We present a proposal to manipulate the Raman process via incoherent pump, tunable intensity, and phase control of the driving fields. It is found that Raman absorptive peaks can become Raman gain peaks by controlling the incoherent pump, and this property leads to the group velocity of a weak probe pulse from subluminal to superluminal with less absorption or gain. Furthermore, it is also shown that Raman gain peaks can be controlled not only by controlling the intensities of the driving fields but also by adjusting phases of the driving fields, and this leads to the strong negative dispersion with less gain.

We propose a scheme to realize a $1\to 2$ universal quantum cloning machine with superconducting quantum interference device qubits, embedded in a high-$Q$ cavity. The controlled-NOT operations are derived to present our scheme, and the two-photon Raman resonance processes are used to increase the operation rate. Compared with previous works, our scheme has advantages in the experimental realization and further utilization.

Spatiotemporal evolution of a confined Bose-Einstein condensate is studied by numerically integrating the time-dependent Gross-Pitaevskii equation. Self-interference between the successively expanding and reflecting nonlinear matter waves results in spiral atomic density profile, which subsequently degenerates into an embedding structure: The inner part preserves memory of the initial states while the outer part forms a sequence of necklacelike rings. The phase plot reveals a series of discrete concentric belts. The large gradients between adjacent belts indicate that the ring density notches are dark solitons. In the dynamical process, a scenario of vortex-antivortex pairs are spontaneously created and annihilated, whereas the total vorticity keeps invariant.

We present time-resolved measurements of light transport through strongly scattering macroporous gallium phosphide at various vacuum wavelengths between 705 nm and 855 nm. Within this range the transport mean free path is strongly wavelength dependent, whereas the observed energy velocity is shown to be independent of the wavelength. We conclude that microscopic resonances, which can strongly slow down the diffusion process in, e.g., granular ${\text{TiO}}_2$, are absent in macroporous gallium phosphide in the wavelength region of concern.

This paper presents a further development of the Siegert-state expansion approach [O. I. Tolstikhin, Phys. Rev. A 73, 062705 (2006)]. Here, we switch to the adiabatic representation. We introduce the adiabatic Siegert states and derive the generalized Born-Fock equations describing the time evolution of coefficients in the expansion of the solution to the time-dependent Schrödinger equation in their terms. By constructing the asymptotic solution to these equations, we develop an adiabatic approximation for transitions to the continuum. The leading-order asymptotic formulas for the spectra of ejected particles in the underbarrier (when the initial state remains bound during all the evolution) and overbarrier (when the initial state is temporarily promoted to the continuum) cases are obtained. These formulas are uniform in the momentum of ejected particles and thus give a complete solution to the problem in the adiabatic approximation. Their relation to previous studies of nonadiabatic transitions to the continuum is discussed. The results are illustrated by calculations for a model describing the dissociation of a diatomic molecule by an electric field pulse.

The Siegert-state expansion approach [O. I. Tolstikhin, Phys. Rev. A 73, 062705 (2006)] is extended to the three-dimensional case. Coupled equations defining the time evolution of coefficients in the expansion of the solution to the time-dependent Schrödinger equation in terms of partial-wave Siegert states are derived, and physical observables (probabilities of transitions to discrete states and the momentum distribution of ejected particles) are expressed in terms of these coefficients. The approach is implemented in terms of Siegert pseudostates and illustrated by calculations of the photodetachment of ${\text{H}}^{-}$ by strong high-frequency laser pulses. The present calculations demonstrate that the interference effect in the laser-atom interaction dynamics found recently in the one-dimensional case [K. Toyota et al., Phys. Rev. A 76, 043418 (2007)] reveals itself in the three-dimensional case as well.

We have investigated the reason for significant discrepancies between the results of two recent, similar computational methods [Zatsarinny et al., Astron. Astrophys. 426, 699 (2004); Gu, Astrophys. J. 590, 1131 (2003)] for dielectronic recombination (DR) of ${\mathrm{Mg}}^{2+}$. It is found that the choice of orbital description can lead to discrepancies by as much as a factor of 2 between total peak DR rate coefficients resulting from otherwise-identical computations. These unexpected differences are attributed to the large sensitivity to bound-orbital relaxation and continuum-orbital description effects on the computed radiative and autoionizing transitions arising from accidental cancellation. In order to obviate these effects, an approach, using a separate, nonorthogonal orbital basis for each configuration, is employed to yield a DR rate coefficient that we assess to be more reliable than all earlier published results.

Multiple electron ionization of diatomic molecular targets is studied for impact of proton beams. It is shown that for these molecular targets, as it was previously predicted for atomic ones, postcollision Auger type emission following inner-shell vacancy production dominates the reaction at high enough impact energies. Moreover, it is proven that this dominance increases as the ionization degree increases.

The best level of ordering and straightening of carbon nanotube arrays is often achieved when they are grown in a dielectric matrix, so such structures present the most suitable candidates for future channeling experiments with carbon nanotubes. Consequently, we investigate here how the dynamic polarization of carbon valence electrons in the presence of various surrounding dielectric media affects the angular distributions of protons channeled through (11,9) single-wall carbon nanotubes. Proton speeds between 3 and 10 a.u., corresponding to energies of 0.223 and 2.49 MeV, are chosen with the nanotube’s length varied between 0.1 and $1\mu \text{m}$. We describe the repulsive interaction between a proton and the nanotube’s atoms in a continuum-potential approximation based on the Doyle-Turner potential, whereas the attractive image force on a proton is calculated using a two-dimensional hydrodynamic model for the dynamic response of the nanotube valence electrons, while assigning to the surrounding medium an appropriate (frequency dependent) dielectric function. The angular distributions of channeled protons are generated using a computer simulation method which solves the proton equations of motion in the transverse plane numerically. Our analysis shows that the presence of a dielectric medium can strongly affect both the appearance and positions of maxima in the angular distributions of channeled protons.

Velocity and kinetic energy distributions (VDs, KEDs) of metal ions generated by nanosecond (ns) pulsed laser ablation under high vacuum have been determined using an electrostatic analyzer plus time-of-flight coupled system. A number of metal and alloy targets, principally involving Fe and Ni, have been studied at different laser fluences. At low fluence, the ion distributions have been shown to fit single Maxwell-Boltzmann-Coulomb (MBC) distributions; for medium and higher fluences, each ion distribution is found to comprise that of the surviving “precursor” ion, itself, overlapped with sidebands which arise from ion-electron recombination and/or ionization. The so-called surviving “precursor” ion of a distribution is that which underwent no change of charge. The Coulomb velocities of the surviving “precursor” ions and those of the ion products resulting from ion-electron collisions have been compared. Ion velocities are correlated with the local electric field resulting from ejection of the photoelectrons following laser ablation. Under identical conditions of laser fluence, the ions are seen to experience an electrical field nearly independent of their charge. The transit times of ions in the plasma have been estimated to be of the order of $1\mathrm{ps}$. An overall quantitative mechanism for metal ablation on this basis is presented, including the ejection time for photoelectrons and differences in ion distributions resulting from employing laser pulses in the nanosecond (ns) and femtosecond (fs) regimes.

Doppler laser cooling of a three-level ladder system using two near-resonant laser fields is analyzed in the case of the intermediate level being metastable while the upper level is short lived. Analytical as well as numerical results, e.g., obtainable scattering rates and achievable temperatures are presented. When appropriate, comparisons with two-level single-photon Doppler laser cooling is made. These results are relevant to recent experimental Doppler laser cooling investigations addressing intercombination lines in alkaline-earth-metal atoms and quadrupole transitions in alkaline-earth-metal ions.

The ionization of atomic hydrogen in intense laser fields is studied theoretically by both quantum-mechanical and classical approaches. In the quantum-mechanical treatment we apply a momentum-space strong-field approximation (MSSFA) and the Coulomb potential is taken into account as a perturbation. The classical calculations are performed within the framework of the classical trajectory Monte Carlo method. The energy and angular distributions of the ionization probabilities of the photoelectrons are presented for different laser pulses. While for the case of low electron energies larger discrepancies can be observed between the theories in the double-differential ionization probabilities, at high electron energies the agreement is excellent. This indicates that the generation of low-energy electrons is of quantum type and it is strongly influenced by the Coulomb potential, while the production of high-energy electrons is of classical type and it is less influenced by the Coulomb interaction. Our MSSFA results are in good agreement with the most reliable calculations based on a numerical solution of the time-dependent Schrödinger equation for high momentum transfers.

The radiative ortho-para transition in molecular hydrogen is studied. This highly forbidden transition is very sensitive to relativistic and subtle nonadiabatic effects. Our result for the transition rate in the ground vibrational level $\Gamma (J=1\to J=0)=6.20\left(62\right)\times {10}^{-14}{\mathrm{yr}}^{-1}$ is significantly lower in comparison to all the previous approximate calculations. Experimental detection of such a weak line by observation of, for example, cold interstellar molecular hydrogen is at present unlikely.

We report the experimental realization of a multibeam atom laser. A single continuous atom laser is outcoupled from a Bose-Einstein condensate via an optical Raman transition. The atom laser is subsequently split into up to five atomic beams with slightly different momenta, resulting in multiple, nearly copropagating, coherent beams which could be of use in interferometric experiments. The splitting process itself is a realization of Bragg diffraction, driven by each of the optical Raman laser beams independently. This presents a significantly simpler implementation of an atomic beam splitter, one of the main elements of coherent atom optics.

The dynamic polarizability of ${}^{87}\mathrm{Rb}$ atoms was measured using a guided-wave Bose-Einstein condensate interferometer. Taking advantage of the large arm separations obtainable in our device, a well-calibrated laser beam is applied to one atomic packet and not the other, inducing a differential phase shift. The technique requires relatively low laser intensity and works for arbitrary optical frequencies. For off-resonant light, the ac polarizability is obtained with a statistical accuracy of 3% and a calibration uncertainty of 6%. On resonance, the dispersion-shaped behavior of the Stark shift is observed, but with a broadened linewidth that is attributed to collective light scattering effects. The resulting nonlinearity may prove useful for the production and control of squeezed quantum states.

We examine the mathematical solutions of the Dirac equation to predict the spontaneous electron-positron pair creation from the vacuum. The Dirac equation contains a position and time-dependent scalar potential to approximate the effect of an external force on the vacuum. We focus on forces that are localized in space as well as in time and find that the resulting creation process is also localized in time but delocalized in space. This illustrates that the Dirac equation can show nonlocal behavior as it predicts that particles can be created even in spatial regions where the force is zero. We also examine the spatial distribution of the created particles and show that for spatially extended force fields it is proportional to the square of the position dependence of the force. But when the force field is narrower than the Compton wavelength, the created electron density approaches a universal shape invariant form that is independent of the strength of the force for sufficiently weak field strength.

A major problem in exploiting microscopic systems for developing a new technology based on the principles of quantum information is the influence of noise, which tends to work against the quantum features of such systems. It thus becomes crucial to understand how noise affects the evolution of quantum circuits: several techniques have been proposed, among which stochastic differential equations (SDEs) can represent a very convenient tool. We show how SDEs naturally map any Markovian noise into a linear operator, which we call a noise gate, acting on the wave function describing the state of the circuit, and we discuss some examples. We show that these gates can be manipulated like any standard quantum gate, thus simplifying in certain circumstances the task of computing the overall effect of the noise at each stage of the protocol. This approach yields equivalent results to those derived from the Lindblad equation; yet, as we show, it represents a handy and fast tool for performing computations, and, moreover, it allows for fast numerical simulations and generalizations to non-Markovian noise. In detail we review the depolarizing channel and the generalized amplitude damping channel in terms of this noise gate formalism and show how these techniques can be applied to any quantum circuit.

We study the possibility of performing perfect teleportation of unknown quantum states from multiple senders to a single receiver with a previously shared stabilizer state. In the model we considered, the utilized stabilizer state is partitioned into several subsystems and then each subsystem is distributed to a distinct party. We present two sufficient conditions for a stabilizer state to achieve a given nonzero teleportation capacity with respect to a given partition plan. The corresponding teleportation protocols are also explicitly given. Interestingly, we find that even mixed stabilizer states are also useful for perfect many-to-one teleportation. Finally, our work provides a perspective from stabilizer formalism to view the standard teleportation protocol and also suggests a technique for analyzing teleportation capability of multipartite entangled states.

We analyze the bipartite and multipartite entanglement for the ground state of the one-dimensional $XY$ model in a transverse magnetic field in the thermodynamical limit. We explicitly take into account the spontaneous symmetry breaking in order to explore the relation between entanglement and quantum phase transitions. As a result we show that while both bipartite and multipartite entanglement can be enhanced by spontaneous symmetry breaking deep into the ferromagnetic phase, only the latter is affected by it in the vicinity of the critical point. This result adds to the evidence that multipartite, and not bipartite, entanglement is the fundamental indicator of long-range correlations in quantum phase transitions.