We show that dynamics can, in general, be used to enhance the coherence of a Hamiltonian ensemble and we analyze the transient coherence using the coarse-grained entropy. We illustrate this concept using a Rydberg atom subject to an impulsive momentum transfer or “kick.” Classical simulations predict that the wave packet generated by the kick undergoes strong transient phase-space localization, which forms an excellent starting point for its further control and manipulation. Moreover, we show that such localized states can be “trapped” for extended periods using a train of subsequent kicks.

In this paper, we have investigated the correlated electron emission of the nonsequence double ionization (NSDI) in an intense linearly polarized field. The theoretical model we employed is the semiclassical rescattering model, the model atom we used is the helium. We find a significant correlation between magnitude and direction of the momentum of two emission electrons, and give a good explanation for this striking phenomenon by observing the classical collisional trajectories. We argue that this correlation phenomenon is universal in NSDI process, as revealed by the recent experiment on the argon.

We give a bound on the minimum number of photons that must be absorbed by any quantum protocol to distinguish between two transparencies. We show how a quantum Zeno method in which the angle of rotation is varied at each iteration can attain this bound in certain situations.

In this paper we consider quantum metastability in a class of moving potentials introduced by Berry and Klein. This class of potential has height and width scaled in a specific way so that it can be transformed into a stationary one. While deriving the nondecay probability of the system, we demonstrate that the appropriate technique to use is the less well known method of scattering states. This method is illustrated through two examples, namely, a moving δ-function potential and a moving barrier potential. For expanding potentials, one finds that a small but finite nondecay probability persists at large times. Generalization to scaling potentials of arbitrary shape is briefly outlined.

Multichannel-quantum-defect theory analyses of $J=0,$ 1, and 2 even-parity Rydberg series $6pnp$ and $6pnf$ of neutral lead have been performed for the experimental energy levels. The channel admixture coefficients were deduced and used to evaluate theoretical lifetimes and Landé ${(g}_J)$ factors. Lifetime and $g_J$ values for higher-lying excited states have also been predicted.

We consider, for systems of indistinguishable fermions, approximate reconstruction of the three- and four-particle reduced density matrices (RDMs) from the one- and two-particle RDMs, $\mathit{\gamma }$ and $\mathit{\Gamma }.$ Our ansatz for reconstructing the four-particle RDM is the linear combination $a\left(\mathit{\Gamma }\wedge \mathit{\Gamma }\right)+b\left(\mathit{\gamma }\wedge \mathit{\gamma }\wedge \mathit{\Gamma }\right)+c\left(\mathit{\gamma }\wedge \mathit{\gamma }\wedge \mathit{\gamma }\wedge \mathit{\gamma }\right),$ where “∧” denotes the antisymmetrized (Grassmann) product. This is a generalization of reconstruction functionals employed recently to perform direct RDM calculations without wave functions via the contracted Schrödinger equation. Here we consider relationships between the parameters a, b, and c that are required in order for the reconstruction functionals to respect the hierarchy of contraction relations between RDMs. To this end we establish several general theorems concerning contractions of antisymmetrized tensor products of $\mathit{\gamma },$ $\mathit{\Gamma },$ and various products thereof. The accuracy of proposed reconstruction functionals is evaluated using accurate density matrices for the ground state of Be.

In the present paper, triply, doubly, singly, and total cross sections of the vapor water-molecule ionization are calculated in the distorted-wave Born approximation, without exchange. In these conditions, fair agreements are found with experimental data, specially in the low and intermediate ejected energy domain ${(E}_e<E_s,$ where $E_e$ and $E_s$ are the ejected and scattered energy, respectively), and an extensive comparison between experimental and theoretical results is given in terms of differential and total cross sections. The present results are also compared to other theoretical ones, specially to results obtained in the plane-wave Born approximation, and in the first-Born approximation frameworks, in order to underline the distortion effect.

We complement a previous article [Harel et al., Phys. Rev. A 55, 287 (1997)] that studied the characteristics of the description of ionization by the molecular approach to atomic collisions, by comparing the wave functions with accurate counterparts. We show how the failure of the basis to describe the phase of the ionizing wave function results in a trapping of the corresponding population in some molecular channels. The time evolution of the molecular wave function then departs from the exact one and the ionization and capture mechanisms appear as interlocked. We thus elucidate the question of the “natural” boundary of the molecular approach and draw further consequences as to the choice of pseudostates and the use of translation factors.

We reconsider nonlinear Compton scattering, induced by a linearly polarized laser field. The radiation power is assumed to be in the relativistic regime such that the ponderomotive energy of an electron in the field is of the order of magnitude of the electron’s rest energy or even above it, i.e., $U_p>~{\mathrm{mc}}^2.$ We investigate in detail for several scattering configurations the angular dependences of the induced nonlinear Compton processes of the order N of absorbed laser photons $\omega ,$ assuming that in general the electron beam and laser beam can cross at an arbitrary angle.

We have studied the loading of gaseous alkali-metal atoms into glass cells held at about 2 K by light-induced atom desorption (LIAD). In LIAD loading, the cell containing dense helium gas and a small amount of alkali metal is irradiated with cw laser light. This technique has two unique features to be investigated: alkali-metal atoms are desorbed, even by weak laser irradiation, from the cell walls not covered visibly with alkali metal, and loading efficiency is much higher below the superfluid transition temperature for the helium inside the cell than above this temperature. On the basis of extensive experimental studies for Rb presented in this paper, we give a consistent picture of the loading mechanism: alkali-metal atoms are optically desorbed from alkali-metal particles on the walls, then transported effectively into the cell volume by helium-gas flow produced by the continuous evaporation of the helium film, which is supplied by the superfluid-film flow on the walls from the bulk liquid helium at the bottom of the cell.

We measured in a molecular-beam experiment the magnetic-field dependence of the last bound levels of the $X{}^1{\Sigma }_g^{+}$ and $a^3{\Sigma }_u^{+}$ potentials close to the $3s+3s$ asymptote of ${\mathrm{Na}}_2.$ The field behavior of these levels in an external magnetic field differs for even or odd nuclear mechanical angular momentum. Coupled-channel calculations using high-precision potentials for the $X{}^1{\Sigma }_g^{+}$ and $a^3{\Sigma }_u^{+}$ potentials give excellent quantitative agreement between experiment and theory. Our data lead to a precise and quantitative description of the magnetic decoupling of the singlet and triplet manifold and of Feshbach resonances in a cold atomic collision.

We investigate the stochastic initiation of superradiant emission from a collection of excited two-state atoms coupled through a low-Q cavity mode. Noncollective emission into other modes of the electromagnetic field is included, allowing us to describe the transition from predominantly noncollective to collective behavior as the number of atoms is increased. We examine the dependence of the photon statistics on the number of atoms and the ratio of collective to noncollective decay rates. Numerical results for a few to ${10}^6$ atoms are obtained using an approximation scheme developed within the framework of quantum trajectory theory. Results are compared with those for a pure collective emission process and an earlier treatment of noncollective effects.

The spectral dependence of the angular distributions of photoelectrons produced by the single-photon detachment of ${\mathrm{Sn}}^{-}\hspace{0.5em}[\mathrm{core},\hspace{0.5em}{(5s}^2,\hspace{0.5em}{5p}^3)]$ ions has been measured at four discrete photon wavelengths ranging from 457.9 to 514.5 nm (2.71–2.41 eV) using a crossed laser-ion beams apparatus. Values of the fine-structure-resolved asymmetry parameters have been determined by fits to the photoelectron yield as a function of the angle between the photon polarization vector and the linear momentum vector of the collected photoelectrons. The measured asymmetry parameters for ${\mathrm{Sn}}^{-}$ are compared to previous asymmetry parameter measurements for negative ions with similar electronic configurations. The measurements were also fit to a previously reported model for photoelectron asymmetry parameters for photodetaching p-orbital electrons from negative ions.

We report a realistic computation of experimentally measured microwave ionization curves of hydrogen Rydberg atoms. The experimental field envelope, the duration of the microwave irradiation, the radial droop of the field in the cavity, and the microcanonical distribution of experimental initial states is completely taken into account. We obtain a strong dependence of the ionization probability on the initial magnetic quantum number. Also, a complete 3D three-way comparison between experiment, quantum, and classical theory in the experimentally relevant parameter regime is presented.

Bloch waves and Bloch bands of Bose-Einstein condensates in optical lattices are studied. We provide further evidence for the loop structure in the Bloch band, and compute the critical values of the mean-field interaction strength for the Landau instability and the dynamical instability.

The cause of decoherence in a quantum system can be traced back to the interaction with the environment. As it has been pointed out by Dicke, in a system of N two-level atoms, where each of the atoms is individually dipole coupled to the environment, there are collective, subradiant states, that have no dipole coupling to photon modes, and therefore, they are expected to decay slower. This property also implies that these type of states decohere slower. We propose a scheme which will create such states. First, the two-level atoms are placed in a strongly detuned cavity and one of the atoms, called the control atom is excited. The time evolution of the coupled atom-cavity system leads to an appropriately entangled state of the atoms. By the aid of a suitable single-atom phase transformation, it is possible to drive the atomic state into the subradiant, i.e., decoherence-free subspace. Up to a certain average number of photons, the result is independent of the state of the cavity. The analysis of the conditions shows that this scheme is feasible with present day techniques achieved in atom-cavity interaction experiments.

We study the dependence on the temperature T of Casimir effects for a range of systems, and, in particular, for a pair of ideal parallel conductors, $l_1\times l_2,$ separated by a vacuum to a distance $l_3$ apart, with $l_1\gg l_3$ and $l_2\gg l_3.$ We find the Helmholtz free energy $A^T$ combining Matsubara’s formalism in which the temperature T appears as a periodic Euclidean fourth dimension of circumference $l_T=ħ{c/k}_{B}T,$ with the semiclassical periodic orbital approximation of Gutzwiller. The approximation was shown to be exact for parallel plates at $T=0.\mathrm{}$ By inspecting the known semiclassical results for the Casimir energy at $T=0\mathrm{}$ in two cases of a rectangular parallelepiped, ${(l}_1\gg l_3$ and $l_2\gg l_3,$ and $l_1\gg l_2$ and $l_1\gg l_3),$ one is led to guess at the expression for $A^T$ of two ideal parallel conductors without performing any calculation. The result, $A^T=-(2\mathrm{}ħ{\mathrm{cl}}_1{l}_2{l}_3/{\pi }^2){\sum }_{n_3=1\mathrm{}}^{\infty }{\sum }_{n_T=-\infty }^{\infty }{L}^{-4}{(n}_3{,n}_T),$ where ${L(n}_3{,n}_T)=[{(2n}_3{l}_3{)}^2{+(n}_T{l}_T{)}^2{]}^{1/2}$ is the length of a classical periodic path on a two-dimensional cylinder section, has been derived previously but never via periodic paths. At $T=0\mathrm{}$ the semiclassical approach provides a finite and systematic approximation scheme in terms of classical paths, which is useful when the normal modes of the cavity cannot be determined either explicitly or implicitly. Slightly extending the domain of applicability of Gutzwiller’s semiclassical periodic-orbit approach, we here evaluate the free energy at $T>\mathrm{}0\mathrm{}$ in terms of periodic classical paths in a four-dimensional cavity, which is the tensor product of the original cavity and a circle. The validity of this approach is at present restricted to particular systems.

We consider a general central-field system in D dimensions and show that the division of the kinetic energy into radial and angular parts proceeds differently in the wave-function picture and the Weyl-Wigner phase-space picture. Thus, the radial and angular kinetic energies are different quantities in the two pictures, containing different physical information, but the relation between them is well defined. We discuss this relation and illustrate its nature by examples referring to a free particle and to a ground-state hydrogen atom.

An ideal and reversible transfer technique for the quantum state between light and metastable collective states of matter is presented and analyzed in detail. The method is based on the control of photon propagation in coherently driven three-level atomic media, in which the group velocity is adiabatically reduced to zero. Form-stable coupled excitations of light and matter (“dark-state polaritons”) associated with the propagation of quantum fields in electromagnetically induced transparency are identified, their basic properties discussed and their application for quantum memories for light analyzed.

We study design challenges associated with realizing a ground-state quantum computer. In such a computer, it is necessary that the energy gap between the ground state and first excited state be sufficiently large to prevent disruptive excitations. Here, an estimate of this gap is provided as a function of computer size. We then address the problem of detecting the output of a ground-state quantum computer. It is shown that the exponential detection difficulties that appear to be present at first can be overcome by small design changes.