We show, by using numerical simulations, that quasisolitons can propagate over a long distance in a fiber Bragg grating that is written in a slow saturable fiber amplifier, such as an erbium-doped fiber amplifier. During the pulse propagation, the front end of the pulse experiences a net gain while the rear end of pulse is attenuated due to the combination of gain saturation and loss. However, the pulse profile almost does not change after propagating over a length of 5 m that is approximately 2500 times larger than the spatial pulse width. The pulse amplitude has an approximately hyperbolic secant profile. We develop a reduced model by using a multiscale analysis to study solitary-wave propagation when nonlinearity and gain are small. When gain saturation also becomes small we find analytically a new family of solitary-wave hyperbolic-secant solutions that approximately solve the reduced model. The solitary waves propagate slightly faster than Bragg solitons that propagate in fiber Bragg gratings without gain and loss.

Conical Bessel-like pulses allow control of the propagation velocity of the main intensity peak. With few-cycle pulses, this leads to a controllable shearing effect with respect to the carrier-phase oscillation and a consequent variation of the instantaneous intensity during propagation. Numerical simulations highlight how this intensity modulation directly controls the atomic dipole phase in the process of high-order harmonic generation and isolates either the long or the short electron-trajectory contributions. We identify a propagation regime in which the harmonic field takes the form of an isolated pulse of 300 as duration.

Measurements to test the neutrality of matter by acoustic means are reported. The apparatus is based on a spherical capacitor filled with gaseous SF${}_6$ excited by an oscillating electric field. The apparatus has been calibrated measuring the electric polarizability. Assuming charge conservation in the $\beta$ decay of the neutron, the experiment gives a limit of ${\epsilon }_{\mathrm{p}-\mathrm{e}}\lesssim 1\times {10}^{-21}$ for the electron-proton charge difference, the same limit holding for the charge of the neutron. Previous measurements are critically reviewed and found to be inaccurate.

We study the Cooper minimum in high-order-harmonic generation from argon atoms by using long wavelength laser pulses. We find that the minimum in high-order-harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semiclassical theoretical approach based on classical trajectory Monte Carlo and quantum electron scattering methods to model the experiment. Our study reveals that the shift between photoionization and high-order-harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wave packet.

Determining the phase in one arm of a quantum interferometer is discussed taking into account the three nonideal aspects in real experiments: nondeterministic state preparation, nonunitary state evolution due to losses during state propagation, and imperfect state detection. A general expression is written for the probability of a measurement outcome taking into account these three nonideal aspects. As an example of applying the formalism, the classical Fisher information and fidelity (Shannon mutual information between phase and measurements) are computed for few-photon Fock and N00N states input into a lossy Mach-Zehnder interferometer. These three nonideal aspects lead to qualitative differences in phase estimation, such as a decrease in fidelity and Fisher information that depends on the true value of the phase.

The ground-state phases of a spin-orbit (SO)-coupled atomic spin-2 Bose-Einstein condensate are studied. Interesting density patterns forme spontaneously are widespread due to the competition between SO coupling and spin-dependent interactions as in an SO-coupled spin-1 condensate. Unlike the case of spin-1 condensates, which are characterized by either a ferromagnetic or a polar phase in the absence of SO, spin-2 condensates can take a cyclic phase, where we find that the patterns formed due to SO are square or triangular in their spin-component densities for axial symmetric SO interaction. Both patterns are found to evolve continuously into striped forms with increased asymmetry of the SO coupling.

We experimentally investigated the dynamic behavior of remote terahertz (THz) generation from two-color femtosecond laser-induced filamentation in air. A record-high THz pulse energy of 570 nJ at frequency below 5.5 THz was measured by optimizing the pump parameters at a controllable remote distance of 16 m, while super-broadband THz (<300 THz) pulse energy was up to 2.8 μJ. A further energy-scaling possibility was proposed. By analyzing simultaneously the fluorescence from both neutral ${\mathrm{N}}_2$ and ${\mathrm{N}}_2$${}^{+}$ in the filament, we found that the enhancement of THz radiation was due principally to guiding of the weak second-harmonic pulse inside the filament of the first strong fundamental pulse.

We show how to compute Casimir forces at nonzero temperatures with time-domain electromagnetic simulations, for example, using a finite-difference time-domain (FDTD) method. Compared to our previous zero-temperature time-domain method, only a small modification is required, but we explain that some care is required to properly capture the zero-frequency contribution. We validate the method against analytical and numerical frequency-domain calculations, and show a surprising high-temperature disappearance of a nonmonotonic behavior previously demonstrated in a pistonlike geometry.

An approximation hierarchy, called the lattice-path-based subsystem (LPSUB$m$) approximation scheme, is described for the coupled-cluster method (CCM). It is applicable to systems defined on a regular spatial lattice. We then apply it to two well-studied prototypical (spin-$\frac{1}{2}$ Heisenberg antiferromagnetic) spin-lattice models, namely, the $\mathit{XXZ}$ and the $\mathit{XY}$ models on the square lattice in two dimensions. Results are obtained in each case for the ground-state energy, the ground-state sublattice magnetization, and the quantum critical point. They are all in good agreement with those from such alternative methods as spin-wave theory, series expansions, quantum Monte Carlo methods, and the CCM using the alternative lattice-animal-based subsystem (LSUB$m$) and the distance-based subsystem (DSUB$m$) schemes. Each of the three CCM schemes (LSUB$m$, DSUB$m$, and LPSUB$m$) for use with systems defined on a regular spatial lattice is shown to have its own advantages in particular applications.

It is demonstrated that two distant quantum wells separated by a reservoir with a continuous spectrum can possess bound eigenstates embedded in the continuum. These represent a linear superposition of quantum states localized in the wells. We show that such a state can be isolated in the course of free evolution from any initial state by a null-result measurement in the reservoir, or it can be reached without such a measurement in the many-body case. The resulting superposition is regulated by the ratio of couplings between the wells and the reservoir. In particular, one can lock the system in one of the wells by enhancing this ratio. By tuning parameters of the quantum wells, many-body entangled states in distant wells can be produced through interactions and statistics.

In the framework of the computational tensor network [Phys. Rev. Lett. 98, 220503 (2007)], the quantum computation is performed in a virtual linear space called the correlation space. It was recently shown [Phys. Rev. Lett. 103, 050503 (2009)] that a state in a correlation space can be downloaded to the real physical space. In this paper, conversely, we study how to upload a state from a real physical space to the correlation space. After showing the impossibility of cloning a state between a real physical space and the correlation space, we propose a simple teleportation-like method of uploading. This method also enables the Gottesman-Chuang gate teleportation trick and entanglement swapping in the virtual-real hybrid setting. Furthermore, compared with the inverse of the downloading method by Cai et al. [Phys. Rev. Lett. 103, 050503 (2009)], which also works to upload, the proposed uploading method has several advantages.

The orbital angular momentum of light (OAM) provides a promising approach for the implementation of multidimensional states (qudits) for quantum-information purposes. In order to characterize the degradation undergone by the information content of qubits encoded in a bidimensional subspace of the orbital angular momentum degree of freedom of photons, we study how the state fidelity is affected by a transverse obstruction placed along the propagation direction of the light beam. Emphasis is placed on the effects of planar and radial hard-edged aperture functions on the state fidelity of Laguerre-Gaussian transverse modes and the entanglement properties of polarization-OAM hybrid-entangled photon pairs.

Generalized quantum measurements are an important extension of projective or von Neumann measurements in that they can be used to describe any measurement that can be implemented on a quantum system. We describe how to realize two nonstandard quantum measurements using cavity QED. The first measurement optimally and unambiguously distinguishes between two nonorthogonal quantum states. The second example is a measurement that demonstrates superadditive quantum coding gain. The experimental tools used are single-atom unitary operations effected by Ramsey pulses and two-atom Tavis-Cummings interactions. We show how the superadditive quantum coding gain is affected by errors in the field-ionization detection of atoms and that even with rather high levels of experimental imperfections, a reasonable amount of superadditivity can still be seen. To date, these types of measurements have been realized only on photons. It would be of great interest to have realizations using other physical systems. This is for fundamental reasons but also since quantum coding gain in general increases with code word length, and a realization using atoms could be more easily scaled than existing realizations using photons.

Algorithmic cooling (AC) of spins applies entropy manipulation algorithms in open spin systems in order to cool spins far beyond Shannon’s entropy bound. Algorithmic cooling of nuclear spins was demonstrated experimentally and may contribute to nuclear magnetic resonance spectroscopy. Several cooling algorithms were suggested in recent years, including practicable algorithmic cooling (PAC) and exhaustive AC. Practicable algorithms have simple implementations, yet their level of cooling is far from optimal; exhaustive algorithms, on the other hand, cool much better, and some even reach (asymptotically) an optimal level of cooling, but they are not practicable. We introduce here semioptimal practicable AC (SOPAC), wherein a few cycles (typically two to six) are performed at each recursive level. Two classes of SOPAC algorithms are proposed and analyzed. Both attain cooling levels significantly better than PAC and are much more efficient than the exhaustive algorithms. These algorithms are shown to bridge the gap between PAC and exhaustive AC. In addition, we calculated the number of spins required by SOPAC in order to purify qubits for quantum computation. As few as 12 and 7 spins are required (in an ideal scenario) to yield a mildly pure spin (60% polarized) from initial polarizations of 1% and 10%, respectively. In the latter case, about five more spins are sufficient to produce a highly pure spin (99.99% polarized), which could be relevant for fault-tolerant quantum computing.

The correlation pattern in the center of mass motion of the three fragments from dissociation of well-defined Rydberg states of ${\mathrm{H}}_3$ and ${\mathrm{D}}_3$ is studied. Dissociation of the molecules is induced by an external electric field. Through a comparison with results obtained in radiative cascading we can show that the correlation pattern is that of the short-lived $2s\hspace{0.5em}{}^2{A}_1^{{}^{\prime }}$ electronic state, of which a tiny amplitude is admixed by the external electric field. A comparison of our results with the predictions by M. Lehner and M. Jungen [J. Phys. B 42, 065101 (2009)] and U. Galster [Phys. Rev. A 81, 032517 (2010)] for predissociation of the $2s\hspace{0.5em}{}^2{A}_1^{{}^{\prime }}$ state is made. We show that the experimental vector correlation maps are direct images of the spatial symmetry of the product of the vibrational wave function and spatial dependence of the nonadiabatic coupling operator.

All 18 bound pure vibrational levels of the HD molecule have been calculated within the framework that does not assume the Born-Oppenheimer (BO) approximation. The nonrelativistic energies of the states have been corrected for the relativistic effects of the order of ${\alpha }^2$ (where $\alpha$ is the fine structure constant), calculated using the perturbation theory with the nonrelativistic non-BO wave functions being the zero-order approximation. The calculations were performed by expanding the non-BO wave functions in terms of one-center explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and by performing extensive optimization of the Gaussian nonlinear parameters. Up to 10 000 basis functions were used for each state.

Electron impact excitation of the $a\hspace{0.5em}{}^3\Pi$ valence state of the carbon-monoxide molecule has been studied in the energy region from threshold to 10 eV. Excitation functions for spin forbidden transitions from the $\nu =0$ level of the ground $X\hspace{0.5em}{}^1{\Sigma }^{+}$ state of CO to the ${\nu }^{\prime }=0$, 1, 2, 3, 4, and 5 levels of the $a\hspace{0.5em}{}^3\Pi$ state are measured. A crossed beam double trochoidal electron spectrometer is used. Forward and backward scattered electrons from the ${\nu }^{\prime }=0$ excitation channel are separated by electron beam modulation and a time-of-flight detection technique. The present results are normalized to the ground state ${}^2\Pi$ resonance vibrational excitation cross sections and absolute values of the differential cross sections at the border angles of $0^{°}$ and ${180}^{°}$ are determined. In this way the differential cross section measurements are completed in the full angular range from $0^{°}$ to ${180}^{°}$. The present results are compared to the existing literature data.

Numerical solutions of the time-dependent Schrödinger equation, using a new unitary numerical method, are used to obtain gauge-independent photoionization angular distributions for two-dimensional (planar) models of aligned ${\mathrm{H}}_2$ and ${{\mathrm{H}}_2}^{+}$ molecules in different electronic states. Simulations are performed with few-cycle attosecond (as) extreme-ultraviolet (xuv) laser pulses at wavelengths $\lambda =$ 10 nm [$\omega =$ 4.56 a.u. (atomic units), $\tau =2\pi /\omega =$ 1.38 a.u. $=$ 33.4 as], and $\lambda =$ 5 nm ($\omega =$ 9.11 a.u., $\tau =$ 16.7 as) for linear and circular polarization ionizations at $R_e=1.675$ a.u., where molecular orbital configurations dominate for which the electron wavelengths ${\lambda }_e⩾R_e$ and at large internuclear distance $R=10$ a.u. (${\lambda }_e<R$) where Heitler-London atomic configurations including ionic states ${\mathrm{H}}^{+}$H${}^{-}$ are more appropriate. The simulations allow one to investigate the effects of electron correlation and entanglement in ${\mathrm{H}}_2$ at different $R$ due to electron spin exchange as compared to the delocalized one-electron ${{\mathrm{H}}_2}^{+}$ system. An ultrashort (delta function) pulse model is used to interpret interferences in momentum distributions.

The complexity of ultrafast molecular photoionization presents an obstacle to the modeling of pump-probe experiments. Here, a simple optimized model of atomic rubidium is combined with a molecular dynamics model to predict quantitatively the results of a pump-probe experiment in which long-range rubidium dimers are first excited, then ionized after a variable delay. The method is illustrated by the outline of two proposed feasible experiments and the calculation of their outcomes. Both of these proposals use Feshbach ${}^{87}\mathrm{Rb}$${}_2$ molecules. We show that long-range molecular pump-probe experiments should observe spin-orbit precession given a suitable pump pulse, and that the associated high-frequency beat signal in the ionization probability decays after a few tens of picoseconds. If the molecule was to be excited to only a single fine-structure state, then a low-frequency oscillation in the internuclear separation would be detectable through the time-dependent ionization cross section, giving a mechanism that would enable observation of coherent vibrational motion in this molecule.