Coherent operations with individual trapped ${\text{Yb}}^{+}$ ions are demonstrated that are robust against variations in experimental parameters and intrinsically indeterministic system parameters. In particular, pulses developed using optimal control theory are demonstrated with trapped ions. Their performance as a function of error parameters is systematically investigated and compared to composite pulses. Such pulses are basic building blocks for single and multiqubit quantum gates.

The Coulomb explosion ionization process of an $\text{H}_2{}^{+}$ wave packet in xuv laser fields has been studied by fully three-dimensional ab initio wave packet calculations, with moving nuclei, beyond the Born-Oppenheimer approximation. Both the cases of parallel and perpendicular orientation of the molecule with respect to the laser field have been investigated. Furthermore, we have examined the applicability of the fixed-nuclei approximation method to obtain the kinetic energy distribution of the released nuclei. It is found that the approximation breaks down already at relatively short pulses. We also show that the relative orientation of the internuclear axis and polarization vector has a crucial effect on the kinetic energy spectrum.

We study the Hong-Ou-Mandel interferometer in the regime of spontaneous parametric down-conversion with high pump beam power at the crystal. In this regime one and two photons from a pump pulsed laser beam generate one and two pairs of photons, respectively. These photons are then directed to the beam splitter of the interferometer and detected at its exit in coincidence. An interesting phenomenon is observed: The reduction of the visibility of the Hong-Ou-Mandel coincidence peak (or dip) with the increase of pump power. We study the relation between the visibility of the fourth-order interference pattern and the power of the pumping laser beam for type I and type II phase-matching crystals. Our theoretical calculations are in good agreement with the experimental results.

We examine the dynamics of bipartite quantum entanglement between a two-level atom and the electromagnetic field. We begin with the Jaynes-Cummings model with a single field mode and examine in detail the exact time evolution of entanglement, including cases where the atomic state is initially mixed and the atomic transition is detuned from resonance. We then explore the effects of other nearby modes by calculating the exact time evolution of entanglement in more complex systems with two, three, and five field modes. For these cases we can obtain exact solutions which include the strong-coupling regime. Finally, we consider the entanglement of a two-level atom with the infinite collection of modes present in the intracavity field of a Fabre-Perot cavity. In contrast to the usual treatment of atom-field interactions with a continuum of modes using the Born-Markov approximation, our treatment in all cases describes the full non-Markovian dynamics of the atomic subsystem under general couplings. Only when an analytic expression for the infinite mode case is desired do we need to make a weak coupling assumption which at late times approximates Markovian dynamics.

On the basis of a general formula obtained earlier via fourth-order perturbation theory within the framework of macroscopic quantum electrodynamics, the van der Waals potential between two neutral, unpolarized, ground-state atoms in the presence of a homogeneous, dispersing and absorbing magnetoelectric sphere is studied. When the radius of the sphere becomes sufficiently large, the result asymptotically agrees with that for two atoms near a planar interface. In the opposite limit of a very small sphere, the sphere can effectively be regarded as being a third ground-state atom, and the nonadditive three-atom van der Waals potential is recovered. To illustrate the effect of a sphere of arbitrary radius, numerical results are presented for the triangular arrangement where the atoms are at equidistance from the sphere, and for the linear arrangement where the atoms and the sphere are aligned along a straight line. As demonstrated, the enhancement or reduction of the interaction potential in the presence of purely electric or magnetic spheres can be physically understood in terms of image charges.

We recently proposed a scheme for the creation of coherent superpositions of vortex states in Bose-Einstein condensates (BECs) using orbital angular momentum (OAM) states of light [Phys. Rev. Lett. 95, 173601 (2005)]. Here we discuss further technical details of the proposal, provide an alternative, time-reversal-symmetric scheme for transfer of a superposition of OAM states of light to the BEC via a procedure analogous to the traditional stimulated Raman adiabatic passage technique, and discuss an alternative trap configuration conducive for sustaining large charge vortices. Superpositions of OAM states of light, that can be created using the existing experimental techniques, can be transferred to an initially nonrotating BEC via a specially devised Raman coupling scheme. The techniques proposed here open up avenues to study coherent interaction of OAM states of light with matter. The study could also be employed for performing various quantum-information-processing tasks with OAM states of light—including a memory for a quantum state of the initial superposition.

We show that a singlet of many multilevel quantum systems arises naturally as the ground state of a physically motivated Hamiltonian. The Hamiltonian simply exchanges the states of nearest neighbors in some network of qudits ($d$-level systems); the results are independent of the strength of the couplings or the network’s topology. We show that local measurements on some of these qudits project the unmeasured qudits onto a smaller singlet, regardless of the choice of measurement basis at each measurement. It follows that the entanglement is highly persistent, and that through local measurements, a large amount of entanglement may be established between spatially separated parties for subsequent use in distributed quantum computation.

Numerical calculations of photodetachment of ${\mathrm{F}}^{-}$ irradiated by a linearly polarized strong short infrared pulse have been performed by evaluating the photoelectron ejection probability in the framework of a Keldysh-type approach modified to account for the temporal behavior of the pulse. The results of the calculations are in good agreement with measurements of energy spectra recently appearing in literature, and show features that may be explained in terms of quantum interferences in time domain.

We analyze the rotation of bright solitary waves formed of atomic Bose-Einstein condensates with attractive atomic interactions. By employing a variational technique and assuming an irrotational quadrupolar flow field, we map out the variational solutions in the rotating frame. In particular, we show that rotation has a considerable stabilizing effect on the system, significantly raising the critical threshold for collapse of the bright solitary waves.

We show that spin correlations of atoms in an optical lattice can be reconstructed by coupling the system to the light, and by measuring correlations between the emitted photons. This principle is the basis for a method to characterize states in quantum computation and simulation with optical lattices. As examples, we study the detection of spin correlations in a quantum magnetic phase, and the characterization of cluster states.

We introduce a time-averaged trap in which the internal state of the atoms is rapidly modulated to modify the magnetic trapping potential. In our experiment, fast radio-frequency linear sweeps flip the spins of atoms at a fast rate, which averages out the magnetic forces. We use this procedure to optimize the accumulation of metastable chromium atoms in an optical dipole trap from a magneto-optical trap. The potential experienced by the metastable atoms is identical to the bare optical dipole potential, so that this procedure allows for trapping all magnetic sublevels, hence increasing by up to 80% the final number of accumulated atoms.

The absolute magnitude of the transition dipole moment function ${\mu }_e\left(R\right)$ of the $A{{}^1\Sigma }_u^{+}-X{{}^1\Sigma }_g^{+}$ band system of ${\text{Na}}_2$ was mapped experimentally over a relatively large range of internuclear distance $R$. The transition dipole moment matrix element of a set of rovibrational transitions between the $A{{}^1\Sigma }_u^{+}$ and $X{{}^1\Sigma }_g^{+}$ states was measured using the Autler-Townes effect. By employing the $R$-centroid approximation, or a fit to a polynomial function involving higher order $R$ centroids, ${\mu }_e$ as a function of the internuclear distance was obtained. These Autler-Townes effect based measurements yield the absolute magnitude of ${\mu }_e$, which can be used to test ab initio theoretical transition dipole moment functions or to “normalize” experimental transition moment functions obtained from intensity measurements, which in general give only the relative behavior of ${\mu }_e\left(R\right)$.

We investigate the vortex patterns appearing in a two-dimensional annular Bose-Einstein condensate rotating in a quadratic plus quartic confining potential. We show that in the limit of small anharmonicity the Gross-Pitaevskii energy can be minimized among the lowest-Landau-level wave functions and use this particular form to obtain theoretical results in the spirit of Aftalion et al. [Phys. Rev. A 73, 011601(R) (2006)]. In particular, we show that the vortex pattern is infinite but not uniform. We also compute numerically the complete vortex structure: It is an Abrikosov lattice strongly distorted near the edges of the condensate with multiply quantized vortices appearing at the center of the trap.

We present measurements of the binding energies of ${}^6\text{L}\text{i}$ $p$-wave Feshbach molecules formed in combinations of the $|F=1/2,m_F=+1/2⟩$ $\left(|1⟩\right)$ and $|F=1/2,m_F=-1/2⟩$ $\left(|2⟩\right)$ states. The binding energies scale linearly with magnetic field detuning for all three resonances. The relative molecular magnetic moments are found to be $113\pm 7\mu \text{K}/\text{G}$, $111\pm 6\mu \text{K}/\text{G}$, and $118\pm 8\mu \text{K}/\text{G}$ for the $|1⟩\text{−}|1⟩$, $|1⟩\text{−}|2⟩$, and $|2⟩\text{−}|2⟩$ resonances, respectively, in good agreement with theoretical predictions. Closed-channel amplitudes and the size of the $p$-wave molecules are obtained theoretically from full closed-coupled calculations.

We explore the zero-temperature phase behavior of a two-dimensional two-component atomic Fermi gas with population and mass imbalance in the regime of the BEC-BCS crossover. Working in the mean-field approximation, we show that the normal and homogeneous balanced superfluid phases are separated by an inhomogeneous superfluid phase of Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type. We obtain an analytical expression for the line of continuous transitions separating the normal and inhomogeneous FFLO phases. We further show that the transition from the FFLO phase to the homogeneous balanced superfluid is discontinuous leading to phase separation. If the species have different masses, the superfluid phase is favored when the lighter species is in excess. We explore the implications of these findings for the properties of the two-component Fermi gas in the atomic trap geometry. Finally, we compare and contrast our findings with the predicted phase behavior of the electron-hole bilayer system.

In this paper, we demonstrate highly directive emission based on anisotropic metamaterials with spatial dispersion of very narrow hyperbola. By embedding the source inside a slab of such material, the emitted energy, both inside and outside the material, is concentrated around the normal of the slab, and a perfect far-field radiation is thus achieved. This effect is verified experimentally in the microwave domain, and a directivity that is far larger than the best one to our knowledge is obtained from the constructed antenna.

By means of optimal control techniques we model and optimize the manipulation of the external quantum state (center-of-mass motion) of atoms trapped in adjustable optical potentials. We consider in detail the cases of both noninteracting and interacting atoms moving between neighboring sites in a lattice of a double-well optical potentials. Such a lattice can perform interaction-mediated entanglement of atom pairs and can realize two-qubit quantum gates. The optimized control sequences for the optical potential allow transport faster and with significantly larger fidelity than is possible with processes based on adiabatic transport.

We present a detailed study of the stability conditions of hydrogen-antihydrogen–like quasimolecules using both variational and finite-size scaling calculations. The stability diagram of the nuclear charge $Z$ as a function of the internuclear distance $R$ shows bound and unbound regions separated by a first-order critical line. Calculations of the leptonic annihilation rate show a peculiar behavior for nuclear charges $Z\ge 2$, which was not observed for the hydrogen-antihydrogen quasimolecule; it goes through a maximum before it decays exponentially for large interhadronic distances. This might have a practical impact on the study of stability of matter-antimatter systems.

Hyperspherical partial wave theory has been applied to calculate $T$-matrix elements and single differential cross-section (SDCS) results for electron-hydrogen ionization processes within the Temkin-Poet model potential. We considered three different values of step length to compute the radial part of final state wave function. Numerical outcomes show that $T$-matrix elements and SDCS values depend on the step length $h$. Here, we have presented $T$-matrix elements and the corresponding SDCS results for 0.0075, 0.009, and 0.01 a.u. values of $h$ and for 27.2, 40.8, and 54.4 eV impact energies. With the help of the calculated data for three different step lengths, we have been able to evaluate a two-term error function depending on the step length $h$. Finally, two-term error corrected $T$-matrix elements and the corresponding SDCS values have been computed. We fitted our two-term error corrected SDCS results by a suitable curve and compared with the benchmark results of Jones et al. [Phys. Rev. A 66, 032717 (2002)]. Our fitted curves agree very well with the calculated results of Jones et al. and two-term error corrected SDCS results show fair agreement with the benchmark results. Two-term error corrected SDCS results are significantly better than the calculated SDCS results of different step lengths.