We prove that the entangling capacity of a two-qubit unitary operator without local ancillas, both with and without the restriction to initial product states, as quantified by the maximum attainable concurrence, is directly related to the distinguishability of a closely related pair of two-qubit unitary operators. These operators are the original operator transformed into its canonical form and the adjoint of this canonical form. The distinguishability of these operators is quantified by the minimum overlap of the output states over all possible input probe states. The entangling capacity of the original unitary operator is therefore directly related to the degree of non-Hermiticity of its canonical form, as quantified in an operationally satisfactory manner in terms of the extent to which it can be distinguished, by measurement, from its adjoint. Furthermore, the maximum entropy of entanglement, again without local ancillas, that a given two-qubit unitary operator can generate is found to be closely related to the classical capacities of certain quantum channels.

We propose a potential scheme to generate entangled photons by manipulating trapped ions embedded in two-mode microcavities assisted by a magnetic field gradient. By means of the spin-spin coupling due to the magnetic field gradient and the Coulomb repulsion between the ions, we show how to efficiently generate entangled photons by detecting the internal states of the trapped ions. We emphasize that our scheme is advantageous to create complete sets of entangled multiphoton states. The requirements and the experimental feasibility of our proposal are discussed in detail.

The collision-induced, rototranslational absorption spectrum of compressed methane gas is computed, based purely on the reliably known, leading multipole-induced dipole components of ${\mathrm{CH}}_4$ molecular pairs. In contrast to previous work of the kind no ad hoc empirical corrections of unknown exchange force-induced dipole components are attempted. Not surprisingly, the calculated spectra show a sizeable absorption defect at virtually all frequencies, when compared to existing laboratory measurements. The defect suggests the presence of dipole-induction mechanisms in addition to those due to the leading multipole-induced dipole terms. The excess absorption, the differences between measured and calculated spectra, resembles in certain ways the excess absorption spectra seen at the same frequencies in methane-$X$ gas mixtures, where $X$ stands for helium, hydrogen, or nitrogen, respectively [Buser and Frommhold, J. Chem. Phys. 122, 024301 (2005)]. To a large extent, the excess absorption seems to be related to collisional distortions of the tetrahedral frame of the unperturbed ${\mathrm{CH}}_4$ molecule.

The hyperspherical $\mathcal{R}$-matrix method with semiclassical outgoing waves, designed to provide accurate double-ionization cross sections, is extended to allow for the computation of ionization-excitation data of comparable quality. Accordingly, it appears now as a complete method for treating the correlated dynamics of two-electron atoms, in particular above their full fragmentation threshold. Cross sections ${\sigma }_n$ and asymmetry parameters ${\beta }_n$ are obtained for single photoionization of helium with excitation of the residual ion up to as high a level as $n=50$ at $0.1\mathrm{eV}$ above the double-ionization threshold. These data are extrapolated to infinite values of $n$ in order to check widespread assumptions regarding this limit. Our data are found consistent with the assumed $n^{-3}$ dependence of the partial ionization cross sections. However, the ${\beta }_{\infty }=-0.636$ obtained still lies far from the $-1$ value expected at the double-ionization threshold.

Vibrationally resolved O $1s$ photoelectron angular distributions from $\mathrm{C}{\mathrm{O}}_2$ molecules, aligned parallel and perpendicular to the electric vector of the incident light, have been measured in the $5{\sigma }_g^{*}$ shape resonance region, with photon energies up to 2 eV above the O $1s$ ionization threshold, using multiple-coincidence electron-ion momentum imaging spectroscopy. The angular distributions depend on the vibrational quanta of the antisymmetric vibrations in the O $1s$ ionized state but do not vary significantly as a function of the photon energy across the $5{\sigma }_g^{*}$ shape resonance.

The $(e,2e)$ cross sections for transitions to the $n=1$, 2, and 3 final state of ${\mathrm{He}}^{+}$ have been measured at impact energies of 1000 and $1600\mathrm{eV}$ by using a newly developed energy and momentum dispersive spectrometer. Binding energy spectra in the range of $8\text{to}86\mathrm{eV}$ and momentum profiles for the transitions to the ground and excited ion states of ${\mathrm{He}}^{+}$ are reported at different impact energies, and the experimental results are compared with plane wave impulse approximation calculations. The impact energy dependence of cross section ratios for the ${\mathrm{He}}^{+}n=1$, 2, and 3 final states are obtained. Some discrepancies are observed between experimental data and theoretical calculations.

We characterize the avoided crossings in a two-parameter, time-periodic system which has been the basis for a wide variety of experiments. By studying these avoided crossings in the near-integrable regime, we are able to determine scaling laws for the dependence of their characteristic features on the nonintegrability parameter. As an application of these results, the influence of avoided crossings on dynamical tunneling is described and applied to the recent realization of multiple-state tunneling in an experimental system.

Various scenarios of self-focusing of axisymmetric few-cycle light pulses in optical media with nonresonant dispersion and nonlinearity are investigated numerically. It is shown that the spatiotemporal dynamics leads to the formation of complex light structures like dumbbells or bubbles depending on the initial intensity. The bubble includes a center area with emptiness, where the electrical field is practically nullified. An analysis is made of the characteristics of spectral supercontinuum generation accompanying the dispersive self-focusing. The development of a broad blueshifted spectral wing during the breakdown of a shock wave of the pulse profile is considered in detail.

Some years ago Peres [Phys. Rev. A 46, 4413 (1992)] described a gedanken experiment for a pair of spatially spin $j$ particles in a singlet state and showed using with a dichotomic observable (essentially a parity operator) that Bell’s theorem in the form of the Clauser-Home-Shimony-Holt (CHSH) inequality is violated by a constant amount (24%) in the limit $j\to \infty$. In this paper we present a scheme for an optical realization of a state that is very close to the spin-$j$ singlet state using two traveling-wave modes of the quantized field using a $50:50$ beam splitter with an input number state. A near-singlet states comes about because the binomial output state of the beam splitter can be written as a sum in terms of states in the form ${\mid j,m⟩}_1\otimes {\mid j,-m⟩}_2$, each state being associated with a Holstein-Primakoff realization of the su(2) spin algebra in terms of the Bose operators of each of the field modes, where $j=N∕2,N$ being the number of photons passing through the beam splitter. The binomial state can violate the CHSH inequality to a greater degree than does the singlet state.

We discuss continuous observation of the momentum of a single atom by employing the high velocity sensitivity of the index of refraction in a driven $\Lambda$-system based on electromagnetically induced transparency. In the ideal limit of unit collection efficiency this provides a quantum-limited measurement with minimal backaction on the atomic motion. A feedback loop, which drives the atom with a force proportional to measured signal, provides a cooling mechanism for the atomic motion. We derive the master equation which describes the feedback cooling and show that in the Lamb-Dicke limit the steady state energies are close to the ground state, limited only by the photon collection efficiency. Outside of the Lamb-Dicke regime the predicted temperatures are well below the Doppler limit.

We report a direct experimental observation of antiphase oscillations in population dynamics in lasers. We show that these population oscillations are intrinsically related to the well-known antiphase polarization dynamics, i.e., the antiphase oscillations of two orthogonal polarization laser field states. We have used a class $B$ Nd:YAG (yttrium aluminum garnet) laser.

Some aspects of plasma kinetics in a Ni-like Ag soft x-ray laser have been analyzed, starting from the real shape of the pump laser pulse measured with a third-order correlator. The role for pump energy reduction of a complex pump pulse structure at a low intensity level has been identified, and the modeling results on the gain lifetime correspond well with the length of the output x-ray pulse measured in the experiment.

Multicharged Kr ions have been measured using monochromatized undulator radiation combined with a coincidence technique. It has been found that a charge-state distribution of Kr ions being coincident with satellite peaks of Kr $2p_{3∕2}$ photoelectron is slightly different from that for the main line. Resonant Auger peaks for $2p^{-1}\mathrm{nl}\to {}^{1}G_4\mathrm{nl}$ transitions generated essentially ${\mathrm{Kr}}^{4+}$ only, which differs from the charge-state distribution for the normal Auger peak. These findings suggest that loosely bound electrons in high Rydberg orbitals are easily shaken-off in electron emission processes.

Composite pulses are a quantum control technique for canceling out systematic control errors. We present a different composite pulse sequence inspired by quantum search. Our technique can correct a wider variety of systematic errors—including, for example, nonlinear over-rotational errors—than previous techniques. Concatenation of the pulse sequence can reduce a systematic error to an arbitrarily small level.

We present the optimal phase estimation for qubits in mixed states, for an arbitrary number of qubits prepared in the same state.

We investigate the optimal distribution of quantum information over multipartite systems in asymmetric settings. We introduce cloning transformations that take $N$ identical replicas of a pure state in any dimension as input and yield a collection of clones with nonidentical fidelities. As an example, if the clones are partitioned into a set of $M_A$ clones with fidelity $F^A$ and another set of $M_B$ clones with fidelity $F^B$, the trade-off between these fidelities is analyzed, and particular cases of optimal $N\to M_A+M_B$ cloning machines are exhibited. We also present an optimal $1\to 1+1+1$ cloning machine, which is an example of a tripartite fully asymmetric cloner. Finally, it is shown how these cloning machines can be optically realized.

Certain quantum-information tasks require entanglement of assistance, namely, a reduction of a tripartite entangled state to a bipartite entangled state via local measurements. We establish that concurrence of assistance (COA) identifies capabilities of and limitations to producing pure bipartite entangled states from pure tripartite entangled states and prove that COA is an entanglement monotone for $(2\times 2\times n)$-dimensional pure states. Moreover, if the COA for the pure tripartite state is at least as large as the concurrence of the desired pure bipartite state, then the former may be transformed to the latter via local operations and classical communication, and we calculate the maximum probability for this transformation when this condition is not met.

A comparative study of the vibrational deactivation of ${}^{14}\mathrm{N}_2{}^{+}$ ($\nu =1$, $j=0$) in collision with ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ atoms is investigated. Close-coupling calculations are carried out in the ${10}^{-6}–2000\text{−}{\mathrm{cm}}^{-1}$ kinetic energy range. At very low kinetic energy, it is found that the cross sections for the collision involving ${}^3\mathrm{He}$ is strongly enhanced by the presence of a virtual state associated with the van der Waals well. The change of mass from ${}^3\mathrm{He}$ to ${}^4\mathrm{He}$ also erases a strong shape resonance at $5\times {10}^{-3}{\mathrm{cm}}^{-1}$ supported by the van der Waals well. As a result, the law of variation of the associated vibrational quenching rate coefficient is found to be very different for these two systems. This behavior is further analyzed by drawing the real part of the scattering length as a function of the variable mass of a fictitious He atom. Two zero energy resonances are identified for a mass of, respectively, about 3.18 and 3.85 atomic mass units.

We calculate the nonlinear effects that appear at the level of the stopping cross section when heavy bare projectiles collide with both atomic hydrogen and a free electron gas. Calculations are carried out by using distorted-wave methods. For hydrogen targets we employ the continuum-distorted-wave eikonal initial-state approximation, while for the free electron gas we use the Coulomb-Lindhard approach. Results are compared with exact results for the quantum harmonic oscillator target. Ratios to the first-order stopping are analyzed in detail and their structures are interpreted in terms of Padé approximants. We also investigate the dependency of the stopping ratios on the impact parameter.