We study propagation and on-off switching of two colliding soliton sequences in the presence of second-order dispersion, Kerr nonlinearity, linear loss, cubic gain, and quintic loss. Employing a Lotka-Volterra (LV) model for dynamics of soliton amplitudes along with simulations with two perturbed coupled nonlinear Schrödinger (NLS) equations, we show that stable long-distance propagation can be achieved for a wide range of the gain-loss coefficients, including values that are outside of the perturbative regime. Furthermore, we demonstrate robust on-off and off-on switching of one of the sequences by an abrupt change in the ratio of cubic gain and quintic loss coefficients, and extend the results to pulse sequences with periodically alternating phases. Our study significantly strengthens the recently found relation between collision dynamics of sequences of NLS solitons and population dynamics in LV models, and indicates that the relation might be further extended to solitary waves of the cubic-quintic Ginzburg-Landau equation.

We propose that parametrically excited patterns, also known as Faraday patterns, can be observed in nonlinear fiber resonators, where the coefficient of Kerr nonlinearity is periodically varying along the fiber in the resonators. We study the parametric instability analytically on the basis of the Floquet theory and also numerically by direct integration of the system. Instead of a classical Faraday wave excitation scenario, where modulation in time causes the formation of patterns in space, here we propose an inverted scenario, where the modulation in space excites the patterns in time.

We have investigated magnetically tunable Feshbach resonances arising from the interaction of Li(${}^{2}S$) with metastable Yb(${}^3{P}_2$) and (${}^3{P}_0$). For Yb(${}^3{P}_2$), all the resonance features are strongly suppressed by inelastic collisions that produce Yb in its lower-lying ${}^3{P}_1$ and ${}^3{P}_0$ states. For Yb(${}^3{P}_0$), sharp resonances exist but they are extremely narrow (widths less than 1 mG).

Spin-orbit-coupled ultracold atoms provide an intriguing new avenue for the study of rich spin dynamics in superfluids. In this Rapid Communication, we observe Zitterbewegung, the simultaneous velocity (thus position) and spin oscillations, of neutral atoms between two spin-orbit-coupled bands in a Bose-Einstein condensate (BEC) through sudden quantum quenches of the Hamiltonian. The observed Zitterbewegung oscillations are perfect on a short time scale but gradually damp out on a long time scale, followed by sudden and strong heating of the BEC. As an application, we also demonstrate how Zitterbewegung oscillations can be exploited to populate the upper spin-orbit band and observe a subsequent dipole motion. Our experimental results are corroborated by a theoretical and numerical analysis and showcase the great flexibility that ultracold atoms provide for investigating rich spin dynamics in superfluids.

We investigate a range of methods to perform tomography in a solid-state qubit device, for which a priori initialization and measurement of the qubit is restricted to a single basis of the Bloch sphere. We explore and compare several methods to acquire precise descriptions of additional states and measurements, quantifying both stochastic and systematic errors, ultimately leading to a tomographically complete set that can be subsequently used in process tomography. We focus in detail on the example of a spin qubit formed by the singlet-triplet subspace of two electron spins in a GaAs double quantum dot, although our approach is quite general.

We investigate the nonrelativistic limit of the gravitationally coupled Dirac equation via a Foldy-Wouthuysen transformation. The relativistic correction terms have immediate and obvious physical interpretations in terms of a gravitational Zitterbewegung and a gravitational spin-orbit coupling. We find no direct coupling of the spin vector to the gravitational force, which would otherwise violate parity. The particle-antiparticle symmetry described recently by one of us [Jentschura, Phys. Rev. A 87, 032101 (2013)] is verified on the level of the perturbative corrections accessed by the Foldy-Wouthuysen transformation. The gravitational corrections to the electromagnetic transition current are calculated.

We study the entangling power of a nanoelectromechanical system (NEMS) simultaneously interacting with two separately trapped ions. To highlight this entangling capability, we consider a special regime where the ion-ion coupling does not generate entanglement in the system and any resulting entanglement will be the result of the NEMS acting as an entangling device. We study the dynamical behavior of the bipartite NEMS-induced ion-ion entanglement as well as the tripartite entanglement of the whole system ($\text{ions}+\text{NEMS}$). We found some quite remarkable phenomena in this hybrid system. For instance, the two trapped ions initially uncorrelated and prepared in coherent states can become entangled by interacting with a nanoelectromechanical resonator (also prepared in a coherent state) as soon as the ion-NEMS coupling achieves a certain value, and this can be controlled by external voltage gate on the NEMS device. We also show that, dynamically, the tripartite entanglement presents a more pronounced robustness against the destructive effects of dissipation when compared to the bipartite content.

This paper presents a method for the efficient numerical computation of Casimir interactions between objects of arbitrary geometries, composed of materials with arbitrary frequency-dependent electrical properties. Our method formulates the Casimir effect as an interaction between effective electric and magnetic current distributions on the surfaces of material bodies and obtains Casimir energies, forces, and torques from the spectral properties of a matrix that quantifies the interactions of these surface currents. The method can be formulated and understood in two distinct ways: (1) as a consequence of the familiar stress-tensor approach to Casimir physics, or, alternatively, (2) as a particular case of the path-integral approach to Casimir physics, and we present both formulations in full detail. In addition to providing an algorithm for computing Casimir interactions in geometries that could not be efficiently handled by any other method, the framework proposed here thus achieves an explicit unification of two seemingly disparate approaches to computational Casimir physics.

We present cross-section results from an experimental and theoretical study on positron scattering from nitrous oxide. Total cross sections (TCSs) have been measured at incident energies between 0.1 and 70 eV with a linear-transmission-based positron spectrometer. Elastic differential and integral cross sections, as well as inelastic integral cross sections and TCSs, have been computed with two different theoretical approaches: the independent atom model with screening-corrected additivity rule (IAM-SCAR), at energies in the 1- to 1000-eV range, and the Schwinger multichannel method at energies between 0.1 and 10 eV. Note that the latter method specifically reports cross sections for the elastic channel. We find good qualitative agreement between the theories and the TCS experiment at all common energies. That level of accord is found to also become quantitative above the ionization energy of nitrous oxide. Electron-impact TCSs calculated with the IAM-SCAR approach are also presented and compared to the existing results in order to uncover any similarities or differences in the scattering processes between these two leptons and nitrous oxide.

We describe theoretical models for the recent experimental observation of macroscopic quantum self-trapping (MQST) in the transverse dynamics of an ultracold bosonic gas in a two-dimensional lattice. The pure mean-field model based on the solution of coupled nonlinear equations fails to reproduce the experimental observations. It greatly overestimates the initial expansion rates at short times and predicts a slower expansion rate of the cloud at longer times. It also predicts the formation of a hole surrounded by a steep square fortlike barrier which was not observed in the experiment. An improved theoretical description based on a simplified truncated Wigner approximation (TWA), which adds phase and number fluctuations in the initial conditions, pushes the theoretical results closer to the experimental observations but fails to quantitatively reproduce them. An explanation of the delayed expansion as a consequence of a type of self-trapping mechanism, where quantum correlations suppress tunneling even when there are no density gradients, is discussed and supported by numerical time-dependent density matrix renormalization group (t-DMRG) calculations performed in a simplified two coupled tubes setup.

The ground-state vibrational wave packet produced by strong-field ionization of Br${}_2$ is characterized by femtosecond high-harmonic transient absorption spectroscopy. Vibrational motion is observed in time by a change in the 3$d$ core-level transition energy with bond length 0.14 eV/pm to higher energies at shorter bond lengths. The wave packet has the expected period of 104 fs for the predominant $v_0{v}_1$ vibrational quantum beat, which is prepared near the outer turning point with a phase of 0.21π±0.05π due to the preferential ionization at short bond lengths. Simultaneous observation of a wave packet on the ${\mathrm{Br}}_2^{+}{}^2{\Pi }_{g,3/2}$ ground state, prepared at the equilibrium bond distance of the ion, confirms preferential ionization near the inner turning point of the neutral. The Br${}_2$ ground-state wave packet has a degree of coherence ranging between 0.19 and 0.24, where unity is perfect coherence. The results utilize the sensitivity of core-level transient absorption spectroscopy to bond length and charge state, providing a means to analyze the formation and evolution of vibrational wave packets.

Shot-by-shot photoelectron spectroscopy has been performed to study the three-photon double ionization of Ar in intense extreme-ultraviolet free-electron laser fields over a photon energy range of 19.6–22.0 eV. It is found that the double ionization to Ar${}^{2+}$ 3$p^{-2}$ proceeds sequentially via the formation of singly charged Ar${}^{+}$ 3$p^{-1}$ states and is enhanced around photon energies of 20.5 and 21.5 eV. Two types of resonance are identified in the two-photon ionization of the Ar${}^{+}$ 3$p^{-1}$ states: (i) resonances to intermediate states at the one-photon energy level and (ii) those to autoionizing states above the Ar${}^{+}$ ionization threshold.

We theoretically investigate the quantum-noise properties of a pulse-pumped high-gain-fiber optical parametric amplifiers (FOPAs) by using the Bogoliubov transformation in multiple frequency modes to describe the evolution of the nondegenerate signal and idler twin beams. The results show that the noise figure of the FOPA is generally greater than the 3 dB quantum limit unless the joint spectral function is factorable and the spectrum of the input signal well matches the gain spectrum in the signal band. However, the intensity-difference noise of the twin beams, which weakly depends on the joint spectral function, can be significantly less than the shot-noise limit when the temporal modes of the pump and the input signal are properly matched. Moreover, to closely resemble real experimental conditions, the quantum noise of twin beams generated from a broadband FOPA is numerically studied by taking the various kinds of experimental imperfections into account. Our study is not only useful for developing a compact fiber source of twin beams, but also helpful for understanding the quantum-noise limit of a pulse-pumped FOPA in the fiber communication system.

We have provided a complete description of light propagation at an oblique angle of incidence in disordered one-dimensional (1D) ultrathin magnetophotonic crystals with an arbitrary number of sheets. We have shown that in the long-wavelength limit, when the parameter $\frac{d{\epsilon }_{L,R}}{\lambda }$ is much less than unity (${\epsilon }_{L,R}$ is the relative permittivity for left- and right-polarized light, $d$ is the thickness of the sheet, and $\lambda$ is the wavelength), the photon transport problem in a 1D magnetophotonic crystal is identical to Anderson's two-channel model. In our discussion we include mode conversion and derive exact and closed analytical expressions for all scattering matrix elements. We have calculated the Faraday and Kerr rotational angles for a periodic system. Our formulas predict correctly the main trends of magneto-optic effects in 1D systems. We also derived analytical expressions for photon localization lengths, in a weak disordered regime, for $s$ and $p$ modes and for circular polarized light. We demonstrate that the presence of coupling modes enhances ${\xi }_s$ and reduces ${\xi }_p$ with respect to the values ${\xi }_s\left(0\right)$ and ${\xi }_p\left(0\right)$ obtained when the coupling modes are absent. Presented analytical expressions for localization lengths are in good agreement with numerical calculations, exact up to order ${\delta }^2$ ($\delta$ being the disorder strength), and valid up to angles of incidence of $1.56$ rad.

When describing field propagation in a ring cavity, longitudinal fields are usually neglected. Although small, the longitudinal component is always present in finite width beams. An equation describing field propagation in Kerr media and inside a ring cavity filled with such medium is derived. Coupling among longitudinal and transverse fields but also other usually neglected terms are considered. Numerical integration results show that coupling with longitudinal field can orientate the resulting pattern along the polarization direction.

We find all spin operators for a Dirac particle satisfying the following very general conditions: (i) spin does not convert positive (negative) energy states into negative (positive) energy states, (ii) spin is a pseudovector, and (iii) eigenvalues of the projection of a spin operator in an arbitrary direction are independent of this direction (isotropy condition). We show that there are four such operators and all of them fulfill the standard su(2) Lie algebra commutation relations. Nevertheless, only one of them has a proper nonrelativistic limit and acts in the same way on negative and positive energy states. We show also that this operator is equivalent to the Newton-Wigner spin operator and Foldy-Wouthuysen mean-spin operator. We also discuss another operator proposed in the literature.

We study the quantum error-correcting codes over mixed alphabets to deal with a more complicated and practical situation in which the physical systems for encoding may have different numbers of energy levels. In particular we investigate their constructions and propose the theory of quantum Singleton bound. Two kinds of code constructions are presented: a projection-based construction for general case and a graphical construction based on a graph-theoretical object composite coding clique dealing with the case of reducible alphabets. We find out some optimal one-error correcting or detecting codes over two alphabets. Our method of composite coding clique also sheds light on constructing standard quantum error-correcting codes, and other families of optimal codes are found.