Absolute single- and double-charge-exchange cross sections for the astrophysically prominent ${\mathrm{O}}^{6+}$ ion with the atomic and molecular targets He and ${\mathrm{H}}_2$ are reported. These collisions give rise to x-ray emissions in the interplanetary medium, planetary atmospheres, and comets as they approach the sun. Measurements have been carried out using the Caltech Jet Propulsion Laboratory electron cyclotron resonance ion source with ${\mathrm{O}}^{6+}$ at energies of 1.17 and 2.33 keV/u characteristic of the slow and fast components of the solar wind. Absolute charge-exchange (CE) data are derived from knowledge of the target gas pressure, target path length, incident ion current, and charge-exchanged ion currents. These data are compared with results obtained using the $n$-electron classical trajectory Monte Carlo method. The radiative and Auger evolution of ion populations following one- and two-electron transfers is calculated with the time-dependent collisional-radiative code nomad using atomic data from the flexible atomic code. Calculated CE emission spectra for $100\AA <\lambda <1400\AA$ are reported as well and compared with experimental sublevel spectra and cross sections.

Solving the time-dependent Schrödinger equation from first principles, the laser-induced breakup dynamics of hydrogen is studied beyond the electric dipole approximation, at very high laser intensities. It is assumed that the atom is being irradiated by an extreme ultraviolet laser light pulse at a wavelength of 13 nm, corresponding to a photon energy of 95 eV. It has already been experimentally demonstrated that the free-electron laser (FEL) FLASH in Hamburg can deliver irradiance levels up to about ${10}^{16}\text{W}/{\mathrm{cm}}^2$ in this wavelength range. Although we will go to even higher intensities in the present work, in order to spot nondipole effects, this merely demonstrates that ultrahigh light intensities can be achieved with present FEL technologies. Furthermore, with new seeding techniques the laser power is expected to go even higher in the future. In our study the atom is exposed to a short attosecond laser pulse, and the role of higher-order corrections to the electric dipole approximation is studied systematically. The main findings are that higher-order corrections beyond the leading first-order term, to a good approximation, can be neglected for all intensities within the nonrelativistic regime, provided the pulse duration is not too long. This means that the effect of second- and higher-order corrections only needs to be accounted for when entering the relativistic regime, within the scope of the Dirac equation. It is further found that the leading first-order correction to the dipole approximation has a great impact on the angular emission pattern of the low-energy photoelectrons.

This paper aims to shed some light on the role of the direct, or nonsequential, ionization channel in the regime in which the sequential channel is open in two-photon double ionization (TPDI) of helium. In this regime the sequential channel dominates any direct contribution unless the laser pulse is of very short duration, in which case their distinction is hard to draw. Based on both a simple model and full solutions of the time-dependent Schrödinger equation, we aim to provide evidence of direct double ionization by identifying a term proportional to the pulse duration in the double ionization yield. Indeed, such a term is identified in the energy-differential yield. When it comes to the total double ionization probability, however, it turns out that the net first-order contribution is negative. The nature of the negative first-order contribution is discussed, and we argue that it is of correlated origin.

Starting from a mean-field model of the Bose-Einstein condensate dimer, we reintroduce classically forbidden tunneling through a Bohr-Sommerfeld quantization approach. We find closed-form approximations to the tunneling frequency more accurate than those previously obtained using different techniques. We discuss the central role that tunneling in the self-trapped regime plays in a quantitatively accurate model of a dissipative dimer leaking atoms to the environment. Finally, we describe the prospects of experimental observation of tunneling in the self-trapped regime, both with and without dissipation.

We present a method for robust timekeeping in which alkali-metal atoms are interrogated in a Ramsey sequence based on stimulated Raman transitions with optical photons. To suppress systematic effects introduced by differential ac Stark shifts and optical intensity gradients, we employ atom optics derived from Raman adiabatic rapid passage (ARP). Raman ARP drives coherent transfer between the alkali-metal hyperfine ground states via a sweep of the Raman detuning through the two-photon resonance. Our experimental implementation of Raman ARP reduced the phase sensitivity of Ramsey sequences to Stark shifts in ${}^{133}\mathrm{Cs}$ atoms by about two orders of magnitude, relative to fixed-frequency Raman transitions. This technique also preserved Ramsey fringe contrast for cloud displacements reaching the $1/e^2$ intensity radius of the laser beam. In a magnetically unshielded apparatus, second-order Zeeman shifts limited the fractional frequency uncertainty to $\sim 3.5\times {10}^{-12}$ after about 2500 s of averaging.

We load a Bose-Einstein condensate into a one-dimensional (1D) optical lattice altered through the use of radio-frequency (rf) dressing. The rf resonantly couples the three levels of the ${}^{87}\mathrm{Rb}$ $F=1$ manifold and combines with a spin-dependent “bare” optical lattice to result in adiabatic potentials of variable shape, depth, and spatial frequency content. We choose dressing parameters such that the altered lattice is stable over lifetimes exceeding tens of ms at higher depths than in previous work. We observe significant differences between the BEC momentum distributions of the dressed lattice as compared to the bare lattice, and find general agreement with a 1D band-structure calculation informed by the dressing parameters. Previous work using such lattices was limited by very shallow dressed lattices and strong Landau-Zener tunneling loss between adiabatic potentials, equivalent to failure of the adiabatic criterion. In this work we operate with significantly stronger rf coupling (increasing the avoided-crossing gap between adiabatic potentials), observing dressed lifetimes of interest for optical lattice-based analog solid-state physics.

We show numerically that both the modified Korteweg–de Vries and the sine-Gordon models are conducive to the generation of supercontinua with spectral bandwidths of several octaves, when an intense short pulse is launched as the initial condition. These models beyond the slowly-varying-envelope approximation could play an important role in modeling supercontinuum generation in gas-filled hollow waveguides.

We introduce a recursive bosonic quantization technique for generating classical parity-time ($\mathcal{PT}$) photonic structures that possess hidden symmetries and higher-order exceptional points. We study light transport in these geometries and we demonstrate that perfect state transfer is possible only for certain initial conditions. Moreover, we show that for the same propagation direction, left and right coherent transports are not symmetric with field amplitudes following two different trajectories. A general scheme for identifying the conservation laws in such $\mathcal{PT}$-symmetric photonic networks is also presented.

We measure the joint $Q$ function of a multispatial-mode field using a charge-coupled device array detector in an unbalanced heterodyne configuration. The intensity pattern formed by interference between a weak signal field and a strong local oscillator is resolved using Fourier analysis and used to reconstruct quadrature amplitude statistics for 22 spatial modes simultaneously. The local oscillator and signal propagate at an angle of $12\mathrm{mrad}$, thus shifting the classical noise to modes that do not overlap with the signal. In this configuration, balanced detection is not necessary.

Coupled-resonator optical waveguides are known to have interesting and useful dispersion properties. Here, we study the transport in these waveguides in the general case where each resonator is open and asymmetric, i.e., is leaky and possesses no mirror-reflection symmetry. Each individual resonator then exhibits asymmetric backscattering between clockwise- and counterclockwise-propagating waves, which, in combination with the losses, induces nonorthogonal eigenmodes. In a chain of such resonators, the coupling between the resonators induces an additional source of non-Hermiticity, and a complex band structure arises. We show that in this situation the group velocity of wave packets differs from the velocity associated with the probability density flux, with the difference arising from a non-Hermitian correction to the Hellmann-Feynman theorem. Exploring these features numerically in a realistic scenario, we find that the complex band structure comprises almost-real branches and complex branches, which are joined by exceptional points, i.e., non-Hermitian degeneracies at which not only the frequencies and decay rates but also the eigenmodes themselves coalesce. The non-Hermitian corrections to the group velocity are largest in the regions around the exceptional points.

We propose two models for the creation of stable dissipative solitons in optical media with the ${\chi }^{\left(2\right)}$ (quadratic) nonlinearity. To compensate spatially uniform loss in both the fundamental-frequency (FF) and second-harmonic (SH) components of the system, a strongly localized “hot spot” carrying the linear gain is added, acting either on the FF or on the SH component. In both systems, we use numerical methods to find families of dissipative ${\chi }^{\left(2\right)}$ solitons pinned to the “hot spot”. The shape of the existence and stability domains may be rather complex. An existence boundary for the solitons, which corresponds to the guided mode in the linearized version of the systems, is obtained in an analytical form. The solitons demonstrate noteworthy features, such as spontaneous symmetry breaking (of spatially symmetric solitons) and bistability.

Entanglement plays a central role in the field of quantum information science. It is well known that the degree of entanglement cannot be increased under local operations. Here, we show that the concurrence of a bipartite entangled state can be increased under the local $\mathcal{PT}$-symmetric operation. This violates the property of entanglement monotonicity. We also use the Bell–Clauser-Horne-Shimony-Holt and steering inequalities to explore this phenomenon.

We have estimated the shift and broadening of the resonance lines in the spectrum of antiprotonic helium atoms $\overline{p}{\mathrm{He}}^{+}$ implanted in solid helium ${}^4\mathrm{He}$. The application of the response function for crystalline helium has enabled determination of the contributions from the collective degrees of freedom to the shift and broadening. It occurs that the broadening due to the collective motion is negligible compared to the natural line width. The available pair-correlation functions for crystalline ${}^4\mathrm{He}$ have been applied for estimating the resonance-line shift due to collisions of $\overline{p}{\mathrm{He}}^{+}$ atom with the surrounding ${}^4\mathrm{He}$ atoms. The dependence of the line shift, which has been calculated in the quasistatic limit, on the solid-${}^4\mathrm{He}$ density is nonlinear.

We demonstrate stable, robust, bright one-dimensional dipolar Bose-Einstein-condensate (BEC) solitons, moving in a direction perpendicular to the polarization direction, formed due to dipolar interaction for repulsive contact interaction. At medium velocity the head on collision of two such solitons is found to be quasielastic with practically no deformation. Upon small perturbation the solitons are found to exhibit sustained breathing oscillation. The findings are illustrated by numerical simulation using the three-dimensional mean-field Gross-Pitaevskii equation and a reduced two-dimensional model in three and two spatial dimensions employing realistic interaction parameters for a dipolar ${}^{164}\mathrm{Dy}$ BEC.

We give explicit expressions for canonical states labeling the vast majority of entanglement equivalent classes of symmetric states of qubits and efficient algorithms for reducing a given state to the representative of the class it belongs. This way, we achieve an almost complete classification under local unitary and local invertible transformations for symmetric states. The main tool is a technique introduced in this work, enabling to decompose in a unique way, spin symmetric states into a superposition of spin-$1/2$ coherent states. For the case of two qubits, the proposed decomposition reproduces the Schmidt decomposition and therefore, in the case of a higher number of qubits, can be considered as its generalization.

We report the results of our ab initio relativistic many-body calculations of the electric dipole moment (EDM) $d_A$ arising from the electron-nucleus tensor-pseudotensor (T-PT) interaction, the interaction of the nuclear Schiff moment (NSM) with the atomic electrons and the electric dipole polarizability ${\alpha }_d$ for ${}^{223}\mathrm{Rn}$. Our relativistic random-phase approximation results are substantially larger than those of lower-order relativistic many-body perturbation theory and the results based on the relativistic coupled-cluster method with single and double excitations are highly accurate for all three properties that we have considered. We obtain $d_A=4.85\left(6\right)\times {10}^{-20}\langle \sigma \rangle C_T\left|e\right|\mathrm{cm}$ from T-PT interaction, $d_A=2.89\left(4\right)\times {10}^{-17}S/\left(\right|e\left|{\mathrm{fm}}^3\right)$ from NSM interaction, and ${\alpha }_d=35.27\left(9\right)e{a}_0^3$. The former two results in combination with the measured value of ${}^{223}\mathrm{Rn}$ EDM, when it becomes available, could yield the best limits for the T-PT coupling constant, EDMs, and chromo-EDMs of quarks and ${\theta }_{\mathrm{QCD}}$ parameter, and would thereby shed light on leptoquark and supersymmetric models that predict $CP$ violation.

Hyperentanglement is a promising resource in quantum information processing, especially for increasing the channel capacity of long-distance quantum communication. Hyperentanglement purification is an important method to obtain high-fidelity nonlocal hyperentangled states from mixed hyperentangled states in a long-distance quantum communication process with noisy channels. Here, we present a two-step hyperentanglement purification protocol for nonlocal mixed hyperentangled states with polarization bit-flip errors and spatial-mode phase-flip errors, resorting to polarization-spatial phase-check quantum nondemolition detectors and the quantum-state-joining method (QSJM). With QSJM, the protocol can preserve the states that are discarded in the previous hyperentanglement purification protocols. It has the advantage of a high efficiency, and it is useful for improving the entanglement of photon systems with several degrees of freedom in long-distance high-capacity quantum communication.

We present the isotope shifts of the $7s_{1/2}$ to $7p_{1/2}$ transition for francium isotopes ${}^{206-213}\mathrm{Fr}$ with reference to ${}^{221}\mathrm{Fr}$ collected from two experimental periods. The shifts are measured on a sample of atoms prepared within a magneto-optical trap by a fast sweep of radio-frequency sidebands applied to a carrier laser. King plot analysis, which includes literature values for $7s_{1/2}$ to $7p_{3/2}$ isotope shifts, provides a field shift constant ratio of 1.0520(10) and a difference between the specific mass shift constants of 170(100) GHz amu between the $D_1$ and $D_2$ transitions, of sufficient precision to differentiate between ab initio calculations.

Intershell, resonant electronic recombination is studied experimentally in an electron-beam ion trap for O-like ${\mathrm{Si}}^{6+}$ to He-like ${\mathrm{Si}}^{12+}$ ions at plasma temperatures in the megakelvin range similar to those found in the solar radiative zone and is compared to extended multiconfiguration Dirac-Fock and relativistic configuration-interaction predictions. For this low-$Z$ ion, the higher-order electronic recombination processes are comparable in strength to the first-order one. The ratio of trielectronic to dielectric recombination for B-like species agrees well with predictions, whereas for C-like ions the measured value is only half as large. This difference is explained by the influence of metastable states populated in the recombining plasma.

We theoretically investigate the use of an isolated attosecond vacuum ultraviolet (VUV) pulse to control the emergence of multiple wave-packet rescatterings in the process of high-harmonic generation (HHG). Through numerical solution of the time-dependent Schrödinger equation for a He atom driven by $0.8–2.0\text{−}\mu \mathrm{m}$ light, we establish the relationship between evidence of multiple rescatterings in HHG and the time delay between the VUV and infrared pulses. We find features of multiple rescatterings present in both the time and frequency domains of emitted HHG, and demonstrate the use of VUV-induced multiple rescatterings for generating trains of ultrashort light pulses.