A scheme for coherent manipulation of collective atomic states is developed such that total subradiant states, in which spontaneous emission is suppressed into all directions due to destructive interference between neighbor atoms, can be created in an extended atomic ensemble. The optimal conditions for creation of such states and suitability of them for quantum storage are discussed. It is shown that in order to achieve the maximum signal-to-noise ratio the shape of a light pulse to be stored and reconstructed using a homogeneously broadened absorption line of an atomic system should be a time-reversed regular part of the response function of the system. In the limit of high optical density, such pulses allow one to prepare collective subradiant atomic states with near flat spatial distribution of the atomic excitation in the medium.

When two-photon interactions are induced by down-converted light with a bandwidth that exceeds the pump bandwidth, they can obtain a behavior that is pulselike temporally, yet spectrally narrow. At low photon fluxes this behavior reflects the time and energy entanglement between the down-converted photons. However, two-photon interactions such as two-photon absorption (TPA) and sum-frequency generation (SFG) can exhibit such a behavior even at high power levels, as long as the final state (i.e., the atomic level in TPA, or the generated light in SFG) is narrow-band enough. This behavior does not depend on the squeezing properties of the light, is insensitive to linear losses, and has potential applications. In this paper we describe analytically this behavior for traveling-wave down conversion with continuous or pulsed pumping, both for high- and low-power regimes. For this we derive a quantum-mechanical expression for the down-converted amplitude generated by an arbitrary pump, and formulate operators that represent various two-photon interactions induced by broadband light. This model is in excellent agreement with experimental results of TPA and SFG with high-power down-converted light and with entangled photons [Dayan et al., Phys. Rev. Lett. 93, 023005 (2004); 94, 043602 (2005); Pe’er et al., ibid. 94, 073601 (2005)].

We study numerically the dynamics of a multi-longitudinal-mode semiconductor laser subject to incoherent optical feedback. The feedback scheme is such that the polarization state of the feedback light is rotated 90°, so that the natural laser mode, TE, is coupled unidirectionally into the orthogonal, unsupported mode, TM. We use traveling-wave equations for the slowly varying complex amplitudes of the two counterpropagating optical fields circulating in the Fabry Pérot cavity, both with TE polarization, coupled to an equation for the carrier population. The carrier equation contains a time-delayed term that takes into account the effect of the incoherent feedback. The model considers a parabolic frequency-dependent gain and does not assume a priori a fixed number of active longitudinal modes. We find that moderate feedback levels reduce the total power and increase the number of oscillating longitudinal modes. Larger feedback levels lead to instabilities at both the external cavity frequency $f_{\mathit{ext}}$ and the relaxation oscillation frequency $f_{\mathit{ro}}$. These findings are in good qualitative agreement with experimental observations by Houlihan et al. [Opt. Commun. 199, 175 (2001)]. For even stronger feedback there is square-wave periodic modulation of the total power, with a repetition period close to twice the delay time. In this regime, which consists of a sequence of turn-on and turn-off events driven by the incoherent feedback, the longitudinal modes show in-phase behavior at a frequency close to $f_{\mathit{ro}}$ accompanied by a slower out-of-phase drift, which is related to variations of the maximum and minimum values of the oscillation amplitude of the modal intensities.

In the weak value formalism of Aharonov et al., the weak value $A_w$ of any observable $A$ is generally a complex number. We derive a physical interpretation of its value in terms of the shift in the measurement pointer’s mean position and mean momentum. In particular, we show that the mean position shift contains a term jointly proportional to the imaginary part of the weak value and the rate at which the pointer is spreading in space as it enters the measurement interaction.

We find a protocol transmitting two quantum states crossly in the butterfly network only with prior entanglement between two senders. This protocol requires only one qubit transmission or two classical bits (cbits) transmission in each channel in the butterfly network. It is also proved that it is impossible without prior entanglement. More precisely, an upper bound of average fidelity is given in the butterfly network when prior entanglement is not allowed. The presented result concerns only the butterfly network, but our techniques can be applied to a more general graph.

While the exact total energy of a separated open system varies linearly as a function of average electron number between adjacent integers, the energy predicted by semilocal density-functional approximations is concave up and the exact-exchange-only or Hartree-Fock energy is concave down. As a result, semilocal density functionals fail for separated open systems of fluctuating electron number, as in stretched molecular ions $\mathrm{A}_2{}^{+}$ and in solid transition-metal oxides. We develop an exact-exchange theory and an exchange-hole sum rule that explain these failures and we propose a way to correct them via a local hybrid functional.

We report total cross sections for single ionization and electron capture in ${\mathrm{H}}^{+}$ collisions with water molecules at impact energies $25\mathrm{keV}<E<5\mathrm{MeV}$. Calculations have been carried out by applying the independent-particle model and the eikonal–classical trajectory Monte Carlo (CTMC) method. We have also estimated fragmentation cross sections by multiplying the partial cross sections by the branching ratios measured in the photoionization experiments of Tan et al. [Chem. Phys. Lett. 29, 299 (1978)].

We propose an entanglement purification protocol for high-dimensional multipartite systems. In the protocol we can select a subensemble in a pure generalized Greenberger-Horne-Zeilinger (GHZ) state. This post-selection can be made by detecting the noise which contaminated the initial pure ensemble when the systems passed through a noisy environment. For the detection of noise we investigate the correlation properties of GHZ states and analyze their possible errors due to a noisy environment. We show that the protocol presented is more efficient than a simple generalization of the purification protocol for a bipartite state in high dimensions.

We study the entanglement capability of the evolution of a pair of qubits subject to unitary dynamics, when the dynamical parameters are fixed, that is they cannot be modified during the time evolution via external control. Unlike the fast local control regime, we find that local and nonlocal contributions to the dynamics are strictly interconnected, and both relevant in determining the entangling capability of the channel. It turns out that it is possible to strongly increase this quantity by suitably initializing the characteristic energies of the two parties; a significative magnification is obtained when these energies are equal. Analytical results are obtained for a subclass of dynamics, and numerical results are presented for more general cases.

We introduce and formalize the concept of information flux in a many-body register as the influence that the dynamics of a specific element receive from any other element of the register. By quantifying the information flux in a protocol, we can design the most appropriate initial state of the system and, noticeably, the distribution of coupling strengths among the parts of the register itself. The intuitive nature of this tool and its flexibility, which allow for easily manageable numerical approaches when analytic expressions are not straightforward, are greatly useful in interacting many-body systems such as quantum spin chains. We illustrate the use of this concept in quantum cloning and quantum state transfer and we also sketch its extension to nonunitary dynamics.

Using the multiconfiguration Dirac-Fock method, including the quantum electrodynamics corrections, especially with the Breit interactions, we calculate the electric quadrupole $\left(E2\right)$ and magnetic dipole $\left(M1\right)$ transition rates for the two transitions ${}^{2}D_{5∕2,3∕2}^o\to {}^{4}S_{3∕2}^o$ of $\mathrm{O}\mathrm{II}$. We show systematically that the correlation effects owing to core electron excitations and the Breit interactions are vitally important for the transition rates. We present a benchmark for the intensity ratio between the two transitions in the limit of high electron density in planetary nebulas, i.e., $r\left(\infty \right)={0.345}_{-0.014}^{+0.028}$, which is in good agreement with modern astronomical observations.

A recently introduced Bethe-Salpeter formalism is applied to the calculation of recoil corrections to the energy levels of all $n=1$, 2, and 3 states of hydrogenic ions. Finite basis set techniques are shown to allow the accurate evaluation of expressions that sum all orders of $Z\alpha$, which are rederived in the formalism using combinatoric techniques. A comparison of the all-order results with one-loop calculations and known terms of the perturbation expansion in $Z\alpha$ is made. Good agreement of the results of the present work with previous calculations is shown, and a discussion of issues that will have to be treated for the many-electron case, where highly accurate experiments have been carried out, is given.

We have performed a kinematically complete experiment for target ionization with simultaneous projectile detachment (TIPD) in $200\text{−}\mathrm{keV}$ ${\mathrm{H}}^{-}+\mathrm{He}$ collisions. From the data we extracted triple-differential cross sections (TDCSs) for each electron separately. These TDCSs closely resemble corresponding data for single ionization by charged-particle impact. Surprisingly, the contributions from higher-order processes to TIPD, proceeding through two independent interactions of each electron with the core of the respective other collision partner, are found to be somewhat larger than the first-order process proceeding through the electron-electron interaction.

The quantum-classical hybrid description of rare-gas clusters interacting with intense light pulses which we have developed is described in detail. Much emphasis is put on the treatment of screening electrons in the cluster which set the time scale for the evolution of the system and form the link between electrons strongly bound to ions and quasifree plasma electrons in the cluster. As an example, we discuss the dynamics of an ${\text{Ar}}_{147}$ cluster exposed to a short vuv laser pulse of 20 eV photon energy.

We investigated the coherence accumulation in Rb atoms excited by a train of femtosecond pulses. The coherence accumulation results in the velocity selective optical pumping of Rb hyperfine levels, observed by a cw probe laser. The effects of the pulse train excitation on the cw probe laser transmission were investigated, depending on the probe laser power. We observed the probe laser absorption increase in the strong probe case, followed by the reduction of the modulations in the probe laser absorption. Results show that accumulation of population and coherence can be effectively reduced and eventually destroyed by increasing the cw laser intensity. A strong cw laser can therefore serve as a switch from the pulse-train to pulse-by-pulse type of interaction of Rb atoms with the fs laser. We developed a density-matrix based theoretical model of the eight-level Rb atoms interacting with the fs and cw laser fields, and present agreement with the experimental results.

We investigate the behavior of dark solitons in a superfluid Fermi gas along the BCS-BEC crossover by solving the Bogoliubov–de Gennes equations and looking for real and odd solutions for the order parameter. We show that in the resonance unitary region, where the scattering length is large, the density profile of the soliton has a deep minimum, differently from what happens in the BCS regime. The superfluid gap is found to be significantly quenched by the presence of the soliton due to the occurrence of Andreev fermionic bound states localized near the nodal plane of the order parameter.

A method for efficiently generating an isolated single-cycle attosecond pulse is proposed. It is shown that the ultraviolet (UV) attosecond (as) pulse can be utilized as a robust tool to control the dynamics of electron wave packets (EWPs). By adding a UV attosecond pulse to an infrared (IR) few-cycle pulse at a proper time, only one return of the EWP to the parent ion is selected to effectively contribute to the harmonics; then, an isolated two-cycle $130\text{−}\mathrm{as}$ pulse with a bandwidth of $45\mathrm{eV}$ is obtained. After complementing the chirp, an isolated single-cycle attosecond pulse with a duration less than $100\mathrm{as}$ seems achievable. In addition, the contribution of the quantum trajectories can be selected by adjusting the delay between the IR and UV fields. Using this method, the harmonic and attosecond pulse yields are efficiently enhanced in contrast to the scheme [G. Sansone et al., Science 314, 443 (2006)] using a few-cycle IR pulse in combination with the polarization gating technique.

We propose a scheme to obtain a highly efficient four-wave mixing (FWM) in a coherent six-level tripod system by using a double-dark resonance and multiphoton destructive interference induced transparency. We show both analytically and numerically that the optical-wave mixing in such a system with a pump wave and an internally generated FWM wave mediated by electromagnetically induced transparency and hence propagating with matched ultraslow group velocities can result in a significant enhancement of FWM conversion efficiency. This interesting scheme may be used to generate coherent short-wavelength radiation in dense optical media with very low pump-wave intensity.

Based on the calculation of the Wigner function, some statistical properties of the superposition of two coherent states with a vacuum state are demonstrated. The distance variation difference function is calculated for these states. Application of homodyne statistics shows that the addition (subtraction) of the vacuum state can improve the classical channel capacity of a noiseless binary symmetric system.

The problem of a source atom radiating into a medium of dielectric atoms is considered using a microscopic approach. The source atom is excited by a pulsed, classical driving field, and the average field amplitude and intensity radiated by the source atom is calculated to first order in the dielectric density. It is shown how both the change in propagation vector and change in propagation speed of the radiation in the medium can be traced to scattering events in the medium.