We show that the group velocity of a weak pulse can be manipulated by controlling the phases of two weak optical fields applied to a V-shaped three-level system. Such control can even cause the probe propagation to change from subluminal to superluminal. We consider two schemes: in the first, the excited states are coupled by decay-induced coherence, which is an inherent property of the medium, and in the second, quantum coherence is created by coupling the excited states to each other by a strong microwave field. We also discuss the group velocity reduction experienced by a single weak propagating probe due to decay-induced coherence.

Twins traveling at constant relative velocity will each see the other’s time dilate leading to the apparent paradox that each twin believes the other ages more slowly. In a finite space, the twins can both be on inertial, periodic orbits so that they have the opportunity to compare their ages when their paths cross. As we show, they will agree on their respective ages and avoid the paradox. The resolution relies on the selection of a preferred frame singled out by the topology of the space.

We provide a canonical form of mixed states in bipartite quantum systems in terms of a convex combination of a separable state and a so-called edge state. We construct entanglement witnesses for all edge states, which allows us to introduce a canonical form of nondecomposable entanglement witnesses and the corresponding positive maps. We also present a nontrivial necessary condition for entanglement witnesses and positive maps to be extremal.

We have simultaneously measured the quantum states of two different spatial modes of the same optical beam by performing quantum-state tomography with an array detector. Both modes are well described by coherent states, but the projection of the signal onto the local oscillator mode contains a mean of 0.09 photons, while a more complicated mode has a mean of 4.3 photons. This demonstrates that for this particular mode the effective detection efficiency when using array detection is over 40 times greater than when using single detectors.

We propose a ground-state approach to realizing quantum computers. This scheme is time-independent and inherently defends against decoherence by possessing an energy barrier to excitation. We prove that our time-independent qubits can perform the same algorithms as their time-dependent counterparts. Advantages and disadvantages of the time-independent approach are described. A model involving quantum dots is provided for illustration.

An iterative algorithm for the reconstruction of an unknown quantum state from the results of incompatible measurements is proposed. It consists of an expectation-maximization step followed by a unitary transformation of the eigenbasis of the density matrix. The procedure has been applied to the reconstruction of the entangled pair of photons.

Expressions for the effective quantum electrodynamics Hamiltonian due to self-energy screening (self-energy correction to the electron-electron interaction) are presented. We use the method of the two-time Green’s function, which handles quasidegenerate atomic states. From these expressions one can evaluate energy corrections to, e.g., $1s2p{}^3{P}_1$ and $1s2p{}^1{P}_1$ in helium and two-electron ions, to all orders in $Z\alpha .$

The decay of orthopositronium into three photons produces a physical realization of a pure state with three-party entanglement. Its quantum correlations are analyzed using recent results on quantum information theory, looking for the final state that has the maximal amount of Greenberger, Horne, and Zeilinger like correlations. This state allows for a statistical dismissal of local realism stronger than the one obtained using any entangled state of two spin one-half particles.

We discuss a simple, experimentally feasible scheme, which elucidates the principles of controlling the reservoir spectrum and the spectral broadening incurred by repeated measurements. This control can yield either the inhibition (Zeno effect) or the acceleration (anti-Zeno effect) of the quasiexponential decay of the observed state by means of frequent measurements. In the discussed scheme, a photon is bouncing back and forth between two perfect mirrors, each time passing a polarization rotator. The horizontal and vertical polarizations can be viewed as analogs of an excited and a ground state of a two level system (TLS). A polarization beam splitter and an absorber for the vertically polarized photon are inserted between the mirrors, and effect measurements of the polarization. The polarization angle acquired in the electrooptic polarization rotator can fluctuate randomly, e.g., via noisy modulation. In the absence of an absorber the polarization randomization corresponds to TLS decay into an infinite-temperature reservoir. The non-Markovian nature of the decay stems from the many round trips required for the randomization. We consider the influence of the polarization measurements by the absorber on this non-Markovian decay, and develop a theory of the Zeno and anti-Zeno effects in this system.

Unlike the ${}^6{\mathrm{Li}}^6\mathrm{Li}$ and ${}^7{\mathrm{Li}}^7\mathrm{Li}$ isotopomers, the ${}^6{\mathrm{Li}}^7\mathrm{Li}$ molecule exhibits surprisingly high dissociation rates for the levels that lie just above the dissociation limit of the $B{}^1{\Pi }_u$ state. As shown by Cacciani and Kokoouline [Phys. Rev. Lett. 84, 5296 (2000)] they can be attributed to a coupling between the $B{}^1{\Pi }_u$ and $1{}^1{\Pi }_g$ states, through the breaking of the ungerade-gerade symmetry. Systematic measurements of these dissociation rates have been performed by discriminating atomic fluorescence from molecular fluorescence in a sub-Doppler experiment involving a beam of lithium dimers and a cw tunable laser beam. Calculations of dissociation rates have been performed in the framework of a perturbative approach, by solving the coupled Schrödinger equations through the use of the mapped Fourier grid representation method. Measured and calculated dissociation rates are in excellent agreement for a unique value of the coupling strength $h=0.144\mathrm{}$ ${\mathrm{cm}}^{-1}.$

Time-dependent quantal density-functional theory (Q-DFT) is a description of the s-system of noninteracting fermions with electronic density equivalent to that of Schrödinger theory, in terms of fields whose sources are quantum-mechanical expectations of Hermitian operators. The theory delineates and defines the contribution of each type of electron correlation to the local electron-interaction potential ${\nu }_{\mathrm{ee}}(\mathbf{r},t)$ of the s system. These correlations are due to the Pauli exclusion principle, Coulomb repulsion, correlation-kinetic, and correlation-current-density effects, the latter two resulting, respectively, from the difference in kinetic energy and current density between the interacting Schrödinger and noninteracting systems. We employ Q-DFT to prove the following sum rules and properties of the s system: (i) the components of the potential due to these correlations separately exert no net force on the system; (ii) the torque of the potential is finite and due solely to correlation-current-density effects; (iii) two sum rules involving the curl of the dynamic electron-interaction kernel defined as the functional derivative of ${\nu }_{\mathrm{ee}}(\mathbf{r},t)$ are derived and shown to depend on the frequency dependent correlation-current-density effect. Furthermore, via adiabatic coupling constant (λ) perturbation theory, we prove: (iv) the exchange potential ${\nu }_x(\mathbf{r},t)$ is the work done in a conservative field representative of Pauli correlations and lowest-order $O\left(\lambda \right)$ correlation-kinetic and correlation-current-density effects; (v) the correlation potential ${\nu }_c(\mathbf{r},t)$ commences in $O\left({\lambda }^2\right),$ and, at each order, it is the work done in a conservative field representative of Coulomb correlations and correlation-kinetic and correlation-current-density effects; (vi) we derive the integral virial theorem relating ${\nu }_{\mathrm{ee}}(\mathbf{r},t)$ to the electron-interaction and correlation-kinetic energy for arbitrary coupling constant strength λ, and show there are no explicit correlation-current-density contributions to the energy. From this integral virial theorem we (vii) obtain the fully interacting $(\lambda =1)$ and exchange-only $(\lambda =0)$ integral virial theorems as special cases, the latter showing there is no explicit correlation-kinetic contribution to the exchange energy; and (viii) write expressions for the electron-interaction and correlation-kinetic actions for arbitrary coupling constant λ in terms of the corresponding fields.

Theories unifying gravity and other interactions suggest the possibility of spatial and temporal variation of physical “constants” in the Universe. Detection of high redshift absorption systems intersecting the sight lines toward distant quasars provides a powerful tool for measuring these variations. In the present paper we demonstrate that high sensitivity to variation of the fine-structure constant $\alpha$ can be obtained by comparing cosmic and laboratory spectra of the Ni II ion. Relativistic effects in Ni II reveal many interesting features. The Ni II spectrum exhibits avoided level crossings under variation of $\alpha$ and the intervals between the levels have strong nonlinear dependencies on relativistic corrections. The values of the transition frequency shifts, due to the change of $\alpha ,$ vary significantly from state to state including changes of sign. This enhances the sensitivity to the variation of $\alpha$ and reduces possible systematic errors. Calculations of the $\alpha$ dependence of the nickel ion spectral lines that are detectable in quasar absorption spectra have been performed using a relativistic configuration interaction method.

The triple differential cross section (3DCS) for the $\mathrm{}(e,2e)\mathrm{}$ ionization of diatomic hydrogen is determined using the first-order transition matrix element of the Born series. A two-effective center continuum wave function which takes into account the diatomic character of the target is introduced to describe the ejected electron in the exit channel. Vertical transitions from the equilibrium position of the ${}^1{\Sigma }_g^{+}$ ground electronic state of the target to the ${}^2{\Sigma }_g^{+}$ ground electronic state of the residual ${\mathrm{H}}_2^{+}$ are considered for relatively high incident electron energy values (∼4 keV). This approach, which needs relatively small computational efforts to apply it to other diatomic systems, produces results that are in good agreement with existing experimental data. The influence on 3DCS of the screening of the nuclear charges produced by the residual bound electron in the exit channel is studied.

We analyze negative-ion resonances in elastic and inelastic total scattering cross sections for slow electron collisions with the heavy-alkali-metal atoms Rb, Cs, and Fr. Our calculations are based on the Dirac $R$-matrix method [Thumm and Norcross, Phys. Rev. A 45, 6349 (1992)]. For incident electrons of up to 2.8 eV kinetic energy, we compare a ${}^3{P}^o$ shape resonance and ${}^3{P}^e,$ ${}^1{P}_1^o,$ and ${}^1{D}_2^o$ Feshbach resonances, located below the first excitation threshold of the atomic target in the low-lying spectra of the ${\mathrm{Rb}}^{-},$ ${\mathrm{Cs}}^{-},$ and ${\mathrm{Fr}}^{-}$ negative ions, with available experimental data and other calculations. We provide the resonance parameters, partial and converged total elastic and inelastic scattering cross sections, and discuss the relative importance of relativistic effects for the three heavy-alkali-metal targets.

We present a quantum-mechanical method to evaluate photoionization spectra of atoms in parallel applied electric and magnetic fields, valid for a wide range of energies and field strengths. We identify different symmetry regions in configuration space and use R-matrix propagation, a frame transformation and a two-dimensional matching procedure to asymptotically defined solutions to solve the Schrödinger equation over all space. We use quantum defect theory to take into account nonhydrogenic atomic cores. We illustrate the method for the hydrogen atom in laboratory strength fields and for strong fields. We analyze the main features of the photoionization spectra for electric and magnetic-field strengths of $51.4\hspace{0.5em}{\mathrm{k}\mathrm{}\mathrm{V}\mathrm{}\mathrm{cm}}^{-1}$ and 470 T, respectively. We identify three types of resonances, defined by their behavior upon varying the external fields and we explain this behavior in terms of the associated wave functions.

Accurate energy levels, transition wavelengths, and dipole oscillator strengths involving the ${1s}^2{2s}_0,$ ${1s}^2{2p}_{-1},$ and ${1s}^2{3d}_{-2}$ states of lithium in strong magnetic field up to $1.27\times {10}^6\mathrm{T}$ are calculated using the modified full core plus correlation method. The contributions to the energy levels from electron correlation is found to be smaller than $0.06\mathrm{a}.\mathrm{u}.\mathrm{}$ in the present calculations. Taking evidence from a detailed convergence study into account, the results for the energy levels are expected to be accurate to within ${10}^{-5}\mathrm{a}.\mathrm{u}.$ The wavelength for the ${1s}^2{2p}_{-1}-{1s}^2{3d}_{-2}$ transition shows a stationary line characteristic, useful for the understanding of observed spectra from white dwarf stars. The absorption oscillator strengths in the length and velocity gauges given in this work are, to the best of our knowledge, the first values for a three-electron atom in a strong magnetic field.

We discuss the final stages of double ionization of atoms in a strong linearly polarized laser field within a classical model. We propose that all trajectories leading to nonsequential double ionization pass close to a saddle in the phase space that we identify and characterize. The saddle lies in a two degree of freedom subspace of symmetrically escaping electrons. The distribution of a longitudinal momenta of ions as calculated within the subspace shows the double hump structure observed in experiments. Including a symmetric bending mode of the electrons allows us to reproduce the transverse ion momenta. We discuss also a path to sequential ionization and show that it does not lead to the observed momentum distributions.

An optical scheme for the error-free transfer of quantum information through a noisy quantum channel is proposed. The scheme is inspired by quantum error-correction schemes, but it avoids the currently unfeasible requirement for a controlled-NOT operation between single photons. The quantum communication scheme presented here rejects bit-flip errors instead of correcting them and combines quantum-measurement properties of three-particle entangled states with properties of the quantum teleportation protocol.