Within Newton-Schrödinger quantum mechanics, which allows gravitational self-interaction, it is shown that a no-split no-collapse measurement scenario is possible. A macroscopic pointer moves at low acceleration, controlled by the Ehrenfest-averaged force acting on it. That makes classicality self-sustaining, resolves Everett’s paradox, and outlines a route to spontaneous emergence of the quantum randomness. Numerical estimates indicate that enhanced short-range gravitational forces are needed for the scenario to work. The scheme fails to explain quantum nonlocality, including two-detector anticorrelations, which points towards the need of a nonlocal modification of the Newton-Schrödinger coupling scheme.

We devise a non-Hermitian Rayleigh-Schrödinger perturbation theory for the single- and the multireference case to tackle both the many-body problem and the decay problem encountered, for example, in the study of electronic resonances in molecules. A complex absorbing potential (CAP) is employed to facilitate a treatment of resonance states that is similar to the well-established bound-state techniques. For the perturbative approach, the full CAP-Schrödinger Hamiltonian, in suitable representation, is partitioned according to the Epstein-Nesbet scheme. The equations we derive in the framework of the single-reference perturbation theory turn out to be identical to those obtained by a time-dependent treatment in Wigner-Weisskopf theory. The multireference perturbation theory is studied for a model problem and is shown to be an efficient and accurate method. Algorithmic aspects of the integration of the perturbation theories into existing ab initio programs are discussed, and the simplicity of their implementation is elucidated.

We extend application of our lowest-order perturbative approach (in electron-electron correlation) for analysis of photo-double-ionization (PDI) of He [A.Y. Istomin et al., J. Phys. B 35, L543 (2002)] to excess energies up to $450\text{eV}$ and to analysis of circular dichroism. We find that account of electron correlation in the final state to first order provides predictions for the triply differential cross section and circular dichroism that are in reasonable agreement with absolute data for excess energies up to $80\text{eV}$. For an excess energy of $450\text{eV}$, account of electron correlation in both initial and final states is necessary and the predicted triply differential cross sections are in agreement with absolute data only for large mutual ejection angles. We find that at excess energies of a few tens of eV, the PDI is dominated by the “virtual” knock-out mechanism, while the “direct” (on-shell) knock-out process gives only small contributions for large mutual ejection angles. As a result, we conclude that the circular dichroism effect at these energies originates from the nonzero electron Coulomb phase shifts.

We discuss rotational excitation of molecules by a pair of left and right circularly polarized laser pulses with opposite chirps. The pulses are supposed to be short enough (picosecond or femtosecond) to prevent relaxation, sufficiently intense to induce adiabatic evolution, and far-off-resonant, e.g., infrared. This technique has been demonstrated recently by Villeneuve et al. [Phys. Rev. Lett. 85, 542 (2000)] in rotational dissociation of molecules. We analyze the properties of this technique by using the concepts of level crossing and adiabatic evolution, which allow us to derive analytically the conditions for efficient excitation. We analyze both the cases of intuitive (divergent frequencies) and counterintuitive (convergent frequencies) chirps and examine various initial conditions, including a single $J$ state, coherent and incoherent superpositions of $J$ states. We propose a technique, which can create superrotors by applying a pair of appropriately timed narrow pulses.

We present a complete set of analytical and invariant expressions for the steady-state density matrix of atoms in a resonant radiation field with arbitrary intensity and polarization. The field drives the closed dipole transition with arbitrary values of the angular momenta $J_g$ and $J_e$ of the ground and excited state. The steady-state density matrix is expressed in terms of spherical harmonics of a complex direction given by the field polarization vector. The generalization to the case of broadband radiation is given. We indicate various applications of these results.

We review various unitary time-dependent perturbation theories and compare them formally and numerically. We show that the Kolmogorov-Arnold-Moser technique performs better owing to both the superexponential character of correction terms and the possibility to optimize the accuracy of a given level of approximation which is explored in detail here. As an illustration, we consider a two-level system driven by short pulses beyond the sudden limit.

We report ab initio correlated relativistic calculations of the effective electric field $W_d$ acting on the electron in two excited electronic states of PbO, required for extracting the electric dipole moment of the electron from an ongoing experiment at Yale, which has the potential of improving accuracy for this elusive property by several orders of magnitude. The generalized relativistic effective core potential and relativistic coupled cluster methods are used, followed by nonvariational one-center restoration of the four-component wave function in the heavy atom core. $W_d$ is $-3.2\times {10}^{24}\text{Hz}∕\left(e\text{cm}\right)$ for the $a\left(1\right)$ state and $-9.7\times {10}^{24}\text{Hz}∕\left(e\text{cm}\right)$ for the $B\left(1\right)$ state. Comparison of calculated and experimental values of the hyperfine constant $A_{\Vert }$ provides an accuracy check for the calculation.

We show that the atom-molecule mixture formed in a degenerate atomic Fermi gas with interspecies repulsion near a Feshbach resonance constitutes a peculiar system where the atomic component is almost nondegenerate but quantum degeneracy of molecules is important. We develop a thermodynamic approach for studying this mixture, explain experimental observations, and predict optimal conditions for achieving molecular Bose-Einstein condensation.

Brightness of high-order harmonics is optimized in the space domain by profile flattening and self-guiding of intense femtosecond laser pulse and in the time domain by controlling the laser chirp. The profile flattening and self-guiding of the laser pulse propagating through a long gas jet effectively increased the phase-matched harmonic generation volume, thereby obtaining strong harmonics with low beam divergence, and the laser chirp control allowed the generation of spectrally sharp harmonics. At optimized conditions, the $61\text{st}$ harmonic, obtained at $134\mathrm{\AA }$ from Ne, had a brightness of about $1\times {10}^{15}\mathrm{W}∕{\text{cm}}^2∕\text{srad}$ with a beam divergence of 0.5 mrad and a spectral bandwidth of $0.7\mathrm{\AA }$.

We describe the formalism for optimally estimating and controlling both the state of a spin ensemble and a scalar magnetic field with information obtained from a continuous quantum limited measurement of the spin precession due to the field. The full quantum parameter estimation model is reduced to a simplified equivalent representation to which classical estimation and control theory is applied. We consider both the tracking of static and fluctuating fields in the transient and steady-state regimes. By using feedback control, the field estimation can be made robust to uncertainty about the total spin number.

For two qubits in a pure state there exists a one-to-one relation between the entanglement measure (the concurrence $\mathcal{C}$) and the maximal violation $\mathcal{M}$ of a Bell inequality. No such relation exists for the three-qubit analog of $\mathcal{C}$ (the tangle $\tau$), but we have found that numerical data is consistent with a simple set of upper and lower bounds for $\tau$ given $\mathcal{M}$. The bounds on $\tau$ become tighter with increasing $\mathcal{M}$, so they are of practical use. The Svetlichny form of the Bell inequality gives tighter bounds than the Mermin form. We show that the bounds can be tightened further if the tangle is replaced by an entanglement monotone that can identify both the W state and the Greenberger-Horne-Zeilinger state.

We introduce and analyze a model system based on a deformation of a spherical pendulum that can be used to reproduce large amplitude bending vibrations of flexible triatomic molecules with two stable linear equilibria. On the basis of our model and the recent vibrational potential [J. Chem. Phys. 115, 3706 (2001)], we analyze the HCN∕CNH isomerizing molecule. We find that HCN∕CNH has no monodromy and introduce the second global bending quantum number for this system at all energies where the potential is expected to work. We also show that LiNC∕LiCN is a qualitatively different system with monodromy.

We have developed a theoretical method for interpretation of photoionization experiments with the ${\mathrm{H}}_3$ molecule. In the present study we give a detailed description of the method, which combines multichannel quantum defect theory, the adiabatic hyperspherical approach, and the techniques of outgoing Siegert pseudostates. The present method accounts for vibrational and rotation excitations of the molecule, deals with all symmetry restrictions imposed by the geometry of the molecule, including vibrational, rotational, electronic, and nuclear spin symmetries. The method was recently applied to treat dissociative recombination of the $\mathrm{H}_3{}^{+}$ ion. Since $\mathrm{H}_3{}^{+}$ dissociative recombination has been a controversial problem, the present study also allows us to test the method on the process of photoionization, which is understood better than dissociative recombination. Good agreement with two photoionization experiments is obtained.

We develop a theoretical model to describe simultaneous ionization excitation and double ionization of a two-electron atomic shell (i.e., $\text{He}1s^2$) by a fast charged-particle impact. The model accounts for the projectile-target interaction to the second order whereas the interaction of the two target electrons ejected into continuum is treated nonperturbatively by the convergent close-coupling (CCC) method. In the second-order term, all intermediate states of the target between two subsequent interactions with the projectile are weighted equally with an average energy denominator (the so-called closure approximation) and only the dipole interaction of the projectile with the target is included. The model is suited to describe two-electron ionization processes with large projectile velocity, small momentum transfer, and small to intermediate energy of the ejected electrons. The model is applied to ionization excitation and double ionization of He by electron and positron impact in kinematics of recent experiments.

The interaction between the beryllium atom and short laser fields is treated on the basis of the nonrelativistic time-dependent Schrödinger equation. We calculate total cross sections and electron energy distributions for the double ionization of the valence electrons through one- and two-photon absorption. We introduce a model potential to represent the $1s^2$ core. The emphasis is put on the double ionization of the valence electrons in the context of ultrashort pulses. Results will be also presented for the ionization of the inner electrons.

The spontaneous spin polarization and bifurcation delay in two-component Bose-Einstein condensates coupled with laser or∕and radio-frequency pulses are investigated. We find that the bifurcation and the spontaneous spin polarization are determined by both physical parameters and relative phase between two condensates. Through bifurcations, the system enters into the spontaneous spin polarization regime from the Rabi regime. We also find that bifurcation delay appears when the parameter is swept through a static bifurcation point. This bifurcation delay is responsible for metastability leading to hysteresis.

Spontaneous transverse pattern formation is experimentally studied in a ${\text{BaTiO}}_3$ photorefractive oscillator under degenerate four-wave mixing conditions. A near self-imaging resonator of high Fresnel number and quasi-one-dimensional in the transverse plane is used. A fine control technique of the cavity detuning, $\Omega$, is described. It allows a precise study of the relation of $\Omega$ with the transverse wave number $k_{\perp }$ of the roll patterns selected by the system. The law $k_{\perp }^2=-\Omega ∕a$ is verified, which evidences that wave-number selection is mainly dictated by the cavity geometry. The experimentally obtained value of the diffraction parameter $a$ matches the theoretical prediction within the error intervals. The bifurcation diagram obtained by varying $\Omega$ shows homogeneous states and domain walls for positive $\Omega$, roll patterns for negative $\Omega$, and nonperiodic patterns for intermediate, negative detuning values. The latter are interpreted also in the frame of the cavity selection mechanism by taking into account the finesse of the cavity.

The intermolecular interaction potentials of methane $\left({\text{CH}}_4\right)$ dimer are calculated within the density functional theory in the local density approximation (LDA). It is found that the calculated potentials have minima when the intermolecular distance of ${\text{CH}}_4$ dimer is about $7.0\text{a.u.}$, which is in good agreement with the experiment. The depth of the potential is $0.017\text{eV}$. The results obtained by our LDA calculations seem to agree well with those obtained by MP2, MP3, and CCSD from the Møller-Plesset and coupled cluster methods by Tsuzuki et al. and with the experimental data.

We report on calculations of the elastic cross section and thermalization rate for collision between two maximally spin-polarized chromium atoms in the cold and ultracold regimes, relevant to buffer-gas and magneto-optical cooling. We calculate ab initio potential-energy curves for ${\text{Cr}}_2$ and the van der Waals coefficient $C_6$, and construct interaction potentials between two colliding Cr atoms. We explore the effect of shape resonances on the elastic cross section, and find that they dramatically affect the thermalization rate. Our calculated value for the $s$-wave scattering length is compared in magnitude with a recent measurement. We propose that a dip in the elastic rate coefficient could be experimentally exploited to obtain an accurate value for the scattering length.

We unify the quantum Zeno effect (QZE) and the “bang-bang” (BB) decoupling method for suppressing decoherence in open quantum systems: in both cases strong coupling to an external system or apparatus induces a dynamical superselection rule that partitions the open system’s Hilbert space into quantum Zeno subspaces. Our unification makes use of von Neumann’ s ergodic theorem and avoids making any of the symmetry assumptions usually made in discussions of BB. Thus we are able to generalize the BB to arbitrary fast and strong pulse sequences, requiring no symmetry, and to show the existence of two alternatives to a pulsed BB: continuous decoupling and pulsed measurements. Our unified treatment enables us to derive limits on the efficacy of the BB method: we explicitly show that the inverse QZE implies that the BB can in some cases accelerate, rather than inhibit, decoherence.