We present spectroscopic data on moderately bound levels in the Cs${}_2$ $a{}^3{\Sigma }_u^{+}$ state. The data have sufficient resolution to be sensitive to rotational and second-order spin-orbit splittings as well as hyperfine and vibrational structure. Quantum numbers are assigned to the levels via selection rules and a global fit to other available data for the $a{}^3{\Sigma }_u^{+}$ and coupled $X{}^1{\Sigma }_g^{+}$ states. The analysis focuses in particular on nearly degenerate pairs of $a$ and $X$ state levels, energy differences between which can be highly sensitive to possible variations in the electron-to-proton mass ratio [cf. DeMille et al., Phys. Rev. Lett. 100, 043202 (2008)]. We also characterize the electric ($E$1) and magnetic ($M$1) dipole transition strengths between nearly degenerate $a$ and $X$ rovibrational levels and find that both types of transitions are feasible with current technology and could give complementary information.

We study the stability of superflow of Bose gases in optical lattices by analyzing the Bose-Hubbard model within the Gutzwiller mean-field approximation. We calculate the excitation spectra of the homogeneous Bose-Hubbard model at unit filling to determine the critical momenta for the Landau and dynamical instabilities. These two critical momenta are shown to approach each other when the on-site interaction increases towards the Mott transition point. In order to make a direct connection with realistic experiments, we next take into account a parabolic trapping potential and compute the real-time dynamics of dipole oscillations induced by suddenly displacing the trap center. We consider the following two cases: standard soft-core bosons, whose interparticle interactions include the on-site one only; and hard-core bosons with long-range dipole-dipole interactions. For both cases, we show that the dipole oscillation is significantly damped when the maximum local momentum exceeds a certain threshold, which quantitatively agrees with the critical momentum for the dynamical instability in the homogeneous system. In the case of dipolar hard-core bosons, the dynamical instability of dipole oscillations leads to the formation of checkerboard density waves in the superfluid phase near the boundary to the supersolid phase.

We propose a simple scalar model for describing pulse phenomena beyond the conventional slowly varying envelope approximation. The generic governing equation has a cubic nonlinearity, and we focus here mainly on contexts involving anomalous group-velocity dispersion. Pulse propagation turns out to be a problem firmly rooted in frames-of-reference considerations. The transformation properties of the model and its space-time structure are explored in detail. Two distinct representations of exact analytical solitons and their associated conservation laws (in both integral and algebraic forms) are presented, and a range of predictions is made. We also report cnoidal waves of the governing nonlinear equation. Crucially, conventional pulse theory is shown to emerge as a limit of the more general formulation. Extensive simulations examine the role of the new solitons as robust attractors.

A simple scalar model for describing spatiotemporal dispersion of pulses, beyond the classic “slowly varying envelopes $+$ Galilean boost” approach, is studied. The governing equation has a cubic nonlinearity, and we focus here mainly on contexts with normal group-velocity dispersion. A complete analysis of continuous waves is reported, including their dispersion relations and modulational instability characteristics. We also present a detailed derivation of exact analytical dark solitons, obtained by combining direct-integration methods with geometrical transformations. Classic results from conventional pulse theory are recovered asymptotically from the spatiotemporal formulation. Numerical simulations test theoretical predictions for modulational instability and examine the robustness of spatiotemporal dark solitons against perturbations to their local pulse shapes.

We report integral cross sections for elastic electron scattering by the lignin subunits phenol, guaiacol, and $p$-coumaryl alcohol. Our calculations employed the Schwinger multichannel method with pseudopotentials and indicate three to four ${\pi }^{*}$ shape resonances for each of these systems, suggesting that low-energy electrons could efficiently transfer energy into the lignin matrix. We also discuss dissociation mechanisms based on the calculated cross sections, available experimental data, virtual orbital analysis, and the knowledge on electron interactions with biomolecules. Our results point out a physical-chemical basis for electron-driven biomass delignification. The latter would be an essential step for efficient biofuel production from lignocellulosic materials.

We study the influence of a chaotic environment in the evolution of an open quantum system. We show that there is an inverse relation between chaos and non-Markovianity. In particular, we remark on the deep relation of the short time non-Markovian behavior with the revivals of the average fidelity amplitude—a fundamental quantity used to measure sensitivity to perturbations and to identify quantum chaos. The long time behavior is established as a finite size effect which vanishes for large enough environments.

The Hubbard model describes interacting itinerant fermion systems and is the simplest model capable of describing the essential physics of strongly correlated electron systems. In this model, both spin correlations and charge correlations are important for the entanglement, thus the magnetic susceptibility is not an appropriate thermal entanglement witness. The specific heat reflects spin correlations and charge correlations. We obtain a relation that permits the specific heat to be used directly as a thermal entanglement witness. We calculate, away from half filling, the entanglement critical temperature $T_E$, below which entanglement is detected, for small linear clusters and the hypercubic lattice in the limit of infinite dimensions by using the exact numerical diagonalization method and the dynamical mean-field theory, respectively. We find, in both cases, that $T_E$ increases with increasing strength of the on-site Coulomb repulsion $U$.

Based on the correlated ground-state wave function of an exactly solvable interacting one-dimensional two-electron model Hamiltonian we address the switch-off of confining and interparticle interactions to calculate the exact time-evolving wave function from a prescribed correlated initial state. Using this evolving wave function, the time-dependent pair probability function $\mathcal{R}(x_1,x_2,t)\equiv n_2(x_1,x_2,t)/\left[n(x_1,t)n(x_2,t)\right]$ is determined via the pair density $n_2(x_1,x_2,t)$ and single-particle density $n(x,t)$. It is found that $\mathcal{R}(0,0,t=\infty )=\mathcal{R}(0,0,t=0)>1$, and $\mathcal{R}(x_1,x_2,t^{*})=1$ at a finite $t^{*}$ for $\Lambda \ne 0$ interparticle interaction strength in the initial two-electron model. By expanding $n(x,t)$ in an infinite sum of closed-shell products of time-dependent normalized single-particle states and time-dependent occupation numbers $P_k(\Lambda ,t)$, the von Neumann entropy $S(\Lambda ,t)=-{\sum }_{k=0}^{\infty }{P}_k\left(t\right)\ln P_k\left(t\right)$ is calculated as well. The such-defined information entropy is zero at $t^{*}\left(\Lambda \right)$ and its maximum in time is $S(\Lambda ,t=\infty )=S(\Lambda ,t=0)$.

Multichannel quantum defect theory (MQDT) can provide an efficient alternative to full coupled-channel calculations for low-energy molecular collisions. However, the efficiency relies on interpolation of the $\mathit{Y}$ matrix that encapsulates the short-range dynamics, and there are poles in $\mathit{Y}$ that may prevent interpolation over the range of energies of interest for cold molecular collisions. We show how the phases of the MQDT reference functions may be chosen so as to remove such poles from the vicinity of a reference energy and dramatically increase the range of interpolation. For the test case of Mg+NH, the resulting optimized $\mathit{Y}$ matrix may be interpolated smoothly over an energy range of several Kelvin and a magnetic field range of over 1000 gauss. Calculations at additional energies and fields can then be performed at a computational cost that is proportional to the number of channels $N$ and not to $N^3$.

The stability of cationic SF${}_5$${}^{+}$(SF${}_6{)}_{n-1}$ clusters was investigated by measuring their blackbody-induced radiative dissociation (BIRD) rates. The clusters were produced in a supersonic expansion ion source and stored in an electrostatic ion-beam trap at room temperature, where their abundances and lifetimes were measured. Using the “master equation” approach, relative binding energies of an SF${}_6$ unit in the clusters could be extracted from the storage-time dependence of the survival probabilities. The results allow for a deeper insight into the effect of a localized charge on the structure and stability of SF${}_6$-based clusters.

We present theoretical studies of above-threshold ionization (ATI) produced by spatially inhomogeneous fields. This kind of field appears as a result of the illumination of plasmonic nanostructures and metal nanoparticles with a short laser pulse. We use the time-dependent Schrödinger equation in reduced dimensions to understand and characterize the ATI features in these fields. It is demonstrated that the inhomogeneity of the enhanced plasmonic field plays an important role in the ATI process and it produces appreciable modifications to the energy-resolved photoelectron spectra. In fact, our numerical simulations reveal that high-energy electrons can be generated. Specifically, using a linear approximation for the spatial dependence of the enhanced plasmonic field and with a near-infrared laser with intensities in the mid ${10}^{14}$ W/cm${}^2$ range, we show it is possible to drive electrons with energies in the near-keV regime. Furthermore, we study how the carrier envelope phase influences the emission of ATI photoelectrons for few-cycle pulses. Our quantum mechanical calculations are supported by their classical counterparts.

We study the guiding of ${}^{87}$Rb 59$D$${}_{5/2}$ Rydberg atoms in a linear, high-gradient, two-wire magnetic guide. Time-delayed microwave ionization and ion detection are used to probe the Rydberg atom motion. We observe guiding of Rydberg atoms over a period of 5 ms following excitation. The decay time of the guided-atom signal is about five times that of the initial state. We attribute the lifetime increase to an initial phase of $l$-changing collisions and thermally induced Rydberg-Rydberg transitions. Detailed simulations of Rydberg-atom guiding reproduce most experimental observations and offer insight into the internal-state evolution.

We investigate the tunneling properties of collective excitations through a domain wall in the ferromagnetic phase of a spin-1 spinor Bose-Einstein condensate. Within the mean-field theory at $T=0$, we show that the transverse spin wave undergoes perfect reflection in the low-energy limit. This reflection property differs considerably from that of a domain wall in a Heisenberg ferromagnet where spin-wave excitations exhibit perfect transmission at arbitrary energy. When the Bogoliubov mode is scattered from this domain wall soliton, the transmission and reflection coefficients exhibit pronounced nonmonotonicity. In particular, we find perfect reflection of the Bogoliubov mode at energies where bound states appear. This is in stark contrast to the perfect transmission of the Bogoliubov mode with arbitrary energy through a dark soliton in a scalar Bose-Einstein condensate.

The electromagnetic field is canonically quantized in the presence of a linear, dispersive, and dissipative medium that is in uniform motion. Specifically we calculate the change in the normal modes of the coupled matter-field system and find a Hamiltonian that contains negative-energy normal modes. We interpret these modes as the origin of phenomena such as quantum friction and find that a detector initially in its ground state and coupled to the electromagnetic field in the vicinity of, or within, a uniformly moving medium has a nonzero probability of excitation at $T=0$ K.

We investigate the dynamics of a driven two-level system (classical Rabi model) using the counter-rotating-hybridized rotating-wave method (CHRW), which is a simple method based on a unitary transformation with a parameter $\xi$. This approach is beyond the traditional rotating-wave approximation (Rabi-RWA) and more importantly, remains the RWA form with a renormalized tunneling strength and a modified driving strength. The reformulated rotating-wave method not only possesses the same mathematical simplicity as the Rabi-RWA but also allows us to explore the effects of counter-rotating (CR) components. We focus on the properties of off-resonance cases for which the Rabi-RWA method breaks down. After comparing the results of different RWA schemes and those of the numerically exact method in a wide range of parameter regime, we show that the CHRW method gives the accurate driven dynamics which is in good agreement with the numerical method. Moreover, the other RWA methods appear as various limiting cases of the CHRW method. The CHRW method reveals the effects of the CR terms clearly by means of coherent destruction of tunneling and Bloch-Siegert shift. Our main results are as follows: (i) the dynamics of the coherent destruction of tunneling is explicitly given and its dependence on $\Delta$ is clarified, which is quantitatively in good agreement with the exact results; (ii) the CR modulated Rabi frequency and the Bloch-Siegert shift are analytically calculated, which is the same as the exact results up to fourth order; (iii) the validity of parameter regions of different RWA methods are given and the comparison of dynamics of these methods are shown. Since the CHRW approach is mathematically simple as well as tractable and physically clear, it may be extended to some complicated problems where it is difficult to do a numerical study.

We study modes trapped in a rotating ring carrying the self-focusing (SF) or self-defocusing (SDF) cubic nonlinearity and double-well potential ${\cos }^2\theta$, where $\theta$ is the angular coordinate. The model, based on the nonlinear Schrödinger (NLS) equation in the rotating reference frame, describes the light propagation in a twisted pipe waveguide, as well as in other optical settings, and also a Bose-Einstein condensate (BEC) trapped in a torus and dragged by the rotating potential. In the SF and SDF regimes, five and four trapped modes of different symmetries are found, respectively. The shapes and stability of the modes and the transitions between them are studied in the first rotational Brillouin zone. In the SF regime, two symmetry-breaking transitions are found, of subcritical and supercritical types. In the SDF regime, an antisymmetry-breaking transition occurs. Ground states are identified in both the SF and SDF systems.

The recent experiments show that the polarization of the terahertz (THz) wave generated from two-color laser-induced gas plasma is a linear polarization, and the polarization angle of the THz wave can be controlled by the optical delay of the two laser beams. In this paper, the polarization property of the THz wave is studied theoretically via a classical and a quantum model, respectively. An analytical expression is obtained to reflect the relationship between the polarization angle of the THz wave and the relative phase of the two-color lasers.

By using different continuation methods, we unveil a wide region in the parameter space of the discrete cubic-quintic complex Ginzburg-Landau equation, where several families of stable vortex solitons coexist. All these stationary solutions have a symmetric amplitude profile and two different topological charges. We also observe the dynamical formation of a variety of “bound-state” solutions composed of two or more of these vortex solitons. All of these stable composite structures persist in the conservative cubic limit for high values of their power content.

We propose a two-dimensional ${\chi }^{\left(2\right)}$ nonlinear photonic crystal capable of satisfying quasi-phase-matching conditions for two or three parametric down-conversions simultaneously. Seeded by a two-mode coherent state, this crystal can produce two-, three-, four-, or five-photon path-entangled states, provided the corresponding number photons are produced, i.e., in a postselection-free way. In particular, up to five-photon NOON state can be generated, which enables phase supersensitive measurements at the Heisenberg limit. The concurrent multiple quasi-phase-matchings provided by the nonlinear photonic crystal open up a way to integrated quantum light sources.

It is shown that a classical optical Fourier processor can be used for the shaping of quantum correlations between two or more photons, and the class of filter functions applicable in the multiphoton Fourier space is identified. This concept is experimentally demonstrated by using two types of periodic phase Fourier filters to manipulate the quantum correlations between path-entangled photon pairs. As many more types of filter functions are applicable, this opens up an alternative avenue for the manipulation of correlations between photons, a key ingredient in optical quantum information processing.