The $E2$ and $M1$ transition rates among the lowest five states (${{[}^4{S}_{3/2}^o]}^{\prime },{{[}^2{D}_{5/2}^o]}^{\prime },{{[}^2{D}_{3/2}^o]}^{\prime },{{[}^2{P}_{3/2}^o]}^{\prime },{{[}^2{P}_{1/2}^o]}^{\prime }$), i.e., the ${{[}^2{D}_{5/2,3/2}^o]}^{\prime }\to {{[}^4{S}_{3/2}^o]}^{\prime }$ and ${{[}^2{P}_{3/2,1/2}^o]}^{\prime }\to {{[}^4{S}_{3/2}^o]}^{\prime }$ transitions, in the $2p^3$ configuration of a nitrogenlike isoelectronic sequence (from $Z=7$ to $Z=79$) are calculated by using a large-scale multiconfiguration Dirac-Fock method. The scaling laws of the excitation energies, fine-structure splittings, forbidden transition rates, line strengths, and ratios of line intensities varying with $Z$ are investigated. The underlying physical mechanism leading to the change of the scaling laws from low $Z$ to high $Z$ are discussed. The calculated transition probabilities and scaling laws are expected to be useful for astrophysical and plasma physical applications.

We study triple-ionization-induced, spatially asymmetric dissociation of ${\mathrm{N}}_2$ using angular streaking in an elliptically polarized laser pulse in conjunction with few-cycle pump-probe experiments. The kinetic-energy-release dependent directional asymmetry in the ion sum-momentum distribution reflects the internuclear distance dependence of the fragmentation mechanism. Our results show that for 5–35-fs near-infrared laser pulses with intensities reaching 10${}^{15}$ W/cm${}^2$, charge exchange between nuclei plays a minor role in the triple ionization of N${}_2$. We demonstrate that angular streaking provides a powerful tool for probing multielectron effects in strong-field dissociative ionization of small molecules.

The negative osmium ion ${\text{Os}}^{-}$ is one of very few atomic anions potentially suitable for laser cooling. We have made a measurement of the Zeeman splitting of a bound-bound transition in ${}^{192}$${\text{Os}}^{-}$ by studying the laser excitation from the 5$d^7$6$s^2$ ${}^4$$F$${}_{9/2}^e$ ground to the 5$d^6$6$s^2$6$p$ ${}^6$$D$${}_{9/2}^o$ excited state in a homogeneous external magnetic field. The experimental Landé factors $g_J=1.31\left(7\right)$ and $g_J=1.50\left(8\right)$, respectively, agree well with calculated values. Both levels are found to split into 10 Zeeman sublevels, resulting in 28 allowed transitions of different relative intensities, in agreement with calculations based on pure and composite $LS$ states. In view of the experimental results, the prospects for laser cooling of ${\text{Os}}^{-}$ are discussed.

We report on high-order harmonic generation (HHG) using a Ti:sapphire laser beam phase shaped with a binary diffractive optical element (DOE) to create two spatially separated synchronized HHG sources at the focus of a lens. Using full three-dimensional computations, we show numerically that the harmonic dipole phase is imprinted in the resulting far-field fringe pattern. Using the corresponding experimental arrangement, we measure HHG phase in aligned carbon dioxide. This arrangement is robust, extremely stable, simple to use, and gives highly resolved fringes. It thus opens new perspectives for combined and refined HHG phase measurements in excited samples.

We implement a four-level semiclassical model of a single pulse interacting with the hyperfine structure in ultracold rubidium aimed at control of population dynamics and quantum state preparation. We discuss a method based on pulse chirping to achieve population inversion between hyperfine states of the $5S$ shell. The results may prove useful for quantum operations with ultracold atoms.

Master equations govern the time evolution of a quantum system interacting with an environment, and may be written in a variety of forms. Time-independent or memoryless master equations, in particular, can be cast in the well-known Lindblad form. Any time-local master equation, Markovian or non-Markovian, may in fact also be written in a Lindblad-like form. A diagonalization procedure results in a unique, and in this sense canonical, representation of the equation, which may be used to fully characterize the non-Markovianity of the time evolution. Recently, several different measures of non-Markovianity have been presented which reflect, to varying degrees, the appearance of negative decoherence rates in the Lindblad-like form of the master equation. We therefore propose using the negative decoherence rates themselves, as they appear in the canonical form of the master equation, to completely characterize non-Markovianity. The advantages of this are especially apparent when more than one decoherence channel is present. We show that a measure proposed by Rivas et al. [Phys. Rev. Lett. 105, 050403 (2010)] is a surprisingly simple function of the canonical decoherence rates, and give an example of a master equation that is non-Markovian for all times $t>0$, but to which nearly all proposed measures are blind. We also give necessary and sufficient conditions for trace distance and volume measures to witness non-Markovianity, in terms of the Bloch damping matrix.

The vacuum-polarization correction for bound electrons or muons is examined. The objective is to formulate a framework for calculating the correction from bound-state quantum electrodynamics entirely in coordinate space, including the Uehling potential, which is usually isolated and treated separately. Pauli-Villars regularization is applied to the coordinate-space calculation and the most singular terms are shown to be eliminated, leaving the physical correction after charge renormalization. The conventional derivation of the Uehling potential in momentum space is reviewed and compared to the coordinate-space derivation.

We study the possibility of using guided photons to generate, control, and measure the entanglement of two qubits that is mediated by a one-dimensional waveguide. We show how entanglement can be generated both with single-photon and with two-photon wave packets. The introduction of a second photon allows for a manipulation of the entanglement between the qubits, and phenomena such as sudden death and revival of entanglement appear. Finally, we propose a procedure for entanglement detection via the scattering output of a single photon over a qubit state.

We develop and demonstrate a technique to engineer universal unitary baths in quantum systems. Using the correspondence between unitary decoherence due to ambient environmental noise and errors in a control system for quantum bits, we show how a wide variety of relevant classical error models may be realized through in-phase or in-quadrature modulation on a vector signal generator producing a resonant carrier signal. We demonstrate our approach through high-bandwidth modulation of the 12.6-GHz carrier appropriate for trapped ${{}^{171}\mathrm{Yb}}^{+}$ ions. Experiments demonstrate the reduction of coherent lifetime in the system in the presence of both engineered dephasing noise during free evolution and engineered amplitude noise during driven operations. In both cases, the observed reduction of coherent lifetimes matches well with quantitative models described herein. These techniques form the basis of a toolkit for quantitative tests of quantum control protocols, helping experimentalists characterize the performance of their quantum coherent systems.

The continuous-variable quantum key distribution (CVQKD) schemes with discrete modulation are proposed to allow distributions of secret keys over long distance due to the sharing of discrete data. However, the nonorthogonal signal states in these schemes can be discriminated by using quantum receivers with low error rates, which may incur state-discrimination (SD) attacks. We consider the feasibility of Eve's attacks on discrete-modulated CVQKD protocol based on the SD receiver. In particular, an intercept-resend attack is proposed on the four-state protocol based on an SD receiver and a heralded noiseless linear amplifier. The security analysis shows that the secret key rate inferred by Alice and Bob can be larger than its true value, which reveals that the original four-state protocol is not secure under the SD attack. Fortunately, the decoy states can be well applied to remedy this defect, but with the cost of complicated practical implementation.

The cross sections of electron impact with Sn${}^{13+}$ resulting in the production of Sn${}^{13+}$-Sn${}^{16+}$ are investigated theoretically by using the fine-structure-level distorted-wave approximation from the threshold to 4000 eV. The electron-impact excitation, ionization, and resonant excitation processes are included in the cross sections. Contributions from different processes are determined by independent processes and isolated resonance approximations. The decay pathways and branching ratios for single and sequentially multiple autoionization for the final autoionized states are determined by detailed calculations. The cross section forming Sn${}^{13+}$ is the largest and next are those for Sn${}^{14+}$ and Sn${}^{15+}$. The double-ionization cross section forming Sn${}^{15+}$ is smaller than that forming Sn${}^{14+}$, yet the relative magnitude is larger at higher incident electron energy. A comparison with a recent measurement for the single-ionization cross section [A. Borovik, Jr. et al., J. Phys. B 46, 175201 (2013)] showed that accurate determination of the branching ratios for single and double autoionization and including contributions of the resonant excitation double autoionization are crucial to obtain accurate results.

Lyman x-ray spectra following charge exchange between C${}^{6+}$ and H${}_2$ are presented for collision velocities between 400 and 2300 km/s (1–30 keV/amu). Spectra were measured by a microcalorimeter x-ray detector capable of fully resolving the C vi Lyman series emission lines though Lyman-$\delta$. The ratios of the measured emission lines are sensitive to the angular momentum $l$ states populated during charge exchange and are used to gauge the effectiveness of different $l$-distribution models in predicting Lyman emission due to charge exchange. At low velocities, we observe that both single-electron-capture and double-electron-capture autoionization contribute to Lyman emission and that a statistical $l$ distribution best describes the measured line ratios. At higher velocities single-electron capture dominates with the $l$ distribution peaked at the maximum $l$.

An ab initio time-dependent convergent close-coupling approach to describing antiproton collisions with molecular hydrogen or the hydrogen molecular ion has been developed. The approach accounts for all possible orientations of the molecular target. Averaging over the molecular orientations is performed fully analytically. For the molecular hydrogen target calculated orientation-averaged total cross sections for single ionization and proton production are compared with several experiments over the energy range of 1–2000 keV. Results for single ionization are in good agreement with experiment, except for the region around the experimental maximum. For proton production reasonable agreement with experiment is observed above 40 keV.

Intense and temporally structured x-ray laser fields enable the controlled generation of strongly coupled nonequilibrium cluster nanoplasmas and the time-resolved investigation of their dynamics. Recent femtosecond pump-probe experiments on xenon clusters revealed subpicosecond relaxation dynamics in the cluster nanoplasma via delay-dependent charge states of emitted atomic ions [M. Krikunova et al., J. Phys. B: At. Mol. Opt. Phys. 45, 105101 (2012)]. Here we report a scheme based on local electron single-particle energy spectra that enables microscopic tracing of the underlying electron-relocalization processes in molecular dynamics simulations up to the strong-coupling regime. We find more efficient recombination in the cluster core and delay-dependent ion charge states, in good agreement with experiments. Our method is applicable to any particle-based plasma simulation and expected to offer insights into correlated relaxation processes in inhomogenous, strongly coupled plasmas.

We consider Dirac quasiparticles, as realized with cold atoms loaded in a honeycomb lattice or in a $\pi$-flux square lattice, in the presence of a weak correlated disorder such that the disorder fluctuations do not couple the two Dirac points of the lattices. We numerically and theoretically investigate the time evolution of the momentum distribution of such quasiparticles when they are initially prepared in a quasimonochromatic wave packet with a given mean momentum. The parallel transport of the pseudospin degree of freedom along scattering paths in momentum space generates a geometrical phase which alters the interference associated with reciprocal scattering paths. In the massless case, a well-known dip in the momentum distribution develops at backscattering (respective to the Dirac point considered) around the transport mean free time. This dip later vanishes in the honeycomb case because of trigonal warping. In the massive case, the dynamical phase of the scattering paths becomes crucial. Its interplay with the geometrical phase induces an additional transient broken reflection symmetry in the momentum distribution. The direction of this asymmetry is a property of the Dirac point considered, independent of the energy of the wave packet. These Berry-phase effects could be observed in current cold-atom lattice experiments.

We propose a form of spontaneous symmetry breaking driven by zero-point quantum fluctuations. To be specific, we consider the low-energy dynamics of a mixture of two species of spin-1 Bose gases. It is demonstrated that the quantum fluctuations lift a degeneracy regarding the relative orientations of the spin directors of the two species and result in correlation or locking between these macroscopic variables. This locking persists in the presence of the trapping potential and weak magnetic fields, allowing, in principle, an experimental probe of this correlated spontaneous symmetry breaking as a macroscopic manifestation of zero-point quantum fluctuations.

Bose-Einstein condensates (BECs) of positronium (Ps) have been of experimental and theoretical interest due to their potential application as the gain medium of a $\gamma$-ray laser. Ps BECs are intrinsically spinor due to the presence of ortho-positronium ($o$-Ps) and para-positronium ($p$-Ps), whose annihilation lifetimes differ by three orders of magnitude. In this paper, we study the spinor dynamics and annihilation processes in the $p$-Ps/$o$-Ps system using both solutions of the time-dependent Gross-Pitaevskii equations and a semiclassical rate-equation approach. The spinor interactions have an O(4) symmetry which is broken to SO(3) by an internal energy difference between $o$-Ps and $p$-Ps. For an initially unpolarized condensate, there is a threshold density of $\approx {10}^{19}$ cm${}^{-3}$ at which spin mixing between $o$-Ps and $p$-Ps occurs. Beyond this threshold, there are unstable spatial modes accompanied by spin mixing. To ensure a high production yield above the critical density, a careful choice of external field must be made to avoid the spin mixing instability.