An embedded selective recoupling method is proposed which is based on the idea of embedding the recently proposed deterministic selective recoupling scheme of Yamaguchi et al. into a stochastic dynamical decoupling method, such as the recently proposed Pauli-random-error-correction scheme. The recoupling scheme enables the implementation of elementary quantum gates in a quantum information processor by partial suppression of the unwanted interactions. The random dynamical decoupling method cancels a significant part of the residual interactions. Thus the time scale of reliable quantum computation is increased significantly. Numerical simulations are presented for a conditional two-qubit swap gate and for a complex iterative quantum algorithm.

A new nuclear quadrupole moment, $Q=0.305\left(2\right)$ b, of ${}^{87}\mathrm{Sr}$ is determined by combining precise measurement of the electric quadrupole hyperfine structure constant of the $4d$ ${}^{2}D_{5∕2}$ state of its ion with calculation. This is a significant improvement over the previous values 0.335(20), 0.327(24), and 0.323(20) b. Relativistic coupled-cluster theory is employed in the calculations and electron correlation effects are included using the single, double, and an important subset of triple excitations. The magnetic dipole and electric quadrupole hyperfine structure constants of a few low-lying states are calculated to a high accuracy. The role of different electron correlation effects in the $4d$ ${}^{2}D_{5∕2}$ state is investigated.

Multielectron processes have been studied by measuring the ${\mathrm{Na}}^{2+}$ and ${\mathrm{Na}}^{3+}$ recoil momenta resulting from $10\mathrm{keV}∕\mathrm{amu}$ ${\mathrm{He}}^{2+}+\mathrm{Na}\left(3s\right)$ collisions. The ${\mathrm{Na}}^{2+}$ $Q$-value spectrum shows that transfer ionization dominates two-electron removal. Double capture populates mostly singly excited $\mathrm{He}\left(1snl\right)$ states. A smaller fraction of double capture leads to doubly excited He. ${\mathrm{Na}}^{3+}$ recoil ions are created by double capture into the He ground state and the emission of a third electron into the continuum. The ${\mathrm{Na}}^{3+}$ recoil ion is not left in its triplet ground state but in one of the low-lying excited singlet terms due to spin conservation.

We have measured and calculated fully differential single-ionization cross sections for the complete three-dimensional space in $75\mathrm{keV}$ $p+\mathrm{He}$ collisions. Several signatures of the projectile–residual-target-ion interaction, some of which are not observable for fast projectiles and for electron impact, are revealed. Some of these features are qualitatively reproduced by our calculations if this interaction is accounted for. However, overall the agreement between theory and experiment is not very good. Thus, our understanding of effects caused by the projectile–residual-target-ion interaction appears to be rather incomplete.

Experiments showing a transverse heating or cooling of channeled ion beams are explained in terms of electron capture and loss processes between the projectile ions and the target. Such processes violate reversibility as the projectile captures electrons from occupied bound states and loses them to unoccupied weakly bound or continuum states. The transition probabilities for the transfer of electrons are calculated in the impact parameter Born approximation. Their dependence on the distance from the crystal strings is determined by scale factors which depend in turn on the relative velocity and the binding energies of the transferred electrons in the projectile and in the crystal, respectively. The appearance of transverse heating and cooling depends on the relative size of the scale factors for capture and loss. The transition from heating to cooling as function of velocity is described in good agreement with the experiments.

In this paper we investigate how Rydberg atoms respond to perturbation by two contrapropagating ultrashort laser pulses. We consider the case where the durations of both pulses ${\tau }_1$ and ${\tau }_2$ are shorter than the inverse of the initial-state energy ${\epsilon }_i^{-1}$. When acting alone such a pulse passes through the atom without noticeable alteration in the atomic state. The situation is different if two such pulses interfere in the region of atom localization. In this case the atomic response is significantly enhanced. This is due to the nonzero momentum transferred to the electron by the interplay of the electric field of one pulse and the magnetic field of the other. The sudden perturbation approximation is used to evaluate the transition probabilities. They are shown to depend on the atom position with respect to the pulse interference region. This dependence is determined by the relationship between the atomic diameter $d_i$ and the interference-region size $l=c({\tau }_1+{\tau }_2)$ ($c$ is the speed of light). If $d_i\ll l$ this dependence is sensitive to the function form of the pulses. For sufficiently strong fields the atoms can be ionized completely. In the opposite case of $d_i\gg l$ the transition probabilities are sensitive to the electron density distribution along the propagation direction. The probabilities of the initial-state destruction and atom ionization drop as $l∕d_i$ irrespective of the characteristics of the pulses.

We study the composite-fermion (CF) construction at and below the single vortex $(L=N)$ state of weakly interacting rotating Bose gases, presenting a new method for handling the large number of derivatives typically occurring via the Slater determinant. Remarkably, the CF wave function at $L=N$ becomes asymptotically exact in the thermodynamic limit, even though this construction is not, a priori, expected to work in the low angular momentum regime. This implies an interesting mathematical identity which may be useful in other contexts.

We demonstrated $\delta$-kick cooling of sodium atoms with a three-dimensional anisotropic harmonic potential. The radius and temperature of the atomic cloud oscillated with kick time, indicating that the elongated atomic distribution in phase space rotates in the Ioffe-Pritchard potential. The temperature of kicked atoms was reduced to below the recoil temperature.

We experimentally demonstrate an approach for manipulating the spectral profiles of entangled-photon pairs. The spectral properties are determined by selecting both the appropriate spatial profile of the pump laser radiation and the geometry of the noncollinear spontaneous parametric downconversion. Both spectra, the spectrum of the individual signal and idler photons, and the joint spectrum of the entangled-photon pairs, can be modified at will over a substantial range of values. This technique is therefore expected to be useful for an array of quantum-optics applications.

We study the possibility of manipulating the focusing properties of a medium with electromagnetically induced transparency. In the focal region of focused ultraslow light pulses, the spectral anomalous behaviors can be actively modified by varying the control field intensity. Unlike the case in free space, we find in slow light focusing that the spectrum bandwidth of the incident field needed to produce observable spectral changes can be reduced by several orders. Numerical simulations with accessible parameters clearly show that spectral anomalies of focused $\mu \mathrm{s}$ pulses are observable.

Using the theory of quantum trajectories, we study the classical capacity $\mathbf{C}$ of noisy quantum channels in the superdense coding protocol. We apply the two models that generalize single-qubit noisy channels, such as amplitude damping and phase flip, to the many-qubit situation. Our numerical results show that the quantum trajectory approach is suitable for modeling the superdense coding protocol in realistic environments.

We propose two schemes for the generation of four-qubit cluster states in ion-trap systems. The first scheme is based on resonant sideband excitation, while the second scheme does not use the vibrational mode as the memory. The schemes can be realized with presently available ion-trap techniques.

We derive pseudopotentials that describe the scattering between two particles in two dimensions for any partial wave $m$, whose scattering strength is parametrized in terms of the phase shift ${\delta }_m$. Using our $m=0$ pseudopotential, we develop a coupled-channel model with two-dimensional (2D) zero-range interactions, which describes the two-body physics across a Feshbach resonance. Our model predicts the scattering length, the binding energy, and the “closed channel molecular fraction” of two particles; these observables can be measured in experiments on ultracold quasi-2D atomic Bose and Fermi gases with present-day technology.

A necessary and sufficient condition is given for general exact remote state preparation (RSP) protocols to be oblivious, that is, no information about the target state can be retrieved from the classical message. A criterion in terms of commutation relations is also derived for the existence of deterministic exact protocols in which Alice’s measurement eigenstates are related to each other by fixed linear operators similar to Bob’s unitaries. For nonmaximally entangled resources, it provides an easy way to search for RSP protocols. As an example, we show how to reduce the case of partially entangled resources to that of maximally entangled ones, and we construct RSP protocols exploiting the structure of the irreducible representations of Abelian groups.

Electron-impact excitation of the $(3p^{5}4s4p{)}^{4}S$ quasimetastable level in potassium has been investigated both experimentally and theoretically for incident energies up to $3\mathrm{eV}$ above the excitation threshold. The ejected-electron excitation function measured with an energy resolution of $0.25\mathrm{eV}$ suggests the presence of several strong resonances just above threshold. Satisfactory agreement with the excitation cross section obtained from a sophisticated $B$-spline $R$-matrix model with nonorthogonal orbitals supports the importance of ${\mathrm{K}}^{-}$ resonances predominantly of the $\left(3p^{5}4s4p^2\right)$ configuration. Good agreement between theory and experiment is also obtained for the optical excitation function measured earlier with a lower energy resolution of $\approx 1.2\mathrm{eV}$. The branching ratios for decay of the $(3p^{5}4s4p{)}^4{S}_{3∕2}$ level into the electron emission and the optical channels were obtained as 0.16 and 0.69, respectively.

The core-excited $\left(1s2s^{2}2p^4{}^{4}P\right)$ negative ion shape resonance of ${\mathrm{C}}^{-}$ near $281.7\mathrm{eV}$ has been investigated using the merged ion beam—photon beam photodetachment technique on the Advanced Light Source beamline 10.0.1. ${\mathrm{C}}^{+}$ ions formed by double detachment were detected as a function of photon energy. Higher resolution spectra yield more precise values for the energy and width of the resonance than our previous measurements [N. D. Gibson et al., Phys. Rev. A 67, 030703(R) (2003)]. The absolute cross section for double detachment from ${\mathrm{C}}^{-}$ following $1s$ photoexcitation is measured for the first time and the spectrum is compared to previous theoretical calculations. These measurements also provide information on the lowest core-excited state of neutral carbon $\left(1s2s^{2}2p^3{}^{5}S\right)$.

The spin response functions measured in multicomponent fermion gases by means of rf transitions between hyperfine states are strongly constrained by the symmetry of the interatomic interactions. Such constraints are reflected in the spin $f$-sum rule that the response functions must obey. In particular, only if the effective interactions are not fully invariant in $SU\left(2\right)$ spin space, are the response functions sensitive to mean field and pairing effects. We demonstrate, via a self-consistent calculation of the spin-spin correlation function within the framework of Hartree-Fock-BCS theory, how one can derive a correlation function explicitly obeying the $f$-sum rule. By contrast, simple one-loop approximations to the spin response functions do not satisfy the sum rule, except in special cases. As we show, the emergence of a second peak at higher frequency in the rf spectrum, as observed in a recent experiment in trapped ${}^6\mathrm{Li}$, can be understood as the contribution from the paired fermions, with a shift of the peak from the normal particle response proportional to the square of the BCS pairing gap.

The interplay of quantum fluctuations with nonlinear dynamics is a central topic in the study of open quantum systems, connected to fundamental issues (such as decoherence and the quantum-classical transition) and practical applications (such as coherent information processing and the development of mesoscopic sensors and amplifiers). With this context in mind, we here present a computational study of some elementary bifurcations that occur in a driven and damped cavity quantum electrodynamics (cavity QED) model at low intracavity photon number. In particular, we utilize the single-atom cavity QED master equation and associated stochastic Schrödinger equations to characterize the equilibrium distribution and dynamical behavior of the quantized intracavity optical field in parameter regimes near points in the semiclassical (mean-field, Maxwell-Bloch) bifurcation set. Our numerical results show that the semiclassical limit sets are qualitatively preserved in the quantum stationary states, although quantum fluctuations apparently induce phase diffusion within periodic orbits and stochastic transitions between attractors. We restrict our attention to an experimentally realistic parameter regime.

We present an experimental and theoretical study of controlling the atomic spatial distributions in a magneto-optical trap (MOT). With a diaphragm we can vary the waist and power of one of the cooling laser beams and we can change parameters of large-diameter, parallelogram-shaped atomic orbits. We show that the radiative force generated by the repumping MOT laser has to be taken into consideration. Computer simulations of atomic trajectories explain the observed spatial structures, and we employ these simulations to present potential applications of controlling the diaphragm diameter as a function of time. A potential use of controlled vortex forces seems to have a great significance in recently presented important new methods to investigate cold atom collisions in the MOT, which were recently published.

We study the dynamics of squeezing and purity exchange between two coupled bosonic modes in the model of the quantum parametric converter, comparing it with the dynamics of entanglement.