The contribution of the light-by-light diagram to the $g$ factor of an electron and muon bound in a Coulomb field is obtained. For an electron in a ground state, our results are in good agreement with the results of other authors obtained numerically for large $Z$. For relatively small $Z$ our results have essentially higher accuracy as compared to the previous ones. For muonic atoms, the contribution is obtained with a high accuracy in the whole region of $Z$.

The symmetry properties of the $S$ matrix in a fully relativistic distorted-wave treatment of electron-impact ionization are investigated. It is shown that the square modulus of the scattering matrix element in which the spin states of all four electrons are determined is not invariant under the reversal of the direction of alignment of all spins. The largest of two contributions to this noninvariance originates from the relativistic modifications of the continuum wave functions induced by the distorting potential of the target atom. A second smaller contribution is manifested on reducing the eight-dimensional matrix elements of the QED covariant propagator to purely spatial two-electron integrals. The triple differential cross section (TDCS) exhibits a spin asymmetry unless the entire scattering process occurs in a single plane. There will be a difference in the TDCS between an $(e,2e)$ event in which the initial beam is polarized parallel or antiparallel with respect to the beam direction even if the target is unpolarized and the final spin states are not determined. The TDCS will remain unchanged if, in addition to reversal of the direction of spin alignment, one appropriate momentum component of one of the two outgoing electrons is reversed.

Lowest-order nondipole effects are studied systematically in double photoionization (DPI) of the He atom. Ab initio parametrizations of the quadrupole transition amplitude for DPI from the ${}^{1}S_0$ state are presented in terms of the exact two-electron radial matrix elements. Analytic expressions for these matrix elements within lowest-order perturbation theory (LOPT) in the interelectron interaction are also given. The corresponding parametrizations for the dipole-quadrupole triply differential cross section (TDCS) are presented for the case of an elliptically polarized photon. A general analysis of retardation-induced asymmetries of the TDCS including the circular dichroism effect at equal energy sharing is presented. Numerical LOPT estimates of nondipole asymmetries in photoelectron angular distributions for the cases of linear and circular polarization and of the circular dichroism effect at equal energy sharing are presented. We find that experimental observation of nondipole effects at excess energies of the order of tens to hundreds of eV should be feasible in TDCS measurements. Our numerical results exhibit a nondipole forward-backward asymmetry in the TDCS for DPI of He at an excess energy of 450 eV that is in qualitative agreement with existing experimental data.

A great deal of effort has been applied to understanding population dynamics within a variety of coherent excitation schemes. The goal in such studies has been to understand the conditions necessary for efficient transfer of population from one state to another. While many theoretical treatments include the effects of natural lifetimes that are present in any given system, some neglect this important aspect when considering specific cases. Adiabatic approximations are also widely made. Additionally, it is often difficult to envision how the different parameters controlling efficient population transfer are interrelated or even which parameters are the most critical, especially when the decay lifetimes are taken into account. This work describes a numerical study of coherent excitation applied to a ${}^{87}\mathrm{Rb}$ ladder system where spontaneous decay rates are included, and adiabaticity is not assumed. A useful method is introduced to explore the interdependence of various excitation parameters. The efficiency of population transfer as a function of several experimentally controllable parameters is explored, and other general trends are summarized.

We investigate the correlations between any number of arbitrarily far-apart regions of the vacuum of the free Klein-Gordon field by means of its finite duration coupling to an equal number of localized detectors. We show that the correlations between any $N$ such regions enable us to distill an $N$-partite $W$ state, and therefore exhibit true $N$-fold entanglement. Furthermore, we show that for $N=3$, the correlations cannot be reproduced by a hybrid local-nonlocal hidden-variable model. For $N⩾4$ the issue remains open.

“Leakage” errors are particularly serious errors which couple states within a code subspace to states outside of that subspace, thus destroying the error protection benefit afforded by an encoded state. We generalize an earlier method for producing leakage elimination decoupling operations and examine the effects of the leakage eliminating operations on decoherence-free or noiseless subsystems which encode one logical, or protected qubit into three or four qubits. We find that by eliminating a large class of leakage errors, under some circumstances, we can create the conditions for a decoherence-free evolution. In other cases we identify a combined decoherence-free and quantum error correcting code which could eliminate errors in solid-state qubits with anisotropic exchange interaction Hamiltonians and enable universal quantum computing with only these interactions.

A Bell-type inequality is proposed in terms of Wigner-Yanase skew information, which is quadratic and involves only one local spin observable at each site. This inequality presents a hierarchic classification of all states of multipartite quantum systems from separable to fully entangled states, which is more powerful than the one presented by quadratic Bell inequalities from two-entangled to fully entangled states. In particular, it is proved that the inequality provides an exact test to distinguish entangled from nonentangled pure states of two qubits. Our inequality sheds considerable light on relationships between quantum entanglement and information theory.

We develop the hydrodynamic theory of Fermi superfluids in the presence of a periodic potential. The relevant parameters governing the propagation of sound (compressibility and effective mass) are calculated in the weakly interacting BCS limit. The conditions of stability of the superfluid motion with respect to creation of elementary excitations are discussed. We also evaluate the frequency of the center-of-mass oscillation when the superfluid gas is additionally confined by a harmonic trap.

We theoretically study the control of a weak quantum light in a resonant three-level condensed medium derived by a pair of resonant counterpropagating laser fields. We analytically demonstrate that the adiabatic switching on of one of the control fields generates an entangled state of photon wave packets moving together at subnormal slow velocity in the medium with relative amplitudes determined by the control laser fields. An abrupt switching-off the control field makes the entangled wave packets propagate in the opposite direction with temporally reversed profiles relative to each other. We have analyzed the quantum control of the entanglement generation in terms of the adiabatic and nonadiabatic switching operations of the coupling laser pulse.

We propose a scheme that allows us to coherently extract cold atoms from a reservoir in a deterministic way. The transfer is achieved by means of radiation pulses coupling two atomic states which are object to different trapping conditions. A particular realization is proposed, where one state has zero magnetic moment and is confined by a dipole trap, whereas the other state with nonvanishing magnetic moment is confined by a steep microtrap potential. We show that in this setup a predetermined number of atoms can be transferred from a reservoir, a Bose-Einstein condensate, into the collective quantum state of the steep trap with high efficiency in the parameter regime of present experiments.

We derive quantum polarization distributions on the Poincaré sphere for two-mode fields as suitable marginals of the standard $s$-ordered quadrature distributions that include the Wigner function. We compare them with previously introduced distributions directly defined on the sphere from first principles. We apply this approach to some relevant field states such as coherent and number states.

Bose-Einstein condensates have been produced in an optical box trap. This optical trap type has strong confinement in two directions comparable to that which is possible in an optical lattice, yet produces individual condensates rather than the thousands typical of a lattice. The box trap is integrated with single-atom detection capability, paving the way for studies of quantum atom statistics.

The coherence properties of a gas of bosonic atoms above the Bose-Einstein-condensation transition temperature were studied. Bragg diffraction was used to create two spatially separated wave packets, which interfere during expansion. Given sufficient expansion time, high fringe contrast could be observed in a cloud of arbitrary temperature. Fringe visibility greater than 90% was observed, which decreased with increasing temperature, in agreement with a simple model. When the sample was “filtered” in momentum space using long, velocity-selective Bragg pulses, the contrast was significantly enhanced in contrast to predictions.

We realize and investigate wavelength-flexible phase-coherent all-optical frequency division by 2. Frequency division is obtained via self-phase-locking in a degenerate continuous-wave (cw) optical parametric oscillator (OPO). The wavelength flexibility of the divider is based on the use of quasi-phase-matching with perpendicular polarizations of the OPO output waves (type II). Mutual injection of the subharmonic waves is achieved by using an intracavity quarter-wave plate. A locking range of up to $160\mathrm{MHz}$ is observed experimentally, and a stable, self-phase-locked operation of the OPO is achieved over typically $15\min$. For the first time, we measure the frequency stability of the divider by recording the relative phases of the subharmonic waves as a function of time. For a measurement time interval of $40\mathrm{s}$, we measure a residual frequency instability of the divider of $8\times {10}^{-18}$. We demonstrate a full control of the OPO’s output-wave phase difference and observe the related change in power ratio of the subharmonic waves in agreement with the theoretically expected behavior. We propose that this possibility to monitor the divider’s drift within the locking range via the power ratio can be used for stabilizing that drift in order to achieve a significant improvement of the long-term stability of the divide-by-2 OPO.

In this Brief Report we present a scheme for the implementation of frequency up- and down-conversion operations in two-mode cavity quantum electrodynamics (QED). This protocol for engineering bilinear two-mode interactions could enlarge perspectives for quantum-information manipulation and also be employed for fundamental tests of quantum theory in cavity QED. As an application we show how to generate a two-mode squeezed state in cavity QED (the original entangled state of Einstein, Podolsky, and Rosen).

The well-known duality relating entangled states and noisy quantum channels is expressed in terms of a channel ket, a pure state on a suitable tripartite system, which functions as a pre-probability allowing the calculation of statistical correlations between, for example, the entrance and exit of a channel, once a framework has been chosen so as to allow a consistent set of probabilities. In each framework the standard notions of ordinary (classical) information theory apply, and it makes sense to ask whether information of a particular sort about one system is or is not present in another system. Quantum effects arise when a single pre-probability is used to compute statistical correlations in different incompatible frameworks, and various constraints on the presence and absence of different kinds of information are expressed in a set of all-or-nothing theorems which generalize or give a precise meaning to the concept of “no-cloning.” These theorems are used to discuss the location of information in quantum channels modeled using a mixed-state environment, the classical-quantum channels introduced by Holevo, and the location of information in the physical carriers of a quantum code. It is proposed that both channel and entanglement problems be classified in terms of pure states (functioning as pre-probabilities) on systems of $p⩾2$ parts, with mixed bipartite entanglement and simple noisy channels belonging to the category $p=3$, a five-qubit code to the category $p=6$, etc., then by the dimensions of the Hilbert spaces of the component parts, along with other criteria yet to be determined.

A two-color $(3+1^{\prime })$ pump-probe scheme is used to create Rydberg wave packets in krypton atoms. The ac-Stark shift, caused by the high intensity pump pulse, allows access to states with principal quantum numbers $n=5–10$ over a broad energy range. States shifted by as much as $3000{\mathrm{cm}}^{-1}$ during the pump pulse are coherently excited, resulting in wave-packet superpositions of field free states. By resolving the kinetic energy of the photoelectrons produced in the ionization probe step, numerous quantum beats are resolved and assigned. In addition, the energies of four previously unmeasured $g$ states are determined.

Among the 14 conformers of the cubic water octamer, there are 114 symmetry-unique pairs of nearest-neighbor water molecules. The pairs can be divided into 5 classes based on hydrogen bonding topology. These 5 possible relative orientations are studied via isotopic replacement. Individual calculations were carried out by systematically replacing selected subsets of the 16 hydrogen atoms of a given ${\left({\mathrm{H}}_2\mathrm{O}\right)}_8$ conformer with deuterium atoms and then performing second-order Møller-Plesset perturbation theory calculations of the clusters. From this analysis, it was found that, for a given orientation, the vibrational frequencies of the covalent $\mathrm{O}\bullet \bullet \mathrm{H}$ bonds for the 5 nearest-neighbor water pairs fall into distinct ranges. This information can be used to spectroscopically identify the different water structures by their unique water topologies and unique spectral signature.

A protocol for quantum secure direct communication with quantum superdense coding is proposed. It combines the ideas of block transmission, the ping-pong quantum secure direct communication protocol, and quantum superdense coding. It has the advantage of being secure and of high source capacity.

As a model, the Pais-Uhlenbeck fourth order oscillator with equation of motion: $(d^{4}q∕d{t}^4)+({\omega }_1^2+{\omega }_2^2)(d^{2}q∕d{t}^2)+{\omega }_1^2{\omega }_2^{2}q=0$ is a quantum-mechanical prototype of a field theory containing both second and fourth order derivative terms. With its dynamical degrees of freedom obeying constraints due to the presence of higher order time derivatives, the model cannot be quantized canonically. We thus quantize it using the method of Dirac constraints to construct the correct quantum-mechanical Hamiltonian for the system, and find that the Hamiltonian diagonalizes in the positive and negative norm states that are characteristic of higher derivative field theories. However, we also find that the oscillator commutation relations become singular in the ${\omega }_1\to {\omega }_2$ limit, a limit which corresponds to a prototype of a pure fourth order theory. Thus the particle content of the ${\omega }_1={\omega }_2$ theory cannot be inferred from that of the ${\omega }_1\ne {\omega }_2$ theory; and in fact in the ${\omega }_1\to {\omega }_2$ limit we find that all of the ${\omega }_1\ne {\omega }_2$ negative norm states move off shell, with the spectrum of asymptotic in and out states of the equal frequency theory being found to be completely devoid of states with either negative energy or negative norm. As a byproduct of our work we find a Pais-Uhlenbeck analog of the zero energy theorem of Boulware, Horowitz, and Strominger, and show how in the equal frequency Pais-Uhlenbeck theory the theorem can be transformed into a positive energy theorem instead.