We compare high-order harmonic generation in gas jets of benzene, cyclohexane, and xenon using ∼800-nm laser pulses of 70 fs and 240 fs duration. The harmonic intensities of the 7th to 15th orders are studied as a function of laser intensity in the range $5\times {10}^{13}$ to $5\times {10}^{15}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ Using 240-fs pulses, the dependence of the harmonic intensity on the laser intensity of the organic molecules exhibits significant deviations from the behavior of atoms, and shows strong evidence for harmonic generation from molecular fragments as well as the parent molecule. When the pulse duration is reduced to 70 fs, the intensity dependences of the harmonics are similar to those of xenon. This is attributed to the significant reduction in dissociation occurring during the shorter pulse and suggests that high-order harmonic generation in the ultrafast regime will be important in the understanding of molecular electron dynamics in strong fields.

We theoretically study reflection and transmission of light in a one-dimensional disordered phase-conjugating medium. Using an invariant imbedding approach a Fokker-Planck equation for the distribution of the probe light reflectance and expressions for the average probabilities of reflection and transmission are derived. A new crossover length scale for localization of light is found, which depends on the competition between phase conjugation and disorder. For weak disorder, our analytical results are in good agreement with numerical simulations.

Intracavity and external third order correlations in the damped nondegenerate parametric oscillator are calculated for quantum mechanics and stochastic electrodynamics (SED), a semiclassical theory. The two theories yield greatly different results, with the correlations of quantum mechanics being cubic in the system’s nonlinear coupling constant and those of SED being linear in the same constant. In particular, differences between the two theories are present in at least a mesoscopic regime. They also exist when realistic damping is included. Such differences illustrate distinctions between quantum mechanics and a hidden variable theory for continuous variables.

The double-minimum $2{}^1{\Sigma }_u^{+}$ state was investigated in ${}^{39}{\mathrm{K}}_2$ and ${}^{39}{\mathrm{K}}^{41}\mathrm{K}$ molecules by analyzing spectra of the $2{}^1{\Sigma }_u^{+}\leftarrow X{}^1{\Sigma }_g^{+}$ band system, simplified by polarization labeling. A wide range of rovibrational levels 21⩽v⩽97, 18⩽J⩽159 was observed. By using the inverted perturbation approach to construct the exotic potential-energy curve, an accurate, isotopically consistent description of the $2{}^1{\Sigma }_u^{+}$ state has been obtained.

A two-state model applicable to top-of-barrier processes in ion-atom collisions at low velocity $v$ is investigated. An expression for the Massey parameter of the transition $\overrightarrow{n}n+1\mathrm{}$ is derived and is used to compute ionization, capture, and excitation from states of high principal quantum number in ${\mathrm{H}}^{+}+\mathrm{H}$ collisions. The Wannier law is obtained at the ionization threshold. The dependence of ionization on the initial n is computed. Total capture cross sections are in good agreement with other calculations, but an unreasonably large fraction goes to resonant capture. It is concluded that multicrossing models, which do not incorporate interference between different paths to the resonant state, overestimate resonant capture.

We report an experiment on recoil-ion momentum spectroscopy of ${\mathrm{Rb}}^{+}$ ions formed by two-photon ionization of ${\mathrm{Rb}}^2{P}_{3/2}$ at 790 nm. Laser-cooled atoms in a magneto-optic trap are used as the target in time-of-flight analysis of the recoil momentum. We demonstrate a resolution of 1 μeV with a limiting momentum resolution below $\delta p=0.05\mathrm{}$ a.u. We also show the use of the photoionization technique for the characterization of cold atomic samples.

We analyze the zero-frequency intensity noise of a single-mode semiconductor laser on the basis of a full quantum-mechanical model which takes into account the carrier distributions over the k states. The theory includes the effects of (i) Coulomb scattering processes, which tend to drive the carriers into intraband Fermi-Dirac quasiequilibrium; (ii) spectral hole burning, i.e., the carrier deviation from quasiequilibrium induced mainly by light-matter interaction; (iii) pump blocking, which, in conformity with the Pauli exclusion principle, prevents the pump carriers whose k state is already occupied from entering the active layer. Our analysis shows that Coulomb scattering and spectral hole burning have only negligible effects on the noise properties of the laser, while pump blocking can bring, under appropriate parametric conditions, a sizable increase of noise intensity. This additional noise can explain why quietly pumped semiconductor lasers fail to reach the perfect squeezing which is expected on the basis of the theory so far available. The squeezing-hindering effect of pump blocking is confirmed as different pumping schemes are employed.

We propose a model for the quantum-state measurement via phase-sensitive amplification. The basic idea is to amplify the quantum state through a two-photon correlated-emission laser, such that there is no noise in the quadrature of interest and all the noise is fed into the conjugate quadrature. The noise-free quadrature is prepared in different phases and then corresponding quadrature distribution is measured. The Wigner function of the initial quantum state is then reconstructed by carrying out inverse Radon transformation familiar in tomographic imaging. This scheme allows us to avoid the deterioration of homodyne detection measurement due to the problem of detector efficiency.

As shown by Hegerfeldt, positivity of energy in quantum mechanics leads to nonlocal effects, where a state that is initially localized immediately develops infinite tails. In a previous paper we have described the mechanism of such effects for relativistic free fields. In this paper we discuss the nonlocal effects for a more complex system that consists of a two-level atom interacting with a radiation field. We study the dressing process of the bare ground state in terms of the photon density. In this process there appears a nonlocal cloud that remains around the atom and “Zeno” photons that move away from the atom with the speed of light. Nonlocality is seen to be to due to a “curtain” effect where the cloud is initially covered by the Zeno photons, and immediately afterwards is uncovered as the photons move away from the atom. No superluminal speeds are involved. We also consider nonlocal effects associated with a partially dressed state where the atom is excited starting from the true ground state. The excitation of the atom produces a nonlocal rearrangement of the photon cloud, which leads to precursors to the usual retarded field.

The average dwell time of an arbitrary wave packet within the interaction region, in nonrelativistic one-dimensional quantum scattering, is obtained. The results are applied to tunneling, yielding a new derivation and reinterpretation of the Büttiker-Landauer barrier interaction times (for a rectangular barrier, of the Larmor times), entirely within the framework of standard quantum measurement theory.

Nonclassical correlations between the quadrature-phase amplitudes of two spatially separated optical beams are exploited to realize a two-channel quantum communication experiment with a high degree of immunity to interception. For this scheme, either channel alone can have an arbitrarily small signal-to-noise ratio (SNR) for transmission of a coherent “message.” However, when the transmitted beams are combined properly upon authorized detection, the encoded message can in principle be recovered with the original SNR of the source. An experimental demonstration has achieved a 3.2 dB improvement in SNR over that possible with correlated classical sources. Extensions of the protocol to improve its security against eavesdropping are discussed.

Bichromatic excitation of the $R_1$ line in ruby at liquid-helium temperature is considered. The frequency difference of the spectral components of the driving field is resonant with the energy gap between the anticrossed spin sublevels $+1/2\mathrm{}$ and $+3/2\mathrm{}$ in the ground state ${}^4{A}_2.$ The condition of the spin coherence trapping in the nonabsorbing state is analyzed. Population trapping is revealed in the reduced spontaneous emission from the excited state and in large spin magnetization of the ground state ${\mathrm{Cr}}^{3+}$ precessing with the frequency difference of the spectral components of the driving field. This magnetization can be detected by a pickup coil. The lifetime of the induced magnetization is strongly dependent on the ratio of the lifetime of the excited state $Ē$ and the spin coherence decay time ${(T}_2{)}_{\mathrm{spin}}.$

The theory of an atom dipole trap composed of a focused, far red-detuned, trapping laser beam, and a pair of red-detuned, counterpropagating, cooling beams is developed for the simplest realistic multilevel dipole interaction scheme based on a model of a (3+5)-level atom. The description of atomic motion in the trap is based on the quantum kinetic equations for the atomic density matrix and the reduced quasiclassical kinetic equation for atomic distribution function. It is shown that when the detuning of the trapping field is much larger than the detuning of the cooling field, and with low saturation, the one-photon absorption (emission) processes responsible for the trapping potential can be well separated from the two-photon processes responsible for sub-Doppler cooling atoms in the trap. Two conditions are derived that are necessary and sufficient for stable atomic trapping. The conditions show that stable atomic trapping in the optical dipole trap can be achieved when the trapping field has no effect on the two-photon cooling process and when the cooling field does not change the structure of the trapping potential but changes only the numerical value of the trapping potential well. It is concluded that the separation of the trapping and cooling processes in a pure optical dipole trap allows one to cool trapped atoms down to a minimum temperature close to the recoil temperature, keeping simultaneously a deep potential well.

We explore extended $\Lambda$ systems driven by two resonant laser fields in a strong regime characterized by the fact that the Rabi frequencies are much bigger than the difference between the two field frequencies. We show that a specific coherent superposition of the lower states is trapped during the dynamics. This extended trapped state is different from the trapped state usually encountered in the rotating-wave-approximation regime. We show the robustness of this trapping in the sense that it is stable with respect to significant deviations of the field parameters (frequencies and amplitudes).

The driven two-photon Jaynes-Cummings model (JCM) exhibits a collapse and revival phenomenon in its mean photon number on a time scale much larger than the periodic revival time for two-state inversion in the usual two-photon JCM. The time scale of revivals for these oscillations is much larger than the corresponding time scale for the driven one-photon JCM and can be explained with the Hamiltonian of type $H_{\eta }=\pm \sqrt{{(J}_{\eta }^{†}+\alpha P_{\eta }{)}^2{(J}_{\eta }+\alpha P_{\eta }{)}^2}$ constructed explicitly for this purpose. The effect of Stark shifts is also studied and it is observed that the dynamics is strongly influenced by such Stark shifts both at early stages as well as at late stages of the evolution but in different ways, and thus Stark shifts play a significant role in determining the dynamical evolution of the system.

Hybrid density-functional calculations are performed to study the structural and electronic properties of ${\mathrm{Si}}_{13}$ clusters. Three isomers with $C_s{,C}_{2v},$ and $C_{3v}$ symmetries are considered. They are and were believed to be the ground state of ${\mathrm{Si}}_{13}$ in literature. Our studies agree with the most recent local-density-approximation calculation, and show that the $C_s$ isomer has the lowest energy among the three isomers. The vertical ionization potential (VIP) and vertical electron affinity (VEA) are calculated and analyzed for the three isomers. The results suggest that the $C_{2v}$ isomer can be distinguished from the other two isomers experimentally by the VIP measurement, and the $C_{3v}$ isomer by the VEA measurement.

I introduce rate-distortion theory for the coding of quantum information, and derive a lower bound, involving the coherent information, on the rate at which qubits must be used to store or compress an entangled quantum source with a given maximum level of distortion per source emission.

The inconclusive rate is considered as a disturbance measure in key distribution in quantum cryptography. Bennett’s two-state protocol is addressed for the case in which a positive operator-valued measure is implemented by the legitimate receiver in the presence of an individual attack by a general unitary disturbing eavesdropping probe. The maximum Renyi information gain by the disturbing probe is calculated for given receiver error and inconclusive rates. It is demonstrated explicitly that less information is available to an eavesdropper at a fixed inconclusive rate and error rate than is available at a fixed error rate only.

Transition amplitudes for allowed transitions in Be-like ions are determined to second order within the framework of relativistic many-body perturbation theory. The calculations start with a $V^{\left(N-2\right)}$ Dirac-Fock potential and include correlation corrections systematically. Multiconfiguration reference states are employed to account for the valence-valence correlations within the valence shell and perturbation theory is applied to systematically improve upon the wave functions. The transition amplitudes obtained in different gauges are in close agreement. The theoretical transition amplitudes agree well with experiment for all ions except for the neutral Be atom.

The multiconfiguration Dirac-Fock method, based on the extended optimal level approximation, is used to calculate magnetic quadrupole transitions $\mathrm{}\left(M2\mathrm{}\right)\mathrm{}$ from ${2s2p(}^3{P}_2)$ to ${2s}^2{(}^1{S}_0)$ states in berylliumlike ions. The calculation of this transition for $Z>\mathrm{}6\mathrm{}$ uses the Dirac-Fock Hamiltonian which is necessary for highly ionized atoms. The Breit interaction is treated as a first-order perturbation, and its non-negligible contributions to the excitation energies for heavier ions are highlighted. Some of the $M2\mathrm{}$ transition probabilities results presented in this paper are calculated for the first time, to our knowledge.