Communication in quantum wireless multihop networks is useful in global quantum networks. We propose a scheme for faithful quantum communication in quantum wireless multihop networks, by performing quantum teleportation between two distant nodes which do not initially share entanglement with each other. The required entanglement among intermediate nodes is established through entanglement swapping based on arbitrary types of Bell pairs. All the measurement outcomes and the types of Bell pairs are sent to the destination node independently. The initial quantum state can be finally recovered by corresponding local operations. Our scheme can reduce end-to-end communication delay by using simultaneous measurements in contrast to the scheme based on sequential entanglement swapping.

We consider quantum signaling between two-level quantum systems in a cavity in the perturbative regime of the earliest possible arrival times of the signal. We present two main results: First, we find that, perhaps surprisingly, the analog of amplitude modulated signaling (Alice using her energy eigenstates $\left|g\right.〉,\left|e\right.〉$, as in the Fermi problem) is generally suboptimal for communication, namely, e.g., phase-modulated signaling (Alice using, e.g., $\{\left|+\right.〉,\left|-\right.〉\}$ states) overcomes the quantum noise already at a lower order in perturbation theory. Second, we study the effect of mode truncations that are commonly used in cavity QED on the modeling of the communication between two-level atoms. We show that, on general grounds, namely for causality to be preserved, the UV cutoff must scale at least polynomially with the desired accuracy of the predictions.

We introduce a modification of the standard entanglement swapping protocol where the generation of entanglement between two distant modes is realized and verified using only local optical measurements. We show, indeed, that a simple condition on the purity of the initial state involving also an ancillary mode is sufficient to guarantee the success of the protocol by local measurements [M. Abdi, S. Pirandola, P. Tombesi, and D. Vitali, Phys. Rev. Lett. 109, 143601 (2012)]. We apply the proposed protocol to a tripartite optomechanical system where the never interacting mechanical modes become entangled and certified using only local optical measurements.

The angular distribution of the $K{\alpha }_1$ ($1s2p_{3/2}{}^{1,3}{P}_{1,2}\to 1s^2{}^1{S}_0$) x-ray emission following the radiative electron capture into initially hydrogenlike ions with nonzero nuclear spin has been studied within the density matrix theory and the multiconfiguration Dirac-Fock method. Emphasis is placed especially upon the hyperfine interaction and how this interaction of the magnetic moment of the nucleus with those of the electrons affects the angular properties of the $K{\alpha }_1$ radiation. Calculations were performed for selected isotopes of heliumlike Sn${}^{48+}$, Xe${}^{52+}$, and Tl${}^{79+}$ ions. A quite sizable contribution of the hyperfine interaction upon the $K{\alpha }_1$ angular emission is found for isotopes with nuclear spin $I=1/2$, while its effect is suppressed for (most) isotopes with nuclear spin $I>1/2$. We therefore suggest that accurate measurements of the $K{\alpha }_1$ angular distribution at ion storage rings can be utilized as a tool for determining the nuclear parameters of rare stable and radioactive isotopes with $I\ge 1/2$.

A fully relativistic multipole scheme is formulated to study two-photon emission processes in hydrogenlike ions with an infinitely heavy, pointlike, and spinless nucleus of charge up to 100. By making use of the Sturmian expansion of the Dirac-Coulomb Green function of the first order constructed by Szmytkowski, closed-form expressions are derived for arbitrary multipole channels. In the nonrelativistic limit, well-known formulas established previously are retrieved. For the sake of assessing the effectiveness of our approach, numerical applications are then carried out for two-photon decay rates of the selected $2s_{1/2}$ and $2p_{1/2}$ atomic states. To this end, radial integrals, the most crucial quantities involved in the matrix elements, are treated with great care by means of two suitable techniques that agree with each other quite closely so that very accurate values are obtained regardless of the choice of parameters, such as radial quantum numbers and orders of spherical Bessel functions of the first kind. In addition, the convergence and stability of computations are checked in connection with the intermediate-state summation, which appears within the second-order perturbation theory. As expected, the gauge invariance of our fully relativistic multipole numbers is confirmed. Relativistic effects, and the influence of the negative spectrum of the complete set of Dirac-Coulomb Sturmians of first order and retardation truncations in the transition operator are examined. Finally, a comparison is undertaken of our two-photon relativistic calculations with refined predictions of other authors based on finite basis-set methods widely employed over the past decades.

Nonadiabatic dynamics simulations are performed to investigate the electronic stopping power of LiF and ${\text{SiO}}_2$-cristobalite-high crystalline thin films when protons and helium ions are hyperchanneling in the $\langle$001$\rangle$ axis. In this theoretical framework, ab initio time-dependent density-functional theory calculations for electrons are combined with molecular dynamics simulations for ions in real time and real space. The energy transfer process between the ions and the electronic subsystem of LiF and ${\text{SiO}}_2$ nanostructures is studied. The velocity-proportional stopping power of LiF and ${\text{SiO}}_2$ for protons and helium ions is predicted in the low-energy range. The measured velocity thresholds of protons in LiF and ${\text{SiO}}_2$, and helium ions in LiF are reproduced. The convergence of the threshold effect with respect to the separation of grid points is confirmed. The underlying physics of the threshold effect is clarified by analyzing the conduction band electron distribution. In addition, the electron transfer processes between the projectile ions and solid atoms in hyperchanneling condition are studied, and its effects on the energy loss is investigated.

We report on the observation of sub-Doppler resonances on the backscattered light from a random porous glass medium with rubidium vapor filling its interstices. The sub-Doppler spectral lines are the consequence of saturated absorption where the incident laser beam saturates the atomic medium and the backscattered light probes it. Some specificities of the observed spectra reflect the transient atomic evolution under confinement inside the pores. Simplicity, robustness, and potential miniaturization are appealing features of this system as a spectroscopic reference.

We demonstrate numerically that when the forward wave approximation of Maxwell's equations is not valid, the hyperbolic secant pulses of self-induced transparency gradually lose energy with increasing propagation distance through the excitation of a backward mode.

A quasimodal expansion method (QMEM) is developed to model and understand the scattering properties of arbitrary shaped two-dimensional open structures. In contrast with the bounded case which has only a discrete spectrum (real in the lossless media case), open resonators show a continuous spectrum composed of radiation modes and may also be characterized by resonances associated to complex eigenvalues (quasimodes). The use of a complex change of coordinates to build perfectly matched layers allows the numerical computation of those quasimodes and of approximate radiation modes. Unfortunately, the transformed operator at stake is no longer self-adjoint, and classical modal expansion fails. To cope with this issue, we consider an adjoint eigenvalue problem whose eigenvectors are biorthogonal to the eigenvectors of the initial problem. The scattered field is expanded on this complete set of modes leading to a reduced order model of the initial problem. The different contributions of the eigenmodes to the scattered field unambiguously appears through the modal coefficients, allowing us to analyze how a given mode is excited when changing incidence parameters. This gives physical insights to the spectral properties of different open structures such as nanoparticles and diffraction gratings. Moreover, the QMEM proves to be extremely efficient for the computation of local density of states.

We have analyzed analytically the Faraday rotation of an electromagnetic wave for a magnetoactive thin metallic film with a nanostructured surface profile. Periodic as well as random surface profiles were considered. The plasmon contribution to the Faraday angle was studied. For the periodic grating case, we have shown that the maximum rotation angle is achieved when the surface plasmon wave number coincides with one of the wave numbers of the inverse lattice. Enhancement of the Faraday angle at plasmonic band edges is predicted. In the case of a random surface profile, it is shown that the diffusion of surface magnetoplasmons gives a dominant contribution to the Faraday rotation. Comparison with experiments is carried out.

We show that when an electron or photon propagates in a cylindrically symmetric waveguide, it experiences both a Zitterbewegung effect and a spin-orbit interaction leading to identical propagation dynamics for both particles. Applying a unified perturbative approach to both particles simultaneously, we find that to first order in perturbation theory, their Hamiltonians each contain identical Darwin (Zitterbewegung) and spin-orbit terms, resulting in the unification of their dynamics. The presence of the Zitterbewegung effect may be interpreted physically as the delocalization of the electron on the scale of its Compton wavelength, or the delocalization of the photon on the scale of its wavelength in the waveguide. The presence of the spin-orbit interaction leads to the prediction of several rotational effects: the spatial or time evolution of either particle's spin or polarization vector is controlled by the sign of its orbital angular momentum quantum number or, conversely, its spatial wave function is controlled by its spin angular momentum.

We predict that the collective excitations of an atomic array become entangled with the light of a high-finesse cavity mode when they are suitably coupled. This entanglement is of the Einstein-Podolsky-Rosen type, is robust against cavity losses, and is a stationary property of the coupled system. It is generated when the atomic array is aligned along the cavity axis and driven transversally by a laser, when coherent scattering of photons into the cavity mode is suppressed because of phase mismatching. We identify the parameter regimes under which entanglement is found and show that these are compatible with existing experimental setups.

It is known that, at optical frequencies, a simple interface between a perfect dielectric and a real metal can sustain the propagation of surface plasmon polaritons only for P-polarized electromagnetic waves, being S-polarized surface plasmons are prohibited. In this work, we formally show that, strictly speaking, both polarization states are in fact prohibited and that only P-polarized pseudosurface waves are allowed, which is what is encountered in the applications. The existence of such pseudosurface modes allows one to reconcile theory and experimental evidence, but also sets limits for them to be considered as modes bound to the interface.

The uncertainty principle can be expressed in entropic terms, also taking into account the role of entanglement in reducing uncertainty. The information exclusion principle bounds instead the correlations that can exist between the outcomes of incompatible measurements on one physical system, and a second reference system. We provide a more stringent formulation of both the uncertainty principle and the information exclusion principle, with direct applications for, e.g., the security analysis of quantum key distribution, entanglement estimation, and quantum communication. We also highlight a fundamental distinction between the complementarity of observables in terms of uncertainty and in terms of information.

We study the radial Schrödinger equation for a particle of mass $m$ in the field of the inverse-square potential $\alpha /r^2$ in the medium–weak-coupling region, i.e., with $-1/4\le 2m\alpha /{\hslash }^2\le 3/4$. By using the renormalization method of Beane et al., with two regularization potentials, a spherical square well and a spherical $\delta$ shell, we illustrate that the procedure of renormalization is independent of the choice of the regularization counterterm. We show that, in the aforementioned range of the coupling constant $\alpha$, there exists at most one bound state, in complete agreement with the method of self-adjoint extensions. We explicitly show that this bound state is due to the attractive square-well and delta-function counterterms present in the renormalization scheme. Our result for $2m\alpha /{\hslash }^2=-1/4$ is in contradiction with some results in the literature.

In 2003, Leggett introduced his model of crypto-nonlocality based on considerations on the reality of photon polarization [A. J. Leggett, Found. Phys. 33, 1469 (2003)]. In this paper, we prove that, contrary to hints in subsequent literature, crypto-nonlocality does not follow naturally from the postulate that polarization is a realistic variable. More explicitly, consider physical theories where (a) faster-than-light communication is impossible, (b) all physical photon states have a definite polarization, and (c) given two separate photons, if we measure one of them and post-select on the result, the measurement statistics of the remaining system correspond to a photon state. We show that the outcomes of any two-photon polarization experiment in these theories must follow the statistics generated by measuring a separable two-qubit quantum state. Consequently, in such experiments any instance of entanglement detection—and not necessarily a Leggett inequality violation—can be regarded as a refutation of this class of theories.

We study the interconnection among steering, uncertainty, and quantum correlation. The practical steering criteria for bipartite quantum entanglement are proposed using the multiple applications of the Maassen-Uffink uncertainty relation. The results show that distant steering causes less uncertainty than that in any local-hidden-variable theories. The reduction of uncertainty in the quantum steering of entangled states leads to quantum correlation. The steering visibility for the mixed states is also discussed. The experimental results on the proposed steering criteria are reported.

A soft-decision decoder for quantum erasure errors with standard depolarizing errors is proposed for concatenated Calderbank-Shor-Steane codes. This decoding is computationally efficient as long as the block size of each concatenation level is sufficiently small. The numerical simulation with a specific quantum code called the $C_4/C_6$ code shows that this decoder can achieve almost optimal performance and the so-called hashing bound for the erasure and depolarizing error model. Since probabilistic-gate errors can be regarded as erasure errors, this decoder is useful for fault-tolerant quantum computation with probabilistic gates. To demonstrate this, we perform numerical simulation with the $C_4/C_6$ code. Consequently, it turns out that the error probability of a logical controlled-not gate with probabilistic physical two-qubit gates is improved by three orders of magnitude by the present decoder compared to a previous hard-decision decoder.