Spatial and spectral properties of the pulsed second-harmonic generation in a periodically poled potassium-titanyl-phosphate waveguide exploiting first-, second-, and third-order quasi-phase-matching are analyzed. Experimental results are interpreted using a model based on the finite-elements method. Correlations between spatial and spectral properties of the fundamental and second-harmonic fields are revealed. Individual nonlinear processes can be exploited combining spatial and spectral filtering. The influence of waveguide parameters on the second-harmonic spectra is also addressed.

The implications of the Lorentz reciprocity theorem for a scatterer connected to waveguides with arbitrary modes, including degenerate, evanescent, and complex modes, are discussed. In general it turns out that a matrix $\mathit{CS}$ is symmetric, where $C$ is the matrix of generalized orthogonality coefficients and $S$ is the scattering matrix. Examples are given, including a scatterer surrounded by waveguides or free space, and discontinuities of waveguides.

We present a detailed theoretical analysis of a weakly driven, multimode optomechanical system, in which two optical modes are strongly and near-resonantly coupled to a single mechanical mode via a three-wave mixing interaction. We calculate one- and two-time intensity correlations of the two optical fields and compare them to analogous correlations in atom-cavity systems. Nonclassical photon correlations arise when the optomechanical coupling $g$ exceeds the cavity decay rate $\kappa$, and we discuss signatures of one- and two-photon resonances as well as quantum interference. We also find a long-lived correlation that decays slowly with the mechanical decay rate $\gamma$, reflecting the heralded preparation of a single-phonon state after detection of a photon. Our results provide insight into the quantum regime of multimode optomechanics, with potential applications for quantum information processing with photons and phonons.

We consider quantum systems in entangled states postselected in nonentangled states. Such systems exhibit unusual behavior, in particular, when weak measurements are performed at intermediate times.

Calculations of the photoionization of endofullerenes often use a square-well potential to represent the confining fullerene. The sensitivity of the photoionization cross sections to the form of this model potential is explored here by the use of a diffuse potential. The cross sections for single photoionization with excitation and double photoionization of He@C${}_{60}$ are found to show damping of the confinement resonance structure as the degree of diffuseness is increased using a commonly adopted well depth of 8.22 eV. For a deeper well depth of 11.00 eV, the double-photoionization cross section is found to be insensitive to the diffuseness of the confining potential, as has also been reported recently for the single photoionization of H@C${}_{60}$ and Xe@C${}_{60}$ [Dolmatov, King, and Oglesby, J. Phys. B 45, 105102 (2012)].

Locally maximally entangleable states (LMESs) constitute a large set of multipartite states, containing, for instance, all stabilizer states. LMESs are uniquely characterized by $(2^n-1)$ phases, where $n$ denotes the number of qubits. We consider here those LMESs whose phases are either 0 or $\pi$ and present a multipartite entanglement purification protocol for arbitrary such states. In contrast to all previously known recurrence protocols, this protocol uses a different ingredient, which is required due to the quantum correlations contained in the various LMESs. We compare this scheme to previously known entanglement purification protocols and show that the direct purification performs better than previously known protocols.

We discuss a $\beta$-dependent family of electronic density scalings of the form $n_{\lambda }\left(\mathbf{r}\right)={\lambda }^{3\beta +1}n\left({\lambda }^{\beta }\mathbf{r}\right)$ in the context of density functional theory. In particular, we consider the following special cases: the Thomas-Fermi scaling ($\beta =1/3$ and $\lambda \gg 1$), which is crucial for the semiclassical theory of neutral atoms; the uniform-electron-gas scaling ($\beta =-1/3$ and $\lambda \gg 1$), that is important in the semiclassical theory of metallic clusters; the homogeneous density scaling ($\beta =0$) which can be related to the self-interaction problem in density functional theory when $\lambda \le 1$; the fractional scaling ($\beta =1$ and $\lambda \le 1$), that is important for atom and molecule fragmentation; and the strong-correlation scaling ($\beta =-1$ and $\lambda \gg 1$) that is important to describe the strong correlation limit. The results of our work provide evidence for the importance of this family of scalings in semiclassical and quantum theory of electronic systems, and indicate that these scaling properties must be considered as important constraints in the construction of new approximate density functionals. We also show, using the uniform-electron-gas scaling, that the curvature energy of metallic clusters is related to the second-order gradient expansion of kinetic and exchange-correlation energies.

Recently, single-cycle THz pulses have been demonstrated in the laboratory to successfully induce field-free orientation in gas-phase polar molecules at room temperature [Phys. Rev. Lett. 107, 163603 (2011)]. In this paper, we examine the maximum attainable field-free molecular orientation with optimally shaped linearly polarized near–single-cycle THz laser pulses of a thermal ensemble. Large-scale benchmark optimal control simulations are performed, including rotational energy levels with the rotational quantum numbers up to $J=100$ for OCS linear molecules. The simulations are made possible by an extension of the recently formulated fast search algorithm, the two-point boundary-value quantum control paradigm, to the mixed-states optimal control problems in the present work. It is shown that a very high degree of field-free orientation can be achieved by strong, optimally shaped near–single-cycle THz pulses. The extensive numerical simulations showed that the maximum attainable $J$-dependent field-free orientation (equal to $0.714$ for $J=60$ and $0.837$ for $J=100$ at 100 K) in the near–single-cycle THz pulse region is close to $92\%$ of the corresponding optimal bound that can be attained by arbitrarily long pulses. It is also found that a smaller amplitude for the optimal control field corresponds to a smaller $J$ (e.g., $\approx 0.005$ a.u. for $J=60$ and $\approx 0.01$ a.u. for $J=100$) in the model simulations. The latter finding may underline the actual experimental performance of the field-free molecular orientation, since presently the available amplitude of single-cycle THz pulses can only reach slightly beyond $20\text{MV}/\text{cm}$ ($\approx 0.0038$ a.u.).

We report experiments on the control of the phase $\eta$ in quantum mechanical superposition states which emerge in electromagnetically induced transparency, $|\psi \rangle =\left(\right|1\rangle +e^{i\eta }|2\rangle )/\sqrt{2}$. We interpret our findings in terms of the measurement role that spontaneous emission and the light fields play in selecting the optically dark and bright superpositions. A phase-switching tool is introduced which enables the rapid measurement of the absolute depth of the dark-state minimum and its absolute position on the frequency scale without detuning the lasers. The phase-switching technique allows one to determine the relative phase $\eta$ in an ensemble of quantum mechanical superposition states of a $\Lambda$ system in a minimally invasive way.

Using a semiclassical model, we evaluate the correlated electron momentum distribution from nonsequential double ionization (NSDI) of neon in an elliptically polarized laser field. The momentum distribution pattern exhibits a nontrivial change with the increase of the laser ellipticity, which can be understood as a combined effect of the extra laser electric field and the ionic Coulomb potential on the tunnel-ionized electron trajectory. Especially for large ellipticities ($\epsilon >0.2$), multiple return collision trajectory contributes dominantly to the NSDI yield and determines the correlated electron momentum distribution, revealing an indispensable role of ionic Coulomb potential in NSDI dynamics under elliptical polarization. Moreover, the nontrivial evolution of the correlated electron momentum distribution with the ellipticity revealed in our model should be testable by a realistic experiment.

We theoretically show that core-excited-state populations can be efficiently manipulated with strong optical fields during their decay, which takes place in a few femtoseconds. We focus on the $1s^{-1}3p$ resonant excitation in neon, where the $1s^{-1}3p$ and $1s^{-1}3s$ core-excited states are coupled by an optical field. By analyzing the Auger electron spectrum we observe the inner-shell population transfer induced by the optical coupling. We also show that the angular anisotropy of the Auger electron is imprinted in the multipeak structure induced by the optical-dressed continuum, namely sidebands.

We study the dynamics of vortices in finite temperature atomic Bose-Einstein condensates, focusing on decay rates, precession frequencies, and core brightness, motivated by a recent experiment [Science 329, 1182 (2010)] in which real-time dynamics of a single vortex was observed. Using the Zaremba, Nikuni, and Griffin (ZNG) formalism based on a dissipative Gross-Pitaevskii equation for the condensate coupled to a semiclassical Boltzmann equation for the thermal cloud, we find a rapid nonlinear increase of both the decay rate and precession frequency with increasing temperatures. The increase, which is dominated by the dynamical condensate-thermal coupling is also dependent on the intrinsic thermal cloud collisional dynamics; the precession frequency also varies with the initial radial coordinate. The integrated thermal cloud density in the vortex core is for the most part independent of the position of the vortex (except when it is near the condensate edge), with its value increasing with temperature. This could potentially be used as a variant to the method of Coddington et al. [Phys. Rev. A 70, 063607 (2004)] for experimentally determining the temperature.

Using perturbation expansion of Maxwell equations, an amplitude equation is derived for nonlinear transverse magnetic (TM) and transverse electric (TE) surface plasmon waves supported by graphene. The equation describes the interplay between in-plane beam diffraction and nonlinearity due to light intensity induced corrections to graphene conductivity and susceptibility of dielectrics. For strongly localized TM plasmons, graphene is found to bring the superior contribution to the overall nonlinearity. In contrast, nonlinear response of the substrate and cladding dielectrics can become dominant for weakly localized TE plasmons.

Effective spin-orbit coupling can be created in cold atom systems using atom-light interaction. We study Bose-Einstein condensates in an optical lattice using the Gross-Pitaevskii equation with spin-orbit coupling. Bloch states for the linear equation are numerically obtained, and compared with stationary solutions to the Gross-Pitaevskii equation with nonlinear terms. Various vortex lattice states are found when the spin-orbit coupling is strong.

We study non-Markovianity and information flow for qubits experiencing local dephasing with an Ohmic class spectrum. We demonstrate the existence of a temperature-dependent critical value of the Ohmicity parameter $s$ for the onset of non-Markovianity and give a physical interpretation of this phenomenon by linking it to the form of the reservoir spectrum. We demonstrate that this link holds also for more general spectra. We unveil a class of initial states for which discord is forever frozen at a positive value. We connect time-invariant discord to non-Markovianity and propose a physical system in which it could be observed.

We present a family of entropic uncertainty relations for pointer-based simultaneous measurements of conjugate observables. The lower bounds of these relations explicitly incorporate the influence of the measurement apparatus. We achieve this by using a mathematical theorem which states that the information entropy of convoluted probability distributions is bound from below. As a consequence of these results we can straightforwardly show that appropriately squeezed states are minimal entropy states for simultaneous measurements.

An unsolved problem in relativistic quantum information research is how to model efficient, directional quantum communication between localized parties in a fully quantum field-theoretical framework. We propose a tractable approach to this problem based on calculating expectation values of localized field observables in the Heisenberg picture. We illustrate our approach by analyzing, and obtaining approximate analytical solutions to, the problem of communicating coherent states between an inertial sender, Alice, and an accelerated receiver, Rob. We use these results to determine the efficiency with which continuous variable quantum key distribution could be carried out over such a communication channel.

Compared to purely atomic collisions, ultracold molecular collisions potentially support a much larger number of Fano-Feshbach resonances due to the enormous number of rovibrational states available. In fact, for alkali-metal dimers we find that the resulting density of resonances cannot be resolved at all, even on the sub-$\mu$K temperature scale of ultracold experiments. As a result, all observables become averaged over many resonances and can effectively be described by simpler, nonresonant scattering calculations. Two particular examples are discussed: nonchemically reactive RbCs and chemically reactive KRb. In the former case, the formation of a long-lived collision complex may lead to the ejection of molecules from a trap. In the latter case, chemical reactions broaden the resonances so much that they become unobservable.

Trapped singly charged ions can crystallize as a result of laser cooling. The emerging structure depends on the number of particles and on the geometry of the trapping potential. In linear multipole radio frequency traps, the geometry of the radial potential can lead to the formation of single-ring structures. We analyze the conditions and stability of single rings as a function of the number of poles. The rings form tubes for a large number of ions and sufficiently small trap aspect ratios. In these structures the arrangement of the ions corresponds to a triangular lattice folded onto a cylinder. The stability of the tubular structures is numerically studied for different lattice constants and their normal-mode spectrum is determined.

The role of the magnetic field of the laser pulse and the importance of relativistic corrections to the equations of motion in the recollision double ionization of He, Li${}^{+}$, and Be${}^{2+}$ are investigated within a classical trajectory model. It is found that the inclusion of the magnetic field is important even at intensities as low as ${10}^{15}$ W/cm${}^2$, but that the relativistic corrections proportional to $1/c^2$ have no impact on either total probabilities or final momentum spectra. Two field configurations with counterpropagating pulses, previously proposed to circumvent the detrimental effect of the magnetic field on the double-ionization probability, are compared with the single-pulse case.