We propose a scheme to realize quantum controlled phase flip (CPF) between two rare-earth ions embedded in the respective microsphere cavity via interacting with a single-photon pulse in sequence. The numerical simulations illuminate that the CPF gate between ions is robust and scalable with extremely high fidelity and low error rate. Our scheme is more applicable than other schemes presented before based on current laboratory cavity-QED technology, and it is possible to be used as an applied unit gate in future quantum computation and quantum communication.

We report on the experimental demonstration of strong quadrature Einstein-Podolsky-Rosen entanglement and squeezing at very low noise sideband frequencies produced by a single type-II, self-phase-locked, frequency degenerate optical parametric oscillator below threshold. The generated two-mode squeezed vacuum state is preserved for noise frequencies as low as $50\mathrm{kHz}$. Designing simple setups able to generate nonclassical states of light in the kHz regime is a key challenge for high sensitivity detection of ultraweak physical effects such as gravitational wave or small beam displacement.

Radiative lifetimes of 11 levels belonging to the $5s5p{}^{1}P^o{}_1$, $5snd{}^{3}D_{1,2}(n=6–9)$ and $5sns{}^{3}S_1(n=7,8)$ series of $\mathrm{Cd}\mathrm{I}$, and of 5 levels of $\mathrm{Cd}\mathrm{II}$ (i.e., $4d^{10}5p{}^{2}P^o{}_{1∕2,3∕2}$, $4d^{10}6s{}^{2}S_{1∕2}$, and $4d^{10}5d{}^{2}D_{3∕2,5∕2}$) have been measured using the time-resolved laser-induced fluorescence technique. Free neutral and singly ionized cadmium atoms have been generated by laser ablation. Single- or two-step excitation processes were considered to populate the levels under study. Branching fractions of $\mathrm{Cd}\mathrm{II}$ transitions have been measured by laser-induced breakdown spectroscopy. Transition probabilities and oscillator strengths for $\mathrm{Cd}\mathrm{I}$ and $\mathrm{Cd}\mathrm{II}$ spectral lines originating from the above states as well as from the $4d^{9}5s^2{}^{2}D_{3∕2,5∕2}$ states of $\mathrm{Cd}\mathrm{II}$ have been deduced by combining the experimental lifetimes and theoretical branching fractions obtained in multiconfigurational relativistic Hartree-Fock calculations taking core-polarization effects into account.

Experimental cross sections are presented for single- and double-electron capture by ${\mathrm{C}}^{q+}$, ${\mathrm{N}}^{q+}$, and ${\mathrm{O}}^{q+}$ $(q=2–6)$ ions from He at incident energies of $(1.0–1800)q\mathrm{eV}$. Measurements were performed by using a mini-electron-beam-ion-source apparatus in combination with an octopole-ion-beam-guide. The cross sections are found to vary significantly depending on the collision energy as well as on the projectile species. Some cross sections reveal minima at incident energies of a few $\mathrm{eV}∕\mathrm{u}$, below which the cross sections increase with decreasing incident energy. To account for the experimental results, we developed a formalism of velocity-dependent capture cross sections within the framework of the classical over barrier model by employing an induced dipole interaction potential between the collision partners. In the calculations, doubly excited levels of projectile ions, formed via simultaneous excitation of the outermost projectile electron, were taken into consideration. It is found that the present model can satisfactorily reproduce the experimentally obtained energy dependence as well as absolute values of the cross sections. We conclude that the attractive induced dipole potential and simultaneous projectile excitation are important in low energy electron-capture collisions.

The structure of vortices in Bose-Einstein condensed atomic gases is studied taking into account many-body correlation effects. It is shown that for excited vortices the particle density in the vortex core increases as the angular momentum of the system increases. The core density can increase by several times with only a few percent change in the angular momentum. This result provides an explanation for the observations in which the measured angular momentum is higher than the estimation based on counting the number of vortices and the visibility of the vortex cores is simultaneously reduced. The calculated density profiles for the excited vortices are in good agreement with experimental measurements of a single vortex.

The influence of intersubsystem coupling on the cyclic adiabatic geometric phase in bipartite systems is investigated. We examine the geometric phase effects for two uniaxially coupled spin-$\frac{1}{2}$ particles, both driven by a slowly rotating magnetic field. It is demonstrated that the relation between the geometric phase and the solid angle enclosed by the magnetic field is broken by the spin-spin coupling, in particular leading to a quenching effect on the geometric phase in the strong coupling limit.

The bound levels and the resonances (energy and width of the excited levels) of $pp\mu$-like exotic molecules for $J=0$ total angular momentum have been computed with an accuracy in the ${10}^{-11}\mathrm{a.u.}$ range, by numerical diagonalization of the complex rotated Hamiltonian in a variational sturmian basis set. For the resonances below the $N=2$ dissociation threshold, the x-ray spontaneous emission spectrum is computed from the wave functions. The radiative decay rate of the first resonance of $pp\mu$ is found to be $0.0713{\mathrm{ps}}^{-1}$, close to half that of a $p\mu \left(2p\right)$ atom, as expected in a simple Born-Oppenheimer picture of a resonance.

It is pointed out that the proper interpretation of a recently published experimental spectrum from the multilaser photoionization of Sr [Eichmann et al., Phys. Rev. Lett. 90, 233004 (2003)] must account for a radiative transition between two autoionizing states. The application of orthonormality selection rules and of configuration-interaction theory involving the continuous spectrum and the quasicontinuum of the upper part of Rydberg series explains quantitatively the appearance, the shape, and the variation of heights of the observed peaks of resonances.

High-resolution spectroscopy of doubly excited states produced by dielectronic recombination (DR) of lithiumlike ${\mathrm{Sc}}^{18+}$ ions was performed by employing the electron-ion merged-beam technique at the heavy-ion storage ring TSR. The experimental procedure for measuring DR resonances with high precision is thoroughly described with an emphasis on the uncertainties of the experimental energy scale. Absolute measurements of recombination rate coefficients were carried out over the center-of-mass energy range $0–50\mathrm{eV}$ that comprises all DR resonances associated with $2s_{1∕2}\to 2p_{1∕2,3∕2}$ excitations. At relative energies below $300\mathrm{meV}$ resonances due to DR via ${\mathrm{Sc}}^{17+}$ $\left(1s^{2}2p_{3∕2}10l_j\right)$ intermediate states were found. Their positions could be measured with an uncertainty of only $\pm 1.8\mathrm{meV}$. The results are compared with theoretical calculations within the framework of relativistic many-body perturbation theory. By combining the precision of the experimental and theoretical results we derive a value for the $2s_{1∕2}\to 2p_{3∕2}$ excitation energy, $44.3107\left(19\right)\mathrm{eV}$, which is by more than an order of magnitude more accurate than the hitherto most precise value obtained from optical spectroscopy.

Alignment and orientation effects in collisions of helium with Rydberg atoms due to the presence of static electric and magnetic fields are predicted. Analytical expressions are obtained for the cross sections of the state-to-state transitions within the manifold with the same principal quantum number. For moderate field-induced inelasticity, the curves of the cross sections vs the fields intensity show modulations that are explained in terms of the phase differences accumulated by the wave functions of the states involved in the collision. Due to the presence of the magnetic field, transitions involving inversion of high magnetic quantum numbers are dramatically quenched. The possibility of observing the predicted effects is discussed. The reported analytical cross sections are expected to help in the understanding of the results.

According to a previous estimation made by Lindhard [Nucl. Instrum. Method Phys. Res. 132, 1 (1976)] on the basis of qualitative arguments and dimensional analysis, the contribution of close collisions to the Barkas effect in the energy loss of swift ions in solids yields a significant fraction of the total effect, being almost equal to the contribution coming from distant collisions. Here the classical estimation by Lindhard is reconsidered and subjected to a tight numerical test. We analyze in quantitative terms the classical description of the Barkas effect following the line of arguments proposed by Lindhard. We consider a swift ion of charge $Z_1$ interacting with the electrons via a screened potential assuming spherical and nonspherical screening models. We calculate by numerical integration the differential and total transport cross sections and find an important asymmetry of both with respect to the sign of $Z_1$, as well as a departure from the $Z_1^2$ behavior. These effects are particularly important for impact parameters in the range of the classical collision radius $(r_{\mathit{cl}}=Z_1{e}^2∕m{v}^2)$. This approach clearly shows the contribution of close collisions to the Barkas effect. We analyze the behavior of the Barkas asymmetry with respect to ion charge and velocity, obtaining a good quantitative agreement with Lindhard’s prediction. However, an additional term predicted for nonspherical potentials cannot be reproduced.

The dynamics of a ring of vortices in two-dimensional Bose-Einstein condensates (with and without an additional vortex at the center) is studied for (1) a uniform condensate in a rigid cylinder and (2) a nonuniform trapped condensate in the Thomas-Fermi limit. The sequence of ground states (within these single-ring configurations) is determined as a function of the external rotation frequency by comparing the free energy of the various states. For each ground state, the Tkachenko-like excitations and the associated dynamical stability are analyzed.

The occurrence of energetic and dynamical instabilities in a Bose-Einstein condensate moving in a one-dimensional (1D) optical lattice is analyzed by means of the Gross-Pitaevskii theory. Results of full 3D calculations are compared with those of an effective 1D model, the nonpolynomial Schrödinger equation, pointing out the role played by transverse degrees of freedom. The instability thresholds are shown to be scarcely affected by transverse excitations, so that they can be accurately predicted by effective 1D models. Conversely, transverse excitations turn out to be important in characterizing the stability diagram and the occurrence of a complex radial dynamics above the threshold for dynamical instability. This analysis provides a realistic framework to discuss the dissipative dynamics observed in recent experiments.

We analyze the scaling laws that govern the production of polarization-entangled photons via type-II spontaneous parametric down-conversion (SPDC). We demonstrate experimentally that thin nonlinear crystals can generate a higher number of entangled photons than thicker crystals, basically because they generate a broader spectrum.

We derive a solution for a two-level system evolving adiabatically under the influence of a driving field, which includes open system effects. This solution, which is obtained by working in the representation corresponding to the eigenstates of the time-dependent Hermitian Hamiltonian, enables the dynamic and geometric phases of the evolving density matrix to be separated. The dynamic phase can be canceled in the limit of weak coupling to the environment, thereby allowing the geometric phase to be readily extracted both mathematically and operationally.

The superfluid–Mott-insulator phase transition of dipolar bosons in optical lattice is analyzed. By using the Bogoliubov approach and decoupling approximation, the energy spectrum and zero-temperature phase diagram of dipolar bosonic atoms in an optical lattice are obtained analytically. The results show that in these systems the superfluid–Mott-insulator phase transition can be induced by tuning the dipole-dipole interaction and the position of the phase boundary can be moved when the on-site interaction is varied. Corresponding to a large on-site interaction, the dipole-dipole interaction is attractive near the critical point of the superfluid–Mott-insulator phase transition.

The embedding method [J. E. Inglesfield, J. Phys. C 14, 3795 (1981)] for computing bound states of the Schrödinger equation is reformulated in terms of the Dirichlet-to-Neumann (DtN) and Neumann-to-Dirichlet (NtD) surface integral operators. Variational principles for energy, allowing the use of trial functions which are discontinuous in values or derivatives, are employed. A method of constructing kernels of the DtN and NtD operators from solutions to an auxiliary Steklov (Stekloff) eigenproblem is presented. Numerical results illustrating the usefulness of the DtN and NtD embedding methods are provided. After necessary modifications, the DtN-NtD formalism presented in this work may constitute the convenient framework for generalizing the embedding method to bound states of the Dirac equation.

We discuss “mode-entangled states” based on the optical transverse modes of the optical field propagating in multimode waveguides, which are classical simulation of the quantum entangled states. The simulation is discussed in detail, including the violation of the Bell inequality and the correlation properties of optical pulses’ group delays. The research on this simulation may be important, for it not only provides useful insights into fundamental features of quantum entanglement, but also yields interesting insights into quantum computation and quantum communication.

Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as $100{\mathrm{cm}}^{-1}$. Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after $1\mu \mathrm{s}$ caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high $n$ states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.

We present time-dependent density functional studies of harmonic generation from ${\mathrm{Ag}}_2$ and ${\mathrm{Ag}}_8$ in pulsed laser fields. The harmonic generation is strongly dependent on the laser frequency. The harmonics are emitted from the clusters much more efficiently when the applied laser field is in tune with the dipole resonance frequency of the system. Such resonance frequency dependence is substantially equal to a resonance phenomenon in a forced oscillator in a sense that the valence $s$-electrons are shaken effectively at the tuned laser frequency and the induced dipole moment continues to oscillate even though the laser field is switched off. Furthermore, we have found that the polarizable core $d$-electrons significantly screen the valence $s$-electrons such that the electron density of the $s$ electrons induced in the laser field is canceled out. The screening effect of the $d$ electrons becomes more important in the system of ${\mathrm{Ag}}_8$ than ${\mathrm{Ag}}_2$.