Table of contents

In this paper, a new decoy state scheme for quantum key distribution with parametric down-conversion source is proposed. We use both three-intensity decoy states and their triggered and nontriggered components to estimate the fraction of single-photon counts and the quantum bit-error rate of single-photon pulses, and then deduce a more accurate value of the key generation rate. The final key rate over transmission distance is simulated, which shows that we can obtain a higher key rate than that of the existing methods, including our own earlier work.

We calculate the energy and the condensate fraction of a system of trapped bosons interacting via a short-range two-body potential with positive scattering length. The potential is attractive and has a two-body bound state. When the scattering length is small compared to the trap length the system is model independent: all potential models —attractive, repulsive and zero-range— provide similar results. When the scattering length is large the attractive model differs qualitatively from the repulsive and zero-range models. In this regime the system with attractive potential becomes independent of the scattering length, with both the energy and the condensate fraction converging towards finite constants.

We investigate the connection between quasi-classical (pointer) states and generalized coherent states (GCSs) within an algebraic approach to Markovian quantum systems (including bosons, spins, and fermions). We establish conditions for the GCS set to become most robust by relating the rate of purity loss to an invariant measure of uncertainty derived from quantum Fisher information. We find that, for damped bosonic modes, the stability of canonical coherent states is confirmed in a variety of scenarios, while for systems described by (compact) Lie algebras, stringent symmetry constraints must be obeyed for the GCS set to be preferred. The relationship between GCSs, minimum-uncertainty states, and decoherence-free subspaces is also elucidated.

For an overdamped particle evolving in a potential and submitted to colored noise, we study numerically the problems of first passage time at the top of the potential and of stochastic resonance. In the limit where the correlation time of the noise is small compared to the other characteristic times of the problem we show that properties of the solution related to long time behavior are controlled by the zero-frequency component of the noise.

It is shown that "negative heat capacity" in nanoclusters is an artifact of applying equilibrium thermodynamic formalism on a "small" system trapped in a metastable state differing from true thermodynamic equilibrium. Trapping occurs due to high-energy barriers separating different regions of the cluster phase space, particularly for cluster sizes near the magic numbers corresponding to closed geometrical shells. Trapping may occur in either the canonical or microcanonical ensemble, but it is unavoidable in the microcanonical and can lead to a dependence on initial conditions of the determined cluster properties.

Measurements in a turbulent round jet at a Taylor microscale Reynolds number R_λ of about 500 indicate that the mixed structure functions of u, the longitudinal velocity fluctuation, and v, the transverse velocity fluctuation, show a larger departure from the Kolmogorov scaling than the u structure functions, and a smaller departure than the v structure functions. Although the scaling exponents of the temperature structure functions are nearly equal to those of the v structure functions at a comparable order, the mixed structure functions of u and the temperature fluctuation θ exhibits a smaller departure from the Kolmogorov scaling than either u or θ structure functions at the same order. The difference between the scaling behaviour of these two mixed structure functions mainly reflects the difference in correlation between (∂u/∂x)² and either (∂v/∂x)² or (∂θ/∂x)², implying that there is an important difference between temperature and velocity fields.

We develop a framework for the interpretation of single-molecule (SM) spectroscopy experiments of probe dynamics in a complex glass-forming system. Specifically, from molecular dynamics simulations of a single probe molecule in a coarse-grained model of a polymer melt, we show the emergence of sudden large angular reorientations (SLARs) of the SM as the mode coupling critical temperature is closely approached. The large angular jumps are intimately related to meta-basin transitions in the potential energy landscape of the investigated system and cause the appearance of stretched exponential relaxations of various rotational observables, reported in the SM literature as dynamic heterogeneity. We show that one can determine parameters predicted by the mode coupling theory from SM trajectory analysis and check the validity of the time temperature superposition principle.

Nanotube alignment has been found to be one of the key parameters to explain the mechanical properties of carbon nanotube fibres. However, the question of the homogeneity of the alignment across the fibre diameter is still open. This letter reports on the first microdiffraction study of the alignment of nanotubes across a fibre, the diameter of which is about 20 μm. We show that the high flux of synchrotron radiation makes the analysis of nanotube alignment from the skin to the core of the fibre possible. The present result opens the way for further micrometer scale analyses of nanotube-based materials by X-ray scattering.

The heavy-fermion metal CePd_1−xRh_x evolves from ferromagnetism at x=0 to a non-magnetic state at some critical concentration x_c. Utilizing the quasiparticle picture and the concept of fermion condensation quantum phase transition (FCQPT), we address the question about non-Fermi liquid (NFL) behavior of ferromagnet CePd_1−xRh_x and show that it coincides with that of both antiferromagnet YbRh₂(Si_0.95Ge_0.05)₂ and paramagnets CeRu₂Si₂ and CeNi₂Ge₂. We conclude that the NFL behavior being independent of the peculiarities of specific alloy, is universal, while numerous quantum critical points assumed to be responsible for the NFL behavior of different HF metals can be well reduced to the only quantum critical point related to FCQPT.

Self-organized chains and stripes of silver nanoparticles have been elaborated by ion-beam sputtering shadow deposition onto faceted alumina substrates. We show that the in-plane organization of the silver nanostructures can be controlled through the grazing-incidence conditions (angle and orientation of the atomic beam with respect to the nanostructured surface). Their optical properties are dominated by a surface-plasmon resonance whose spectral position depends on the polarization of the incident light (parallel or perpendicular to the facets of the alumina template) and that can be attributed to a strong electromagnetic coupling between individual nanoparticles.

We introduce a new type of Gutzwiller variational wave function for correlated electrons coupled to phonons, able to treat on equal footing electronic and lattice degrees of freedom. We benchmark the wave function in the infinite-U Hubbard-Holstein model away from half-filling on a Bethe lattice, where we can directly compare with exact results by the Dynamical Mean-Field Theory. For this model, we find that variational results agree perfectly well with the exact ones. In particular the wave function correctly describes the crossover to a heavy polaron gas upon increasing the electron-phonon coupling.

We propose a design for a one-dimensional quantum box device where the charge fluctuations are described by an anisotropic two-channel Kondo model. The device consists of a quantum box in the Coulomb blockade regime, weakly coupled to a quantum wire by a single-mode point contact. The electron correlations in the wire produce strong backscattering at the contact, significantly increasing the Kondo temperature as compared to the case of non-interacting electrons. By employing boundary conformal field theory techniques we show that the differential capacitance of the box exhibits manifest two-channel Kondo scaling with temperature and gate voltage, uncontaminated by the one-dimensional electron correlations. We discuss the prospect to experimentally access the Kondo regime with this type of device.

We present a theory and experiment demonstrating the optical readout of charge and spin in a single InAs/GaAs self-assembled quantum dot. By applying a magnetic field we create the filling-factor–2 quantum Hall singlet phase of the charged exciton. Increasing or decreasing the magnetic field leads to electronic spin-flip transitions and increasing spin polarization. The increasing total spin of electrons appears as a manifold of closely spaced emission lines, while spin flips appear as discontinuities of the emission lines. The number of multiplets and discontinuities measures the number of carriers and their spin. We present a complete analysis of the emission spectrum of a single quantum dot with N=4 electrons and a single hole, calculated and measured in magnetic fields up to 23 tesla.

A magnetic inclusion inside a superconductor gives rise to a fascinating complex of vortex loops. Our calculations, done in the framework of the Ginzburg-Landau theory, reveal that loops always nucleate in triplets around the magnetic core. In a mesoscopic superconducting sphere, the final superconducting state is characterized by those confined vortex loops and the ones that eventually spring to the surface of the sphere, evolving into vortex pairs piercing through the sample surface.

We report results of dc magnetization measurements in the intermetallic compound MnSi, highlighting a magnetic-field–induced metamagnetic transition and thermomagnetic history effects. The metamagnetic transition is found to be irreversible in nature; in the field reducing cycle the sample does not go back to the original zero-field cooled magnetic state. We interpret this result in terms of kinetic arrest of the reverse phase transition from the field-induced higher-field magnetic state, and draw analogy with the similar phenomenon observed recently in various classes of magnetic materials including CMR manganites.

We investigated the magnetic properties of the Pr_0.5−xNd_xSr_0.5MnO₃ (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) system. Since the two end samples Pr_0.5Sr_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ are A-type and CE-type AFM at low temperatures, respectively, due to their different orbital ordering d_(x²−y²) and d_(3x²−r²)/d_(3y²−r²), the magnetic structure of Pr_0.5−xNd_xSr_0.5MnO₃ is expected to be a mixture of A- and CE-type AFM. Our experiment results show that T_C remains almost constant, while T_N decreases dramatically as Pr³⁺ ions are replaced by Nd³⁺ ions. We suggest that this originates from the orbital ordering instability of the different magnetic structure type rather than from the ionic radius at the A-site.

The field dependence of the magnetic entropy change of ferromagnetic lanthanide-based materials has been studied. The recently proposed master curve for the field dependence of the magnetocaloric effect of Fe-based amorphous alloys can also be constructed for these lanthanide-based crystalline materials, suggesting a universal behavior. The exponent n controlling the field dependence of the magnetic entropy change can be used for the interpretation of results in the case of multiple magnetic ordering phenomena.

We propose a topological quantum phase transition for quantum states with different Berry phases in hole-doped III-V semiconductor quantum wells with bulk and structure inversion asymmetry. The Berry phase of the occupied Bloch states can be characteristic of topological metallic states. It is found that the adjustment of the thickness of the quantum well may cause a transition of the Berry phase in a two-dimensional hole gas. Correspondingly, the jump of the spin Hall conductivity accompanies the change of the Berry phase. This property is robust against the impurity potentials in the system. Experimental detection of this topological quantum phase transition is discussed.

We measure the current-voltage (J-V) characteristics of organic composites as a function of carbon nanotubes concentration dispersed in a poly-3-hexilthiophene (P3HT) matrix. From a drift-diffusion-space-charge model and adapting the general effective medium and classical percolation theories, we quantify the system transport features. We find a drastic increase of the injection current due to drain channels provided by the nanotubes, probably a universal mechanism for charge injection in such type of system. We identify a percolation transition (with t=0.3, the lowest critical exponent so far reported in the literature) and a fractal structure for transport after the percolation. This nearly 1D structure is surprising since the composites do not have any peculiar orientation along some preferential direction. Supported by transmission electron microscopy we explain the fractal behavior in terms of the morphology of the conductivity paths.

Multiple Zhang-Rice type spectral features have been observed in resonant inelastic X-ray scattering (RIXS) from the quasi–one-dimensional cuprate charge transfer insulator Li₂CuO₂. The first feature appears at constant emission energy, and is associated with a Zhang-Rice singlet final state. The second is an interplaquette charge transfer excitation that results in a novel triplet Zhang-Rice–type final state. It is accompanied by the presence of a O 2p nonbonding to upper Hubbard band excitation at an energy close to that of a calculated triplet charge transfer Zhang-Rice–type excitation. The site selectivity and polarization rules associated with RIXS allows these two excitations to be distinguished.

Robustness against noise is characteristic for biological networks of a living cell. Engineering of artificial noise-robust networks is important for various industrial and logistic applications. Here, flow distribution (pipeline) networks, representing a simplification of biological signal transduction systems or a toy model of logistic transportation systems, are investigated. By running evolutionary optimization, networks having prescribed output patterns and robust against structural noise are constructed. Statistical properties of such networks, including their motif distributions, are determined and discussed.

We predict the nature (attractive or repulsive) and range (exponentially screened or long-range power law) of the electrostatic interactions of oppositely charged, planar plates as a function of the salt concentration and surface charge densities (whose absolute magnitudes are not necessarily equal). An analytical expression for the crossover between attractive and repulsive pressure is obtained as a function of the salt concentration. This condition reduces to the high-salt limit of Parsegian and Gingell where the interaction is exponentially screened and to the zero salt limit of Lau and Pincus in which the important length scales are the inter-plate separation and the Gouy-Chapman length. In the regime of low salt and high surface charges we predict —for any ratio of the charges on the surfaces— that the attractive pressure is long-ranged as a function of the spacing. The attractive pressure is related to the decrease in counter-ion concentration as the inter-plate distance is decreased. Our theory predicts several scaling regimes with different scaling expressions for the pressure as a function of salinity and surface charge densities. The pressure predictions can be related to surface force experiments of oppositely charged surfaces that are prepared by coating one of the mica surfaces with an oppositely charged polyelectrolyte.

We present a simple one-parameter model for spatially localised evolving agents competing for spatially localised resources. The model considers selling agents able to evolve their pricing strategy in competition for a fixed market. Despite its simplicity, the model displays extraordinarily rich behaviour. In addition to "cheap" sellers pricing to cover their costs, "expensive" sellers spontaneously appear to exploit short-term favourable situations. These expensive sellers "speciate" into discrete price bands. As well as variety in pricing strategy, the "cheap" sellers evolve a strongly correlated spatial structure, which in turn creates niches for their expensive competitors. Thus an entire ecosystem of coexisting, discrete, symmetry-breaking strategies arises.

We present a study of the application of a variant of a recently introduced heuristic algorithm for the optimization of transport routes on complex networks to the problem of finding the optimal routes of communication between nodes on wireless networks. Our algorithm iteratively balances network traffic by minimizing the maximum node betweenness on the network. The variant we consider specifically accounts for the broadcast restrictions imposed by wireless communication by using a different betweenness measure. We compare the performance of our algorithm to three other known algorithms and find that our algorithm achieves the highest transport capacity both for minimum-node-degree geometric networks, which are directed geometric networks that model wireless communication networks, and for configuration model networks that are uncorrelated scale-free networks.