The absorption and dispersion properties of a driven two-level atomic system probed by a weak field have been measured simultaneously with a phase modulated Mach-Zehnder interferometer. We find that the resonant absorption and the negative dispersion are reduced with respect to the nondriving case by a factor of 490 and 47, respectively, which is in good agreement with the theory. This means a $\sim 10$-fold enhancement of the amount of dispersion provided per amount of residual absorption. The experimental values are compared to the requirements that have to be fulfilled for the realization of a broadband, high finesse optical cavity. Finally, we propose a specific degenerate three-level system that overcomes some of the shortcomings of the driven two-level atom.

For a system with three identical atoms, the dependence of the s-wave virtual state energy on the weakly bound dimer and trimer binding energies is calculated in the form of a universal scaling function. The scaling function is obtained from a renormalizable three-body model with a pairwise Dirac-$\delta$ interaction. The threshold condition for the appearance of the trimer virtual state was also discussed.

Using a density-functional based algorithm, the full infrared and Raman spectra are calculated for the neutral ${\mathrm{Ti}}_8{\mathrm{C}}_{12}$ cluster assuming geometries of $T_h,$ $T_d,$ $D_{2d},$ and $C_{3v}$ symmetry. The $T_h$ pentagonal dodecahedron is found to be dynamically unstable. The calculated properties of the relaxed structure having $C_{3v}$ symmetry are found to be in excellent agreement with experimental gas-phase infrared results, ionization potential and electron affinity measurements. Consequently, the results presented may be used as a reference for further experimental characterization using vibrational spectroscopy.

Cold-atom interferometers use guiding potentials that split the wave function of the Bose-Einstein condensate and then recombine it. We present a theoretical analysis of the wave-function recombination instability that is due to the weak nonlinearity of the condensate. It is most pronounced when the accumulated phase difference between the arms of the interferometer is close to an odd multiple of $\pi$ and consists in exponential amplification of the weak ground state mode by the strong first excited mode. The instability exists for both trapped-atom and beam interferometers.

We measured the angular distribution of spontaneous emission radiation from dye molecules near a plane boundary. We found a systematic change of the angular distribution depending on the pump beam angle. By comparing the experimental results obtained by normal pumping and evanescent pumping, we could discern between the characteristics of radiation from the molecules within subwavelength distance from the boundary and that from the molecules relatively far from the boundary. It is found that the exact shape of the angular distribution of radiation is determined by the local geometrical factor related to the pump beam angle and the radiation detection angle. The characteristics of the spontaneous emission beyond critical angle and the measured polarization dependence of the radiation are in good agreement with the triplet mode quantum theory, as well as semiclassical theory.

Single-mode microcavities filled with liquid crystals are predominantly nonlinear refractive optical elements. Pumped by lasers tuned above the filled cavity resonance frequency, we show that the phenomena of bistability, spatial pattern formation, and “optical bullet holes” appear even in a single-mode microcavity configuration. The solitons have an unexpected dark spatial profile and appear only beyond the bistability threshold.

We consider a class of models of self-interacting bosons hopping on a lattice. We show that properly tailored space-temporal coherent control of the single-body coupling parameters allows for the universal quantum computation in a given sector of the global Fock space. This general strategy for encoded universality in bosonic systems has, in principle, several candidates for physical implementation.

Our quantum search protocol indicates that entanglement is not required for $\sqrt{N}$ speedup. We also reemphasize the quantum error correction mechanism in our scheme.

We discuss the range of validity of a modal description for the spatiotemporal dynamics of the optical field in vertical-cavity surface-emitting lasers. We focus on the secondary pulsations that appear during the turn-off transients when the injection current is modulated by a square-wave signal. We compare the results obtained with both a full spatiotemporal model [J. Mulet and S. Balle, IEEE J. Quantum. Electron. $38,$ 291 (2002)] and a modal expansion derived from this model. We find that the results obtained from the two descriptions agree for strong lateral guiding. However, for weak lateral guiding we find differences because the optical-field profile changes significantly due to spatial changes in the refractive index induced by the carrier density. The reason is that in the full spatiotemporal model a shrinkage of the mode profile occurs, which leads to an enhancement of the secondary pulsations. This effect is not included in the modal expansion, and it determines the limits of validity of such an approach for gain-guided devices.

We introduce a very general approximation to the quantum propagator that is based on the assumption that the most important contributions to the complex semiclassical propagator evolve from real classical trajectories that almost satisfy the desired boundary conditions. Our results for two systems — the autocorrelation function for the quartic anharmonic oscillator and the photodissociation spectrum of ${\mathrm{CO}}_2$ — show that these nearly real contributions yield an excellent approximation to the quantum propagator for quite long times. The approach taken here is applicable to problems with many (e.g., several hundred) degrees of freedom, and hence promises to provide an accurate and useful representation of the quantum dynamics for a wide variety of physically interesting systems.

Measurements of relative multiple ionization cross sections have been performed for 50–300-keV ${\mathrm{H}}^{+},$ ${\mathrm{D}}^{+},$ and ${\mathrm{He}}^{+}$ impact on ${\mathrm{N}}_2,$ ${\mathrm{O}}_2,$ CO, and NO molecules. Fragment ions with total charges up to ${Q=q}_1{+q}_2=5\mathrm{}$ have been detected in coincidence using a position- and time-sensitive detector. Dependence of the cross section on the molecular orientation with respect to the ion beam is observed for all targets. The experimental data are compared with theoretical calculations based on the statistical energy deposition model.

We establish a connection between quantum inequalities, known from quantum-field theory on curved spacetimes, and the degree of squeezing in quantum-optical experiments. We prove an inequality which binds the reduction of the electric-field fluctuations to their duration. The bigger the level of fluctuation suppression the shorter its duration. As an example of an application of this inequality is the case of squeezed light whose phase is controlled with $1\%$ accuracy for which we derive a limit of $-15\mathrm{dB}$ on the allowed degree of squeezing.

We present a scheme utilizing position-dependent ac Stark shifts for doing quantum logic with trapped ions. By a proper choice of direction, position, and size, as well as power and frequency of a far-off-resonant Gaussian laser beam, specific ac Stark shifts can be assigned to the individual ions, making them distinguishable in frequency space. In contrast to previous all-optical based quantum gates with trapped ions, the present scheme enables individual addressing of single ions and selective addressing of any pair of ions for two-ion quantum gates, without using tightly focused laser beams. Furthermore, the decoherence rate due to off-resonant excitations can be made negligible as compared with other sources of decoherence.

We investigate a simple scheme for autocorrelation measurement of an xuv pulse. It is based on double ionization of He. We have found that, in a certain photon energy range, the detection of doubly charged positive ions instead of energy-resolved photoelectrons is sufficient for autocorrelation, which greatly simplifies the detection system for practical use.

The forerunners preceding the main tunneling signal of the wave created by a source with a sharp onset or by a quantum shutter have been generally associated with over-the-barrier (nontunneling) components. We demonstrate that, while this association is true for distances that are larger than the penetration length, for smaller distances the forerunner is dominated by under-the-barrier components. We find that its characteristic arrival time is inversely proportional to the difference between the barrier energy and the incidence energy, a tunneling time scale different from both the phase time and the Büttiker-Landauer time.

It is shown that different distinguishability measures impose different orderings on ensembles of N pure quantum states. This is demonstrated using ensembles of equally probable, linearly independent, symmetrical pure states, with the maximum probabilities of correct hypothesis testing and unambiguous state discrimination being the distinguishability measures. This finding implies that there is no absolute scale for comparing the distinguishability of any two ensembles of N quantum states, and that distinguishability comparison is necessarily relative to a particular discrimination strategy.

Entangled coherent states can be used to determine the entanglement fidelity for a device that is designed to teleport coherent states. This entanglement fidelity is universal in that the calculation is independent of the use of entangled coherent states and applies generally to the teleportation of entanglement using coherent states. The average fidelity is shown to be a poor indicator of the capability of teleporting entanglement; i.e., very high average fidelity for the quantum teleportation apparatus can still result in low entanglement fidelity for one-mode of the two-mode entangled coherent states.

We study the entanglement properties of a closed chain of harmonic oscillators that are coupled via a translationally invariant Hamiltonian, where the coupling acts only on the position operators. We consider the ground state and thermal states of this system, which are Gaussian states. The entanglement properties of these states can be completely characterized analytically when one uses the logarithmic negativity as a measure of entanglement.

We show that two electrons confined in a square semiconductor quantum dot have two isolated low-lying energy eigenstates, which have the potential to form the basis of scalable computing elements (qubits). Initialization, one-qubit and two-qubit universal gates, and readout are performed using electrostatic gates and magnetic fields. Two-qubit transformations are performed via the Coulomb interaction between electrons on adjacent dots. Choice of initial states and subsequent asymmetric tuning of the tunneling energy parameters on adjacent dots control the effect of this interaction.