We study the influence of electronic scattering on the angular distributions of ions and atoms moving through a free-electron gas. We present a general formulation based on the multiple-scattering formalism, using two alternative descriptions: a linear approach based on Lindhard’s dielectric function, and a nonlinear treatment based on density functional calculations. We obtain analytical expressions for the multiple-scattering function and for the half-width of the distribution. We show the results of several calculations for slow hydrogen and helium and their isotopes. We analyze the results and obtain analytical approximations for all of the cases. The magnitude of the electronic effects on the multiple scattering distribution is determined and compared with previous estimations.

Absolute measurements are presented for the positron-impact cross sections for positronium formation, direct ionization, and total ionization of the diatomic molecules ${\mathrm{N}}_2$, CO, and ${\mathrm{O}}_2$, in the range of energies from threshold to $90\mathrm{eV}$. Cross sections for the electronic excitation of the $a{}^1\Pi$ and $a^{\prime }{}^1\Sigma$ state in ${\mathrm{N}}_2$ and the $A{}^1\Pi$ state in CO near threshold are also presented. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. In ${\mathrm{O}}_2$, a feature previously seen in the total ionization cross section is observed in both the positronium formation and total ionization cross sections. The possible origin of this feature and its relationship to positron-induced dissociation is discussed. In ${\mathrm{N}}_2$, the near-threshold electronic excitation cross section is larger than that for positronium formation. This likely explains the relatively high efficiency of this molecule when used for buffer-gas positron trapping.

The excitation lineshift and linewidth of the principal resonance line of Na embedded in liquid He have been obtained using combined Monte Carlo (MC) simulation and quantum mechanics (QM) calculations. The metropolis MC simulation used interatomic potentials obtained from high-level quantum mechanics results. Using the structures of the simulation statistically relevant configurations are sampled for subsequent QM calculations. The transition wavelengths for the $3s\to 3p$ principal resonance line of atomic Na were obtained using time-dependent density-functional-theory calculations of the central alkali-metal atom surrounded by the first solvation shell composed of 42 He atoms. The widths are obtained by the statistical distribution of calculated transitions. Three different functionals were used. Statistically converged results using the $\mathrm{PBE}1\mathrm{PBE}∕6\text{−}311++\mathrm{G}(\mathrm{d},\mathrm{p})$ give a blueshift of 14.7 nm and a width at half maximum of 7.8 nm. Similar results are obtained using the Beeke-Lee-Yang-Parr three parameter exchange functional and the Becke three-parameter exchange with the Perdew nonlocal correlation functional models. These results seem to give a reasonable statistical representation of the structure of the He cavity enclosing the host Na atom and the consequent solvent effect on the $3s\to 3p$ excitation.

We report calculated cross sections for the electronic excitation of carbon monoxide by positron impact. The calculations were carried out with the Schwinger multichannel method and included six collision channels, namely the ground $\left(X{}^1\Sigma ^{+}\right)$ and five electronically excited ($A{}^1\Pi$, $I{}^1\Sigma ^{-}$ and $D{}^1\Delta$) states. Present calculated excitation cross sections to the $A{}^1\Pi$ state did not present any resonant features, being in this sense consistent with previous calculations for the isoelectronic nitrogen molecule. The experimental $a{}^1\Pi _g$ excitation cross section of ${\mathrm{N}}_2$ presented a resonantlike structure which would also be expected in the $A{}^1\Pi$ excitation of CO. We discuss possible reasons for the disagreement between experiment and theory.

We propose an approach to all-optical frequency standard design, based on a counterintuitive combination of the coherent population trapping effect and signal discrimination at the maximum of absorption for the probe radiation. The short-term stability of such a standard can achieve the level of ${10}^{-14}∕\sqrt{\tau }$. The physics beyond this approach is a dark resonance splitting caused by the interaction of the nuclear magnetic moment with the external magnetic field.

We have studied collisional perturbations of radio-frequency (rf) electric-dipole $\left(E1\right)$ transitions between the nearly degenerate opposite-parity levels in atomic dysprosium (Dy) in the presence of 10 to $80\mathrm{\mu }\mathrm{Torr}$ of ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, He, Ar, Ne, Kr, and Xe. Collisional broadening and shift of the resonance, as well as the attenuation of the signal amplitude are observed to be proportional to the foreign-gas density with the exception of ${\mathrm{H}}_2$ and Ne, for which no shifts were observed. Corresponding rates and cross sections are presented. In addition, rates and cross sections for ${\mathrm{O}}_2$ are extracted from measurements using air as foreign gas. The primary motivation for this study is the need for accurate determination of the shift rates, which are needed in a laboratory search for the temporal variation of the fine-structure constant [A. T. Nguyen, D. Budker, S. K. Lamoreaux, and J. R. Torgerson, Phys. Rev. A 69, 22105 (2004)].

We present a review of properties of ultracold atomic Fermi-Bose mixtures in inhomogeneous and random optical lattices. In the strong interacting limit and at very low temperatures, fermions form, together with bosons or bosonic holes, composite fermions. Composite fermions behave as a spinless interacting Fermi gas, and in the presence of local disorder they interact via random couplings and feel effective random local potential. This opens a wide variety of possibilities of realizing various kinds of ultracold quantum disordered systems. In this paper we review these possibilities, discuss the accessible quantum disordered phases, and methods for their detection. The discussed quantum phases include Fermi glasses, quantum spin glasses, “dirty” superfluids, disordered metallic phases, and phases involving quantum percolation.

We show theoretically that it is possible to generate laser light based on two-photon and other high-order multiphoton processes when an atomic beam of optically driven potassium atoms crosses a high-finesse optical cavity. We use a rigorous model that takes into account all the atomic substates involved in the optical interactions and is valid for any drive and lasing field intensities. The polarizations of the drive and lasing fields are assumed to be fixed. Stable and unstable laser emission branches are obtained, which are represented as a function of cavity detuning and are analyzed in terms of the fundamental quantum processes yielding them. Closed-curve laser-emission profiles are obtained for multiphoton lasing based on processes involving more than one lasing photon. Two-photon laser emission branches show relatively long segments of stationary emission, combined in general with some segments of nonstationary emission, or with segments of mixture with three-photon emission processes. Rayleigh and hyper-Rayleigh processes can become simultaneously resonant, entailing in such case a large and fast transfer of population from the atomic initial ground sublevel to other ground sublevels with different $z$ components of the total angular momentum. They could be useful in generating multiphoton correlated field states. In all cases the largest laser emission intensities are obtained from the highest-order processes, rather than the lowest. These results open the way to the understanding of experiments performed in the past years and suggest possibilities for more efficient and varied types of multiphoton laser operation.

We show experimentally how the spatial shape of the pump beam controls the ellipticity of the spatial mode function in noncollinear spontaneous parametric down-conversion. The degree of ellipticity depends on the pump beam width, especially for highly focused beams. We introduce an effective length, the so-called noncollinear length, that determines the importance of the ellipticity of the spatial mode function. We also discuss the ellipticity induced by the spectrum of the pump beam.

Recent work has shown that a simple chain of interacting spins can be used as a medium for high-fidelity quantum communication. We describe a scheme for quantum communication using a spin system that conserves $z$ spin, but otherwise is arbitrary. The sender and receiver are assumed to directly control several spins each, with the sender encoding the message state onto the larger state space of her control spins. Given that the encoding for the “zero” message basis state is chosen to be the all-spin-down state, we show how to find the encoding for the “one” basis state that maximizes the fidelity of communication, using a simple method based on the singular-value decomposition. Also, we show that this solution can be used to increase communication fidelity in a rather different circumstance: where no encoding of initial states is used, but where the sender and receiver control exactly two spins each and vary the interactions on those spins over time. The methods presented are computationally efficient, and numerical examples are given for systems having up to 300 spins.

In the paper published in Phys. Rev. A 65, 052331 (2002), an entanglement-based quantum key distribution protocol for $d$-level systems was proposed. However, in this Comment, it is shown that this protocol is insecure for a special attack strategy.

In the preceding Comment it has been claimed that an entangled-based quantum key distribution protocol proposed by Zhang, Li, and Gao [Phys. Rev. A 64, 024302 (2001)] and its generalization to $d$-level systems by Karimipour, Bahraminasab, and Bagherinezhad [Phys. Rev. A 65, 052331 (2002)] are insecure against an attack devised by the authors of the Comment. We invalidate the arguments of the Comment and show that the protocols are still secure.

Two noninteracting quantum systems which couple to a common environment with many degrees of freedom initially in thermal equilibrium can become entangled due to the indirect interaction mediated through this heat bath. I examine here the dynamics of reservoir-induced entanglement for a heat bath consisting of a thermal electromagnetic radiation field, such as blackbody radiation or the cosmic microwave background, and show how the effect can be understood as result of an effective induced interaction.

We have implemented an experimental setup in order to demonstrate the feasibility of time-coding protocols for quantum key distribution. Alice produces coherent 20-ns faint pulses of light at 853 nm. They are sent to Bob with delay 0 ns (encoding bit 0) or 10 ns (encoding bit 1). Bob directs at random the received pulses to two different arms. In the first one, a 300 ps resolution Si photon counter allows Bob to precisely measure the detection times of each photon in order to establish the key. Comparing them with the emission times of the pulses sent by Alice allows one to evaluate the quantum bit error rate (QBER). The minimum obtained QBER is 1.62%. The possible loss of coherence in the setup can be exploited by Eve to eavesdrop the line. Therefore, the second arm of Bob setup is a Mach-Zehnder interferometer with a 10 ns propagation delay between the two paths. Contrast measurement of the output beams allows one to measure the autocorrelation function of the received pulses that characterizes their average coherence. In the case of an ideal setup, the value expected with the pulses sent by Alice is 0.576. The experimental value of the pulses autocorrelation function is found to be 0.541. Knowing the resulting loss of coherence and the measured QBER, one can evaluate the mutual information between Alice and Eve and the mutual information between Alice and Bob, in the case of intercept-resend attacks and in the case of attacks with intrication. With our values, Bob has an advantage on Eve of 0.43 bit per pulse. The maximum possible QBER corresponding to equal informations for Bob and Eve is 5.8%. With the usual attenuation of fibres at 850 nm, it shows that secure key distribution is possible up to a distance of 2.75 km, which is sufficient for local links.

We studied mutual ionization in $200\text{−}\mathrm{keV}$ ${\mathrm{H}}^{-}+\mathrm{He}$ collisions in a kinematically complete experiment by measuring the fully momentum-analyzed recoil ions and both active electrons in coincidence. Comparison of the data to our calculations, based on various theoretical models, show that mutual ionization proceeds predominantly through the interaction between both electrons. The post-collision interaction between the outgoing ejected electrons as well as higher order processes involving the interaction between the core of both collision partners are also important.

Recent experiments have demonstrated that highly charged ions can be guided through insulating nanocapillaries along the direction of the capillary axis for a surprisingly wide range of injection angles. Even more surprisingly, the transmitted particles remain predominantly in their initial charge state, thus opening the pathway to the construction of ion-optical elements without electric feedthroughs. We present a theoretical treatment of this self-organized guiding process. We develop a classical trajectory transport theory that relates the microscopic charge-up with macroscopic material properties. Transmission coefficients, angular spread of transmitted particles, and discharge characteristics of the target are investigated. Partial agreement with experiment is found.

A system of trapped ions under the action of off-resonant standing waves can be used to simulate a variety of quantum spin models. In this work, we describe theoretically quantum phases that can be observed in the simplest realization of this idea: quantum Ising and $XY$ models. Our numerical calculations with the density matrix renormalization group method show that experiments with ion traps should allow one to access general properties of quantum critical systems. On the other hand, ion trap quantum spin models show a few unusual features due to the peculiarities of induced effective spin-spin interactions which lead to interesting effects like long-range quantum correlations and the coexistence of different spin phases.