the EBIT-II electron-beam-ion trap and a flat-field spectrometer, the extreme UV resonance lines ${4s}_{1/2}-{4p}_{3/2}$ of the Cu-like ions of Yb, W, Pb, Th, and U have been observed and their wavelengths measured. Our results differ substantially from earlier measurements performed with plasmas produced by the OMEGA and NOVA lasers and have higher accuracy. Our results are in good agreement with theory in all ions but those of the highest charge states.

We predict the existence of a self-sustained one-electron wave packet moving on a circular orbit in the helium atom. The wave packet is localized in space, but does not spread in time. This is a realization within quantum theory of a classical object that has been called a “Rutherford atom,” a localized planetary electron on an unquantized circular orbit under the influence of a massive charged core.

In this paper we analyze theoretically the “signal background” observed in experimental studies of the radiative decay of hydrogen atoms following resonant coherent excitation (RCE) in grazing collisions with a LiF surface. The radiation intensity is evaluated by treating the RCE within a time-dependent (classical-trajectory) quantal approach, and by assuming that the excited electronic states are quenched whenever the atom-surface distance drops below a certain “switching distance.” Associating the mechanism of excited-state quenching with the charge transfer to the LiF conduction band we show that the hereto unexplained signal background can be attributed to the dependence of the switching distance on the incident energy.

We have carried out a numerical investigation of the influence of photoionization, causing neutral atom depletion and accompanying electron impact dephasing, on a stimulated Raman adiabatic passage (STIRAP) scheme in a dense medium of three-level atoms. The nonlinear mixing from the large (maximal) coherence generated during the population transfer is calculated within the framework of a time-dependent model for a single atom with simplified propagation appropriate to a thin slab of material. For definiteness and consistency of the atomic parameters employed we have treated a scheme in atomic calcium. For generation of intense vacuum ultraviolet fields, laser intensities in the range from 100 ${\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ to 1 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ are required, and these are found to cause significant ionization of the atom. We find that at atomic densities $>{10}^{16}/{\mathrm{cm}}^3$ the effects of electron impact dephasing on the coherence evolution is very significant and will dominate over other dephasing mechanisms. In contrast at lower densities $\sim {10}^{14}/{\mathrm{cm}}^3$ (compatible with a sealed vapor cell) the conversion efficiency is relatively unaffected by electron impact dephasing. We conclude that in general photoionization depletion and electron impact dephasing will be significant in maximal coherence wave mixing and STIRAP population transfer schemes operated at high density.

We study a harmonically confined Bose-Einstein condensate under rotation. Vortex lattice configurations are investigated through a variational approach. Vortices with more than a unit of angular momentum are not stable. We explicitly show that the critical rotational frequency is quite small, as was reported in a recent experiment, with an asymmetric harmonic confining potential. The width of the condensate along the z direction (the anisotropy direction) and in the xy plane and the vortex density are determined as functions of the rotational frequency. The aspect ratio becomes small for large rotational frequencies, making the condensate an effectively two-dimensional system.

Quantum electrodynamic (QED) electron self-energy and vacuum polarization corrections to the binding energy of the $8s$ electron in the eka-radon $\mathrm{}(Z=118\mathrm{})\mathrm{}$ negative ion are calculated. This ion was found recently to be stable in the Dirac-Coulomb-Breit Hamiltonian; an improved basis and correlating all 119 electrons give a binding energy of $-0.064\left(2\right)\mathrm{}\mathrm{eV}.$ The QED contribution is 0.0059(5) eV, amounting to a 9% reduction of the binding energy. This is the largest relative QED effect reported for neutral or weakly ionized atoms, and confirms the importance of QED corrections for superheavy elements.

Metastable calcium atoms, produced in a magneto-optic trap (MOT) operating within the singlet system, are continuously loaded into a magnetic trap formed by the magnetic quadrupole field of the MOT. At MOT temperatures of 3 mK and 240 ms loading time, we observe $1.1\times {10}^8$ magnetically trapped ${}^3{P}_2$ atoms at densities of $2.4\times {10}^8{\mathrm{cm}}^{-3}$ and temperatures of 0.61 mK. In a modified scheme, we first load a MOT for metastable atoms at a temperature of 0.18 mK and subsequently release these atoms into the magnetic trap. In this case, 240 ms of loading yields $2.4\times {10}^8$ trapped ${}^3{P}_2$ atoms at a peak density of $8.7\times {10}^{10}{\mathrm{cm}}^{-3}$ and a temperature of 0.13 mK. The temperature decrease observed in the magnetic trap for both loading schemes can be explained only in part by trap size effects.

We show that when cold collisions account for just a few percent of the total loss in magneto-optical traps, as for alkaline-earth metals, their contribution can be obtained by comparing the load and decay quasiexponential curves. We exemplify it by measuring the collision rate coefficient β for calcium atoms at small trap laser detuning, which confirms the role of the long-lived ${}^1{\Pi }_g$ state at small internuclear separation. Systematic and simpler measurements of β for alkaline-earth metals are important for cold collision theory, since these elements have nondegenerate ground state and no hyperfine structure. Our method has general applicability and should considerably reduce the experimental challenges associated with these important measurements.

We study the dynamics of the relative phase following the connection of the two independently formed Bose-Einstein condensates. Dissipation is assumed to be due to the creation of quasiparticles induced by a fluctuating condensate particle number. The coherence between different values of the phase, which is characteristic of the initial Fock state, is quickly lost after the net exchange of a few atoms has taken place. This process effectively measures the phase and marks the onset of a semiclassical regime in which the system undergoes Bloch oscillations around the initial particle number. These fast oscillations excite quasiparticles within each condensate and the system relaxes at a longer time scale until it displays low-energy, damped, Josephson plasma oscillations, eventually coming to a halt when the equilibrium configuration is finally reached.

Maximally entangled mixed states are those states that, for a given mixedness, achieve the greatest possible entanglement. For two-qubit systems and for various combinations of entanglement and mixedness measures, the form of the corresponding maximally entangled mixed states is determined primarily analytically. As measures of entanglement, we consider entanglement of formation, relative entropy of entanglement, and negativity; as measures of mixedness, we consider linear and von Neumann entropies. We show that the forms of the maximally entangled mixed states can vary with the combination of (entanglement and mixedness) measures chosen. Moreover, for certain combinations, the forms of the maximally entangled mixed states can change discontinuously at a specific value of the entropy. Along the way, we determine the states that, for a given value of entropy, achieve maximal violation of Bell’s inequality.

We propose a spatiotemporal discretization of the Schrödinger equation for field operators of noninteracting bosons. The scheme, being based on the associated space of the creation and annihilation operators and on a discretization of the Schrödinger equation, preserves equal time commutation relations, possesses Hermitian action, unitary evolution operator, and all relevant integrals which are counterparts of the respective conservation quantities in the continuum limit. Although the scheme is implicit, it allows a reduction to a nonlocal but explicit scheme. In the limit of small time steps the scheme is reduced to a model studied earlier. Some particular examples of the discrete-time Schrödinger equation in the presence of external potentials are given.

Analyzing the properties of entanglement in many-particle spin-1/2 systems is generally difficult because the system’s Hilbert space grows exponentially with the number of constituent particles, N. Fortunately, it is still possible to investigate a many-particle entanglement when the state of the system possesses sufficient symmetry. In this paper, we present a practical method for efficiently computing various bipartite entanglement measures for states in the symmetric subspace and perform these calculations for $N\sim {10}^3.$ By considering all possible bipartite splits, we construct a picture of the multiscale entanglement in large symmetric systems. In particular, we characterize dynamically generated spin-squeezed states by comparing them to known reference states (e.g., Greenberger-Horne-Zeilinger and Dicke states), and families of states with near-maximal bipartite entropy. We quantify the trade-off between the degree of entanglement and its robustness to particle loss, emphasizing that substantial entanglement need not be fragile.

The temperature correction to the Casimir force is considered for real metals at low temperatures. With the temperature decrease, the mean free path for electrons becomes larger than the field penetration depth. In this condition, description of metals with the impedance of anomalous skin effect is shown to be more appropriate than with the permittivity. The effect is crucial for the temperature correction. It is demonstrated that in the zero-frequency limit, the reflection coefficients should coincide with those of ideal metal if we demand the entropy to be zero at $T=0.\mathrm{}$ All the other prescriptions discussed in the literature for the $n=0\mathrm{}$ term in the Lifshitz formula give negative entropy. It is shown that the temperature correction in the region of anomalous skin effect is not suppressed as it happens in the plasma model. This correction will be important in the future cryogenic measurements of the Casimir force.

The Casimir force between macroscopic bodies depends strongly on their shape and orientation. To study this geometry dependence in the case of two deformed metal plates, we use a path-integral quantization of the electromagnetic field which properly treats the many-body nature of the interaction, going beyond the commonly used pairwise summation (PWS) of van der Waals forces. For arbitrary deformations we provide an analytical result for the deformation induced change in the Casimir energy, which is exact to second order in the deformation amplitude. For the specific case of sinusoidally corrugated plates, we calculate both the normal and the lateral Casimir forces. The deformation induced change in the Casimir interaction of a flat and a corrugated plate shows an interesting crossover as a function of the ratio of the mean plate distance H to the corrugation length $\lambda :$ For $\lambda \ll H$ we find a slower decay $\sim H^{-4},$ compared to the $H^{-5}$ behavior predicted by PWS which we show to be valid only for $\lambda \gg H.\mathrm{}$ The amplitude of the lateral force between two corrugated plates which are out of registry is shown to have a maximum at an optimal wavelength of $\lambda \approx 2.5H.\mathrm{}$ With increasing $H/\lambda \gtrsim 0.3$ the PWS approach becomes a progressively worse description of the lateral force due to many-body effects. These results may be of relevance for the design and operation of novel microelectromechanical systems (MEMS) and other nanoscale devices.

We present different algorithms for mixed-state multicopy entanglement distillation for pairs of qubits. Our algorithms perform significantly better than the best-known algorithms. Better algorithms can be derived that are tuned for specific initial states. These algorithms are based on a characterization of the group of all locally realizable permutations of the $4^n$ possible tensor products of n Bell states.

We show that quantum games are more efficient than classical games and provide a saturated upper bound for this efficiency. We also demonstrate that the set of finite classical games is a strict subset of the set of finite quantum games. Our analysis is based on a rigorous formulation of quantum games, from which quantum versions of the minimax theorem and the Nash equilibrium theorem can be deduced.

We propose a decoupling-free nuclear-spin quantum computer installed on a quantum electron spin chain with a singlet ground state and a finite spin gap. Qubits are $I=1/2\mathrm{}$ nuclear spins implanted periodically along the quantum spin chain. A magnetic field gradient is applied parallel to the chain, which allows individual access to each qubit. A single-qubit operation (rotation gate) is realized with the rf field tuned to the nuclear Larmor frequency at the qubit of interest, while a two-qubit operation (controlled-NOT gate) is achieved using the Suhl-Nakamura interaction through a packet of triplet magnons, which are excited by a microwave tuned to the spin gap energy $(SN$ gate). The interaction can be switched off by turning off the microwave, and a decoupling-free quantum computer is realized. The initialization is achieved with an optical pumping qubit initializer, which has a multilayered structure of the quantum spin chain and a semiconductor. Spin polarizations created by the optical pumping in the semiconducting layers are transferred to the spin chain layers through a cross polarization and a spin diffusion. The scheme allows us to separate the initialization process from the computation, enabling us to optimize the latter independently of the former.

We introduce a refinement of the standard continuous variable teleportation measurement and displacement strategies. This refinement makes use of prior knowledge about the target state and the partial information carried by the classical channel when entanglement is nonmaximal. This gives an improvement in the output quality of the protocol. The strategies we introduce could be used in current continuous variable teleportation experiments.

The discrete formulation of adiabatic quantum computing is compared with other search methods, classical and quantum, for random satisfiability (SAT) problems. With the number of steps growing only as the cube of the number of variables, the adiabatic method gives solution probabilities close to 1 for problem sizes feasible to evaluate via simulation on current computers. However, for these sizes the minimum energy gaps of most instances are fairly large, so the good performance scaling seen for small problems may not reflect asymptotic behavior where costs are dominated by tiny gaps. Moreover, the resulting search costs are much higher than for other methods. Variants of the quantum algorithm that do not match the adiabatic limit give lower costs, on average, and slower growth than the conventional GSAT heuristic method.

We present a constructive proof that anyonic magnetic charges with fluxes in a nonsolvable finite group can perform universal quantum computations. The gates are built out of the elementary operations of braiding, fusion, and vacuum pair creation, supplemented by a reservoir of ancillas of known flux. Procedures for building the ancilla reservoir and for correcting leakage are also described. Finally, a universal qudit gate set, which is ideally suited for anyons, is presented. The gate set consists of classical computation supplemented by measurements of the X operator.