We present a layered hybrid-system approach to quantum communication that involves the distribution of a topological cluster state throughout a quantum network. Photon loss and other errors are suppressed by optical multiplexing and entanglement purification. The scheme is scalable to large distances, achieving an end-to-end rate of 1 kHz with around 50 qubits per node. We suggest a potentially suitable implementation of an individual node composed of erbium spins (single atom or ensemble) coupled via flux qubits to a microwave resonator, allowing for deterministic local gates, stable quantum memories, and emission of photons in the telecom regime.

The elastic scattering wave function for electrons scattered from the $N$th excited state of hydrogen is the final state of the matrix element for excitation of that state. This paper deals with the solution of that problem primarily in the context of the Temkin-Poet (TP) model [A. Temkin, Phys. Rev. 126, 130 (1962); R. Poet, J. Phys. B 11, 3081 (1978)], wherein only the radial parts of the interaction are included. The relevant potential for the outer electron is dominated by the Hartree potential, $V_N^{\mathrm{H}}\left(r\right)$. In the first part of the paper, $V_N^{\mathrm{H}}\left(r\right)$ is approximated by a potential $W_N^{}\left(r\right)$, for which the scattering equation can be analytically solved. The results allow formal analytical continuation of $N$ into the continuum, so that the ionization threshold law can be deduced. Because the analytic continuation involves going from $N$ to an imaginary function of the momentum of the inner electron, the threshold law turns out to be an exponentially damped function of the available energy $E$, in qualitative accord with the result of Macek and Ihra [J. H. Macek and W. Ihra, Phys. Rev. A 55, 2024 (1997)] for the TP model. Thereafter, the scattering equation for the Hartree potential $V_N^{\mathrm{H}}\left(r\right)$ is solved numerically. The numerical aspects of these calculations have proven to be challenging and required several developments for the difficulties to be overcome. The results for $V_N^{\mathrm{H}}\left(r\right)$ show only a simple energy-dependent shift from the approximate potential $W_N^{}\left(r\right)$, which therefore does not change the analytic continuation and the form of the threshold law. It is concluded that the relevant optical potential must be included in order to compare directly with the analytic result of Macek and Ihra. The paper concludes with discussions of (a) a quantum mechanical interpretation of the result, and (b) the outlook of this approach for the complete problem.

We perform a detailed analysis of above-threshold ionization (ATI) in atoms within the strong-field approximation by considering spatially inhomogeneous monochromatic laser fields. The locally enhanced field induced by resonance plasmons is an example for such inhomogeneous fields. We investigate how the individual pairs of quantum orbits contribute to the photoelectron spectra and the angular electron momentum distributions. We demonstrate that the quantum orbits have a very different behavior in the spatially inhomogeneous field when compared to the homogeneous field. In the case of inhomogeneous fields, the ionization and rescattering times differ between neighboring cycles, despite the field being monochromatic. Indeed, the contributions from one cycle may lead to a lower cutoff, while another may develop a higher cutoff. Within our model, we show that the ATI cutoff extends far beyond the semiclassical cutoff, as a function of inhomogeneity strength. Furthermore, the angular momentum distributions have very different features compared to the homogeneous case. For the neighboring cycles, the electron momentum distributions do not share the same absolute momentum, and they do not have the same yield.

We present an interpretation of the terahertz (THz) generation in atomic gases interacting with two-color laser fields. Within the presented interpretation the process of the THz generation is associated with purely intra-atomic nonlinearity and the energy of emission is determined by the work (executed by the laser field) on the atomic electron motion in the nonharmonic intra-atomic field. To illustrate the specific features of this generation mechanism we present the results of numerical simulations on an argon atom interaction with a two-color laser field formed by the arbitrary polarized pulses at fundamental frequency and second harmonic of Ti:sapphire laser. The intensity of the laser field has been limited by $I_0<6.77\times {10}^{12}\frac{W}{{\mathrm{cm}}^2}$ (for $\lambda =800$ nm) that enables us to model the ionization-free regime of the light-atom interaction. It is demonstrated that the spectrum of the THz emission, integral efficiency of generation, and polarization state of spectral components strongly depend on the angle between the polarization vectors of laser field components. So, the variation of this angle provides the effective method of the THz generation control.

Phase resonances in multilayer metal-dielectric compound gratings are numerically modeled and described. Additional grating layers allow for more complex types of phase resonances in which light travels in different, circuitous routes through the structure. It is shown that phase resonances produce highly enhanced fields, have a very narrow bandwidth, are Fano resonances with asymmetric line shapes of their reflectance, and produce a complete inversion in the transmittivity and opacity of low loss structures. Several multigrating structures are numerically modeled that show the addition of more complicated phase resonances as more layers are added to the structure. The dispersion curves of the multiple bands of phase resonances are predicted using an analytical approach and numerical modeling. Applications of the resonance effects are described for optical filters.

We investigate the electromagnetic force experienced by a microparticle supporting high-quality whispering gallery modes that are excited by a surface wave. Our theoretical approach is based on an analytical representation of the solution of the scattering problem with a subsequent numerical treatment. It accounts rigorously for the interaction of the microparticle with the waveguiding surface and allows us to establish the balances of electromagnetic power and momentum flow for the system. We show that the resonant excitation of the whispering gallery modes and suppression of the transmitted surface wave lead to an almost complete transformation of the momentum flow of the initial surface wave into the propelling force on the microparticle. The validation of the momentum balance justifies the definition of the momentum flow of the surface wave as the ratio of carried power and phase velocity. A simple approximate relation between the propelling force and the power of the transmitted surface wave is also introduced. The transverse force can be either attractive or repulsive depending on the particle-to-surface distance, particle size, and operating frequencies, and it can significantly exceed the value of the propelling force. A comparison with a microparticle excited by a plane wave is also included.

We investigate theoretically an optomechanical system in which a cavity with a moving mirror is driven by two external fields of different frequencies. Under a resonance condition in the resolved-sideband limit, we show that there are motional states of the mirror such that the system is decoupled from the external fields if the single-photon optomechanical coupling strength is sufficiently strong. The decoupling is a quantum interference effect associated with the coherence in the mirror's mechanical degrees of freedom. We discuss the properties of such dark states and indicate how they can be generated by optical pumping due to amplitude damping of the cavity field.

Coherent diffractive imaging (CDI) is a means of overcoming the resolution and image contrast limitations of electron and x-ray lenses. The central tenet of the method is that the intensity of the diffraction pattern must be measured on a sufficiently fine pixel pitch, which is inversely related to the size of the illuminated region of the object. We show here that ptychography—a form of CDI that uses many diffraction patterns—is not subject to this constraint. Using a variant of the ePIE inversion algorithm, we demonstrate experimentally that the sampling requirement in ptychography is independent of illumination size.

We study the intensity dependence of NaI predissociation in midinfrared femtosecond laser fields. We find that the predissociation probability does not increase monotonically with the laser intensity and shows strong oscillation as the intensity of the control pulse is increased. In the presence of the control laser field, nonadiabatic transition induces or resists the population inversion between the ground state $X$(${}^1$Σ${}^{+}$) and the first excited state $A$(0${}^{+}$) at certain intensity. By controlling the wave-packet interference at the laser-induced crossings, NaI predissociation can be enhanced or suppressed. These results suggest that the laser intensity should be carefully employed in controlling molecular reactions by strong laser fields.

We introduce a general formalism to describe the effects of Markovian noise which is spatially correlated, typically decaying over some finite correlation length. For any system of interest, this formalism describes spatial correlations without the necessity to choose a particular microscopic model for the environment. We present a method of mapping the equations to Lindblad form and discuss functional forms for homogeneous spatial correlation functions. We also discuss two example microscopic models for the environment which exhibit nontrivial spatial-temporal correlation functions.

We address the dynamics of entanglement and quantum discord for two noninteracting qubits initially prepared in a maximally entangled state and then subjected to a classical colored noise, i.e., coupled with an external environment characterized by a noise spectrum of the form $1/f^{\alpha }$. More specifically, we address systems in which the Gaussian approximation fails, i.e., mere knowledge of the spectrum is not enough to determine the dynamics of quantum correlations. We thus investigate the dynamics for two different configurations of the environment: in the first case, the noise spectrum is due to the interaction of each qubit with a single bistable fluctuator with an undetermined switching rate, whereas in the second case we consider a collection of classical fluctuators with fixed switching rates. In both cases, we found analytical expressions for the time dependence of entanglement and quantum discord, which may also be extended to a collection of fluctuators with random switching rates. The environmental noise is introduced by means of stochastic time-dependent terms in the Hamiltonian, and this allows us to describe the effects of both separate and common environments. We show that the non-Gaussian character of the noise may lead to significant effects, e.g., environments with the same power spectrum, but different configurations give rise to the opposite behavior for quantum correlations. In particular, depending on the characteristics of the environmental noise considered, both entanglement and discord display either a monotonic decay or the phenomena of sudden death and revivals. Our results show that the microscopic structure of the environment, in addition to its noise spectrum, is relevant for the dynamics of quantum correlations and may be a valid starting point for the engineering of non-Gaussian colored environments.

Measurement-device-independent quantum key distribution (MDI-QKD) is immune to all the detection attacks; thus when it is combined with the decoy-state method, the final key is unconditionally secure, even if a practical weak coherent source is used by Alice and Bob. However, until now, the analysis of decoy-state MDI-QKD with a weak coherent source is incomplete. In this paper, we derive, with only vacuum+weak decoy state, some tight formulas to estimate the lower bound of yield and the upper bound of error rate for the fraction of signals in which both Alice and Bob send a single-photon pulse to the untrusted third party Charlie. The numerical simulations show that our method with only vacuum+weak decoy state can asymptotically approach the theoretical limit of the infinite number of decoy states. Furthermore, the statistical fluctuation due to the finite length of date is also considered based on the standard statistical analysis.

We show that the Hong-Ou-Mandel effect from quantum optics is equivalent to the swap test, a quantum information primitive which compares two arbitrary states. We first derive a destructive swap test that does not need the ancillary qubit that appears in the usual quantum circuit. Then we study the Hong-Ou-Mandel effect for two photons meeting at a beam splitter and prove it is, in fact, an optical implementation of the destructive swap test. This result offers both an interesting simple realization of a powerful quantum information primitive and an alternative way to understand and analyze the Hong-Ou-Mandel effect.

Kruszynska and Kraus [Phys. Rev. A 79, 052304 (2009)] have recently introduced the so-called locally maximally entangleable (LME) states of $n$ qubits which can be maximally entangled with local auxiliary qubits using controlled operations. We characterize the local entangleability of hypergraph states and $W$ states using the approach of Kruszynska and Kraus. We show that (i) all hypergraph states are LME; (ii) hypergraph states and LME states are not equivalent under local unitaries; (iii) a $W$ state of $n$ qubits is not LME; and (iv) no hypergraph state of $n$ qubits can be converted into the $W$ state under local unitary transformations. Moreover, we also present an approach for encoding weighted hypergraphs into LME states.

Ternary electron-assisted collisional radiative recombination (E-CRR) and neutral-assisted radiative recombination (N-CRR) have very strong negative temperature dependencies and hence dominate electron recombination in some plasmas at very low temperatures. However, there are only few data for molecular ions and almost none for atomic ions at temperatures much below 300 K. In this experimental study we used a cryogenic afterglow plasma experiment (Cryo-FALP II) at temperatures from 50 K to 100 K to measure ternary recombination rate coefficients for ${\mathrm{Ar}}^{+}$ ions in ambient helium gas. The measured magnitudes and the temperature dependencies of the ternary recombination rate coefficients for electron-assisted and neutral-assisted collisional radiative recombination agree well with theoretical predictions ($K_{\mathrm{E}-\mathrm{CRR}}\sim T^{-4.5}$ and $K_{\mathrm{He}-\mathrm{CRR}}\sim T^{-2.5}$ to $T^{-2.9}$) over a broad range of electron and He densities.

Charge-transfer cold Yb${}^{+}$$+$ Rb collision dynamics is investigated theoretically using high-level ab initio potential energy curves, dipole moment functions, and nonadiabatic coupling matrix elements. Within the scalar-relativistic approximation, the radiative transitions from the entrance $A{}^1{\Sigma }^{+}$ to the ground $X{}^1{\Sigma }^{+}$ state are found to be the only efficient charge-transfer pathway. The spin-orbit coupling does not open other efficient pathways, but alters the potential energy curves and the transition dipole moment for the $A-X$ pair of states. The radiative, as well as the nonradiative, charge-transfer cross sections calculated within the ${10}^{-3}–10$ cm${}^{-1}$ collision energy range exhibit all features of the Langevin ion-atom collision regime, including a rich structure associated with centrifugal barrier tunneling (orbiting) resonances. Theoretical rate coefficients for two Yb isotopes agree well with those measured by immersing Yb${}^{+}$ ions in an ultracold Rb ensemble in a hybrid trap. Possible origins of discrepancy in the product distributions and relations to previously studied similar processes are discussed.

We study interacting dipolar atomic bosons in a triple-well potential within a ring geometry. This system is shown to be equivalent to a three-site Bose-Hubbard model. We analyze the ground state of dipolar bosons by varying the effective on-site interaction. This analysis is performed both numerically and analytically by using suitable coherent-state representations of the ground state. The latter exhibits a variety of forms ranging from the SU(3) coherent state in the delocalization regime to a macroscopic catlike state with fully localized populations, passing for a coexistence regime where the ground state displays a mixed character. We characterize the quantum correlations of the ground state from the bipartition perspective. We calculate both numerically and analytically (within the previous coherent-state representation) the single-site entanglement entropy which, among various interesting properties, exhibits a maximum value in correspondence to the transition from the catlike to the coexistence regime. In the latter case, we show that the ground-state mixed form corresponds, semiclassically, to an energy exhibiting two almost-degenerate minima.

In recent years, matter-wave interferometry has attracted growing attention due to its unique suitability for high-precision measurements and the study of fundamental aspects of quantum theory. Diffraction and interference of matter waves can be observed not only at a spatial aperture (such as a screen edge, slit, or grating), but also at a time-domain aperture (such as an absorbing barrier, or “shutter,” that is being periodically switched on and off). The wave phenomenon of the latter type is commonly referred to as “diffraction in time.” Here, we introduce a versatile, exactly solvable model of diffraction in time. It describes time evolution of an arbitrary initial quantum state in the presence of a time-dependent absorbing barrier, governed by an arbitrary aperture function. Our results enable a quantitative description of diffraction and interference patterns in a large variety of setups, and may be used to devise new diffraction and interference experiments with atoms and molecules.

We study imbalanced fermionic superfluids in an array of one-dimensional tubes at the incipient dimensional crossover regime, wherein particles can tunnel between neighboring tubes. In addition to single-particle tunneling (ST), we consider pair tunneling (PT) that incorporates the interaction effect during the tunneling process. We find that with an increase of PT strength, a system of low global polarization evolves from a structure with a central Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state to one with a central BCS-like fully paired state. For the case of high global polarization, the central region exhibits pairing zeros embedded in a fully paired order. In both cases, PT enhances the pairing gap, suppresses the FFLO order, and leads to spatial separation of fully paired and fully polarized regions, the same as in higher dimensions. Thus we show that PT beyond second-order ST processes is of relevance to the development of signatures characteristic of the incipience of the dimensional crossover.

A link to the Dicke-model type quantum phase transition (QPT) is established by considering a time-dependent driving of the atom-field coupling and nonlinear atom-photon interaction as well, which make a notable impact on the ground-state properties. Motivated by the recent pioneer experiment [K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger, Nature (London) 464, 1301 (2010)], in this paper we demonstrate theoretically a rich multistable structure and nonequilibrium QPTs in terms of the Floquet theory and generalized rotating wave approximation. The positive nonlinear interaction does not lead to the change of the order-parameter curve except for a slight shift of the critical point. However, an additional full atomic population of the excited state is generated by the negative interaction resulting in the coexistence region of normal and superradiant phases. When both driving and nonlinear interaction exist the single order-parameter curve of the superradiant phase spreads out and a wide area of multiple local minima emerges in addition.