With suitably directed initial states, we show how the time-averaged density of an evolving wave packet localizes on a mirror plane. A classical analog of this behavior can sometimes be found with trajectories weighted according to a Wigner distribution of the initial quantum state. However, in the limit of strongly chaotic classical dynamics, no such classical analog exists and the quantum localization in the density tends to be stronger by a factor of 2. Two very different systems are used to illustrate this effect, one being a three-dimensional model for a lithium atom moving within a ${\text{C}}_{60}$ cage and the other being a two-dimensional double well problem.

The quantum dynamics of a spin chain interacting with multiple bosonic baths is described in a mixed Wigner-Heisenberg representation. The formalism is illustrated by simulating the time evolution of the reduced density matrix of two coupled spins, where each spin is also coupled to its own bath of harmonic oscillators. In order to prove the validity of the approach, an analytical solution in the Born-Markov approximation is found. The agreement between the two methods is shown.

For certain joint measurements on a pair of spatially separated particles, we ask how much entanglement is needed to carry out the measurement exactly. For a class of orthogonal measurements on two qubits with partially entangled eigenstates, we present upper and lower bounds on the entanglement cost. The upper bound is based on a recent result by Berry [Phys. Rev. A 75, 032349 (2007)]. The lower bound, based on the entanglement production capacity of the measurement, implies that for almost all measurements in the class we consider, the entanglement required to perform the measurement is strictly greater than the average entanglement of its eigenstates. On the other hand, we show that for any complete measurement in $d\times d$ dimensions that is invariant under all local generalized Pauli operations, the cost of the measurement is exactly equal to the average entanglement of the states associated with the outcomes.

We derive the fully retarded energy shift of a neutral atom in two different geometries that are useful for modeling etched microstructures. First, we calculate the energy shift due to a reflecting cylindrical wire, and then we work out the energy shift due to a semi-infinite reflecting half-plane. We analyze the results for the wire in various limits of the wire radius and the distance of the atom from the wire, and obtain simple asymptotic expressions useful for estimates. For the half-plane we find an exact representation of the Casimir-Polder interaction in terms of a single fast converging integral, which is easy to evaluate numerically.

Fisher divergences (FDs) and Jensen-Shannon divergences (JSDs) are used in this work to quantify the informational discrepancies between the one-particle electron densities of neutral atoms, singly charged ions, and isoelectronic series. These dissimilarity magnitudes, computed for a set of 319 atomic systems in both position and momentum spaces, provide relevant information concerning pattern, structure, and periodicity properties of the ionization processes. In particular an apparent correlation between extremal values of the atomic ionization potential and the divergences is found. Results are compared with those obtained by quantum similarity techniques.

We compute the $T=0\text{K}$ Van der Waals (nonretarded Casimir) interaction energy $E$ between two infinitely long, crossed conducting wires separated by a minimum distance $D$ much greater than their radius. We find that, up to a logarithmic correction factor, $E\propto -D^{-1}{\left|\text{sin}\theta \right|}^{-1}f\left(\theta \right)$, where $f\left(\theta \right)$ is a smooth bounded function of the angle $\theta$ between the wires. We recover a conventional result of the form $E\propto -D^{-4}{\left|\text{sin}\theta \right|}^{-1}g\left(\theta \right)$ when we include an electronic energy gap in our calculation. Our prediction of gap-dependent energetics may be observable experimentally for carbon nanotubes either via atomic force microscopy detection of the Van der Waals force or torque or indirectly via observation of mechanical oscillations. This shows that strictly parallel wires, as assumed in previous predictions, are not needed to see a unique effect of this type.

Absolute partial cross sections for dissociative electron attachment to $\text{H}–\text{C}\equiv \text{C}–\text{H}$ and $\text{D}–\text{C}\equiv \text{C}–\text{D}$ have been measured using a time-of-flight ion spectrometer combined with a trochoidal electron monochromator. For ${\text{C}}_2{\text{H}}_2$ the present data are in good agreement with the total cross sections measured previously in our laboratory using the total ion-collection method. The present measurements on deuterated acetylene revealed a pronounced isotope effect at the first dissociative electron attachment band at 3 eV, with the cross section for ${\text{C}}_2{\text{D}}^{-}/{\text{C}}_2{\text{D}}_2$ being 14.4 times smaller than that for ${\text{C}}_2{\text{H}}^{-}/{\text{C}}_2{\text{H}}_2$. The dominant fragments at the second dissociative attachment band around 8 eV are the light fragments ${\text{H}}^{-}$ or ${\text{D}}^{-}$. These bands exhibit weaker isotope effects, in agreement with their assignment to Feshbach resonances with narrow autodetachment widths.

The momentum distributions of electrons ionized from H atoms by chirped few-cycle attosecond pulses are investigated by numerically solving the time-dependent Schrödinger equation. The central carrier frequency of the pulse is chosen to be 25 eV, which is well above the ionization threshold. The asymmetry (or difference) in the yield of electrons ionized along and opposite to the direction of linear laser polarization is found to be very sensitive to the pulse chirp (for pulses with fixed carrier-envelope phase), both for a fixed electron energy and for the energy-integrated yield. In particular, the larger the pulse chirp, the larger the number of times the asymmetry changes sign as a function of ionized electron energy. For a fixed chirp, the ionized electron asymmetry is found to be sensitive also to the carrier-envelope phase of the few-cycle pulse.

We consider a system of three identical bosons near a Feshbach resonance in the universal regime with large scattering length usually described by model-independent zero-range potentials. We employ the adiabatic hyperspherical approximation and derive the rigorous large-distance equation for the adiabatic potential for finite-range interactions. The effective range correction to the zero-range approximation must be supplemented by a new term of the same order. The nonadiabatic term can be decisive. Efimov physics is always confined to the range between effective range and scattering length. The analytical results agree with numerical calculations for realistic potentials.

We model single-photon nonlinearities resulting from the dipole-dipole interactions of cold polar molecules. We propose utilizing “dark state polaritons” to effectively couple photon and molecular states; through this framework, coherent control of the nonlinearity can be expressed and potentially used in an optical quantum computation architecture. Due to the dipole-dipole interaction the photons pick up a measurable nonlinear phase even in the single-photon regime. A manifold of protected symmetric eigenstates is used as basis. Depending on the implementation, major sources of decoherence result from nonsymmetric interactions and phonon dispersion. We discuss the strength of the nonlinearity per photon and the feasibility of this system.

We address the problem of efficient modeling of photon pairs generated in spontaneous parametric down-conversion and coupled into single-mode fibers. It is shown that when the range of relevant transverse wave vectors is restricted by the pump and fiber modes, the computational complexity can be reduced substantially with the help of the paraxial approximation, while retaining the full spectral characteristics of the source. This approach can serve as a basis for efficient numerical calculations or can be combined with analytically tractable approximations of the phase-matching function. We introduce here a cosine-Gaussian approximation of the phase-matching function that works for a broader range of parameters than the Gaussian model used previously. The developed modeling tools are used to evaluate characteristics of the photon pair sources such as the pair production rate and the spectral purity quantifying frequency correlations. Strategies to generate spectrally uncorrelated photons, necessary in multiphoton interference experiments, are analyzed with respect to trade-offs between parameters of the source.

Rates of the two-, three-, four-, and five-photon annihilations of the electron-positron pairs are determined numerically for the three-body positronium ion ${\text{Ps}}^{-}\left(e^{-}{e}^{+}{e}^{-}\right)$ and four-body bipositronium “molecule” ${\text{Ps}}_2\left(e^{-}{e}^{+}{e}^{-}{e}^{+}\right)$. The values obtained in our computations are ${\Gamma }_{2\gamma }\left({\text{Ps}}^{-}\right)\approx 2.08048530525\times {10}^9{\text{s}}^{-1}$, ${\Gamma }_{3\gamma }\left({\text{Ps}}^{-}\right)\approx 5.6364151550\times {10}^6{\text{s}}^{-1}$, ${\Gamma }_{4\gamma }\left({\text{Ps}}^{-}\right)\approx 3.075\times {10}^3{\text{s}}^{-1}$, ${\Gamma }_{5\gamma }\left({\text{Ps}}^{-}\right)\approx 5.383{\text{s}}^{-1}$, and ${\Gamma }_{2\gamma }\left({\text{Ps}}_2\right)\approx 4.4385952\times {10}^9{\text{s}}^{-1}$, ${\Gamma }_{3\gamma }\left({\text{Ps}}_2\right)\approx 1.202497\times {10}^7{\text{s}}^{-1}$, ${\Gamma }_{4\gamma }\left({\text{Ps}}_2\right)\approx 6.562\times {10}^3{\text{s}}^{-1}$, ${\Gamma }_{5\gamma }\left({\text{Ps}}_2\right)\approx 11.484{\text{s}}^{-1}$. The four- and five-photon annihilation rates are significantly smaller than the corresponding two- and three-photon annihilation rates known for these systems. We also determine the rates of one- and zero-photon annihilation for the ${\text{Ps}}^{-}$ ion and ${\text{Ps}}_2$ system. The corresponding numerical values are ${\Gamma }_{1\gamma }\left({\text{Ps}}^{-}\right)\approx 3.82491\times {10}^{-2}{\text{s}}^{-1}$, ${\Gamma }_{1\gamma }\left({\text{Ps}}_2\right)\approx 1.94188\times {10}^{-1}{\text{s}}^{-1}$, and ${\Gamma }_{0\gamma }\left({\text{Ps}}_2\right)\approx 2.32197\times {10}^{-9}{\text{s}}^{-1}$.

In this work we analyze the dynamical Casimir effect for a massless scalar field confined between two concentric spherical shells considering mixed boundary conditions. We thus generalize a previous result in literature [Phys. Rev. A 78, 032521 (2008)], where the same problem is approached for the field constrained to the Dirichlet-Dirichlet boundary conditions. A general expression for the average number of particle creation is deduced considering an arbitrary law of radial motion of the spherical shells. This expression is then applied to harmonic oscillations of the shells, and the number of particle production is analyzed and compared with the results previously obtained under Dirichlet-Dirichlet boundary conditions.

The photon Green function for a system consisting of a bounded medium surrounded by vacuum is analyzed on the basis of the Dyson equation. It is found that the fluctuations of the light field decompose universally into contributions arising from generation and recombination processes in the medium, and those arising from an external electromagnetic stimulation, i.e., light incident from the vacuum. This proves to be advantageous for further quantum-kinetically exact analysis of the interplay in the coupled system of light and matter. Particularly, it is shown that arbitrary (even nonclassical) externally induced light always propagates according to the classical wave propagation problem. These results are valid for arbitrary nonequilibrium situations, regardless of specific optical properties or geometrical shapes of the matter. For demonstration, exact relations for the scattering of squeezed light by a steady-state semiconductor slab are derived on this footing.

We study light scattering by spheres made of negative refractive index materials. We demonstrate that scattering efficiency of the light does not always obey the well-known Rayleigh’s law $Q_{\text{sca}}\sim {\kappa }^4$ for small values of size parameter $\kappa$, but could be constant $Q_{\text{sca}}\sim \text{const}$, or even inversionally proportional $Q_{\text{sca}}\sim 1/{\kappa }^2$. In addition, it is found that spheres with radius comparable with the input wavelength may exhibit suppression of the scattering in any directions, particularly, in forward one. It can be considered as analogs of Brewster’s angles for spheres.

We discuss possibilities of pumping an atomic inner-shell x-ray laser with an x-ray free-electron laser (XFEL). Self-consistent gain calculations show that with the first available XFEL, the Linac Coherent Light Source at Stanford, it will become possible to produce subfemtosecond x-ray pulses at intensities reaching $6\times {10}^{16}\text{W}/{\text{cm}}^2$. Small-signal gain calculations indicate that saturation of more than one lasing line is possible, resulting in temporally separated femtosecond x-ray pulses of different wavelengths. The presented lasing scheme creates broad capability for advancing the field of high-intensity ultrashort x-ray physics.

Rotational analogs to magnetic fluxons in conventional Josephson junctions are predicted to emerge in the ground state of rotating tunnel-coupled annular Bose-Einstein condensates (BECs). Such topological condensate-phase structures can be manipulated by external potentials. We determine conditions for observing macroscopic quantum tunneling of a fluxon. Rotational fluxons in double-ring BECs can be created, manipulated, and controlled by external potentials in different ways than is possible in the solid-state system, thus rendering them a promising candidate system for studying and utilizing quantum properties of collective many-particle degrees of freedom.

Secret sharing is a protocol in which a “boss” wants to send a classical message secretly to two “subordinates,” such that none of the subordinates is able to know the message alone, while they can find it if they cooperate. Quantum mechanics is known to allow for such a possibility. We analyze tolerable quantum bit error rates in such secret sharing protocols in the physically relevant case when the eavesdropping is local with respect to the two channels of information transfer from the boss to the two subordinates. We find that using entangled encoding states is advantageous to legitimate users of the protocol. We therefore find that entanglement is useful for secure quantum communication. We also find that bound entangled states with positive partial transpose are not useful as a local eavesdropping resource. Moreover, we provide a criterion for security in secret sharing—a parallel of the Csiszár-Körner criterion in single-receiver classical cryptography.

We present quantum networks for a $n$-qubit controlled gate $C^{n-1}\left(U\right)$ which use a higher-dimensional (qudit) ancilla as a catalyzer. In its simplest form the network has only $n$ two-particle gates (qubit-qudit)—this is the minimum number of two-body interactions needed to couple all $n+1$ subsystems ($n$ qubits plus one ancilla). This class of controlled gates includes the generalized Toffoli gate $C^{n-1}\left(X\right)$ on $n$ qubits, which plays an important role in several quantum algorithms and error correction. A particular example implementing this model is given by the dispersive limit of a generalized Jaynes-Cummings Hamiltonian of an effective spin $s$ interacting with a cavity mode.