We present a general framework for sensitivity optimization in quantum parameter estimation schemes based on continuous (indirect) observation of a dynamical system. As an illustrative example, we analyze the canonical scenario of monitoring the position of a free mass or harmonic oscillator to detect weak classical forces. We show that our framework allows the consideration of sensitivity scheduling, as well as estimation strategies for nonstationary signals, leading us to propose corresponding generalizations of the standard quantum limit for force detection.

We construct a set of $2^{2^n}$ independent Bell-correlation inequalities for n-partite systems with two dichotomic observables each, which is complete in the sense that the inequalities are satisfied if and only if the correlations considered allow a local classical model. All these inequalities can be summarized in a single, albeit nonlinear inequality. We show that quantum correlations satisfy this condition provided the state has positive partial transpose with respect to any grouping of the n systems into two subsystems. We also provide an efficient algorithm for finding the maximal quantum-mechanical violation of each inequality, and show that the maximum is always attained for the generalized GHZ state.

Decoherence effects associated to the damping of a tunneling two-level system are shown to dominate the tunneling probability at short times in strong-coupling regimes in the context of a soluble model. A general decomposition of tunneling rates in dissipative and unitary parts are implemented. Master equation treatments fail to describe the model system correctly when more than a single relaxation time is involved.

The Wigner functions on the one-dimensional lattice are studied. Contrary to the previous claim in literature, Wigner functions exist on the lattice with any number of sites, whether it is even or odd. There are infinitely many solutions satisfying the conditions which reasonable Wigner functions should respect. After presenting a heuristic method to obtain Wigner functions, we give the general form of the solutions. Quantum-mechanical expectation values in terms of Wigner functions are also discussed.

The phase conjugation of an unknown Gaussian state cannot be realized perfectly by any physical process. A heuristic argument is used to derive a tight lower bound on the noise that must be introduced by an approximate phase conjugation operation. A universal transformation achieving the optimal imperfect phase conjugation is then presented, which is the continuous counterpart of the universal-NOT transformation for quantum bits. In addition, it is shown that more information can be encoded into a pair of conjugate Gaussian states than by using the same state twice.

We propose the use of ponderomotive forces to entangle the motions of different atoms. Two situations are analyzed: one where the atoms belong to the same optical cavity and interact with the same radiation field mode; the other where each atom is placed in own optical cavity and the output field of one cavity enters the other.

Single-differential and total cross sections of electron-impact ionization are calculated in a relativistic formulation for ions in the helium isoelectronic sequence: He, ${\mathrm{Li}}^{+},$ ${\mathrm{B}}^{3+},$ ${\mathrm{C}}^{4+},$ ${\mathrm{N}}^{5+},$ ${\mathrm{Na}}^{9+},$ ${\mathrm{Fe}}^{24+},$ and ${\mathrm{Ag}}^{45+}.$ Transition amplitudes are evaluated in the two-potential distorted-wave approximation. Relativistic effects are investigated by taking the nonrelativistic limit. Both the rearrange and capture amplitudes are found to be important near the ionization threshold. Results are compared with other theoretical calculations and experimental data. The systematics of ionization cross sections along the isoelectronic sequence is also studied.

Calculated generalized oscillator strengths for monopole, dipole, and quadrupole discrete and continuous spectrum excitations in Ne, Ar, Kr, and Xe are presented. The results cover the broad range of transferred energy $\omega ,\hspace{0.5em}0<~\omega <120\mathrm{}\hspace{0.5em}\mathrm{Ry}$ and momentum $q,\hspace{0.5em}0<~q<\mathrm{}2\mathrm{}\hspace{0.5em}\mathrm{a}.\mathrm{u}.$ The calculations were performed in the one-particle Hartree-Fock approximation and with account of many-electron correlations. The latter effects are included via the random phase approximation with exchange, proving to be important in all dipole, monopole, and quadrupole channels as well as in all the domains of the transferred energies and momenta considered. Particularly important are the many-electron correlations at high q values, where new additional purely correlational maxima and minima appear. These results, particularly the maxima and minima are expected to stimulate experimental activity in this domain of atomic physics.

Dissociative ionization of highly charged ${\mathrm{CS}}_2^{q+}$ $(q=3\mathrm{}–10)$ molecular ions produced in collisions of ${\mathrm{CS}}_2$ with ${\mathrm{Ar}}^{8+}$ ions is studied at 120 keV energy. Triple coincidence techniques have been applied to measure the velocity components of all three ionic fragments produced in the fragmentation process. The velocity measurements have been used to deduce the total kinetic energy released (KER) in the fragmentation of the molecular ion for each particular product channel. KER values were found to be similar to those predicted by a pure Coulomb explosion model, revealing the apparent unimportance of binding electronic interactions in highly charged, multielectron molecular systems. The propensity of fragmentation into symmetric and asymmetric channels depends only on energetics, with the most exothermic channel being favored. The velocity vectors of the ion fragments yield information on ion structure prior to fragmentation; results indicate that the ionization is impulsive and fragmentation is simultaneous.

We employ optimal control theory to discover an efficient information-retrieval algorithm that can be performed on a Rydberg-atom data register using a shaped terahertz pulse. The register is a Rydberg wave packet with one consituent orbital phase reversed from the others (the “marked bit”). The terahertz pulse that performs the decoding algorithm does so by driving electron probability density into the marked orbital. Its shape is calculated by modifying the target of an optimal-control problem so that it represents the direct product of all correct solutions to the algorithm.

We present an analysis of the ionization and excitation dynamics of $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ when exposed to an ultrashort and intense laser pulse. The studies are based on quantum close-coupling calculations and classical trajectory Monte Carlo simulations. The quantum and classical approaches are compared at three different levels: (i) Total (ii) differential ionization probabilities, and (iii) a direct comparison of the dynamics during the laser pulse from snapshots of the spatial electronic probability density. For short and intense pulses, the results of the two methods are in fair agreement. We show that the direction of ejection of the ionized electron is very asymmetric and strongly sensitive to the initial phase of the electric field.

We investigate the mechanism of damping and heating of trapped ions associated with the polarization of the residual background gas induced by the oscillating ions themselves. Reasoning by analogy with the physics of surface electrons in liquid helium, we demonstrate that the decay of Rabi oscillations observed in experiments on ${}^9{\mathrm{Be}}^{+}$ can be attributed to the polarization phenomena investigated here. The measured sensitivity of the damping of Rabi oscillations with respect to the vibrational quantum number of a trapped ion is also predicted in our polarization model.

Implementation of the adiabatic rapid passage scheme using two chirped pulses in a three-state $\Lambda$ system, where the final state is a manifold of closely spaced levels is proposed. We show how for a proper choice of pulse parameters the method can either select a single level or a superposition state in the manifold. The efficiency, robustness, and potential use of both possibilities are explored.

Pump-probe experiments are performed in which cold colliding Na atoms are photoassociated to form ${\mathrm{Na}}_2$ molecules, and subsequently ionized. The experiments are performed in a regime where the pulsed photoassociation to an intermediate potential takes place at long range, and to several electronic, vibrational, and rotational states. A probe pulse then produces an ionization signal by further exciting these molecules to autoionizing doubly excited states. The time dependence of the ion signal shows a dramatic flux-enhancement effect on a nanosecond time scale due to the motion on the intermediate potentials. This time dependence can be viewed as monitoring the inward motion and dephasing of a population wave packet formed at long range.

We extend an earlier semiclassical model to describe the dissipative motion of N atoms coupled to M modes inside a coherently driven high-finesse cavity. The description includes momentum diffusion via spontaneous emission and cavity decay. Simple analytical formulas for the steady-state temperature and the cooling-time for a single atom are derived and show surprisingly good agreement with direct stochastic simulations of the semiclassical equations for N atoms with properly scaled parameters. A thorough comparison with standard free-space Doppler cooling is performed and yields a lower temperature and a cooling-time enhancement by a factor of M times the square of the ratio of the atom-field coupling constant to the cavity decay rate. Finally, it is shown that laser cooling with negligible spontaneous emission should indeed be possible, especially for relatively light particles in a strongly coupled field configuration.

Based on a tunneling Hamiltonian description, we calculate the Josephson, normal, and interference currents between two Bose-Einstein condensates described by the Bogoliubov theory. The dominant Josephson term is of first order in the tunneling with a critical current density proportional to the ground state pressure. In contrast to superconductors, the normal current remains finite at zero temperature. We discuss the dynamics of the relative phase in a semiclassical approximation derived from an exact functional integral approach, which includes the interaction effects at fixed total particle number. It is shown that the normal current leads to a damping of the Josephson oscillations and, at long times, eliminates the macroscopic quantum self-trapping predicted by Smerzi et al. Finally, we give estimates for an experimental realization of Josephson tunneling in cold atomic gases, which indicate that coherent transfer of atoms might be realized with a ${}^{23}\mathrm{Na}$ condensate.

Instabilities for boson-fermion mixed condensates of trapped alkali-metal atoms due to the boson-fermion attractive interaction are studied using a variational method. A stable, metastable, and unstable region as a function of the boson-fermion interaction exists. The stability condition is obtained analytically from the asymptotic expansion of the variational total energy. The lifetime of metastable states is discussed for tunneling decay and is estimated to be very long. It suggests that, except near the instability border, metastable mixed condensate should be almost stable against tunneling decay. The critical border between metastable and unstable phases is calculated numerically and is shown to be consistent with the Mølmer scaling condition.

A model for spontaneous emission in active dielectric microstructures is given in terms of the classical electric field Green’s tensor and the quantum-mechanical operators for the generating currents. A formalism is given for calculating the Green’s tensor, which does not rely on the existence of a complete power orthogonal set of electromagnetic modes, and the formalism may therefore be applied to microstructures with gain and/or absorption. The Green’s tensor is calculated for an optical fiber amplifier, and the spontaneous emission in fiber amplifiers is studied with respect to the position, transition frequency, and vector orientation of a spatially localized current source. Radiation patterns are studied using a Poynting vector approach taking into account amplification or absorption from an active medium in the fiber.

We analyze instantaneous profiles of intense femtosecond optical pulses propagating in helium gas measured using femtosecond time-resolved optical polarigraphy (FTOP). Considering the characteristics of the optical Kerr effect, relations linking images obtained by FTOP with intensity distributions of optical pulses are derived under approximations applicable to a relatively wide range of experimental conditions. By inversely applying the derived equations, images proportional to instantaneous intensity distributions of propagating light pulses are constructed from snapshot images taken in the experiment. From an analysis of the images obtained, we discuss the temporal changes in the optical-pulse profiles. We can directly observe the energy reduction of the optical pulses after they pass through the simultaneously generated laser plasma. It is confirmed that each filament can be separately investigated by this method even under multifilament conditions. Furthermore, the images clearly reveal intensity distributions along the propagation axis as well as cross-section distributions of femtosecond-pulse filaments, and show temporal shape modulations shorter than the incident pulse width in the middle of the propagation. We specify the volume where the light energy concentrates at a particular instant of time, and directly observe the cross-section expansion of the volume at the focal point brought about due to nonlinear effects.