We present the results of a comprehensive analytical and numerical study of ultrawide spectral broadening and compression of isolated extremely short visible, uv-vuv and middle infrared (MIR) pulses by high-order stimulated Raman scattering in hollow waveguides. Spectral and temporal characteristics of the output pulses and the mechanism of pulse compression using dispersion of the gas filling and output glass window are investigated without the slowly varying envelope approximation. Physical limitations due to phase mismatch, velocity walk off, and pump-pulse depletion as well as improvements through the use of pump-pulse sequences and dispersion control are studied. It is shown that phase-locked pulses as short as $\sim 2\mathrm{fs}$ in the visible and uv-vuv, and 6.5 fs in the MIR can be generated by coherent scattering in impulsively excited Raman media without the necessity of external phase control. Using pump-pulse sequences, shortest durations in the range of about 1 fs for visible and uv-vuv probe pulses are predicted.

We present the results of a theoretical study of coherent x radiation induced by relativistic electrons in crystals for observation angles exceeding ${\gamma }^{-1},$ where $\gamma$ is the projectile Lorentz factor. The assumption for the process includes both elastic and inelastic radiation mechanisms. The results were compared with available experimental data.

We outline a procedure for Heisenberg-limited phase resolution between two Bose-Einstien condensates (BECs) defined as different hyperfine levels. The method involves first establishing a maximally entangled state using the ideas of nonlinear interferometry previously discussed in the optical domain [C. C. Gerry et al., Phys. Rev. A 66, 013804 (2002)]. In the case of the condensates, the nonlinear interactions are realized by the interatomic interactions within each condensate. Quarter cycle Raman pulses between hyperfine levels act as beam splitters. Parity measurements of one of the components of the BEC resolve the phase at the Heisenberg limit. We point out that parity measurements can be made by coupling the mode of interest with a third condensate where both components evolve under nonlinear interatomic interactions. After another Raman pulse, the components are populated according to parity. One need only determine which component is populated to determine the parity.

In a recent paper, Mihara [Phys. Rev. A 65, 052326 (2002)] presented several cryptographic protocols that were claimed to be quantum mechanical in nature. In this Comment, it is pointed out that these protocols can be described in purely classical terms. Hence, the security of these schemes does not rely on the usage of entanglement or any other quantum-mechanical property.

We show that the coined quantum walk on a line can be understood as an interference phenomenon, can be classically implemented, and indeed already has been. The walk is essentially two independent walks associated with the different coin sides, coupled only at initiation. There is a simple analogy between the evolution of walker positions and the propagation of light in a dispersive optical fiber.

By shaking a magnetic trap spatially, trapped cold Rb atoms are either heated or further cooled, depending on the shaking frequency. The atom temperature as a function of shaking frequency exhibits a dispersion-shaped resonance, which is caused by frequency selective energy absorption due to the anharmonicity of the trap potential. Selective heating of high-energy atoms enhances their loss, which can result in cooling rather than heating of a trapped atomic cloud.

Joint atom-photon states can become nonseparably entangled by momentum conservation in scattering events. The consequences are open for observation in interference experiments in which one particle is used to monitor the evolution of the other. We have previously quantified the degree of available recoil entanglement in spontaneous emission, and present here the extensions to Rayleigh scattering and Raman scattering, with an emphasis on the similarity among the three cases. It is found that such scattering processes have the potential to create a higher degree of entanglement between the scattered photon and the recoiled atom than any reported to date.

We report our theoretical and experimental investigations into errors in quantum-state estimation, putting a special emphasis on their asymptotic behavior. Tomographic measurements and maximum likelihood estimation are used for estimating several kinds of identically prepared quantum states (biphoton polarization states) produced via spontaneous parametric down-conversion. Excess errors in the estimation procedures are eliminated by introducing an estimation strategy utilizing Akaike’s information criterion. We make a quantitative comparison between the errors of the experimentally estimated states and their asymptotic lower bounds, which are derived from the Cramér-Rao inequality. Our results reveal the influence of entanglement on the errors in the estimation. An alternative measurement strategy employing inseparable measurements is also discussed, and its performance is numerically explored.

The separability and entanglement of quantum mixed states in $C^2\otimes C^3\otimes C^N$ composite quantum systems are investigated. It is shown that all quantum states $\rho$ with positive partial transposes and rank $r\left(\rho \right)<~N$ are separable.

We consider quantum teleportation as a tool to investigate the thermally entangled state of a three-qubit Heisenberg $\mathrm{XX}$ ring. Our investigation reveals interesting aspects of quantum entanglement not reflected by the pairwise thermal concurrence of the state. In particular, two mixtures of different pairs of W states, which result in the same concurrence, could yield very different average teleportation fidelities.

Differential-phase-shift quantum key distribution based on two nonorthogonal states is described. A weak coherent pulse train is sent from Alice to Bob, in which the phase of each pulse is randomly modulated by $\{0,\pi \}.$ Bob measures the differential phase by a one-bit delay circuit. The system has a simple configuration without the need for an interferometer and a bright reference pulse in Alice’s site, unlike the conventional QKD system based on two nonorthogonal states, and has an advantage of improved communication efficiency. The principle of the operation is successfully demonstrated in experiments.

A relationship between a recently introduced multipartite entanglement measure, state mixedness, and spin-flip symmetry is established for any finite number of qubits. It is also shown that, for those classes of states invariant under the spin-flip transformation, there is a complementarity relation between multipartite entanglement and mixedness. A number of example classes of multiple-qubit systems are studied in light of this relationship.

Spectral lines from the $3s-{3p}_{3/2}$ transitions in Na-like to P-like uranium have been measured at the Lawrence Livermore National Laboratory EBIT-I electron-beam ion trap facility. The measured x-ray energies are among the most accurate spectroscopic data available for highly charged heavy ions and provide important tests of relativistic atomic structure theory as well as bound-state quantum electrodynamic (QED) corrections in strong fields. Relativistic configuration-interaction (RCI) calculations based on the no-pair Hamiltonian with B-spline basis functions have been carried out to study these transitions. Our RCI energies, corrected by many-electron screened QED corrections also calculated here, yield theoretical results in very good agreement with experiment.

The polarization of the emitted photons is studied for the radiative recombination of free electrons into the bound states of bare, highly charged ions. We apply density matrix theory in order to investigate how the photon polarization is affected if the incident electrons are themselves spin polarized. For K-shell electron capture, for instance, the linear polarization of the light, which is measured out of the reaction plane, is defined by the degree of polarization of the electrons and may be used as a tool for studying the polarization properties of the electron targets and/or the projectile ions. Detailed computations of the Stokes parameters of x-ray emission following the radiative recombination of bare uranium ions ${\mathrm{U}}^{92+}$ are carried out for a wide range of projectile energies and for different polarization states of the incident electrons.

The time-dependent close-coupling method is used to calculate electron-impact excitation cross sections for the $\mathrm{Li}\mathrm{}\left(2s\right)\mathrm{}\to \mathrm{Li}\left(n\mathcal{l}\right)$ and $\mathrm{Li}\mathrm{}\left(2p\right)\mathrm{}\to \mathrm{Li}\left(n\mathcal{l}\right)$ transitions at incident energies just above the ionization threshold. The implementation of the time-dependent close-coupling method on a nonuniform lattice allows the study of continuum-coupling effects in excitations to high principal quantum number, i.e., $n<~10.$ Good agreement is found with R-matrix with pseudostates calculations, which also include continuum-coupling effects, for excitations to low principal quantum number, i.e., $n<~4.$ Poor agreement is found with standard distorted-wave calculations for excitations to all principal quantum numbers, with differences still at the 50% level for $n=10.\mathrm{}$ We are able to give guidance as to the accuracy expected in the $n^3$ extrapolation of nonperturbative close-coupling calculations of low n cross sections and rate coefficients.

We report on an experiment at the Paul Scherrer Institute, Villigen, Switzerland measuring x rays from muon transfer from deuterium to helium. Both the ground-state transfer via the exotic $(d{\mu }^{3,4}\mathrm{He}{)}^{*}$ molecules and the excited-state transfer from $(\mu {d)}^{*}$ were measured. The use of charge-coupled device detectors allowed x rays from 1.5 keV to 11 keV to be detected with sufficient energy resolution to separate the transitions to different final states in both deuterium and helium. The x-ray peaks of the $(d{\mu }^3\mathrm{He}{)}^{*}$ and $(d{\mu }^4\mathrm{He}{)}^{*}$ molecules were measured with good statistics. For the ${\mathrm{D}}_2{+}^3\mathrm{He}$ mixture, the peak has its maximum at $E_{d{\mu }^3\mathrm{He}}=6768\mathrm{}\pm 12\mathrm{eV}$ with full width at half maximum (FWHM) ${\Gamma }_{d{\mu }^3\mathrm{He}}=863\mathrm{}\pm 10\mathrm{eV}.$ Furthermore, the radiative branching ratio was found to be ${\kappa }_{d{\mu }^3\mathrm{He}}=0.301\mathrm{}\pm \mathrm{0.061.}$ For the ${\mathrm{D}}_2{+}^4\mathrm{He}$ mixture, the maximum of the peak lies at $E_{d{\mu }^4\mathrm{He}}=6831\mathrm{}\pm 8\mathrm{eV}$ and the FWHM is ${\Gamma }_{d{\mu }^4\mathrm{He}}=856\mathrm{}\pm 10\mathrm{eV}.$ The radiative branching ratio is ${\kappa }_{d{\mu }^4\mathrm{He}}=0.636\mathrm{}\pm \mathrm{0.097.}$ The excited-state transfer is limited by the probability to reach the deuterium ground state, $q_{1s}.$ This coefficient was determined for both mixtures: $q_{1s}^{{}^3\mathrm{He}}=68.9\mathrm{}\pm 2.7$% and $q_{1s}^{{}^4\mathrm{He}}=90.1\mathrm{}\pm 1.5\%.$

The angle-resolved energy spectra of the electrons emitted from a metal surface during specular reflection of heavy ions have been studied by means of the first-order, time-dependent perturbation theory, with the electron states obtained from a finite step-barrier potential model. The surface response has been described by the dielectric formalism within the specular reflection model, where the local-field correction and the plasmon-pole approximation have been used for the dielectric function in the cases of low and high ion velocities, respectively. The ion trajectory has been evaluated based on the surface continuum potential and the ion image potential, whereas the screening of the ion by the bound electrons has been taken into account by means of the Brandt-Kitagawa model. It has been found that the electron spectra are dominated by the decay of surface plasmons for fast incident ions, whereas the electron emission is dominated by the single-electron excitation mechanism for slow incident ions.

Clusters were produced as a result of argon gas cooling during expansion through a supersonic nozzle. A two-dimensional model was set up in order to calculate gas expansion and partial condensation into clusters. Calculations were validated by experimental measurements using Mach-Zehnder interferometry and Rayleigh scattering, and performed with two types of nozzles (Laval and conical nozzles). These optical diagnostics together with numerical simulations led to the cluster size and density determination with spatial resolution through the gas and cluster jet. Cluster production was observed to be very sensitive to the nozzle geometry. Homogeneous gas and cluster jets were produced and characterized using conical nozzle geometry, with cluster density about ${10}^{12}{\mathrm{per}\mathrm{}\mathrm{cm}}^3.$ Due to the fast valve-nozzle connecting geometry, shock waves have been observed at the Laval nozzle throat that strongly affected cluster production on the jet axis. Averaged cluster radius was observed to be easily tunable from 180 to 350 Å by varying the upstream gas pressure $P_0$ from 20 to 60 bars. A different scaling law, versus $P_0,$ has been observed for this regime of large cluster, compared to Hagena’s predictions for the small cluster regime.

We have probed the fragmentation dynamics in a bent triatomic molecule (water), a nonplanar molecule (methanol), and a planar ring-structured molecule (benzene), using 100 fs duration pulses of linearly and circularly polarized, infrared, intensity-selected laser light. At laser intensities larger than ${10}^{15}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2},$ the yield of singly and multiply charged atomic fragments from these molecules is suppressed when the light is circularly polarized. At lower intensities, the fragment ion yield is not significantly polarization dependent. This hitherto-unobserved intensity-dependent effect of the polarization state of light on the fragmentation dynamics is rationalized using a simple electron-rescattering model. Circular polarization switches “off” electron rescattering and leads to suppression of multiple ionization and molecular fragmentation. Moreover, the degree of suppression is dependent upon the amount of energy transfer from the optical field to the molecule: the larger the energy transfer that is required for a particular fragmentation channel, the more marked is its suppression when circular polarization is used.