In a recent work [Phys. Rev. A 84, 033804 (2011)] we considered elastic scattering of a twisted particle from a plane wave and studied how the orbital helicity of the twisted particle changes after scattering. We showed that for realistic beams the relative change can be small, $|m^{\prime }-m|/m\ll 1$, thus resolving a controversy which existed in the literature. In this Addendum, using a more accurate analysis, we argue that in the kinematics of Compton backscattering a stronger statement is true: in fact $m^{\prime }\approx m$, and the difference $|m^{\prime }-m|$ even by a single unit is already suppressed.

Quantum control in high-order-harmonic generation is considered theoretically by using a spatial distribution of static electric fields along the propagation direction of the driving field. It is shown that the trajectories of the electrons during its acceleration by the laser field in the high-harmonics-generation process can be controlled by periodically distributed static electric fields, which conveniently shape the driving laser field during propagation. Applying this mechanism, a quasi-phase-matching scheme that leads to filtered enhanced high harmonics is achieved. The harmonics in the plateau region are enhanced due to periodical phase variations in the long quantum trajectories as a consequence of the faster change experienced by the intensity-dependent phase along the longer electron trajectories. This effect should be observed in all quasi-phase-matching schemes based on perturbation of the microscopic quantum phase. The richness of adding a static-electric-field spatial pattern in the interaction region suggests a general scheme for feedback loop control in high-order-harmonic generation.

We have experimentally demonstrated high-resolution compressive ghost imaging at the single-photon level using entangled photons produced by a spontaneous parametric down-conversion source and using single-pixel detectors. For a given mean-squared error, the number of photons needed to reconstruct a two-dimensional image is found to be much smaller than that in quantum ghost imaging experiments employing a raster scan. This procedure not only shortens the data acquisition time, but also suggests a more economical use of photons for low-light-level and quantum image formation.

Starting from a Lagrangian, an electromagnetic field is quantized in the presence of a medium in thermal equilibrium and also in a medium with time-varying temperature. The vector potential for both equilibrium and nonequilibrium cases is obtained and vacuum fluctuations of the fields are calculated. As an illustrative example, the finite-temperature decay rate and level shift of an atom in a polarizable medium are calculated in this approach.

We investigate system-environment correlations based on the exact dynamics of a qubit and its environment in the framework of pure decoherence (phase damping). We focus on the relation of decoherence and the buildup of system-reservoir entanglement for an arbitrary (possibly mixed) initial qubit state. In the commonly employed regime where the qubit dynamics can be described by a Markov master equation of the Lindblad type, we find that for almost all qubit initial states inside the Bloch sphere, decoherence is complete while the total state is still separable—no entanglement is involved. In general, both “separable” and “entangling” decoherence occurs, depending on the temperature and initial qubit state. Moreover, we find situations where classical and quantum correlations periodically alternate as a function of time in the regime of low temperatures.

Starting from a Lagrangian and using functional-integration techniques, series expansions of Green's function of a real scalar field and electromagnetic field, in the presence of a medium, are obtained. The parameter of expansion in these series is the susceptibility function of the medium. Relativistic and nonrelativistic Langevin-type equations are derived. Series expansions for Lifshitz energy in finite temperature and for an arbitrary matter distribution are derived. Covariant formulations for both scalar and electromagnetic fields are introduced. Two illustrative examples are given.

We advance a perspective of the entanglement issue that appeals to the Schlienz-Mahler measure [Phys. Rev. A 52, 4396 (1995)]. Related to it, we propose a criterium based on the consideration of convex subsets of quantum states. This criterium generalizes a property of product states to convex subsets (of the set of quantum states) that is able to uncover an interesting geometrical property of the separability property.

The operation of a frequency comb at extreme ultraviolet (xuv) wavelengths based on pairwise amplification and nonlinear upconversion to the 15th harmonic of pulses from a frequency-comb laser in the near-infrared range is reported. It is experimentally demonstrated that the resulting spectrum at 51 nm is fully phase coherent and can be applied to precision metrology. The pulses are used in a scheme of direct-frequency-comb excitation of helium atoms from the ground state to the $1s4p$ and $1s5p$ ${{}^{1}P}_1$ states. Laser ionization by auxiliary 1064 nm pulses is used to detect the excited-state population, resulting in a cosine-like signal as a function of the repetition rate of the frequency comb with a modulation contrast of up to 55$\%$. Analysis of the visibility of this comb structure, thereby using the helium atom as a precision phase ruler, yields an estimated timing jitter between the two upconverted-comb laser pulses of 50 attoseconds, which is equivalent to a phase jitter of $0.38\left(6\right)$ cycles in the xuv at 51 nm. This sets a quantitative figure of merit for the operation of the xuv comb and indicates that extension to even shorter wavelengths should be feasible. The helium metrology investigation results in transition frequencies of $5740806993\left(10\right)$ and $5814248672\left(6\right)$ MHz for excitation of the $1s4p$ and $1s5p$ ${}^1{P}_1$ states, respectively. This constitutes an important frequency measurement in the xuv, attaining high accuracy in this windowless part of the electromagnetic spectrum. From the measured transition frequencies an eight-fold-improved ${}^4$He ionization energy of $5945204212\left(6\right)$ MHz is derived. Also, a new value for the ${}^4$He ground-state Lamb shift is found of $41247\left(6\right)$ MHz. This experimental value is in agreement with recent theoretical calculations up to order $m{\alpha }^6$ and $m^2/M{\alpha }^5$, but with a six-times-higher precision, therewith providing a stringent test of quantum electrodynamics in bound two-electron systems.

Nonperturbative close-coupling and perturbative distorted-wave methods are used to calculate electron-impact ionization cross sections for excited states of C${}^{+}$. Both the time-dependent close-coupling and the time-independent distorted-wave methods are used to calculate direct ionization cross sections for the $1s^{2}2s^{2}3l$ configurations. An $R$ matrix with pseudostates method is used to calculate total ionization cross sections for the $1s^{2}2s^{2}3l$ configurations, including both direct and indirect excitation-autoionization contributions. Comparison of the results shows both a substantial reduction of the direct ionization cross sections due to nonperturbative effects and a large contribution to the total ionization cross sections due to indirect contributions coming from $2s\to 2p$ excitations. The ionization cross-section results will permit the generation of more accurate generalized collisional-radiative ionization coefficients needed for the modeling of moderately dense carbon plasmas.

We investigated elastic collisions of low-energy electrons with the hydrogen-bonded formic-acid dimer, formamide dimer, and formic-acid–formamide complex. We focused on how the ${\pi }^{*}$ shape resonances of the isolated monomers are affected when bonded to another molecule. The scattering cross sections were computed with the Schwinger multichannel method with pseudopotentials in the static-exchange and static-exchange-plus-polarization approximations, for energies ranging from 1 to 6 eV. The present results support the existence of two low-lying ${\pi }^{*}$ shape resonances for the formic-acid dimer, as suggested in previous theoretical and experimental studies. We also found low-lying ${\pi }^{*}$ shape resonances for the formamide dimer and for the formic-acid–formamide complex. For the dimers, the presence of a center of inversion is key to understanding how these resonances arise from linear combinations of the ${\pi }^{*}$ anion states of the respective monomers. For the formic-acid–formamide complex, the resonances are more localized on each unit, lying at lower energies with respect to the isolated monomers. The present results suggest that if there is no delocalization of the ${\pi }^{*}$ resonances over the pair for hydrogen-bonded molecules, then their positions would lie below those of the units.

Dissociation of CO${{}_2}^{3+}$ formed in fast, heavy-ion-induced ionization of CO${}_2$, using 5 MeV u${}^{-1}$ Si${}^{12+}$ ions as projectiles, has been studied by the technique of multi-ion coincidence momentum imaging. From the momentum correlation between the ejected ions, we conclude that dissociation of CO${{}_2}^{3+}$ into ${\text{C}}^{+}+{\text{O}}^{+}+{\text{O}}^{+}$ is a concerted process involving linear and bent conformations of CO${{}_2}^{3+}$. The kinetic-energy-release spectrum of the dissociation channel $\text{CO}{{}_2}^{3+}\to {\text{C}}^{+}+{\text{O}}^{+}+{\text{O}}^{+}$ is measured and the possible excited states of CO${{}_2}^{3+}$ are computed ab initio.

A merged-beam technique is used to measure charge transfer (CT) cross sections for the (${\mathrm{O}}_2{}^{+},\mathrm{D}$) and (${\mathrm{CO}}^{+},\mathrm{D}$) systems over a wide range of collision energy from 20 eV/u to 2 keV/u. At the higher energies where the collision is rovibrationally frozen and the differences in the $Q$ values of the CT process can be neglected, the cross sections all converge to $\left(7.5\pm 0.5\right)\times {10}^{-16}$ cm${}^2$ at 2 keV/u and are consistent with a rovibrational frozen (${\mathrm{H}}_2{}^{+},\mathrm{H}$) calculation. Toward lower velocities, (${\mathrm{O}}_2{}^{+},\mathrm{D}$) and (${\mathrm{CO}}^{+},\mathrm{D}$) have consistently similar cross sections but diverge below 60 eV/u. In contrast, previously reported merged-beam measurements for (${\mathrm{D}}_2{}^{+},\mathrm{H}$), a system with fewer electrons on the molecular core, no electronic excited states, and relatively fewer charge transfer channels, shows a decreasing cross section toward lower energies. These different trends are compared to previous merged-beam measurements of charge transfer with H for several atomic 4$+$ ions (Si${}^{4+}$, Ne${}^{4+}$, N${}^{4+}$, and C${}^{4+}$) which have a variety of electrons on the core.

Interference and coincidence detection of two photons emitted by two remote ions can lead to an entangled state, which is a critical resource for scalable quantum-information processing. Currently the success probabilities of experimental realizations of this protocol are mainly limited by low coupling efficiency of a photon emitted by an ion into a single-mode fiber. Here we consider two strategies to enhance the collection probability of a photon emitted from a trapped Yb${}^{+}$ ion, using analytic methods that can be easily applied to other types of ions or neutral atoms. Our analysis shows that we can achieve fiber coupling efficiency of over 30$\%$ with an optical cavity made of a flat fiber tip and a spherical mirror. We also investigate ways to increase the fiber coupling efficiency using high-numerical-aperture optics, and show that collection probability of over 15$\%$ is possible with proper control of aberration.

We revisit in detail the non-mean-field ground-state phase diagram for a binary mixture of spin-1 Bose-Einstein condensates including quantum fluctuations. The noncommuting terms in the spin-dependent Hamiltonian under the single-spatial-mode approximation make it difficult to obtain exact eigenstates. Utilizing spin-z-component conservation and total spin angular momentum conservation, we numerically derive information on the building blocks and evaluate the von Neumann entropy to quantify the ground states. The mean-field phase boundaries are found to remain largely intact, yet the ground states show fragmented and entangled behaviors within large parameter spaces of interspecies spin-exchange and singlet-pairing interactions.

We investigate the collapse and revival of first-order coherence in deep optical lattices when long-range interactions are turned on and find that the first few revival peaks are strongly attenuated already for moderate values of the nearest-neighbor interaction coupling. It is shown that the conventionally employed Gutzwiller wave function, with only on-site number dependence of the variational amplitudes, leads to incorrect predictions for the collapse and revival oscillations within the extended Bose-Hubbard model. We provide a modified variant of the Gutzwiller ansatz, reproducing the analytically calculated time dependence of first-order coherence in the limit of zero tunneling.

We study energy transfer and orbital angular momentum of supermodes in a double-ring array of evanescently coupled monomode optical fibers. The structure of supermodes and the spectra of their propagation constants are obtained. The geometrical parameters of the array, at which the energy is mostly confined within the layers, are determined. The developed method for finding the supermodes of concentric arrays is generalized for the case of multiring arrays. The orbital angular momentum carried by a supermode of a double-ring array is calculated. The discrete lattice current is introduced. It is shown that the sum of discrete currents over the array is a conserved quantity. The connection of the total discrete current with orbital angular momentum of discrete optical vortices is made.

We report on the comparative experimental investigation on third- and fifth-harmonic generation (THG and FHG) in atomic and molecular gases driven by midinfrared ultrafast laser pulses at a wavelength of ∼1500 nm. We observe that the conversion efficiencies of both the THG and FHG processes saturate at similar peak intensities close to ∼1.5 × 10${}^{14}$ W/cm${}^2$ for argon, nitrogen, and air, whose ionization potentials are close to each other. Near the saturation intensity, the ratio of yields of the FHG and THG reaches ∼10${}^{-1}$ for all the gases. Our results show that high-order Kerr effect seems to exist; however, contribution from the fourth-order Kerr refractive index coefficient alone is insufficient to balance the Kerr self-focusing without the assistance of plasma generation.

We study partially coherent fields that have a coherent-mode representation in the orbital-angular-momentum-mode basis. For such fields, we introduce the concepts of the angular coherence function and the coherence angle. Such fields are naturally produced by the process of parametric down-conversion—a second-order nonlinear optical process in which a pump photon breaks up into two entangled photons, known as the signal and idler photons. We show that the angular coherence functions of the signal and idler fields are directly related to the angular Schmidt (spiral) spectrum of the down-converted two-photon field and thus that the angular Schmidt spectrum can be measured directly by measuring the angular coherence function of either the signal or the idler field, without requiring coincidence detection.

The photoassociation (PA) dynamics of ultracold cesium atoms steered by the picosecond laser pulse with cubic-phase modulation has been investigated theoretically by solving numerically the time-dependent Schrödinger equation using the mapped Fourier grid method. The temporal duration of the pulse is prolonged, and the pulse area is enlarged by modulating the cubic phase. The PA efficiency is related to the pulse area. When the pulse area is larger than a certain value, the cubic-phase pulses noticeably enhance the PA efficiency, although they have lower energies.