In this paper, we report on the effects of $\mathrm{LS}$ term dependence on radiative rates and electron-impact excitation cross sections in He-like ions. In particular, we examine the variation of the $2p$ orbital between the $1s2p$ ${}^{3}P$ and $1s2p$ ${}^{1}P$ terms in ${\mathrm{Li}}^{+},$ ${\mathrm{C}}^{4+},$ ${\mathrm{F}}^{7+},$ and ${\mathrm{Mg}}^{10+}.$ We find that for transitions between the $1s2p$ ${}^1{P}_1$ level and the ground state, term dependence causes a variation in both the radiative rate and the excitation cross section at nearly the twenty percent level; however, by ${\mathrm{Mg}}^{10+}$ this variation is less than five percent.

A scheme is proposed to generate an arbitrary entangled state of the two-dimensional vibrational motion of a trapped ion. In the scheme the ion is excited by a sequence of laser pulses tuned to the appropriate vibrational sidebands. In order to generate a pure state with upper phonon numbers M and N in the X and Y directions, respectively, we require no more than $\mathrm{}(M+2\mathrm{})\mathrm{}(N+1\mathrm{})\mathrm{}$ operations.

A general relation between the Moyal formalisms for a spin and a particle is established. Once the formalism has been set up for a spin, the phase-space description of a particle is obtained from contracting the group of rotations to the oscillator group. In this process, turn into a spin Wigner kernel turns into the Wigner kernel of a particle. In fact, only one out of $2^{2s}$ different possible kernels for a spin shows this behavior.

The dynamical-algebraic structure underlying all the schemes for quantum information stabilization is argued to be fully contained in the reducibility of the operator algebra describing the interaction with the environment of the coding quantum system. This property amounts to the existence of a nontrivial group of symmetries for the global dynamics. We provide a unified framework that allows us to build systematically additional classes of error correcting codes and noiseless subsystems. It is shown that by using symmetrization strategies one can artificially produce noiseless subsystems supporting universal quantum computation.

An earlier measurement of the $2^2{S}_{1/2}–2^2{P}_{1/2}$ Lamb shift in ${\mathrm{He}}^{+}$ by the anisotropy method is repeated in order to either verify or remove a significant discrepancy between theory and experiment. The principal change from our previous measurement is a redesigned photon detection system to eliminate a residual polarization sensitivity of the photon detectors. The result of the measurement corresponds to a Lamb shift of 14 041.13(17) MHz, in excellent agreement with the theoretical value 14 041.18(13) MHz. The good agreement between theory and experiment provides a clear test of the recently calculated two-loop binding correction of $-1.339\hspace{0.5em}\mathrm{MHz}.$

Electron-ion recombination of completely stripped ${\mathrm{Bi}}^{83+}$ has been investigated at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. Absolute recombination-rate coefficients have been measured for relative energies between ions and electrons from 0 up to about 125 eV. In the energy range from 15 meV to 125 eV, the experimental result is well described by the theory for radiative recombination (RR). However, below 15 meV the experimental rate increasingly exceeds the RR calculation by up to a factor of 5.2. This puzzling enhancement phenomenon, which has already been observed at other storage rings and single-pass merged-beams setups, is reported here from the ESR and for a bare ion heavier than argon. The enhancement is insensitive to changes of the electron density, which in subsequent measurements was set to $1.6\times {10}^6,$ $3.2\times {10}^6,$ and $4.7\times {10}^6{\mathrm{cm}}^{-3}.$ An additional variation of the magnetic guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted in oscillations with a period of 7.6 mT of the rate that are accompanied by considerable changes of the transverse electron temperature.

We investigate a three-step population scheme to efficiently transfer atoms to a particular metastable state of atomic dysprosium, and present evidence of adiabatic passage in the first step. This scheme will be employed in an ongoing search for parity nonconservation (PNC) in dysprosium. In order to excite a large fraction of the transverse velocity distribution, laser beams with divergences matched to the atomic beam are applied in the first and second steps. The total efficiency of the population transfer is analyzed, and the implications for the PNC work are discussed.

We present an approach to laser cooling of internal molecular degrees of freedom for vibrationally hot molecules using a sequence of ultrashort laser pulses. It is assumed that the molecules initially occupy different vibrational states with a substantial portion of the molecules being in excited vibrational states. We show that the ultimate aim of increasing the vibrational ground-state population through a reduction of the system’s entropy can be achieved through a multistep process. In the first step, we design an ultrashort laser pulse that selectively transfers most of the population of the excited vibrational states to an excited electronic surface; then the field is switched off and the system allowed to relax until most of the excited electronic state population has decayed due to spontaneous emission. By repeating this procedure a few times, the entropy of the system can be substantially reduced and the population of the vibrational ground state increased considerably, even if the lifetimes of the excited electronic states are much greater than the length of a control pulse, i.e., if dissipative effects are negligiable on the time scales of coherent control.

Feedback in compound quantum systems is effected by using the output from one subsystem (“the system”) to control the evolution of a second subsystem (“the ancilla”) that is reversibly coupled to the system. In the limit where the ancilla responds to fluctuations on a much shorter time scale than does the system, we show that it can be adiabatically eliminated, yielding a master equation for the system alone. This is very significant as it decreases the necessary basis size for numerical simulation and allows the effect of the ancilla to be understood more easily. We consider two types of ancilla: a two-level ancilla (e.g., a two-level atom) and an infinite-level ancilla (e.g., an optical mode). For each, we consider two forms of feedback: coherent (for which a quantum-mechanical description of the feedback loop is required) and incoherent (for which a classical description is sufficient). We test the master equations we obtain using numerical simulation of the full dynamics of the compound system. For the system (a parametric oscillator) and feedback (intensity-dependent detuning) we choose, good agreement is found in the limit of heavy damping of the ancilla. We discuss the relation of our work to previous work on feedback in compound quantum systems, and also to previous work on adiabatic elimination in general.

A system of N two-dimensional weakly interacting bosons in a harmonic trap is considered. When the two-particle potential is a δ function, Smith and Wilkin have proved analytically that the elementary symmetric polynomials of particle coordinates measured from the center of mass are exact eigenstates. In this study, we point out that their proof works equally well for an arbitrary two-particle potential that possesses translational and rotational symmetries. We find that the interaction energy associated with the eigenstate with angular momentum L is equal to $aN\left(N-1\right)/2+\left(b-a\right)NL/2,$ where a and b are the interaction energies of two bosons in the lowest-energy one-particle state with zero and one unit of angular momentum, respectively. Additionally, we study briefly the case of attractive quartic interactions. We prove rigorously that the lowest-energy state is the one in which all angular momentum is carried by the center of mass motion.

We investigate two-body loss in an optical dipole trap for ${}^{87}\mathrm{Rb}$ atoms. In the presence of additional near-resonant light, such as from a magneto-optical trap during the trap loading, the two-body loss is strongly enhanced by long-range radiative escape. We suppressed this loss by a factor of 15 by adding a sideband to the optical dipole trap laser. This allows more atoms to be loaded into the optical dipole trap.

Angular momentum can be transferred to a trapped Bose-Einstein condensate by distorting its shape with an external rotating field, provided the rotational frequency is larger than a critical frequency fixed by the energy and angular momentum of the excited states of the system. By using the Gross-Pitaevskii equation and sum rules, we explore the dependence of such a critical frequency on the multipolarity of the excitations and the asymmetry of the confining potential. We also discuss its possible relevance for vortex nucleation in rotating traps.

Relativistic and leading QED corrections to the static electric dipole polarizability of helium are calculated. The resulting theoretical uncertainty is estimated to be under 2 ppm.

We demonstrate the efficient generation of Raman sidebands in a medium coherently prepared in a dark state by continuous-wave low-intensity laser radiation. Our experiment is performed in sodium vapor excited in $\Lambda$ configuration on the $D_1$ line by two laser fields of resonant frequencies ${\omega }_1$ and ${\omega }_2,$ and probed by a third field ${\omega }_3.$ First-order sidebands for frequencies ${\omega }_1,$ ${\omega }_2$ and up to the third-order sidebands for frequency ${\omega }_3$ are observed. The generation starts at a power as low as 10 μW for each input field. Dependences of the intensities of both input and generated waves on the frequency difference $\left({\omega }_1-{\omega }_2\right),$ on the frequency ${\omega }_3,$ and on the optical density are investigated.

The arguments of Cohen [Phys. Rev. A 60, 80 (1999)] against the “ignorance interpretation” of mixed states are questioned. The physical arguments are shown to be inconsistent and the supporting example illustrates the opposite of the original statement. The operational difference between two possible definitions of mixed states is exposed and the inadequacy of one of them is stressed.

Two objections have been raised to the arguments presented by Cohen [Phys. Rev. A 60, 80 (1999)]. It is pointed out that neither objection has anything to do with the main subject matter of that paper, and shown that both objections are based on misunderstandings of the examples to which they relate.

Projectile elastic close-coupling method is employed to investigate the positronium-hydrogen scattering using different basis sets to find the rate of convergence with added eigenstates and pseudostates. We report s-, p-, and d- wave phase shifts below the first positronium excitation threshold and also integrated elastic and excitation cross sections along with the corresponding Born-Oppenheimer results up to the incident positronium energy 200 eV. The present pseudostate singlet and triplet s-wave phase shifts are in good agreement with those of Drachman and Houston [R. J. Drachman and S. K. Houston, Phys. Rev. A 12, 885 (1975)]; [R. J. Drachman, ibid. 19, 1900 (1979)] and 22-state target elastic pseudostate close-coupling predictions of Campbell et al. [C. P. Campbell et al., Phys. Rev. Lett. 80, 5097 (1998)]. The effect of the inelastic channels of the target atom on the elastic one at low energies is found to be significant in studying positronium-atom scattering.

We study the energy difference $\Delta E\left(n\right)\mathrm{}$ between the ${4f}^{n+1\mathrm{}}{5d}^0{6s}^2$ and ${4f}^n{5d}^1{6s}^2$ configurations of the entire neutral lanthanide series, where n runs from 0 (La) through 13 (Yb). The $\Delta E\left(n\right)\mathrm{}$ values obtained by numerical multiconfigurational Dirac-Fock (MC-DF) calculations are lower than experiment, while those given by nonrelativistic self-consistent-field (NR-SCF) calculations are larger than experiment. The difference between the MC-DF and NR-SCF $\Delta E\left(n\right)\mathrm{}$ values is about $3–5\mathrm{eV}.$ If we add the correlation energy corrections obtained by nonrelativistic second-order perturbation calculations to $\Delta E\left(n\right)\mathrm{}$ given by MC-DF calculations, the resulting values of $\Delta E\left(n\right)\mathrm{}$ for the neutral lanthanides agree with experiment to within 1.0 eV, except for La and Yb.

There is a need in spectral line shapes for a realistic treatment of the translational motion in the case of arbitrary ratio of the mass of the perturber to the mass of the active molecule. Here we use a one-dimensional rigid-sphere collision kernel to determine the translational motion. By discretizing the velocity we are able to exploit the analogy between Dicke narrowing and line mixing. The validity of using a one-dimensional versus three-dimensional kernel is explored. A comparison of the results with the well-known soft and hard collision models is also carried out.

Molecular spectroscopy in the infrared region is pushed to new limits by using the Ramsey technique of separated fields on a two-photon transition in ${\mathrm{SF}}_6$ excited using a ${\mathrm{CO}}_2$ laser. The geometry employed elegantly compensates all Doppler problems, while giving progressively higher resolution for simple optical adjustments. In addition, the molecular beam can be characterized by interrogation of a quasiresonant intermediate level. Fringes are obtained with a periodicity down to 1.5 kHz and a statistical uncertainty of a few ${10}^{-13}{\mathrm{Hz}}^{\mathrm{-}1/2}.$ The projection for a frequency standard is in the ${10}^{-15}$ range.