In this paper, we consider a generalized measurement where one particular quantum signal is unambiguously extracted from a set of noncommutative quantum signals and the other signals are filtered out. Simple expressions for the maximum detection probability and its positive operator valued measure are derived. We apply such unambiguous quantum state filtering to evaluation of the sensing of decoherence channels. The bounds of the precision limit for a given quantum state of probes and possible device implementations are discussed.

Energies of the ground and a number of even- and odd-parity excited states of multi-valence-electron ions have been computed by a computational method in the framework of relativistic multireference many-body perturbation theory. Relativistic multireference perturbation calculations are reported for the ground and over 80 low-lying odd- and even-parity excited states of siliconlike argon $\left({\mathrm{Ar}}^{4+}\right)$ and aluminumlike iron $\left({\mathrm{Fe}}^{13+}\right)$ to demonstrate the unprecedented accuracy of the method. The theory deviates from experiment by less than 0.2% for all but a few excited levels in siliconlike argon. For the more highly ionized aluminumlike iron, the deviations are reduced to within 0.06%. Theoretical magnetic dipole and electric quadrupole transition rates of the lowest-lying ${}^2{P}_{3/2}^o$ state of aluminumlike Fe and Mn are evaluated, and lifetimes are compared with a recent ion trap experiment.

The dissociative recombination of an ${\mathrm{H}}_3^{+}$ ion after it is struck by a low-energy electron is important for understanding observations of ${\mathrm{H}}_3^{+}$ in diffuse interstellar clouds. At the same time, it is the simplest triatomic ion and for this reason its theoretical description can serve as a prototype for other polyatomic ions. Meanwhile, experimental determinations of the recombination rate have varied widely, which has resulted in some controversy and confusion. Until recently, it seemed unlikely that this problem could be resolved by theoretical studies because the mechanism of ${\mathrm{H}}_3^{+}$ dissociative recombination remained unclear. A recent study, however [Kokoouline et al., Nature (London) 412, 891 (2001)], provided evidence that the inclusion of the Jahn-Teller coupling can produce a dissociative recombination rate that overlaps the range of experimental observations. Here, we propose a theoretical description of the coupling between nuclear and electronic degrees of freedom in a polyatomic molecule, which describes the competition between autoionization and predissociation of the Rydberg states formed after an incident electron is captured. The method treats the vibrational and rotational excitations of the ion, accounts for all symmetry restrictions imposed by the geometry of the molecule, including vibrational, rotational, and electronic and nuclear-spin symmetries. The framework combines the multichannel quantum-defect theory, the adiabatic hyperspherical approach, and the techniques of outgoing-wave Siegert pseudostates. The proposed method can be applied to studies of dissociative recombination of other triatomic ions, including all the degrees of freedom quantum mechanically. Our calculations of the cross section and the recombination rate confirm that the Jahn-Teller effect is responsible for the large rate in ${\mathrm{H}}_3^{+}.$ Theoretical results for dissociative recombination of ${\mathrm{H}}_3^{+}$ are in agreement with storage-ring experiments.

Charge transfer processes due to collisions of ground state ${\mathrm{B}}^{2+}\left(2s{}^{2}S\right)$ ions with atomic hydrogen are investigated using the quantum-mechanical molecular-orbital close-coupling (MOCC) method. The MOCC calculations utilize ab initio adiabatic potentials and nonadiabatic radial and rotational coupling matrix elements obtained with the spin-coupled valence-bond approach. Total and state-selective cross sections and rate coefficients are presented. Comparison with the existing experiments shows our results to be in good agreement. When $E<\mathrm{}80\mathrm{}\mathrm{e}\mathrm{V}/\mathrm{u},$ the differences between the current total MOCC cross sections with and without rotational coupling are small $\mathrm{}(<3\%).$ Rotational coupling becomes more important with increasing energy: for collision energies $E>\mathrm{}400\mathrm{}\mathrm{e}\mathrm{V}/\mathrm{u},$ inclusion of rotational coupling increases the total cross section by 50%–80%, improving the agreement between the current calculations and experiments. For state-selective cross sections, rotational coupling induces mixing between different symmetries; however, its effect, especially at low collision energies, is not as important as had been suggested in previous work.

We reexamined charge-transfer cross sections for protons colliding with $\mathrm{Na}\mathrm{}\left(3s\right)\mathrm{}$ atoms for collision energies from the threshold at 1.7 eV to 40 eV using the recently developed hyperspherical close-coupling method. Our results disagree with the recent calculations by Dutta et al. [Phys. Rev. A 63, 022709 (2001)], but are in good agreement with the earlier calculations of Croft and Dickinson [J. Phys. B 29, 57 (1996)] except at energies below 3 eV. Our calculations support the doubt on the experimental data of Kushawaha.

By using density functional calculation with generalized gradient approximation, we have studied the structural and electronic properties of the zinc clusters. The lowest-energy structures of ${\mathrm{Zn}}_n$ $(n=2\mathrm{}–20)$ clusters are determined. Three kinds of growth pathways are obtained in the small zinc clusters from ${\mathrm{Zn}}_4$ to ${\mathrm{Zn}}_8$ and tetrahedron-based structures have favorable energy. The zinc clusters with 7–16 atoms are semiconductorlike. A structural transition from low coordination cagelike to high coordination compact structures is obtained around ${\mathrm{Zn}}_{17}.$ The ${\mathrm{Zn}}_n$ clusters with $n=4,$ 7, 9, 10, 14, 18, 20 show relatively high stability, consistent with the electron shell model and mass spectra. The ionization potentials of the ${\mathrm{Zn}}_n$ clusters are calculated and compared with conducting sphere droplet model. The size evolution of zinc clusters from van der Waals to covalent and bulk metallic behavior is discussed. The Zn clusters show stronger metallicity than the Cd and Hg clusters with same size.

We discuss the response of a Bose-Einstein condensate to a change in the scattering length, which is experimentally realized by tuning the magnetic field near a Feshbach resonance. In particular, we consider the collapse of the condensate induced by a sudden change in the scattering length from a large positive to a small negative value. We also consider the condensate dynamics that results from a single pulse in the magnetic field, due to which the scattering length is rapidly increased from zero to a large value and then after some time rapidly decreased again to its initial value. We focus primarily on the consequences of the quantum evaporation process on the dynamics of the Bose-Einstein condensate, but also discuss the effects of atom-molecule coherence.

We consider optimally spin-squeezed states that maximize the sensitivity of the Ramsey spectroscopy, and for which the signal-to-noise ratio scales as the number of particles N. Using the variational principle we prove that these states are eigensolutions of the Hamiltonian $H\left(\lambda \right)=\lambda S_z^2-S_x,$ and that, for large N, the states become equivalent to the quadrature squeezed states of the harmonic oscillator. We present numerical results that illustrate the validity of the equivalence.

We propose a scheme for transferring quantum states from the propagating light fields to macroscopic, collective vibrational degree of freedom of a massive mirror by exploiting radiation pressure effects. This scheme may prepare an Einstein-Podolsky-Rosen state in position and momentum of a pair of distantly separated movable mirrors by utilizing the entangled light fields produced from a nondegenerate optical parametric amplifier.

We determine the entanglement capability of a self-inverse Hamiltonian evolution, which reduces to the known result for the Ising Hamiltonian, and identify optimal input states for yielding the maximal entanglement rate. We introduce the concept of the operator entanglement rate, and find that the maximal operator entanglement rate gives a lower bound on the entanglement capability of a general Hamiltonian.

Capture and loss rates for protons moving in an electron gas are calculated using many-body perturbation theory. The role of the form of the bound-state wave function for weakly bound states around the proton is analyzed. We find significant differences (up to a factor of 2 higher) in the values of Auger capture and loss rates when using Hulthen-type instead of hydrogenic wave functions. Its relevance in stopping power is briefly discussed.

A technique is proposed to increase multiplicatively the pulse spacing of a periodic subfemtosecond-pulse train based on molecular modulation, with more pairs of driving lasers to control the higher-order stimulated Raman scattering. We demonstrate that the pulse spacing can be readily doubled, tripled, or even quadrupled by using pairs of off-resonant two-photon Raman transitions driven by laser beams of sufficient intensities and relatively shifted carrier frequencies. A periodic subfemtosecond-pulse train with a pulse width down to 0.53 fs and pulse spacing up to 376 fs can be generated with appropriate molecular coherence control of the fundamental rotational transition in molecular hydrogen.

In this Comment we question the recent analysis of two dense coding protocols presented by Lee, Ahn, and Hwang [Phys. Rev. A 66, 024304 (2002)]. We argue that in the case of two-party communication protocol, there is no reason for using a maximally entangled state of more than two qubits.

In a recent Brief Report, L. Lee, D. Ahn, and S. W. Hwang [Phys. Rev. A 66, 024304 (2002)] have claimed that using pairwise entangled qubits gives rise to an exponentially more efficient dense coding when two parties are involved than using maximally entangled qubits shared among N parties. Here we show that their claim is not true.

We present the results of a pump-probe laser spectroscopic investigation of the Doppler-broadened sodium $D1\mathrm{}$ resonance line. We find 13 resonances in the resulting spectra. These observations are well described by the numerical predictions of a four-level atomic model of the hyperfine structure of the sodium $D1\mathrm{}$ line. We also find that many, but not all, of these features can be understood in terms of processes originating in a two-level or three-level subset of the full four-level model. The processes we observed include forward near-degenerate four-wave mixing and saturation in a two-level system, difference-frequency crossing and nondegenerate four-wave mixing in a three-level V system, electromagnetically induced transparency and optical pumping in a three-level lambda system, cross-transition resonance in a four-level double-lambda system, and conventional optical pumping. Most of these processes lead to sub-Doppler or even subnatural linewidths. The dependence of these resonances on the pump intensity and pump detuning from atomic resonance are also studied.

The spectra of the hydrogen and antihydrogen atoms in the presence of an external electric field are compared. It is shown that the nonresonant corrections to the transition frequency may contain terms linear in the electric field. The existence of these terms does not violate space and time parity and leads to a difference in the resonant spectroscopic measurements for the hydrogen and antihydrogen atoms in an external electric field.

An interpretation is proposed for the dynamics of light filaments formed when a femtosecond laser pulse propagates in air. The pulse evolves as a set of coupled nonlinear oscillators forming a coherent structure over several Rayleigh lengths. The plasma generated by photoionization is numerically shown to be the single saturation mechanism of the beam self-focusing. Other physical processes such as group velocity dispersion and the quintic susceptibility ${\chi }^{\mathrm{}\left(5\right)\mathrm{}}$ for the polarization promote different propagation regimes. From theoretical expressions of the oscillation period for the spatial profile of the pulse, we show that the electron density in a femtosecond filament may be estimated.

We present the correct way to obtain the general solution of the Schrödinger equation for a particle in a time-dependent linear potential following the approach used in the paper of Guedes [Phys. Rev. A 63, 034102 (2001)]. In addition, we show that, in this case, the solutions (wave packets) are described by the Airy functions.

In this reply I discuss further the misconception in my paper [Phys. Rev. A 63, 034102 (2001)] related to the solution of the Schrödinger equation for a particle in a time-dependent linear potential.

We prepare a degenerate Fermi gas of potassium atoms by sympathetic cooling with rubidium atoms in a one-dimensional optical lattice. In a tight lattice, we observe a change of the density of states of the system, which is a signature of quasi two-dimensional confinement. We also find that the dipolar oscillations of the Fermi gas along the tight lattice are almost completely suppressed.