We demonstrate an electron-hole symmetry in the method of stimulated Raman adiabatic passage (STIRAP), and point out the existence of separate electron and hole regimes in atoms and molecules. In particular, we study hole STIRAP’s in the triplet states of ${\mathrm{N}}_2.$ We show that when electron and hole STIRAP’s coexist, the competition between them leads to a powerful method of controlling bifurcating processes in molecular systems.

We produce polarization entangled states with variable degree of entanglement for twin photons. Entanglement in polarization is coupled to entanglement in position that produces transverse coincidence interference fringes. We show both theoretically and experimentally that we can use the position interference pattern to measure the polarization degree of entanglement. The coupling between polarization and transverse degrees of freedom are demonstrated to be useful in the manipulation of the entanglement of the twin photons.

We propose an optical double-cavity resonator whose response to a signal is similar to that of an electromagnetically induced transparency (EIT) medium. A combination of such a device with a four-level EIT medium can serve for achieving large cross-Kerr modulation of a probe field by a signal field. This would offer the possibility of building a quantum logic gate based on photonic qubits. We discuss the technical requirements that are necessary for realizing a probe-photon phase shift of $\pi$ caused by a single-photon signal. The main difficulty is the requirement of an ultralow reflectivity beam splitter, and we must be able to operate a sufficiently dense cool EIT medium in a cavity.

To investigate the effect of the environment on a quantum-mechanical system we consider two two-level atoms in a free radiation field in the presence of a screen. By assuming that the screen causes continuous ideal measurements on the free radiation field we derive a quantum jump description for the state of the atoms. Our results are consistent with the master equation for dipole interacting atoms, but give more insight in the time evolution of a single system. To illustrate this we derive a necessary and sufficient criterion for interference in a two-atom double-slit experiment and analyze bunching in the statistics of photons emitted in a certain direction.

A time-dependent perturbation calculation for a four-level system is presented. A resonantly enhanced Kerr nonlinearity is produced with a combination of long, short, and delayed laser pulses in the presence of electromagnetically induced transparency. We show the enhanced Kerr nonlinearity and vanishingly small linear susceptibility due to the induced transparency, both are favorable for cross-phase modulation. In addition, we show that possible constructive and destructive interference between different excitation pathways could also lead to enhancement and suppression of the Kerr nonlinearity.

We analyze the dynamics of a semiconductor laser with phase conjugate optical feedback, using numerical simulations based on rate equations for the complex amplitude of the electric field and the carrier density. From this analysis we observe the presence of low-frequency fluctuations which are similar to those observed in a semiconductor laser with conventional optical feedback. The similarities and differences between phase conjugate and conventional optical feedback are discussed, and a mechanism for the appearance of low-frequency fluctuations in a semiconductor laser with phase conjugate feedback is suggested.

For potential tails consisting of an inverse-square term and an additional attractive ${1/r}^m$ term, $V\left(r\right)\mathrm{}\sim [{ħ}^2/(2\mathrm{}\mathcal{M}\left)\right]\left[\right(\gamma {/r}^2\left)-\right({\beta }^{m-2}{/r}^m\left)\right],$ we derive the near-threshold quantization rule $n=n\left(E\right)\mathrm{}$ which is related to the level density via $\rho =dn/dE.\mathrm{}$ For a weak inverse-square term, $-\frac{1}{4}<\gamma <\frac{3}{4}$ (and $m>\mathrm{}2),$ the leading contributions to $n\left(E\right)\mathrm{}$ are $n\stackrel{E\to 0}{=}A-B(-{E)}^{\sqrt{\gamma +1/4\mathrm{}}},$ so ρ has a singular contribution proportional to $(-{E)}^{\sqrt{\gamma +1/4\mathrm{}}-1}$ near threshold. The constant B in the near-threshold quantization rule also determines the strength of the leading contribution to the transmission probability through the potential tail at small positive energies. For $\gamma =0\mathrm{}$ we recover results derived previously for potential tails falling off faster than ${1/r}^2.$ The weak inverse-square tails bridge the gap between the more strongly repulsive tails, $\gamma >~3/4,$ where $n\left(E\right)\mathrm{}\stackrel{E\to 0}{=}A+O\left(E\right)\mathrm{}$ and ρ remains finite at threshold, and the strongly attractive tails, $\gamma <-1/4,$ where $n\stackrel{E\to 0}{=}B\ln (-E/A),$ which corresponds to an infinite dipole series of bound states and connects to the behavior $n\stackrel{E\to 0}{=}A+B{E}^{\mathrm{}(1/2)\mathrm{}-\mathrm{}(1/m)\mathrm{}},$ describing infinite Rydberg-like series in potentials with longer-ranged attractive tails falling off as ${1/r}^m,$ $0<m<\mathrm{}2.\mathrm{}$ For $\gamma =-1/4$ (and $m>\mathrm{}2)$ we obtain $n\left(E\right)\mathrm{}\stackrel{E\to 0}{=}A+C/\ln (-E/B),$ which remains finite at threshold.

We have studied the ionization and fragmentation of ${\mathrm{C}}_{60}$ in collisions with protons and deuterons at impact energies of 50–300 keV. The time-of-flight spectra of all fragment ions were measured in coincidence with one of the emitted electrons. The cross sections of multiple ionization as well as relative yields of even-mass fullerene-type ions of various charges are deduced from experimental spectra. A theoretical model based on a statistical approach is developed for the explanation of the experimental data. The model describes the $p+{\mathrm{C}}_{60}$ collision as proceeding in three steps: (i) energy deposition by the projectile, (ii) emission of prompt electrons, and (iii) ${\mathrm{C}}_2$ evaporation and delayed electron emission. The observed distribution of fragment fullerenes indicates that the delayed electron emission is an essential part of the process and that it occurs before the equilibration is achieved.

In this work we point out a theoretical picture accounting for some unpredicted trapping conditions that have been observed—but not satisfactorily explained—in the past and have been recently confirmed by our experiments. We have realized a sodium magneto-optical trap working on the $3^2{S}_{1/2}\mathrm{}(F=1\mathrm{})\mathrm{}\to 3^2{P}_{3/2}{(F}^{\prime }=0)$ transition that, according to the usually accepted model, should not work. Our results, with respect to the previous unexplained observations, support more stringent conclusions because our experimental setup gives us the possibility to repump atoms from the $F=2\mathrm{}$ state using $D_1$ transitions. This definitely excludes that the repumping frequency may play a role in the trap dynamics. A peculiar perturbation approach allows us to demonstrate that the confinement force originates, in this case, from a magnetically induced level-mixing effect. Moreover, we describe separately the nature of damping and confining forces and we recognize that in this case they are due to different transitions. Trap simulations based on a dynamical three-dimensional model are presented, which quantitatively reproduce our experimental results.

The stability region of a far-off-resonance dipole-atom trap (FORT) composed of a single trapping laser beam and a ${\sigma }^{+}-{\sigma }^{-}$ cooling field configuration is discussed for the simplest $\mathrm{}(3+5)\mathrm{}$-level atomic scheme. The ultimate parameters of the stable FORT are evaluated by extending the theory of the FORT to the fourth-order rate-equation approximation. It is shown that when the detuning of the trapping field is much larger than the detuning of the cooling field, and with a proper choice of a low optical saturation due to the trapping and cooling field, the mixing of the one-photon processes responsible for the trapping potential with the two-photon processes responsible for the sub-Doppler cooling of atoms in the trap remains weak. It is concluded that the FORT with superimposed cooling fields possesses a stability region where the diffusive heating is suppressed by the cooling processes. Numerical evaluations of the stability region for a model of a $\mathrm{}(3+5)\mathrm{}$-level atom are given.

We demonstrate how the freedom in the definition of the positive-$P$ representation may be used to reduce stochastic sampling errors, using the single-mode anharmonic oscillator as an example. In the modeling of this system, which is well known for being problematic, we achieve an improvement of more than 20 orders of magnitude in the distribution of the trajectories.

Quantum computers can, in principle, exceed the speed of ordinary computers by taking advantage of quantum coherence and entanglement. It is possible to find a “needle in a haystack” of $N-1$ straws in only $\sqrt{N}$ searches by utilizing coherent superposition states. In the present paper we show how modern quantum optics may provide a simple and practicable quantum search procedure, which may also yield insights into quantum search algorithms in general.

Total spin eigenstates can be used to intrinsically encode a direction, which can later be decoded by means of a quantum measurement. We study the optimal strategy that can be adopted if, as is likely in practical applications, only product states of N spins are available. We obtain the asymptotic behavior of the average fidelity, which provides a proof that the optimal states must be entangled. We also give a prescription for constructing finite measurements for general encoding eigenstates.

The $3p$ photoelectron spectra of free Fe and Co atoms have been measured with the use of monochromatized synchrotron radiation and the atomic beam technique. These spectra reveal complex ${3p}^{-1}$ multiplet structures, which are discussed based on the predictions of configuration-interaction calculations. Both the strong $3p-3d$ exchange interaction and the term-dependent lifetime broadening by super-Coster-Kronig decay determine the shape of the $3p$ photoelectron spectra. The $3p-3d$ interaction gives rise to an $\mathrm{LS}$ multiplet structure, which spans an energy range of about 20 eV, whereas the term-dependent lifetime broadening causes the almost complete suppression of low-spin components.

Rotational and vibrational distributions of zero-temperature collisional rate coefficients for atom-diatom scattering are used together with effective range theory to obtain lifetimes for predissociation. High-order indirect potential coupling in the quantum-mechanical calculation is interpreted using a simple classical picture that describes the quasiresonant dynamics of atom-diatom collisions by the conservation of classical action. The importance of closed channel thresholds in determining the structure of the distributions and the balance between momentum gap and near-resonant effects is discussed.

We present a detailed experimental study of the effects of strong ionization in the process of medium-order harmonic generation. The intensity-dependent density of free electrons significantly modifies the conversion efficiency due to the combined effect of depletion of the neutral atoms and defocusing of the pump beam. We find that the harmonic yield is maximized for particular values of the pump-laser intensity and that, for the range of experimental parameters we used, such optimum intensities do not depend on the harmonic order or on the focusing geometry, but only on the ionization potential of the gas. Finally we show how the defocusing due to ionization is effective in guiding the pump beam along channels of optimum intensity, thus enhancing the overall conversion efficiency for particular focusing conditions. A simple model is presented that, though relying on very crude approximations of the processes at play, is nevertheless able to reproduce the experimental results in a surprisingly good way.

The quantized vortex state is investigated in a Bose-Einstein condensate, confined in a multiply connected geometry formed by a Laguerre-Gaussian optical trap. Solving the Gross-Pitaevskii equation variationally, we show that the criterion for vortex stability is that the interatomic interaction strength must exceed a critical interaction strength. The time evolution of a freely expanding Laguerre-Gaussian condensate with a vortex is calculated and used to derive the interference pattern of such a condensate overlapping with a parabolically trapped condensate.

We have observed transverse pattern formation in type-II continuous-wave triply resonant optical parametric oscillators (OPOs). Different kinds of cavities with degenerate transverse modes have been studied: confocal, concentric, semiconcentric, and planar. We have investigated the OPO oscillation threshold and the transverse intensity distribution of the emitted fields as a function of the distance from the degeneracy position. The experimental configurations for which the system is truly multimode have been determined.

We report extensions and corrections to the measurement of the Feshbach resonance in ${}^{85}\mathrm{Rb}$ cold atom collisions reported earlier [J. L. Roberts et al., Phys. Rev. Lett. 81, 5109 (1998)]. In addition to a better determination of the position of the resonance peak [154.9(4) G] and its width [11.0(4) G], improvements in our techniques now allow the measurement of the absolute size of the elastic-scattering rate. This provides a measure of the s-wave scattering length as a function of magnetic field near the Feshbach resonance and constrains the Rb-Rb interaction potential.

The present work revisits the calculations of nanoscopic quantum processes from a beam of low-energy positrons impinging on molecular hydrogen gas and focuses on the vibrational excitation processes from energy-transfer collisions. Earlier calculations were found to contain a numerical error in the chosen interaction forces, while the present corrected results are seen to agree very well with recently published experimental data.