We show that there exists a class of inequalities between correlation functions of different orders of a chaotic electron field. These inequalities lead to the antibunching effect and are a consequence of the fact that electrons are fermions: indistinguishable particles with antisymmetric states. The derivation of the inequalities is based on the known form of the correlation functions for the chaotic state and on the properties of matrices and determinants.

In a recent paper, Coutinho, Nogami, and Tomio [Phys. Rev. A 59, 2624 (1999)] presented an example in which, they claim, Feynman’s prescription of disregarding the Pauli principle in intermediate states of perturbation theory fails. We show that, contrary to their claim, Feynman’s prescription is consistent with the exact solution of their example.

In a recent paper, we raised a question on the validity of Feynman’s prescription of disregarding the Pauli principle in intermediate states of perturbation theory. In the preceding Comment, Cavalcanti correctly pointed out that Feynman’s prescription is consistent with the exact solution of the model that we used. This means that the Pauli principle does not necessarily apply to intermediate states. We discuss implications of this puzzling aspect.

We consider an $\overrightarrow{N}M$ quantum cloning transformation acting on pure two-level states lying on the equator of the Bloch sphere. An upper bound for its fidelity is presented, by establishing a connection between optimal phase-covariant cloning and phase estimation. We give the explicit form of a cloning transformation that achieves the bound for the case $N=1,$ $M=2,$ and find a link between this case and optimal eavesdropping in the quantum cryptographic scheme BB84.

We have observed the deexcitation x-ray spectra of the exotic molecules $(d{\mu }^4\mathrm{He}{)}^{*}$ and $(d{\mu }^3\mathrm{He}{)}^{*}$ with good statistics and low background. From the time distributions of these x rays, we have directly determined the muon transfer rates from ground-state muonic deuterium atoms to helium nuclei. The obtained transfer rates in gaseous mixtures at $\sim 30$ K are ${\lambda }_{d^3\mathrm{He}}=(1.856\mathrm{}\pm 0.077)\times {10}^8\hspace{0.5em}{\mathrm{s}}^{-1}$ to ${}^3\mathrm{He},$ and ${\lambda }_{d^4\mathrm{He}}=(10.50\mathrm{}\pm 0.21)\times {10}^8\hspace{0.5em}{\mathrm{s}}^{-1}$ to ${}^4\mathrm{He}.$ In liquid mixtures, we measured the muon transfer rates ${\lambda }_{d^3\mathrm{He}}=(2.77\mathrm{}\pm 0.73)\times {10}^8\hspace{0.5em}{\mathrm{s}}^{-1}$ and ${\lambda }_{d^4\mathrm{He}}=(14.2\mathrm{}\pm 1.4)\times {10}^8\hspace{0.5em}{\mathrm{s}}^{-1}.$ These transfer rates have to be multiplied with the target density and the atomic helium concentration to obtain the “effective transfer rates” for a specific target composition. The expected isotopic effect between mixtures containing ${}^3\mathrm{He}$ and mixtures containing ${}^4\mathrm{He},$ as well as with respect to the hydrogen-helium case, is clearly confirmed. A density effect was observed for both isotopic compositions. We investigated the widths and the energies of the intensity maxima of the observed energy spectra of the molecular x rays. From the comparison of the measured energy spectra with calculated ones, we conclude that decay from the rotational state $J=1\mathrm{}$ of the muonic molecule dominates over decay from $J=0\mathrm{}$ at the investigated experimental conditions.

The strong-interaction limit of density-functional (DF) theory is simple and provides information required for an accurate resummation of DF perturbation theory. Here we derive the point-charge-plus-continuum (PC) model for that limit, and its gradient expansion. The exchange-correlation (xc) energy $E_{\mathrm{xc}}\left[\rho \right]\equiv {\int }_0^{1}d\alpha W_{\alpha }\left[\rho \right]$ follows from the xc potential energies $W_{\alpha }$ at different interaction strengths $\alpha >~0$ [but at fixed density $\rho \left(\mathbf{r}\right)].$ For small $\alpha \approx 0,$ the integrand $W_{\alpha }$ is obtained accurately from perturbation theory, but the perturbation expansion requires resummation for moderate and large $\alpha .$ For that purpose, we present density functionals for the coefficients in the asymptotic expansion $W_{\alpha }\to W_{\infty }{+W}_{\infty }^{\prime }{\alpha }^{-1/2}$ for $\overrightarrow{\alpha }\infty$ in the PC model. $W_{\infty }^{\mathrm{PC}}$ arises from strict correlation, and $W_{\infty }^{\prime \mathrm{PC}}$ from zero-point vibration of the electrons around their strictly correlated distributions. The PC values for $W_{\infty }$ and $W_{\infty }^{\prime }$ agree with those from a self-correlation-free meta-generalized gradient approximation, both for atoms and for atomization energies of molecules. We also (i) explain the difference between the PC cell and the exchange-correlation hole, (ii) present a density-functional measure of correlation strength, (iii) describe the electron localization and spin polarization energy in a highly stretched ${\mathrm{H}}_2$ molecule, and (iv) discuss the soft-plasmon instability of the low-density uniform electron gas.

Measurements of the linear polarization degree of the $5p{}^2{P}_{3/2}\to 8p{}^2{P}_{j^{\prime }}$ transitions ${(j}^{\prime }=1/2,3/2)$ in atomic ${}^{87}\mathrm{Rb}$ are reported. The radiative transitions are electric-dipole forbidden, but electric-quadrupole allowed. In addition, because of the relatively strong spin-orbit interaction in Rb, the $p-p$ transitions can acquire a magnetic-dipole transition strength that may be a measurable fraction of the electric-quadrupole strength. The polarization measurements can generally provide a different and sensitive way to extract the relative transition strengths. We have measured the linear polarization degree of the $5p{}^2{P}_{3/2}\to 8p{}^2{P}_{j^{\prime }}$ transitions over a wide range of Rb density. We report here observations of a different depolarization effect, whereby the measured polarization of the $5s{}^2{S}_{1/2}\to 5p{}^2{P}_{3/2}\to 8p{}^2{P}_{1/2}$ transition increases, while that for the $5s{}^2{S}_{1/2}\to 5p{}^2{P}_{3/2}\to 8p{}^2{P}_{3/2}$ transition decreases with increasing Rb density. The behavior is attributed to strong laser field effects associated with the $5s{}^2{S}_{1/2}\to 5p{}^2{P}_{3/2}$ resonance transition.

Absolute inclusive and exclusive cross sections for ionization of ${\mathrm{H}}^{-}$ and He by Compton scattering are presented and analyzed. The exclusive cross section for ionization of one electron, leaving the second electron in the ground state, is $20\%$ smaller for ${\mathrm{H}}^{-}$ than for He. Furthermore, the exclusive cross section for ionization with excitation to the first excited state is seven times larger for ${\mathrm{H}}^{-}$ than for He. However, at high energies the inclusive Compton cross sections for both ${\mathrm{H}}^{-}$ and He are within $1\%$ of two times the Thomson cross section for elastic scattering of a free electron, even when ground-state correlation is accurately included. A general analysis based on quasielastic scattering is applied to both shake and correlated calculations.

The angular distribution of the photoelectrons produced from a ground-state atom ionized under the combined action of two (or several) radiation fields can exhibit circular dichroism effects, when reversing the polarization state (right versus left, for instance) of one of the fields. We address the question of such dichroic effects in a currently explored scenario in which the atom is ionized in the presence of a strong laser field and of one (or several) of its higher harmonics.

A model is developed to explain the temperature dependence of the group velocity as observed in the experiments of Hau et al. [Nature (London) 397, 594 (1999)]. The group velocity is quite sensitive to the change in the spatial density. The inhomogeneity in the density and its temperature dependence are primarily responsible for the observed behavior.

A setup is proposed in which the number-state expansion of a one-mode traveling-wave optical field is truncated so as to leave its vacuum, one-, and two-photon components. The process is a realization of quantum teleportation on a three-state basis. The possibility of generalization to the first n components is also considered.

Multiple differential cross sections for $\mathrm{}(e,2e)\mathrm{}$ reactions are computed for ${\mathrm{H}}_2,$ ${\mathrm{D}}_2,$ and ${\mathrm{T}}_2$ targets at high-impact energies by using a first-order Born approximation within the formal theory. Results are compared with previously existing calculations. The influence of vibrational states in obtaining differential cross sections is determined for an asymmetric coplanar geometry.

The photoelectron-photoion coincidence method is shown to be very successful for the quantitative investigation of the ratio of radiative to nonradiative transitions and for the ratio of single to double Auger transitions. These decay probabilities are important parameters for analytical methods as well as for theoretical descriptions. The method is demonstrated for the neon $1s$ decay. Especially the fluorescence yield and the yield for the double Auger process have been determined both experimentally and theoretically.

We consider the model of a nondegenerate parametric oscillator, in the adiabatic limit of strongly damped pump mode, and find an exact analytic solution for the joint two-mode Wigner function in the steady-state regime. The one-mode Wigner functions for the individual signal and idler modes are derived as well. This gives an explicit and complete phase-space representation of this two-mode nonequilibrium quantum system. Simple examples are given illustrating the phase-space images of optical bistability and the phase diffusion effect.

Recently, in a very interesting article [Phys. Rev. A 56, 796 (1997)], Bhattacherjee has discussed the influence of nonlinearity in one-photon processes on the relationship between field and dipole squeezing in the two-level thermal Jaynes-Cummings model. Here we correct Eqs. (14)–(17) of that paper and exactly show that the squeeze period and duration in the case of $\mathrm{n̄}=0\mathrm{}$ and $\lambda \ne 0$ are inversely proportional to the nonlinear parameter $\lambda .$

We examine interference phenomena involving entangled pairs of quantum coherent states. Two-coherent-state interferometry, which can involve macroscopic numbers of photons, is shown to share several characteristics of two-particle quantum interferometry in such a macroscopic limit. These include the complementarity between one-system and two-system interference visibilities in the extreme cases of product and maximally entangled quantum states, and the violation of a Bell-type inequality.

Let $\{|{\psi }_1〉,\dots ,|{\psi }_n〉{;p}_1,\dots {,p}_n\}$ be an ensemble of pure quantum states. We show that it is possible to increase all of the pairwise overlaps $|〈{\psi }_i|{\psi }_j〉|,$ i.e., make each constituent pair of the states more parallel (while keeping the prior probabilities the same), in such a way that the von Neumann entropy S is increased, and dually, make all pairs more orthogonal while decreasing S. We show that this phenomenon cannot occur for ensembles in two dimensions but that it is a feature of almost all ensembles of three states in three dimensions. It is known that the von Neumann entropy characterizes the classical and quantum information capacities of the ensemble and we argue that information capacity, in turn, is a manifestation of the distinguishability of the signal states. Hence, our result shows that the notion of distinguishability within an ensemble is a global property that cannot be reduced to considering distinguishability of each constituent pair of states.

The photoionization cross sections of $5s$ electrons in Xe atom in the ${\mathrm{Xe}\mathrm{@}\mathrm{C}}_{60}$ endohedral complex have been calculated. It is shown that within the wide frequency region, from the threshold to the giant resonance, the cross section is qualitatively modified, acquiring additional resonances due to inner reflection of the photoelectron from the surrounding carbon atoms. The strong dependence of this effect upon the radius of the fullerene shell has been predicted.

We analyze the dependence on the exit angle of the energy loss suffered by the fragment protons resulting from the dissociation of swift ${\mathrm{H}}_2^{+}$ molecules after traversing thin carbon foils. The trajectory of each proton was followed by numerically solving the corresponding equations of motion, using the dielectric formalism to describe the forces due to electronic excitations in the medium, whereas the repulsion between protons from the same molecule was taken into account by using a Coulomb potential. Nuclear collisions with target nuclei were incorporated through a Monte Carlo code. The contribution of each one of the above interactions to the joint angle and energy-loss spectra of the fragment protons is discussed as a function of the molecule incident energy and foil thickness.

We consider the interaction of a traveling optical wave with a resonant two-level atom, which is initially in its general superposition state, i.e., in the superposition state of the ground and excited internal energy levels and simultaneously has mutually different momentum distributions, corresponding to each of these internal energy levels. We show that during the interaction, the atom periodically gets large-scale changes in these momentum distributions. As a consequence, a portion of the atomic momentum of the states, corresponding to each internal energy level, gets large-scale changes too, much more than the momentum $ħk$ of an absorbed/emitted photon. The amount of momentum that a photon transfers between the atomic internal energy levels is in general more than its own momentum $ħk.\mathrm{}$ The special case is discussed in which the atom’s preliminary superposition state is created as a result of interaction of the atom with the resonant standing wave. Also it is pointed out that in appropriate time intervals the mentioned phenomenon can be presented as a transformation of the resonant Kapitza-Dirac splitting of atomic states into the Stern-Gerlach-type splitting.