We have measured the spectrum of the amplified spontaneous emission of two coupled vertical-cavity semiconductor lasers. The spectrum shows a triplet structure consisting of a central peak at the lasing frequency and two sidebands. Theoretical analysis shows that the sidebands are due to spatial relaxation oscillations; the triplet structure is a result of spatial hole burning which occurs due to interference between the lasing supermode and the fluctuations in the nonlasing supermode.

We study the influence of the transverse structure of pump and lasing fields and of the width of the doped region on the conditions for the appearance of the multimode instability in an ${\mathrm{Er}}^{3+}$-doped ring fiber laser. We show that the instability can be inhibited while maintaining a large output power when the radius of the doped region is a fraction of the core radius.

Calculations are presented of the time-dependent dispersion of a cluster medium interacting with an intense, femtosecond laser, where the clusters are “exploding” on the time scale of the laser pulse. It is shown that the dispersion of the clusters can transiently compensate (or buffer) the dispersion of free electrons to dramatically improve the phase matching of water-window high-order harmonic generation in the presence of strong ionization. A practical implementation of this phase-matching scheme is described.

We present an alternative scheme for the generation of a two-qubit quantum gate interaction between laser-cooled trapped ions. The scheme is based on the ac Stark shift (light shift) induced by laser light resonant with the ionic transition frequency. At specific laser intensities, the shift of the ionic levels allows the resonant excitation of transitions involving the exchange of motional quanta. We compare the performance of this scheme with respect to that of related ion-trap proposals and find that, for an experimental realization using traveling-wave radiation and working in the Lamb-Dicke regime, an improvement of over an order of magnitude in the gate switching rate is possible.

We suggest a scheme of conditional teleportation of quantum states of optical fields using squeezers and photon counting. Alice feeds the mode whose state is desired to be teleported and one mode of a two-mode squeezed vacuum into a parametric amplifier, and detects output photon numbers. The result is then communicated to Bob, who shifts the photon number of his part accordingly. We show that for some classes of states the method can yield, with reasonable success probability, a teleportation fidelity close to unity. The method represents a modification of a recently proposed scheme [G. J. Milburn and S. L. Braunstein, Phys. Rev. A 60, 937 (1999)], where measurements of the photon-number difference and the phase sum are considered. The present scheme brings the method closer to the current experimental techniques.

The time evolution of the muon spin in fully anisotropic muonium $(\mathrm{Mu}={\mu }^{+}{+e}^{-})$ in the presence of Heisenberg spin exchange has been investigated theoretically. First, the energy levels of anisotropic Mu as a function of field are investigated analytically with a particular emphasis on the crossing and avoidance of energy levels at certain magnetic fields, which have important consequences in muon spin dynamics. Second, the knowledge of the energy levels is applied to investigate the muon spin depolarization due to electron spin exchange with spin-$\frac{1}{2}$ paramagnetic species, where the muon spin depolarization rate and the precession amplitude observed by the muon-spin-rotation (μSR) technique are explicitly expressed solely in terms of the matrix that diagonalizes the anisotropic Mu hyperfine Hamiltonian. The treatment presented here represents a special systematic and practical method that allows one to investigate the time evolution of the muon spin in anisotropic Mu in the presence of electron spin exchange. Several concrete examples are discussed in detail, including those in which all the μSR observables can be obtained analytically. The method developed in this work is used to explain the relaxation rate maximum in anisotropic Mu in semiconductors observed at the longitudinal fields at which two of the Mu energy levels avoid each other due to a strong level mixing or avoidance, where the present formalism takes the tensor nature of the anisotropic hyperfine interaction fully into account without invoking the convenient but not necessarily correct notion of an effective magnetic field in an anisotropic Mu. Also discussed is the possibility of observing additional relaxation maximum at a low-avoidance field, where the effective magnetic-field approximation completely breaks down. Observation of such a maximum will provide valuable information on the parameters characterizing the anisotropic Mu in question. The formalism presented here can also be applied to anisotropic positronium on surfaces, anisotropic Mu undergoing both charge exchange and spin exchange, and fast spin exchange.

We have measured hyperfine-structure splittings and isotope shifts in the ${2p}^{3}3s{}^5{S}_2\to {2p}^{3}3p{}^5{P}_{1,2,3}$ transitions of atomic oxygen using Doppler-free saturated absorption spectroscopy near 778 nm. Values for the hyperfine-structure coupling constants of the ${}^5{S}_2$ and ${}^5{P}_{1,2,3}$ states in ${}^{17}\mathrm{O}$ and the ${}^{18}{\mathrm{O}\mathrm{-}}^{17}\mathrm{O}$ and ${}^{18}{\mathrm{O}\mathrm{-}}^{16}\mathrm{O}$ isotope shifts of the ${2p}^{3}3s{}^5{S}_2\to {2p}^{3}3p{}^5{P}_{1,2,3}$ transitions were found. The set of values for hyperfine-structure coupling constants and isotope shifts is more complete and generally more precise than previously reported results.

Resonant ion-pair formation from collisions of electrons with electronic and vibronic ground-state diatomic molecular ions has been studied in the present work for ${\mathrm{HD}}^{+}$ and ${\mathrm{OH}}^{+}.$ The cross section for ${\mathrm{HD}}^{+}$ has a magnitude of the order of $3\times {10}^{-19}\hspace{0.5em}{\mathrm{cm}}^2$ and is characterized by an energy threshold and 14 resolved peaks in the energy range up to 16 eV. A theoretical study confirms that the structures derive primarily from quantum interference of the multiple dissociation pathways. Measurements for ${\mathrm{OH}}^{+}$ reveal that the cross section for ${\mathrm{H}}^{+}$ and ${\mathrm{O}}^{-}$ formation is lower than ${10}^{-21}\hspace{0.5em}{\mathrm{cm}}^2$ at energies of 6 and 12 eV.

Dissociative ionization of the ${\mathrm{D}}_2$ molecule was studied using linearly polarized 400- and 800-nm laser pulses. At 800 nm, the ${\mathrm{D}}^{+}$ ions resulting from the dissociation of the molecule are observed to be parallel to the polarization of the laser field. At the shorter wavelength (400 nm), we observe that a considerable number of ${\mathrm{D}}^{+}$ ions are ejected perpendicular to the polarization of the laser field. This observation is interpreted in terms of the radiative coupling between the ground $1s{\sigma }_g$ and the high-lying $3d{\sigma }_g$ and $3p{\pi }_u$ electronic states of ${\mathrm{D}}_2^{+}.$ This interpretation was verified by comparing the experimentally measured and theoretically predicted kinetic energy of the ${\mathrm{D}}^{+}$ ions. Our results indicate that the radiative coupling between the ground and highly excited electronic states must be considered when intense laser fields with high frequencies are used.

The rate coefficients for the dissociative charge-transfer reaction of ${\mathrm{He}}^{+}+{\mathrm{N}}_2\to$ products and ${\mathrm{He}}^{+}+{\mathrm{CH}}_4\to$ products are measured using ion storage. Helium ions were produced by electron bombardment of helium gas. The ion population in the trap was measured as a function of the storage time to give the reaction rate. The rate coefficient for ${\mathrm{He}}^{+}$ and ${\mathrm{N}}_2$ is $1.07(\pm 0.11)\times {10}^{-9}\hspace{0.5em}{\mathrm{cm}}^3{\mathrm{s}}^{-1},$ and the rate coefficient for ${\mathrm{He}}^{+}$ and ${\mathrm{CH}}_4$ is $1.73(\pm 0.29)\times {10}^{-9}\hspace{0.5em}{\mathrm{cm}}^3{\mathrm{s}}^{-1}.$ The equivalent temperature for the two reactions was estimated to be 8700 and 8000 K, respectively. Our results are essentially the same as those obtained previously at 300 K.

We investigated the emission of K Auger electrons from collisions of hydrogenlike ions ${\mathrm{C}}^{5+},$ ${\mathrm{N}}^{6+},$ and ${\mathrm{O}}^{7+}$ with graphite as well as fullerene covered gold surfaces. Besides the quite well understood Auger electrons emitted from the projectile, an extremely high yield of Auger electrons originating from surface atoms is observed. Remarkably the target Auger spectra show discrete (atomic) KLL lines superimposed on the broad KVV spectra originating from bulk carbon. This indicates high-sputtering yields of highly excited carbon ions/atoms with K-shell vacancies.

In this work we treat inelastic scattering of fast electrons on metal clusters in the range of transferred energies above the ionization threshold. We demonstrate that in this energy range many-electron collective excitations, namely, the volume plasmons, provide dominating contribution to the differential cross section resulting in its resonance behavior. The volume-plasmon resonances excited in the cluster during the collision decay via the ionization process. We determine the resonance frequency and the autoionization width of the volume plasmon excitations. In order to elucidate the role of plasmon excitations, we calculate the differential cross section of the process using two different approaches. We calculate the cross section in the random-phase approximation using the Hartree-Fock wave functions as a basis and accounting simultaneously for all single-particle and collective excitations in the cluster. We compare the results of this approach with those obtained in the plasmon resonance approximation.

We present a direct comparison between calculated and measured ionization in Li and Rb when an initially bound Rydberg electron is stripped from the atom by ramping an electric field [0.415 V/(cm ns)]. We describe the method used to calculate the field dependence of the ionization; the method can be used to evolve the population from field strengths where three or more n manifolds cross to the fields where the electron is stripped from the atom. The essential feature of the method is the repeated application of the Landau-Zener approximation for all of the level crossings as the field is ramped. We also give a description of the dynamics of Li and Rb in the ramping field. We observe that the Li selective field ionization is sensitive to the Stark levels within an n manifold, while for Rb only an n dependence is observed.

In experiments conducted recently at MIT on Na Bose-Einstein condensates [S. Inouye et al., Nature (London) 392, 151 (1998); J. Stenger et al., Phys. Rev. Lett. 82, 2422 (1999)], large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of a pair of atoms in the condensate. A collisional deactivation mechanism affecting a temporarily formed molecular condensate [see V. A. Yurovsky, A. Ben-Reuven, P. S. Julienne and C. J. Williams, Phys. Rev. A 60, R765 (1999)], studied here in more detail, accounts for the results of the slow-sweep experiments. A best fit to the MIT data yields a rate coefficient for deactivating atom-molecule collisions of $1.6\times {10}^{-10}\hspace{0.5em}{\mathrm{cm}}^3/\mathrm{s}.$ In the case of the fast-sweep experiment, a study is carried out of the combined effect of two competing mechanisms, the three-atom (atom-molecule) or four-atom (molecule-molecule) collisional deactivation versus a process of two-atom trap-state excitation by curve crossing [F. H. Mies, P. S. Julienne, and E. Tiesinga, Phys. Rev. A 61, 022721 (2000)]. It is shown that both mechanisms contribute to the loss comparably and nonadditively.

A fully quantum model is used to derive both the intensity noise of a laser operating with an electronic feedback loop and the nonlinear corrections to the noise of the free-running laser. The implementation of a nonstandard feedback loop on a ${\mathrm{N}\mathrm{d}:\mathrm{Y}\mathrm{V}\mathrm{O}}_4$ microchip laser pumped with an amplitude squeezed diode laser permits us to decrease the intensity of the relaxation oscillation noise peak while leaving the low-frequency part of the spectrum unaffected. This procedure also results in a decrease of the noise in the low-frequency region, showing evidence for nonlinear effects in the noise spectrum. The latter are found to be in good agreement with the theoretical predictions. Good agreement between the experimental results and theoretical predictions is also found over the complete frequency range.

We report on an initial squeezing of 3.3 dB below the shot noise level, generated in bulk, quasi-phase-matched lithium niobate. This squeezing was generated by degenerate parametric amplification, at 1047 nm, of a mode-locked pulse train. We present a detailed comparison of the process performed in both the quasi-phase-matched material and in ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_3,$ which was phase matched using conventional birefringent methods. The single-pass gain measured was 2.5 from ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_3$ and 3.9 from periodically poled lithium niobate (PPLN), although the former was affected by photorefractive damage. We directly recorded $11\%$ of noise reduction in the vacuum using the PPLN parametric amplifier, purely because the detection efficiency was so low in our experiment. The experimental data was fitted to a model which includes the initial squeezing level and the homodyne detector efficiency as free parameters. The observed squeezing agrees well with the a total detection efficiency of just $16\%$ and an initial squeezing level of 3.3 dB (53%) in PPLN.

We report on progress in the optimization and understanding of the collisional pumping of x-ray lasers in the transient regime using an ultrashort subpicosecond heating pulse. An irradiation scheme using a frequency-doubled 600-ps laser pulse to preform the plasma is tested. The effect of traveling-wave irradiation on the time-integrated and time-resolved lasing signal at the $4d{}^1{S}_0\to 4p{}^1{P}_1$ Ni-like Ag line is studied in detail. Under specific irradiation conditions, strong lasing is also obtained on another spectral line at 16.05 nm that is identified as the $4f{}^1{P}_1\to 4d{}^1{P}_1$ transition in Ni-like Ag.

The dynamics of the driven Jaynes-Cummings model where a two-level atom is coupled to a single mode of cavity fields and is driven by an external coherent field is investigated. We find that in this model quantum collapse and revivals of the atomic population inversion and squeezing of the cavity field can be generated from a thermal photon state. We also notice that the squeezing is very sensitive to the mean number of thermal photons initially in the cavity, but the quantum collapse and revivals are not. The influence of the amplitude of the driving field and various detuning parameters on the nonclassical effects is discussed. The result shows that one can use the external field to control nonclassical effects in the Jaynes-Cummings model.

We propose a reliable scheme for engineering a general cavity-field state. This is different from recently presented strategies, where the cavity is supposed to be initially empty and the field is built up photon by photon through resonant atom-field interactions. Here, a coherent state is previously injected into the cavity. So, the Wigner distribution function of the desired state is constructed from that of the initially coherent state. Such an engineering process is achieved through an adaptation of the recently proposed technique of projection synthesis to cavity QED phenomena.

We consider a special kind of mixed states—a Werner derivative, which is the state transformed by nonlocal unitary—local or nonlocal—operations from a Werner state. We show the following: (i) The amount of entanglement of Werner derivatives cannot exceed that of the original Werner state. (ii) Although it is generally possible to increase the entanglement of a single copy of a Werner derivative by local quantum operations and classical communication, the maximal possible entanglement cannot exceed the entanglement of the original Werner state. The extractable entanglement of Werner derivatives is limited by the entanglement of the original Werner state.