With growing success in experimental implementations it is critical to identify a “gold standard” for quantum information processing, a single measure of distance that can be used to compare and contrast different experiments. We enumerate a set of criteria that such a distance measure must satisfy to be both experimentally and theoretically meaningful. We then assess a wide range of possible measures against these criteria, before making a recommendation as to the best measures to use in characterizing quantum information processing.

Multiphoton entanglement is an important resource for linear optics quantum computing. Here we show that a wide range of highly entangled multiphoton states, including $W$-states, can be prepared by interfering single photons inside a Bell multiport beam splitter and using postselection. A successful state preparation is indicated by the collection of one photon per output port. An advantage of the Bell multiport beam splitter is that it redirects the photons without changing their inner degrees of freedom. The described setup can therefore be used to generate polarization, time-bin, and frequency multiphoton entanglement, even when using only a single photon source.

Measurements of differential cross sections for the electron-impact excitation of molecular nitrogen from the ground $X{}^1\Sigma _{\mathrm{g}}^{+}(v^{″}=0)$ level to the $A{}^3\Sigma _{\mathrm{u}}^{+}\left(v^{\prime }\right)$, $B{}^3\Pi _{\mathrm{g}}\left(v^{\prime }\right)$, $W{}^3\Delta _{\mathrm{u}}\left(v^{\prime }\right)$, $B^{\prime }{}^3\Sigma _{\mathrm{u}}^{-}\left(v^{\prime }\right)$, $a^{\prime }{}^1\Sigma _{\mathrm{u}}^{-}\left(v^{\prime }\right)$, $a{}^1\Pi _{\mathrm{g}}\left(v^{\prime }\right)$, $w{}^1\Delta _{\mathrm{u}}\left(v^{\prime }\right)$, and $C{}^3\Pi _{\mathrm{u}}\left(v^{\prime }\right)$ levels are presented. The data are obtained at the incident energies of 10, 12.5, 15, 17.5, 20, 30, 50, and 100 eV over the angular range of 5°–130° in 5° intervals. The individual electronic state excitation differential cross sections are obtained by unfolding electron energy-loss spectra of molecular nitrogen using available semiempirical Frank-Condon factors. The data are compared to previous measurements and to available theory. We also make several important suggestions regarding future work that, like the present, relies on the unfolding of electron energy-loss spectra for obtaining differential cross sections.

We report the observation of cone emission generated when $\sim 10\text{−}\mathrm{nJ}$, 100-fs laser pulses in the 730–770 nm wavelength range are transmitted through a dense cesium vapor. The spatial and spectral characteristics of the observed cone emission were studied experimentally. Cone angle dependence on the laser wavelength, laser average power, and Cs atomic and molecular concentrations were investigated. The cone emission (CE) was only observed when the laser beam self-focused after passing through the medium. We identify the self-phase modulation as a dominant mechanism for CE generation in our experiment.

The momentum transfer from light to a dielectric material in the photon drag effect is calculated by evaluation of the relevant Lorentz force. In accordance with measurements on Si and Ge, the material is taken as a two-component optical system, with charge carriers described by an extinction coefficient $\kappa$ in a host semiconductor described by real refractive indices ${\eta }_p$ (phase) and ${\eta }_g$ (group). The calculated momentum transfer to the charge carriers alone has the value ${\eta }_p\hslash \omega ∕c$ per photon, the so-called Minkowski value, found experimentally. The time-dependent Lorentz force is calculated for light in the form of a narrow-band single-photon pulse. When the pulse is much shorter than the attenuation length, which is much shorter than the sample thickness, there is a clear separation in time between surface and bulk contributions to the forces. The total bulk momentum transfer (charges plus host) in this case is found to be $\hslash \omega ∕{\eta }_{g}c$, the so-called Abraham value.

We demonstrate a quantum error correction scheme that protects against accidental measurement, using a parity encoding where the logical state of a single qubit is encoded into two physical qubits using a nondeterministic photonic controlled-NOT gate. For the single qubit input states $\mid 0⟩$, $\mid 1⟩$, $\mid 0⟩\pm \mid 1⟩$, and $\mid 0⟩\pm i\mid 1⟩$ our encoder produces the appropriate two-qubit encoded state with an average fidelity of $0.88\pm 0.03$ and the single qubit decoded states have an average fidelity of $0.93\pm 0.05$ with the original state. We are able to decode the two-qubit state (up to a bit flip) by performing a measurement on one of the qubits in the logical basis; we find that the 64 one-qubit decoded states arising from 16 real and imaginary single-qubit superposition inputs have an average fidelity of $0.96\pm 0.03$.

We reexamine the back-action of emitted photons on the wave packet evolution about the relative position of two cold atoms. We show that photon recoil resulting from the spontaneous emission can induce the localization of the relative position of the two atoms through the entanglement between the spatial motion of individual atoms and their emitted photons. The obtained result provides a more realistic model for the analysis of the environment-induced localization of a macroscopic object.

Newtonian adiabatics is the consistent truncation of the adiabatic approximation to second order in small velocities. To be complete, it must unify two hitherto disjoint intellectual streams in the study of adiabatic motion. The newer stream focuses on Berry’s induced vector potential, or geometric magnetism, and Provost and Vallée’s induced scalar potential, reflecting geometry in Hilbert space. The older stream focuses on Inglis’ induced inertia, influencing the geometry of adiabatic-parameter space. Starting with the Hamiltonian of the newer stream, unification is simple: A naive or primitive inertia, whose inverse appears in two terms of that Hamiltonian, is replaced by the convention-independent sum of primitive and induced inertia tensors.

In spite of many results in quantum information theory, the complex nature of compound systems is far from clear. In general the information is a mixture of local and nonlocal (“quantum”) information. It is important from both pragmatic and theoretical points of view to know the relationships between the two components. To make this point more clear, we develop and investigate the quantum-information processing paradigm in which parties sharing a multipartite state distill local information. The amount of information which is lost because the parties must use a classical communication channel is the deficit. This scheme can be viewed as complementary to the notion of distilling entanglement. After reviewing the paradigm in detail, we show that the upper bound for the deficit is given by the relative entropy distance to so-called pseudoclassically correlated states; the lower bound is the relative entropy of entanglement. This implies, in particular, that any entangled state is informationally nonlocal—i.e., has nonzero deficit. We also apply the paradigm to defining the thermodynamical cost of erasing entanglement. We show the cost is bounded from below by relative entropy of entanglement. We demonstrate the existence of several other nonlocal phenomena which can be found using the paradigm of local information. For example, we prove the existence of a form of nonlocality without entanglement and with distinguishability. We analyze the deficit for several classes of multipartite pure states and obtain that in contrast to the GHZ state, the Aharonov state is extremely nonlocal. We also show that there do not exist states for which the deficit is strictly equal to the whole informational content (bound local information). We discuss the relation of the paradigm with measures of classical correlations introduced earlier. It is also proved that in the one-way scenario, the deficit is additive for Bell diagonal states. We then discuss complementary features of information in distributed quantum systems. Finally we discuss the physical and theoretical meaning of the results and pose many open questions.

We propose a scheme for teleporting an arbitrary superposition of entangled Dicke states of any number of atoms (qubits) between two distant cavities. Our method relies on adiabatic passage using multi-atom dark states in each cavity, and a conditional detection of photons leaking out of both cavities. The ideal success probability of the protocol decreases polynomially in the number of atoms. The fidelity is unity for a single Dicke state, and can be optimized for the superposition by unitary postprocessing. Issues of experimental feasibility and applications to quantum informatics are discussed.

In this paper we develop a unified framework to study the coherent control of trapped ions subject to state-dependent forces. Taking different limits in our theory, we can reproduce previous designs of quantum gates and propose a different design of fast gates based on continuous laser beams. We demonstrate how to simulate Ising Hamiltonians in a many ions setup, and how to create highly entangled states and induce squeezing. Finally, in a detailed analysis we identify the physical limits of this technique and study the dependence of errors on the temperature.

In this paper we present integral and differential cross sections for electron scattering by the ${\mathrm{C}}_3{\mathrm{H}}_4$ isomers allene and propyne for energies from $1\text{to}10\mathrm{eV}$. To compute the scattering cross sections we employ the Schwinger multichannel method with pseudopotential at the static exchange plus polarization approximation. Our results are in good agreement with experimental total cross sections and differential cross sections. We also discuss the role of the isomer effect in the cross sections of these isomers in the $1–10\mathrm{eV}$ energy range.

We report the results of calculations which were performed to investigate equilibrium structures, electronic and magnetic properties of small ${\mathrm{Ni}}_n\mathrm{B}$ clusters with $n=1–8,12$ within the framework of density functional theory. The calculated results find that doping of boron enhances the binding energy but reduces the magnetic moments of Ni clusters. The boron prefers to maximize the number of $\mathrm{Ni}–\mathrm{B}$ bonds by selecting the site which increases the coordination of nickel atoms with B, and is seen to induce significant changes in the geometries of the host clusters for $n<4$. This study also reveals that some of the ${\mathrm{Ni}}_n\mathrm{B}$ clusters considered have a substantial higher highest ocupied-lowest unoccupied molecular orbital gap of spin-up electrons as compared to that of spin-down electrons. This may have interesting consequences in the case of the spin-polarized transport, where there will be no conductance for spin-up electrons in such ${\mathrm{Ni}}_n\mathrm{B}$ clusters.

A methodology to calculate generalized coherent states for a periodically driven system is presented. We study wave packets constructed as a linear combination of suitable Floquet states of the three-dimensional Rydberg atom in a microwave field. The driven coherent states show classical space localization, spreading, and revivals and remain localized along the classical trajectory. The microwave strength and frequency have a great effect in the localization of Floquet states, since quasienergy avoided crossings produce delocalization of the Floquet states, showing that tuning of the parameters is very important. Using wavelet-based time-frequency analysis, the classical phase-space structure is determined, which allows us to show that the driven coherent state is located in a large regular region in which the $z$ coordinate is in resonance with the external field. The expectation values of the wave packet show that the driven coherent state evolves along the classical trajectory.

We study theoretically the interference patterns produced by the overlap of an array of Bose-Einstein condensates that have no phase coherence among them. We show that density-density correlations at different quasimomenta, which play an important role in two-condensate interference, become negligible for large $N$, where $N$ is the number of overlapping condensates. In order to understand the physics of this phenomenon, it is sufficient to consider the periodicity of the lattice and the statistical probability distribution of a random-walk problem. The average visibility of such interference patterns decreases as $N^{-1∕2}$ for large $N$.

We discuss a Bosonic channel model with memory effects. It relies on a multimode squeezed (entangled) environment’s state. The case of lossy Bosonic channels is analyzed in detail. We show that in the absence of input energy constraints the memory channels are equivalent to their memoryless counterparts. In the case of input energy constraint we provide lower and upper bounds for the memory channel capacity.

We numerically study the quantum adiabatic algorithm for propositional satisfiability. A new class of previously unknown hard instances is identified among random problems. We numerically find that the running time for such instances grows exponentially with their size. The worst case complexity of the quantum adiabatic algorithm therefore seems to be exponential.

We demonstrate that multiple copies of a bipartite entangled pure state may serve as a catalyst for certain entanglement transformations while a single copy cannot. Such a state is termed a “multiple-copy catalyst” for the transformations. A trade-off between the number of copies of source state and that of the catalyst is also observed. These results can be generalized to probabilistic entanglement transformations directly.

Based on the idea that four excited electrons arrange themselves around the nucleus in the corners of a pyramid in order to minimize their mutual repulsion, we present an analytical model of quadruply excited states. The model shows excellent comparison with ab initio results and provides a clear physical picture of the intrinsic motion of the four electrons. The model is used to predict configuration-mixing fractions and spectra of these highly correlated states.

We describe a method to project photonic two-qubit states onto the symmetric and antisymmetric subspaces of their Hilbert space. This device utilizes an ancillary coherent state, together with a weak cross-Kerr nonlinearity, generated, for example, by electromagnetically induced transparency. The symmetry analyzer is nondestructive, and works for small values of the cross-Kerr coupling. Furthermore, this device can be used to construct a nondestructive Bell-state detector.