A method is described which allows the detection of atomic quantum beats or wave-packet evolution in low-lying states, phenomena which are usually difficult to observe. The excitation method consists of a phase-sensitive two-pulse technique. Detection of the excited-state population after the two pulses is achieved by monitoring rubidium dimer ions which are formed in the associative ionization reaction, Rb*(n)p+Rb →${\mathrm{Rb}}_2^{+}$+${\mathit{e}}^{\mathrm{-}}$. The dimer ion yield as a function of the wavelength showed strong features, which coincided with the 8p to 15p atomic resonances. The two-pulse technique revealed atomic fine-structure quantum beats due to the coherent excitation of the ${\mathit{np}}^2$${\mathit{P}}_{1/2,3/2}$ fine-structure states of the atom. © 1996 The American Physical Society.

We show that the teleportation of an unknown quantum state can be achieved without the irreversible amplification of an intermediate detector, as required in the original scheme. This allows us to show how the quantum information is ‘‘hidden’’ within the correlations between the system and the environment while being wholly absent from any of the individual subsystems. This revival of correlations from the environment is quite surprising since it seems to go against the usual intuition of the environment irreversibly destroying information. By developing a description of quantum teleportation at the amplitude level we can see why the relevant information is robust to such irreversible actions of the environment. © 1996 The American Physical Society.

We have developed a direct self-consistent-field (SCF) molecular-orbital (MO) method based on the density-functional linear combinations of atomic-orbital methods, which is efficient for obtaining total energies of large molecules. In this method, we introduce the Schwartz inequality and efficiently reduce the number of atomic-orbital pairs, which must be considered for calculating the matrix elements and electron density. The MO calculations for ${\mathrm{C}}_{60}$ confirm that the total number of calculation steps for obtaining the matrix elements and electron density becomes 1/6 of that for an ordinary SCF-MO method, keeping an eight-digit accuracy in total energy. © 1996 The American Physical Society.

Variational solutions are given for problems involving diverse fermionic and bosonic N=2–7-body systems. The trial wave functions are chosen to be combinations of correlated Gaussians, which facilitate a fully analytical calculation of the matrix elements. The nonlinear parameters of the trial function are selected by a stochastic technique. The method has proved efficient, accurate, and seems feasible for any few-body bound-state problems emerging in atomic or nuclear physics. © 1996 The American Physical Society.

We have examined available data for the odd-parity energy-level structures in Zr and Hf, stimulated by the designations of four predicted 6d7${\mathit{s}}^2$7p levels in the homologous atom rutherfordium (Rf, Z=104) by Eliav et al. [Phys. Rev. Lett. 74, 1079 (1995)]. We point out some errors and deficiencies in the Zr data and give the results of Hartree-Fock calculations for Hf 5d6${\mathit{s}}^2$6p and Rf 6d7${\mathit{s}}^2$7p levels. Configuration interactions within the (d+s${)}^3$p complexes were included. The resulting eigenvectors allow meaningful LS-coupling designations for most of the levels belonging mainly to Hf 5d6${\mathit{s}}^2$6p and for most of the predicted Rf levels belonging mainly to 6d7${\mathit{s}}^2$7p. Some changes in the designations assigned to these levels in the literature are suggested: in particular, the lowest level of both Hf 5d6${\mathit{s}}^2$6p and Rf 6d7${\mathit{s}}^2$7p is most appropriately designated ${}_{}{}^3{}_{}{}^{}\mathrm{F}_2^{\mathit{o}}{}_{}{}^{}$. We point out the need for systematic whole-row studies of the low odd-parity configurations in 4d- and 5d-shell spectra. © 1996 The American Physical Society.

Recently, Greenwood et al. [Phys. Rev. Lett. 75, 1062 (1995)] reported measurements of large-angle elastic scattering of electrons from singly ionized argon at an energy of 3.3 eV. They compared their results for the differential cross section with cross sections determined using phase shifts obtained from two different scattering potentials and found large discrepancies between theory and experiment at large angles. They state that these differences may be due to the effects of polarization of the target, which are not included in their calculations, as well as inaccurate representations of electron exchange in the local scattering potentials that are employed to determine the phase shifts. In order to test these proposed explanations of the discrepancies, we have carried out calculations of elastic scattering from ${\mathrm{Ar}}^{+}$ using the R-matrix method. We compare both a single-state calculation, which does not include polarization, and a 17-state calculation, in which the effects of dipole polarizability are included through the use of polarization pseudostates within the close-coupling expansion, to each other and with the measurements. We find some differences between the two calculations at intermediate scattering angles, but very close agreement at angles above 100°. Although the calculated cross sections agree with experiment between 120° and 135°, large discrepancies persist at angles above 135°. We conclude that the differences between the measurements and theory cannot be explained on the basis of an inaccurate representation of electron exchange or polarization of the target. © 1996 The American Physical Society.

We have performed spectroscopy on sodium atoms that are optically channeled in the single node of a laser standing wave set up across a parallel-plate cavity. Using this technique we have extended our previous measurement of the Lennard-Jones van der Waals energy-level shift [Sandoghdar et al., Phys. Rev. Lett. 68, 3432 (1992)] down to a cavity width of ∼500 nm. We discuss the applications of this technique to the precise measurement of atom-surface distances. © 1996 The American Physical Society.

The Dirac contour representation and its properties are studied. It provides an elegant formalism for an extension of the harmonic-oscillator Hilbert space into a larger space that is suitable for the description of oscillator systems at both positive and negative temperatures. The analytic continuation of various physical quantities into the negative temperature region is examined in detail, and several interesting connections are revealed.

We present the experimental demonstration of a Bell-state analyzer employing two-photon interference effects. Photon pairs produced by parametric down-conversion allowed us to generate momentum-entangled Bell states and to demonstrate the properties of this device. The performance obtained indicates its readiness for use with quantum communication schemes and in experiments on the foundations of quantum mechanics.

The double-differential cross section for inelastic x-ray scattering by polarized bound electrons has been calculated within the so-called impulse approximation. In this way the analysis of Ribberfors [Phys. Rev. B 12, 2067 (1975)] has been extended to the case of magnetic Compton scattering.

We have performed photoassociative spectroscopy of ultracold potassium atoms in a high-density vapor-cell magneto-optical trap (MOT) in which atoms are confined predominantly in "dark" hyperfine levels that do not interact with the trapping light. Three well-resolved vibrational series (one strong and two weak) of the ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}_2^{}{}_{}{}^{}$ molecule are observed within 8 ${\mathrm{cm}}^{-1\mathrm{}}$ below the $4{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ atomic limit. One of the series is identified as the $0_g^{-}$ pure long-range state and the other two are assigned to the $0_u^{+}$ and $1_g$ states. These photoassociative spectra are obtained by measuring reductions in the trap fluorescence due to trap loss induced by an intense photoassociation laser. By using a vapor-cell-dark MOT technique, we achieve atomic densities greater than ${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$, increasing our trap loss signals by an order of magnitude compared to a normal MOT.

We have studied how the presence of a strongly coupled off-resonant molecular potential affects cold collisions in a laser field. To do this, we have performed a fully quantal time-dependent three-state calculation that includes both spontaneous emission and laser-driven dynamics of the collision complex. Our results show that although the presence of the off-resonant state has an effect on the collision process, two-state calculations will be valid unless the Rabi coupling greatly exceeds the detuning. We have investigated how this physics impinges on trap loss and optical shielding in a magneto-optical trap. Our results indicate that the experimentally observed saturation of optical shielding cannot be explained adequately by off-resonant effects.

We present a calculation of cooling of Rydberg atoms in bichromatic standing light waves. The atoms in our model are assumed to have a cascade three-level transition scheme, with the second excited level being the Rydberg state. The cooling fields, which also excite the atoms to the Rydberg state, consist of two standing waves whose frequencies are tuned near the two transition frequencies of the cascade atomic system. It is shown that the mechanisms of Sisyphus cooling and velocity-selective coherent population trapping can co-exist in such a configuration, and under certain conditions the majority of the laser-cooled atoms are in the Rydberg state.

The time evolution of a two-level atom that is simultaneously exposed to the field of a running laser wave and a homogeneous gravitational field is studied. The result of the coupled dynamics of internal transitions and center-of-mass motion is worked out exactly. Neglecting spontaneous emission and performing the rotating-wave approximation, we derive the complete time-evolution operator in an algebraical way by using commutation relations. The result is discussed with respect to the physical implications. In particular the long-time and short-time behaviors are physically analyzed in detail. The breakdown of the Magnus perturbation expansion is shown. © 1996 The American Physical Society.

We present a general formalism for studying the interaction between laser fields and degenerate-Zeeman-sublevel atoms correct up to the first order in the weak probe field. We derive from this theory the equations of motion for a Λ-type system involving ${\mathit{S}}_{1/2}$, F=2⇆${\mathit{P}}_{1/2}$, F′=1⇆${\mathit{S}}_{1/2}$, F=1 transitions in ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ atoms. These equations are used to numerically investigate the coherent population trapping (CPT) schemes in the ${\mathit{S}}_{1/2}$, F=2⇆${\mathit{P}}_{1/2}$, F′=1 transition induced by a linearly polarized pumping field and the electromagnetically induced transparency exhibited in the weak probe spectrum in the ${\mathit{S}}_{1/2}$, F=1⇆${\mathit{P}}_{1/2}$, F′=1 transition. We discuss the effects of the CPT on the probe spectrum with and without the Doppler broadenings by comparing the probe spectrum derived from the real system with that derived from an ideal Λ-type system. We show that the atoms in the CPT states are shielded from interacting with the probe field while those in the absorbent states make contributions to the weak probe by the Raman anti-Stokes process, and the effect of the real system on the probe field becomes equivalent to the ideal Λ system only in the strong CPT parameter regime. © 1996 The American Physical Society.

We examine the nonclassical property of a Schrödinger cat state, i.e., a superposition of two coherent states, when it is amplified by either a phase-sensitive or a phase-insensitive amplifier. In both cases the corresponding P representation remains nonpositive in some area of the complex plane, for any amplification factor. The contribution to the P function from these areas decreases exponentially with an amplification factor. With a phase-sensitive amplifier the onset of decay can be postponed to a larger amplification factor. Thus the nonclassical nature of the field can be preserved under a wide range of conditions, in spite of the noise introduced in the amplification process. © 1996 The American Physical Society.

The vacuum field in a dielectric cavity where the medium has a rapidly changing time-dependent refractive index exhibits nonclassical features that are directly related to the distortion of the vacuum fluctuations of the field. For appropriate parameters, these nonclassical effects are appreciable enough that a suitable measure of the change in the zero-point noise of the cavity field could provide a way to probe distortions of the vacuum fluctuations. © 1996 The American Physical Society.

Using a continuous basis set of optical modes, the spontaneous emission into the lowest-order resonant quasimode of a planar cavity is analyzed for the case of a laterally distributed multiemitter system. We show that for a spatially distributed, randomly phased multiemitter system, the spontaneous rate into the lowest-order cavity quasimode contains a Purcell effect, increases with increasing mode quality factor Q, and saturates when the mode loss becomes less than the dipole dephasing rate. Cooperative effects leading to superradiance for a properly phased multiemitter system are also considered, for which the change in the cavity-confined total spontaneous emission rate can be sizable. © 1996 The American Physical Society.

The third-order optical susceptibility ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ is calculated for a system of N cold atoms trapped in a three-dimensional harmonic potential. The dipole-dipole interaction between atoms contributes to ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ reflecting quantum statistical properties of atoms. This contribution in Bose condensed systems is proportional to ${\mathit{N}}^2$ and is much larger than that in the system of Fermi atoms. © 1996 The American Physical Society.

We perform stochastic simulations of the quantum Zeno effect experiment of the type realized with trapped cooled ions. The results are carefully examined for the case where the experiment is performed on a single atomic particle. The results of the simulations for a single ion exhibit the probabilistic behavior that is required to satisfy the collapse of the wave-packet hypothesis. When a large ensemble of ions is described by the simulations, the results are the same as those produced using the density-matrix method. In this way the stochastic simulation methods provide a link between these two approaches, as they are compatible with each in the appropriate regime. The results are also discussed in terms of what may be observed experimentally given the practical limitations of performing experiments with individual ions. © 1996 The American Physical Society.