In the present work, we consider the dynamics of dark solitons as one mode of a defocusing photorefractive lattice coupled with bright solitons as a second mode of the lattice. Our investigation is motivated by an experiment that illustrates that such coupled states can exist with both components in the first gap of the linear band spectrum. This finding is further extended by the examination of different possibilities from a theoretical perspective, such as symbiotic ones where the bright component is supported by states of the dark component in the first or second gap, or nonsymbiotic ones where the bright soliton is also a first-gap state coupled to a first or second gap state of the dark component. While the obtained states are generally unstable, these instabilities typically bear fairly small growth rates, which enable their observation for experimentally relevant propagation distances.

The slowing down of fast electrons emitted from insulators [Mylar, polypropylene (PP)] irradiated with swift ion beams (C, O, Kr, Ag, Xe; 20–64 MeV/u) was measured by the time-of-flight method at LNS, Catania and GANIL, Caen. The charge buildup, deduced from both convoy- and binary-encounter electron peak shifts, leads to target material-dependent potentials (6.0 kV for Mylar, 2.8 kV for PP). The number of projectiles needed for charging up (charging-up time constant) is inversely proportional to the electronic energy loss. After a certain time, a sudden decharging occurs. For low beam currents, charging-up time, energy shift corresponding to maximum charge buildup, and time of decharging are regular. For high beam currents, the time intervals become irregular (chaotic).

We propose a polarization to orbital angular momentum teleportation scheme using entangled photon pairs generated by spontaneous parametric down-conversion. By making a joint detection of the polarization and angular momentum parity of a single photon, we are able to detect all the Bell states and perform, in principle, perfect teleportation from a discrete to a continuous system using minimal resources. The proposed protocol implementation demands experimental resources that are currently available in quantum optics laboratories.

In this paper we use different techniques from the fractional and pseudo-operators calculus to solve partial differential equations involving operators with noninteger exponents. We apply the method to equations resembling generalizations of the heat equations and discuss the possibility of extending the procedure to the relativistic Schrödinger and Dirac equations.

Recently several more efficient versions of quantum state tomography have been proposed, with the purpose of making tomography feasible even for many-qubit states. The number of state parameters to be estimated is reduced by tentatively introducing certain simplifying assumptions on the form of the quantum state, and subsequently using the data to rigorously verify these assumptions. The simplifying assumptions considered so far were (i) the state can be well approximated to be of low rank, or (ii) the state can be well approximated as a matrix product state, or (iii) only the permutationally invariant part of the density matrix is determined. We add one more method in that same spirit: We allow in principle any model for the state, using any (small) number of parameters (which can, e.g., be chosen to have a clear physical meaning), and the data are used to verify the model. The proof that this method is valid cannot be as strict as in the above-mentioned cases, but is based on well-established statistical methods that go under the name of “information criteria.” We exploit here, in particular, the Akaike information criterion. We illustrate the method by simulating experiments on (noisy) Dicke states.

We demonstrate the formation of many-particle interferences of different degrees to determine the transmission of four photons of tunable indistinguishability through a four-port beam splitter array. The probability of certain output events depends nonmonotonically on the degree of distinguishability, due to distinct multiparticle interference contributions to the transmission signal.

We present two economical one-step error-correction protocols for multipartite polarization-entangled systems in a Greenberger-Horne-Zeilinger state. One uses spatial entanglement to correct errors in the polarization entanglement of an $N$-photon system, resorting to linear optical elements. The other uses frequency entanglement to correct errors in the polarization entanglement of an $N$-photon system. The parties in quantum communication can obtain a maximally entangled state from each $N$-photon system transmitted with one step in these two protocols, and both of their success probabilities are $100\%$, in principle. That is, they both work in a deterministic way, and they do not largely consume the less-entangled photon systems, which is far different from conventional multipartite entanglement purification schemes. These features may make these two protocols more useful for practical applications in long-distance quantum communication.

In the study of open quantum systems, one typically obtains the decoherence dynamics by solving a master equation. The master equation is derived using knowledge of some basic properties of the system, the environment, and their interaction: One basically needs to know the operators through which the system couples to the environment and the spectral density of the environment. For a large system, it could become prohibitively difficult to even write down the appropriate master equation, let alone solve it on a classical computer. In this paper, we present a quantum algorithm for simulating the dynamics of an open quantum system. On a quantum computer, the environment can be simulated using ancilla qubits with properly chosen single-qubit frequencies and with properly designed coupling to the system qubits. The parameters used in the simulation are easily derived from the parameters of the system $+$ environment Hamiltonian. The algorithm is designed to simulate Markovian dynamics, but it can also be used to simulate non-Markovian dynamics provided that this dynamics can be obtained by embedding the system of interest into a larger system that obeys Markovian dynamics. We estimate the resource requirements for the algorithm. In particular, we show that for sufficiently slow decoherence a single ancilla qubit could be sufficient to represent the entire environment, in principle.

We present a modified scheme of differential-phase-shift (DPS) quantum key distribution (QKD) for improving its performance. A transmitter sends a weak coherent pulse train segmented with vacant pulses. Then, a receiver can find eavesdropping by monitoring the photon detection rate at particular time slots. Simulations show that the proposed scheme is robust against a sequential attack and a general individual attack.

We experimentally generate and tomographically characterize a mixed, genuinely non-Gaussian bipartite continuous-variable entangled state. By testing entanglement in 2$\times$2-dimensional two-qubit subspaces, entangled qubits are localized within the density matrix, which, first, proves the distillability of the state and, second, is useful to estimate the efficiency and test the applicability of distillation protocols. In our example, the entangled qubits are arranged in the density matrix in an asymmetric way, i.e., entanglement is found between diverse qubits composed of different photon number states, although the entangled state is symmetric under exchanging the modes.

Two different measures of operator entanglement of two-qubit gates, namely, Schmidt strength and linear entropy, are studied. While these measures are shown to have a one-to-one relation between them for the Schmidt number 2 class of gates, no such relation exists for the Schmidt number 4 class, implying that the measures are inequivalent in general. Further, we establish a simple relation between linear entropy and local invariants of two-qubit gates. The implication of the relation is discussed.

In tasks where multipartite entanglement plays a central role, state purification is, due to inevitable noise, a crucial part of the procedure. We consider a scenario exploiting the multipartite entanglement in a straightforward multipartite purification algorithm and compare it to bipartite purification procedures combined with state teleportation. While complete purification requires an infinite amount of input states in both cases, we show that for an imperfect output fidelity the multipartite procedure exhibits a major advantage in terms of input states used.

Based on a rigorous quantum electrodynamics (QED) approach, a theoretical analysis is performed for the two-photon transitions in heavy He-like ions. Special attention is paid to the interelectronic-interaction corrections to the decay rates that are taken into account within the two-time Green-function method. Detailed calculations are carried out for the two-photon transitions $2\hspace{0.5em}{}^1{S}_0\to 1\hspace{0.5em}{}^1{S}_0$ and $2\hspace{0.5em}{}^3{S}_1\to 1\hspace{0.5em}{}^1{S}_0$ in He-like ions within the range of nuclear numbers $Z=28$–92. The total decay rates together with the spectral distributions are given. The obtained results are compared with experimental values and previous calculations.

High-resolution studies of the fine-structure pattern in high-L n $=$ 37 levels of Th${}^{3+}$ have been carried out using radio-frequency (rf) spectroscopy detected with resonant excitation Stark ionization spectroscopy (RESIS). Intervals separating $L$ $=$ 9 to $L$ $=$ 15 levels have been measured, and the results analyzed with the long-range effective potential model. The dipole polarizability of Th${}^{4+}$is determined to be $\alpha$${}_{\mathrm{D}}$ $=$ 7.720(7) a.u. The quadrupole polarizability is found to be 21.5(3.9) a.u. Both measurements represent significant tests of a priori theoretical descriptions of this highly relativistic ion.

We report measured and calculated differential elastic cross sections for collisions of low-energy electrons with diacetylene (1,3-butadiyne). A generally satisfactory agreement between theory and experiment has been found. The calculated cross sections provide interesting insight into the underlying resonant structure.

We calculated the yield of outgoing hydrogen negative ions after the reflection of 1-keV neutral hydrogen atoms from a $(2\times 1)$-reconstructed Si(100) surface. We find that the charge-transfer dynamics at the reconstructed surface is dependent on both the surface-electronic structure and orientation of the projectile trajectory relative to the crystal azimuthal directions. Our results are in good quantitative agreement with the measured $\mathrm{H}{}^{-}$ fractions of Maazouz and Esaulov [Surf. Sci. 398, 49 (1998)] for scattering trajectories that are aligned perpendicularly to rows of silicon dimers.

Recent experiments showed that the alignment-dependent ionization probabilities of hydrogen molecules vary as a function of laser intensity, and the anisotropy deviates from the prediction of the molecular tunneling ionization model (MO-ADK). To investigate the physical origins of the deviation, we systematically studied the anisotropy of hydrogen molecules in intense laser fields for three wavelengths, 400 nm (multiphoton ionization), 1850 nm (tunneling ionization), and 800 nm (in between), by solving the time-dependent Schrödinger equation with a model potential. The calculated ratio of the ionization probabilities for laser fields parallel and perpendicular to the molecular axis are in reasonable agreement with experiment. Furthermore, by analyzing the molecular wave function and the model potential, we found that the discrepancies between experiment and the MO-ADK prediction originated from the inaccurate coefficients used in the model and that the assumption of isotropy of the effective potential in the tunneling region is invalid.

Optical pumping of alkali-metal atoms in vapor cells causes spin currents to flow to the cell walls where excess angular momentum accumulates in the wall nuclei. Experiments reported here indicate that the substantial enhancement of the nuclear-spin polarization of salts at the cell walls is primarily due to the nuclear-spin current, with a lesser contribution from the electron-spin current of the vapor.

We study harmonically trapped ultracold Bose gases with attractive interparticle interactions under external rotation in three spatial dimensions and determine the critical value of the attraction strength where the gas collapses as a function of the rotation frequency. To this end, we examine the stationary ground state in the corotating frame with a many-body approach as well as within the Gross-Pitaevskii theory of systems in traps with different anisotropies. In contrast to recently reported results [N. A. Jamaludin, N. G. Parker, and A. M. Martin, Phys. Rev. A 77, 051603(R) (2008)], we find that the collapse is not postponed in the presence of rotation. Unlike repulsive gases, the properties of the attractive system remain practically unchanged under rotation in isotropic and slightly anisotropic traps.

Correlations between successive detections in beams of free electrons are studied with a transmission electron microscope. For incoherent illumination of the detectors, a certain random coincidence probability is observed, indicative for uncorrelated arrival times of the electrons. When the illumination is changed from incoherent to coherent, a reduction of the random coincidence probability is observed, indicative for antibunching in the arrival times of the electrons. However, the amount of reduction is larger than the theoretically expected value calculated from the Pauli principle, forbidding more than one identical fermion to occupy the same quantum state. For a certain coherent illumination of the detectors, where we use magnetic lenses in electron microscopes for magnifications of the coherence length, we find an enhanced coincidence probability, indicative for bunching in the arrival times of the electrons. This originates from correlations in beams of free electrons due to Coulomb interactions.