Within the framework of quantization of the macroscopic electromagnetic field, equations of motion and an effective Hamiltonian for treating both the resonant dipole-dipole interaction between two-level atoms and the resonant atom-field interaction are derived, which can suitably be used for studying the influence of arbitrary dispersing and absorbing material surroundings on these interactions. The theory is applied to the study of the transient behavior of two atoms that initially share a single excitation, with special emphasis on the role of the two competing processes of virtual- and real-photon exchange in the energy transfer between the atoms. In particular, it is shown that for weak atom-field interaction there is a time window, where the energy transfer follows a rate regime of the type obtained by ordinary second-order perturbation theory. Finally, it is shown that the resonant dipole-dipole interaction can change the singlet line of the emitted light to a doublet spectrum for weak atom-field interaction and the doublet spectrum to a triplet spectrum for strong atom-field interaction.

Starting from a fully microscopic spin-flip model for a vertical cavity surface emitting laser, we derive a macroscopic model that includes the frequency dependence of the complex gain and the gain nonlinearities due to spectral hole burning. The macroscopic model reproduces well the predictions of the microscopic one. The gain nonlinearities affect especially the switching from the higher to the lower frequency, and their effects are more manifest when the spin-flip rate is large.

Due to interaction with the vacuum of the radiation field, near-degenerate lower levels in a $\Lambda$ system have an additional coherence term, the spontaneously generated coherence term. In this paper, we investigate the effects of this kind of coherence on the transient process in the presence of a weak probe and a strong coherent field, with or without an incoherent pump. We find that the transient gain (absorption) properties can be dramatically affected by the spontaneously generated coherence. The transient gain can be greatly enhanced due to the effect of spontaneously generated coherence, and the transient absorption can be completely eliminated just by choosing the proper relative phase between the two fields.

We propose a scheme to implement a single-mode quantum filter, which selectively eliminates the one-photon state in a quantum state $\alpha |0〉+\beta |1〉+\gamma |2〉.$ The vacuum state and the two-photon state are transmitted without any change. This scheme requires single-photon sources, linear optical elements, and photon detectors. Furthermore, we demonstrate how this filter can be used to realize a two-qubit projective measurement and to generate multiphoton polarization entangled states.

A secret key shared through quantum key distribution between two cooperative players is secure against any eavesdropping attack allowed by the laws of physics. Yet, such a key can be established only when the quantum channel error rate due to eavesdropping or imperfect apparatus is low. Here, a practical quantum key distribution scheme by making use of an adaptive privacy amplification procedure with two-way classical communication is reported. Then, it is proven that the scheme generates a secret key whenever the bit error rate of the quantum channel is less than $0.5-0.1\sqrt{5}\approx 27.6\%,$ thereby making it the most error resistant scheme known to date.

We propose a scheme for realizing two-qubit quantum phase gates with atoms in a thermal cavity. The photon-number-dependent parts in the evolution operator are canceled with the assistant of a strong classical field. Thus the scheme is insensitive to the thermal field. In the scheme the detuning between the atoms and the cavity is equal to the atom-cavity coupling strength and thus the gates operate at a high speed, which is also important in view of decoherence. The scheme can be generalized to generate multiatom-entangled states with a thermal cavity.

The asymptotics of the Boltzmann-Shannon information entropy as well as the Renyi entropy for the quantum probability density of a single-particle system with a confining (i.e., bounded below) power-type potential ${V\left(x\right)=x}^{2k}$ with $k\in N$ and $x\in R,$ is investigated in the position and momentum spaces within the semiclassical (WKB) approximation. It is found that for highly excited states both physical entropies, as well as their sum, have a logarithmic dependence on its quantum number not only when $k=1\mathrm{}$ (harmonic oscillator), but also for any fixed k. As a by-product, the extremal case $\overrightarrow{k}\infty$ (the infinite well potential) is also rigorously analyzed. It is shown that not only the position-space entropy has the same constant value for all quantum states, which is a known result, but also that the momentum-space entropy is constant for highly excited states.

Recently, entanglement teleportation has been investigated by Lee and Kim [Phys. Rev. Lett. 84, 4236 (2000)]. In this paper we study entanglement teleportation via two separate thermally entangled states of a two-qubit Heisenberg $\mathrm{XX}$ chain. We established the condition under which the parameters of the model have to satisfy in order to teleport entanglement. The necessary minimum amount of thermal entanglement for some fixed strength of exchange coupling is a function of the magnetic field and the temperature.

We derive a lower bound on the inconclusive probability of unambiguous discrimination among n linearly independent quantum states by using the constraint of no signaling. It improves the bound presented in the paper of Zhang, Feng, Sun, and Ying [Phys. Rev. A 64, 062103 (2001)], and when the optimal discrimination can be reached, these two bounds coincide with each other. An alternative method of constructing an appropriate measurement to prove the lower bound is also presented.

Some of the most promising proposals for scalable solid-state quantum computing, e.g., those using electron spins in quantum dots or donor electron or nuclear spins in Si, rely on a two-qubit quantum gate that is ideally generated by an isotropic exchange interaction. However, an anisotropic perturbation arising from spin-orbit coupling is inevitably present. Previous studies focused on removing the anisotropy. Here we introduce a new universal set of quantum logic gates that takes advantage of the anisotropic perturbation. The price is a constant but modest factor in additional pulses. The gain is a scheme that is compatible with the naturally available interactions in spin-based solid-state quantum computers.

The measured angular differential cross section (DCS) for the elastic scattering of electrons from ${\mathrm{Ar}}^{+}{(3s}^2{3p}^5{}^{2}P)$ at the collision energy of 16 eV is presented. By solving the Hartree-Fock equations, we calculate the corresponding theoretical DCS including the coupling between the orbital angular momenta and spin of the incident electron and those of the target ion and also relaxation effects. Since the collision energy is above one inelastic threshold for the transition ${3s}^2{3p}^5{}^{2}P–{3s3p}^6{}^{2}S,$ we consider the effects on the DCS of inelastic absorption processes and elastic resonances. The measurements deviate significantly from the Rutherford cross section over the full angular range observed, especially in the region of a deep minimum centered at approximately 75°. Our theory and an uncoupled, unrelaxed method using a local, spherically symmetric potential by Manson [Phys. Rev. 182, 97 (1969)] both reproduce the overall shape of the measured DCS, although the coupled Hartree-Fock approach describes the depth of the minimum more accurately. The minimum is shallower in the present theory owing to our lower average value for the d-wave non-Coulomb phase shift ${\sigma }_2,$ which is due to the high sensitivity of ${\sigma }_2$ to the different scattering potentials used in the two models. The present measurements and calculations therefore show the importance of including coupling and relaxation effects when accurately modeling electron-ion collisions. The phase shifts obtained by fitting to the measurements are compared with the values of Manson and the present method.

The time-dependent close-coupling theory is applied to the study of the electron-impact ionization of helium from the excited $\mathrm{}\left(1s2s\right)\mathrm{}$ configuration. Calculations are made in an effort to resolve the discrepancy between theoretical calculations and existing experimental measurements for this cross section. We find good agreement with the existing convergent close-coupling calculations of Bray and Fursa [J. Phys. B 28, L197 (1995)], but are in substantial disagreement with the experimental measurements of this quantity by Dixon et al. [J. Phys. B 9, 2617 (1976)].

We report studies of the magnetic decoupling of Rb spin relaxation in the presence of ${\mathrm{H}}_2$ buffer gas at high pressures $(>~1.5\mathrm{atm}).\mathrm{}$ At low Rb number densities $(<~5\times {10}^{13}{\mathrm{cm}}^{-3}),$ we find that the observed magnetic-field dependence of the spin relaxation is almost solely due to binary collisions between Rb atoms and hydrogen molecules, and that it is very well described by three rates: the Rb-Rb spin-exchange rate, the S-damping rate, and the Carver rate associated with the hyperfine pressure-shift interaction in collisions of Rb with ${\mathrm{H}}_2$ molecules. At higher Rb densities, the contribution to the relaxation from Rb triplet dimers becomes significant.

The influence of the magnetic-field component of the incident pulse on the emission of photons by multiply charged ions interacting with intense, near-infrared laser pulses is investigated theoretically using a strong-field approximation that treats the coupling of the atom with the incident field beyond the dipole approximation. For peak pulse intensities approaching ${10}^{17}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2},$ the electron drift in the laser propagation direction due to the magnetic-field component of the incident pulse strongly influences the photon emission spectra. In particular, emission is reduced and the plateau structure of the spectra modified, as compared to the predictions in the dipole approximation. Nondipole effects become more pronounced as the ionization potential of the ion increases. Photon emission spectra are interpreted by analysing classical electron trajectories within the semiclassical recollision model. It is shown that a second pulse can be used to compensate the magnetic-field induced drift for selected trajectories so that, in a well-defined spectral region, a single attosecond pulse is emitted by the ion.

A theoretical model that predicts the unexpectedly high photocurrents that were reported by Farkas et al. [J. Phys. B $31,$ L461 (1998)] for the photoelectric effect in gold at midinfrared laser wavelengths $\left(12\mathrm{}\mu \mathrm{m}\right)$ is presented. It is shown that the observed effect, a nominally 46-photon process at threshold, is a stepwise and multiple-order process, which is strongly saturated at the moderate laser intensity of about $\sim 10{\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ The $\approx 30$-photon-wide energy spectrum of the observed photoelectron current is due to above threshold photoemission. This relatively efficient above threshold effect is due to laser absorption in the long range Coulombic image surface potential.

We analyze the dynamical stability of gap solitons formed in a quasi-one-dimensional Bose-Einstein condensate in an optical lattice. Using two different numerical methods we show that, under realistic assumptions for experimental parameters, a gap soliton is stable only in a truly one-dimensional situation. In two and three dimensions, resonant transverse excitations lead to dynamical instability. The time scale of the decay is numerically calculated and shown to be large compared to the characteristic time scale of solitons for realistic physical parameters.

The maximum coincidence rate in intensity correlation experiments with hard x rays from a synchrotron radiation source is considered quantitatively. Special emphasis is given to a correction that is caused by path-length differences from asymmetric Bragg reflections in a monochromator. Applications of such intensity correlation measurements are suggested.

In a recent paper, Nesbet [Phys. Rev. A 65, 010502(R) (2001)] has proposed dropping “the widespread but unjustified assumption that the existence of a ground-state density functional for the kinetic energy, $T_{\mathrm{s}}\left[\rho \right],$ of an N-electron system implies the existence of a density-functional derivative, $\delta T_{\mathrm{s}}\left[\rho \right]/\delta \rho \left(\mathbf{r}\right),$ equivalent to a local potential function,” because, according to his arguments, this derivative “has the mathematical character of a linear operator that acts on orbital wave functions.” Our Comment demonstrates that the statement called by Nesbet an “unjustified assumption” happens, in fact, to be a rigorously proven theorem. Therefore, his previous conclusions stemming from his different view of this derivative, which undermined the foundations of density-functional theory, can be discounted.

The method of complex-coordinate rotation is applied to investigate resonances in positron scattering by helium ions. Here we present results for high-angular-momentum states $(L=2\mathrm{}–6).$ Together with the earlier values for the $L=0\mathrm{}$ and 1 states, these stabilized complex eigenvalues exhibit a regular pattern. We also calculate some narrow S-wave resonances below the excited positronium $\mathrm{}(N=2\mathrm{})\mathrm{}$ threshold. Our converged results for the first S state lying above the ${\mathrm{He}}^{+}$ $\mathrm{}(n=5\mathrm{})\mathrm{}$ threshold differ substantially from the calculation by Igarashi and Shimamura [Phys. Rev. A 56, 4733 (1997)].

We investigate the properties of a dilute boson-fermion mixture at zero temperature. The ground-state energy of the system is calculated to second order in the Bose-Fermi coupling constant. The Green’s function formalism is applied to obtain results for the effect of Bose-Fermi interactions on the spectrum of phonon excitations, the momentum distribution, the condensate fraction, and the superfluid density of the bosonic component. A quantitative discussion of these effects is presented in the case of mixtures with attractive interspecies interaction.