The nonclassicality of single-mode quantum states is studied in relation to the entanglement created by a beam splitter. It is shown that properly defined quantifications—based on the quantum superposition principle—of the amounts of nonclassicality and entanglement are strictly related to each other. This can be generalized to the amount of genuine multipartite entanglement, created from a nonclassical state by an $N$ splitter. As a consequence, a single-mode state of a given amount of nonclassicality is fully equivalent, as a resource, to exactly the same amount of entanglement. This relation is also considered in the context of multipartite entanglement and multimode nonclassicality.

Inherent channel losses during photon transmission limit the efficiency of optical quantum communication. In this paper, we construct a hyperdistillation scheme which can be used to distill the spatial-mode and polarization states of a photon simultaneously from photon loss in a heralded strategy. Exploiting linear-optical devices, the fidelity of a single-photon two qubit in a lossy signal mode can be increased to one with a certain success probability. By using a four-photon entanglement resource, the success probability of the hyperdistillation operation could be improved. The proposed scheme provides a realistic solution to overcome the transmission losses and exhibits unique properties, which shows important applications to quantum information processing.

We present a theoretical study of the electronic and nuclear flux densities of a vibrating ${\text{H}}_2$ molecule after an electronic excitation by a short femtosecond laser pulse. The final state, a coherent superposition of the electronic ground state $X{}^1{\Sigma }_g^{+}$ and the electronic excited state $B{}^1{\Sigma }_u^{+}$, evolves freely and permits the partition of the electronic flux density into two competing fluxes: the adiabatic and the transition flux density. The nature of the two fluxes allows us to identify two alternating dynamics of the electronic motion, occurring on the attosecond and the femtosecond time scales. In contradistinction to the adiabatic electronic flux density, the transition electronic flux density shows a dependence on the carrier-envelope phase of the laser field, encoding information of the interaction of the electrons with the electric field. Furthermore, the nuclear flux density displays multiple reversals, a quantum effect recently discovered by Manz et al. [J. Manz, J. F. Pérez-Torres, and Y. Yang, Phys. Rev. Lett. 111, 153004 (2013)], calling for investigation of the electronic flux density.

We report cross-section results from experimental and theoretical studies on electron collisions with 1-butyne ($\text{HC}\equiv \text{C}–{\text{CH}}_2{\text{CH}}_3$) and acetylene ($\text{HC}\equiv \text{CH}$) molecules and from computations for a propyne ($\text{HC}\equiv \text{C}–{\text{CH}}_3$) molecule. Absolute $grand$-total electron-scattering cross sections (TCSs) were measured at impact energies ranging from about 0.5 to 300 eV using the linear electron-transmission method. The TCS energy curve for 1-butyne has a very broad enhancement on which some distinct features are superimposed, namely a resonant-like maximum (located near 3.2 eV), a broad hump (centered around 7.5 eV), and a shoulder (spanned between 12 and 26 eV). The shape of our experimental TCS curve for acetylene closely resembles that reported earlier, while its magnitude is usually larger. As no previous calculations for electron collisions with 1-butyne and propyne are disclosed in the literature, we computed the elastic (ECS) and ionization (ICS) cross sections for these molecules. Similar calculations were also performed for acetylene molecules. The additivity rule was employed to calculate the ECSs from 50 to 3000 eV, while the binary-encounter-Bethe approach was used for computation of the ICSs, from the threshold up to 3000 eV. The sum (ECS + ICS) of these two computed cross sections reasonably reproduces the TCS measurements above 50 eV. Furthermore, the experimental TCS curves, obtained in our laboratory, for a series of acetylenic compounds—acetylene ($\text{HC}\equiv \text{CH}$), propyne ($\text{HC}\equiv \text{C}–{CH}_3$), and 1-butyne ($\text{HC}\equiv \text{C}–{\text{CH}}_2{\text{CH}}_3$)—are compared to study the substitutional effect. Finally, the influence of the structural differences on the electron-scattering TCS for isomers of the ${\text{C}}_4{\text{H}}_6$ molecule (1-butyne, 2-butyne, and 1,3–butadiene) is indicated and discussed.

We present a simple model for calculating strong-field atomic and molecular ionization dominated by Freeman resonances. Our model combines multiphoton coupling between bound states, including dynamic Stark shifts, with coupling to a discretized continuum. The simplicity of the model allows us to interpret pulse-shape-dependent strong-field ionization yields and to demonstrate the relevance of strong-field atomic or molecular phase matching to ionization, as well as bound-state population transfer. Comparison with experimental measurements demonstrates that the calculations capture the essence of the pulse-shape-dependent ionization yields.

We use ultrafast laser pulse shaping to discriminate between different pathways to multiple continua in strong-field molecular ionization. Shaping the laser pulse which ionizes the molecule allows us to control the photoelectron spectrum, which we interpret using a recently developed model of resonantly enhanced multiphoton ionization. Our measurements and calculations allow us to distinguish between a single intermediate resonance leading to multiple continua and multiple intermediate resonances each leading to a separate continuum.

The triple differential cross sections of the photo-double-ionization of the nitrogen molecule to the $X$ ${}^1{\Sigma }^{\mathrm{+}}{}_g$ and $a^3$Π${}_u$ ${{\mathrm{N}}_2}^{2+}$ states have been measured at about 20 eV above their respective ionization thresholds in the equal energy sharing kinematics and calculated using a model which makes use of correlated two-center double continuum wave functions. The comparison of the results with those obtained by the Gaussian parametrization method applied in the past with success to heliumlike targets shows the influence of the molecular nature of the ${\mathrm{N}}_2$ target in the photo-double-ionization.

Motivated by recent experiments on circular dichroism in the photoelectron momentum distributions from strong-field ionization of chiral molecules [C. Lux et al., Angew. Chem. Int. Ed. 51, 5001 (2012); C. S. Lehmann et al., J. Chem. Phys. 139, 234307 (2013)], we investigate the origin of this effect theoretically. We show that it is not possible to describe photoelectron circular dichroism with the commonly used strong-field approximation due to its plane-wave nature. We therefore apply the Born approximation to the scattering state and use this as a continuum-state correction in the strong-field approximation. We obtain electron distributions for the molecules camphor and fenchone. In order to gain physical insight into the process, we study the contributions of individual molecular orientations.

We introduce a time-dependent R-matrix theory generalized to describe double-ionization processes. The method is used to investigate two-photon double ionization of He by intense XUV laser radiation. We combine a detailed B-spline-based wave-function description in an extended inner region with a single-electron outer region containing channels representing both single ionization and double ionization. A comparison of wave-function densities for different box sizes demonstrates that the flow between the two regions is described with excellent accuracy. The obtained two-photon double-ionization cross sections are in excellent agreement with other cross sections available. Compared to calculations fully contained within a finite inner region, the present calculations can be propagated over the time it takes the slowest electron to reach the boundary.

Sped-up protocols that drive a system quickly to the same populations that can be reached by a slow adiabatic process may involve Hamiltonian terms which are difficult to realize. We use the dynamical symmetry of the Hamiltonian to find, by means of Lie transforms, alternative Hamiltonians that achieve the same goals without the problematic terms. We apply this technique to three-level systems (two interacting bosons in a double well, and beam splitters with two and three output channels) driven by Hamiltonians that belong to the four-dimensional algebra U3S3.

The BCS-BEC (Bose-Einstein condensation) crossover in a lattice is a powerful paradigm that describes how a superconductor deviates from the Bardeen-Cooper-Schrieffer physics as the attractive interaction increases. Optical lattices loaded with binary mixtures of cold atoms allow one to access this phenomenon experimentally in a clean and controlled way. We show that, however, the possibility to study this phenomenon in actual cold-atoms experiments is limited by the effect of the trapping potential. Real-space dynamical mean-field theory calculations show indeed that interactions and the confining potential conspire to pack the fermions in the center of the trap, which approaches a band insulator when the attraction becomes sizeable. Interestingly, the energy gap is spatially more homogeneous than the superfluid condensate order parameter. We show how this physics reflects in several observables, and we propose an alternative strategy to disentangle the effect of the harmonic potential and measure the intrinsic properties resulting from the interaction strength.

The process of a cascade light emission by an excited multilevel quantum system has been described exactly. A multiphoton wave function of the system at an arbitrary time has been derived, which allows one to find any characteristic of the emitted field. Using this wave function, we have obtained a general expression for the cascade emission spectrum. This expression is of special interest when some transitions have close frequencies; this case has not been treated before. We present several examples of application of the obtained general expression.

We study the dynamics of a two-level system described by a slowly varying Hamiltonian and weakly coupled to the Ohmic environment. We follow the Bloch-Redfield perturbative approach to include the effect of the environment on qubit evolution and take into account modification of the spectrum and matrix elements of qubit transitions due to the time dependence of the Hamiltonian. We apply this formalism to two problems. (1) We consider a qubit, or a spin-$\frac{1}{2}$, in a rotating magnetic field. We show that once the rotation starts, the spin has a component perpendicular to the rotation plane of the field that initially wiggles and eventually settles to the value proportional to the product of angular rotation velocity of the field and the Berry curvature. (2) We reexamine the Landau-Zener transition for a system coupled to the environment at arbitrary temperature. We show that as temperature increases, the thermal excitation and relaxation become leading processes responsible for transition between states of the system. We also apply the Lindblad master equations to these two problems and compare results with those obtained from the Bloch-Redfield equations.

The OH molecule in its ground state presents a versatile platform for precision measurement and quantum information processing. These applications vitally depend on the accurate measurement of transition energies between the OH levels. Significant sources of systematic errors in these measurements are shifts based on the geometric phase arising from the magnetic and electric fields used for manipulating OH. In this article, we present these geometric phases for fields that vary harmonically in time, as in the Ramsey technique. Our calculation of the phases is exact within the description provided by our recent analytic solution of an effective Stark-Zeeman Hamiltonian for the OH ground state. This Hamiltonian has been shown to model experimental data accurately. We find that the OH geometric phases exhibit rich structure as a function of the field rotation rate. Remarkably, we find rotation rates where the geometric phase accumulated by a specific state is zero, or where the relative geometric phase between two states vanishes. We expect these findings to be of importance to precision experiments on OH involving time-varying fields. More specifically, our analysis quantitatively characterizes an important item in the error budget for precision spectroscopy of ground-state OH.

Atom-molecule collisions in helium–helium–alkali-metal triatomic systems at cold energies are studied using the adiabatic hyperspherical representation. We consider the elastic collision processes ${}^4\mathrm{He}X+{}^4\mathrm{He}\to {}^4\mathrm{He}X+{}^4\mathrm{He}$ and ${}^4\mathrm{He}_2+X\to {}^4\mathrm{He}_2+X$, as well as the reactive collision processes ${}^4\mathrm{He}X+{}^4\mathrm{He}\rightleftharpoons {}^4\mathrm{He}_2+X$, where $X$ is one of the bosonic alkali-metal atoms: ${}^7\mathrm{Li}$, ${}^{23}\mathrm{Na}$, ${}^{39}\mathrm{K}$, ${}^{85}\mathrm{Rb}$, or ${}^{133}\mathrm{Cs}$. The elastic and reactive collision cross sections are calculated at nonzero collision energies by including not only zero atom-molecule relative angular momentum, $L=0$, processes but also $L>0$ processes. The total cross section for ${}^4\mathrm{He}X+{}^4\mathrm{He}\to {}^4\mathrm{He}X+{}^4\mathrm{He}$ is found to increase with the scattering length between ${}^4\mathrm{He}$ and $X$, while the other collision processes are not found to have any systematic dependence on the alkali-metal species $X$.

We use general concepts of statistical mechanics to compute the quantum frictional force on an atom moving at constant velocity above a planar surface. We derive the zero-temperature frictional force using a nonequilibrium fluctuation-dissipation relation, and we show that in the large-time, steady-state regime, quantum friction scales as the cubic power of the atom's velocity. We also discuss how approaches based on Wigner-Weisskopf and quantum regression approximations fail to predict the correct steady-state zero-temperature frictional force, mainly due to the low-frequency nature of quantum friction.

A systematic study and comparison of the photoionization cross sections of the outer and inner shells of the noble-gas atoms He, Ne, Ar, Kr, and Xe confined endohedrally inside a ${\text{C}}_{60}$ molecule are calculated by employing a time-dependent local-density-approximation formulation. Confinement resonances are found to be a general feature of these cross sections and dramatic interchannel coupling effects, significantly increasing the atomic cross sections, are exhibited in all cases in the vicinity of the ${\text{C}}_{60}$ plasmons. Hybridization effects, the mixing of the atomic and cage bound-state wave functions, are also found, but no systematics of the hybridization present themselves.

We present a study for the dispersion relation and character of the excitations of a single- and two-component Bose-Einstein condensate (BEC). We study the single-component dispersion for a finite BEC system and look at examples of quasiparticles to understand and characterize the dispersion relation. Next we present the dispersion relation for a two-component BEC in both the miscible and immiscible parameter regimes. Then we present examples of the quasiparticles for both regimes.

The stability of persistent currents in a two-component Bose-Einstein condensate in a toroidal trap is studied in both the miscible and the immiscible regimes. In the miscible regime we show that superflow decay is related to linear instabilities of the spin-density Bogoliubov mode. We find a region of partial stability, where the flow is stable in the majority component while it decays in the minority component. We also characterize the dynamical instability appearing for a large relative velocity between the two components. In the immiscible regime the stability criterion is modified and depends on the specific density distribution of the two components. The effect of a coherent coupling between the two components is also discussed.